Coax Trap dipole for 12m/17m/30m/40m/80m — 30 April 2021

Coax Trap dipole for 12m/17m/30m/40m/80m

This story should begin with “when I was young” 🙂

Well, about 20 years ago, when I lived in a building with a small roof and I couldn’t install log wire antennas, I decided to build a coax trap dipole for HF bands, based on a project reported in a well known italiank book named “Costruiamo le Antenne Filari” (let’s build wire antenna) by Nerio Neri I4NE and Rinaldo Briatta I1UW.

I was attending high school and I was at the firs experience with my hamradio activity, so I didn’t have a grid dip meter, which was highly reccomended also by the authors to verfy the exact trap resonance.

I wanted to build this antenna anyway, so I tried to replicate exactly the project respecting every measure with millimetric precision, with the hope to do only little adjustment on the dipole arms lenght. But everyone who have worked with RF, knows that in practice it’s very difficult to do a trap dipole with a SWR meter only!

I built the antenna using 2,5mmq electric cable, a 1:1 balun, and built the traps using 40mm diameter plastic pipes for hydraulic systems and RG58 cable.

The coax traps were assembled in the classic configuration of the picture below:

The construction details of the trap were not published for copyright reasons.

Since the plastic pipe was not too strong, I decided to insert a small piece of 16mm pvc pipe inside the trap, and to fix the dipole to this pipe, avoiding the stress or break of the trap (see pictures later in the article).

I installed the dipole on top of a 10m pole and kept 120 degrees as arms aperture. As usual, with trap dipole the tuning should start from the higher band, which corresponds to the lenght between the traps closest to the feeding point, and then go on to the lower bands. I spent a lot of time searching for the first resonance, without any success. With much disappointment, I uninstalled the dipole and left it in a bag in my garage.

When I moved to my actual home, I went in the old garage and… I saw this old project and I remembered how much time I spent trying to let it work! But in the meantime I gained theoretical and practical experience. I don’t have a grid dip meter, but I have a NanoVNA, sto…I decided to begin another battle with this dipole!

I mad some serarch on the internet and I decided to try the following setup for searching the trap resonance frequency:

The coax trap act as parallel resonance circuit, so it looks like an open circuit at resonance frequency, while its impedance decreases at other frequencies. If the VNA channel is loaded on a 50 Ohm load and the trap is connected in parallel to this load, the return loss should show a minimum (optimal) at resonance frequency.

I did the setup as reported in the figure, with SMA to BNC transition, a BNC “T” connector and a BNC 50Ohm load. I soldered the trap terminal to a BNC male connector, and I put this connector to the free terminal of the “T” connector.

I measured all the trap resonance and I immediately understood why the dipole was not working. The 24MHz trap was resonating at about 28MHz!! All the other traps where resonating quite near the expected frequencies, but the 24MHz was completely wrong.

After different tests, I came to the conclusion that I misunderstood (or it was not clear in the book) how to wrap the trap. The plastic pipe lenght is actually too long compared to the coil, if it’s wrapped with no spacing between the turns. So I gradually started to add space between the turns and the resonance started to decrease rapidly. At the end, I reached 24MHz with all the turns spaced to the entire plastic tube, as shown below.

Then I slightly tuned all the other traps, adding some more spacing between turns and fixing the trap with plastic tape at the end. For the 24MHz trap, I decided to add some bi-component epossidic glue for fixing the turns, because the spacing was critical.

I was happy and almost sure to have found the reason the missing resonance when I was a pretty new OM, so I installed the new dipole and started to analize it with my VNA.

As soon as I did the first scan 0-30MHz with my VNA, I saw the first resonance at 24MHz! I just shortened some cm every arm, because I added some centimeters (it’s easy to cut, but not so easy to add lenght…). I was very happy for this first comfroting result which I never had during first experiments, and this told me that I was on the right way because the traps were really “isolating” the arms after the traps themselves.

Although I respected the lenght declared on my book, the resonance at 18MHz was too high, so I had to extend the wires between the 1st and the 2nd trap in order to move the resonance in the range 18.098-18.118MHz.

I had to spend a lot of time an patience to tune and optimize all the resonances, but at the end, I got good VSWR an all the bands except 40meters, on which I never get less than 1.8:1 . I tried to slightly change the spacing between the turns of the trap, but I never got values better than 1.8:1.

Here below, I attach all the measurements done with my NanoVNA on all the design bands.

In the center of the grpahs, there is a windows which shows the results of the bandwidth analysis.

Another behaviour that can be observed, is that the bandwidth drops with frequency. This probabily happens because at lower bands, the RF pass through more traps, and the “Q” factor of the traps affects the dipole bandwidth.

Here below, some picture attached.

In the picture below, I reported the original project lenghts and my experimental lenghts written by hand.

I also write some conclusions here below:

  • A 1:1 balun is always advisable. I used a commercial ferrite balun.
  • The project is quite “time-consuming” and need a lot of patience for the tuning. If you have not enough time and you need something “plug&play”, you should be better to buy a commercial trap dipole.
  • Considering the previuous point, I understand why the multiband trap dipole are quite expensive compared to the material costs 🙂
  • Always leave more wire length than the project. I suggest 30cm minimum. It’s easy to cut but difficult to add wire (I had to solder the wire and cover the soldering joint with heat-shrink tube)
  • During the tuning, always care about the simmetry between the lenght of the two arms
  • Last but not the least: the resonance of the traps shall be verified in lab before dipole installation!

73’s

Alfredo IZ7BOJ

Measurement of a NOOELEC FM bandstop filter with NanoVNA — 21 March 2021

Measurement of a NOOELEC FM bandstop filter with NanoVNA

I recently bought NanoVNA. It was a great choice!

This tiny device measures everything that an OM needs for his experiments and there is a ton of help on the NanoVNA forums and YouTube.

I got a lot of controversial feedbacks on the instrument, especially when used at frequency higher that UHF (theoretically the instrument reaches 3GHz..) so I decided to do a simple test with a 2 ports measurement.

Please note that the following tests don’t claim to be a complete characterization of NanoVNA. I simply write some results after some simple test.

I tested a Nooelec notch filter for FM broadcasst (88-108MHz) and compared the results with the Nooelec measurements. Nooelec made S parameters csv files available on the website, from DC to 200MHz and from DC to 1200MHz. I appreciated this files very much, because it’s unusual and I always find pdf plots.

I accurately calibrated the VNA with the calibration kit in bundle.

Important Note: As suggested by NanoVNASaver during the guided calibration procedure, it’s necessary to calibrate the instrument in “stand alone” mode and store the results in one internal memories (I used “Save 0”), for better results! If I skip this step, the S21 calibration becomes really bad at higher frequencies (about 3-4 dB instead of 0dB with the through connection)! I don’t understand very well why this happens, but at the end it’s mandatory to follow the suggestion of calibration assistant.

After the verification of the S21 calibration with the SMA through connector (flat line around 0dB), I started to measure the filter with NanoVNASaver.

With NanoVNASaver it’s possible to export the graph in S2P csv file. This file is in “touchstone” format and data are stored as Real and Imaginary part for every S parameter. I imported this file in excel and then calculated the S21 in dB with the well known formula 20*log[sqrt(Re^2+Im^2)] and I easily got a result comparable to the Nooelec files.

I set the NanoVNA for 1000 points sweep, while the Nooelec includes only 600 points, but Excel can plot different series on the same graph although the “X” axis are different.

The result from DC to 200MHz is here below:

Here below, the results from 0 to 1200MHz, plotted in the same way:

It’s not too bad, considering that the Nooelec measurement is not taken on my filter and one specimen can different from other due to component tolerances. Moreover, as previously explained , the frequency resolution is different between the plots and this could giustify the deeper “hole” in the plot.

In the second figure, a ripple is present in the NOOELEC plot at higher frequencies, so I don’t know if their VNA is properly calibrated.

I’m going to do some other tests up to 3GHz. For the moment, I hope these results could be helpful to those are going to buy ad NanoVNA or a NOOELEC Filter.

73’s de Alfredo IZ7BOJ

Building the Lightweight V/U Yagi kit by Tino’S Funkshop — 24 January 2021

Building the Lightweight V/U Yagi kit by Tino’S Funkshop

I wanted to build a lightweight yagi for portable operation with my SATNOGS Az/El rotator. I surfed on the web and I found the reference site for yagi builders: DK7ZB. In the 2m/70cm-Yagis ultralight session, I found a lot of pictures of very interesting home-made yagis. I discovered that some pieces like central dipole insulator and elements supports are available on Tino’s Funkshop online store (former NUXOCOM.de) and I also discovered that, besides single yagi parts, also complete antenna kits are available.

The material for lightweight yagis is very common, so I tried to compare the price of the Tino’s Kit for 2m/70cm 13 elements Lightweight Yagi kit with the price of the same parts available on my local or online shops. I made a Bill of Material in excel with prices and… the Kit wins! Moreover, I would spent much less time for collecting all the parts and I would have the custom central dipole insulator, which is the most difficult part in my opinion.

I ordered the kit for only 21,49€ (!), payed with PayPal and I got it in a week . The shipment expenses outside Germany are about 15€ and the courier is DHL. In case of small purchases, the shipment is not negligible. Anyway, The packaging was perfect and tha materials inside were unharmed. The kit content is shown below:

The kit is composed by the following parts:

  • PVC tubes for Boom, 20mm diameter
  • 20mm PVC Clips, for elements support
  • 3,2mm aluminum rods, for all elements except radiator
  • 4mm aliminum rod, for radiator element
  • 7.5 x 4.5 x 4 cm PVC box, for feeding point
  • N panel connector
  • Small piece of 16mm PVC tube for RF choke
  • Small Piece of RG188 cable for RF choke
  • 2 metal lugs to be soldered to RG188
  • brass terminal strip with screws, 4mm inner diameter
  • All needed screws
  • Detailed instructions and datasheet

I’m going to describe main steps for building the antenna

  1. Cut all the elements to the lenght indicated on the datasheet. Be precise at mm! After the cut, remove the aluminum burrs using a flat lime.

Since the VHF reflector is longer than 1meter and the aluminum rods are 1m long, it’s necessary to lengthen it with one rest of piece from other reflectors and using the mammut connector, as the image below:

2. Drill holes in the PVC clips using 3,2mm tip. You can also try to use a 3mm tip and enlarge the hole moving slighthly the drill back and forth. In any case, you can use some glue if the element will not stay in its position. I used a stand for the drill, in order to to perpendicular holes. Anyway, don’t care if you don’t have a stand and the holes are not perfectly parallel to the clip edge, you can do small rotation once the clip is mounted on the boom.

3. Let’s insert the elements in the clamps and mark the element name in order to make the positioning easier.

4. Now, let’s proceed to the most particular part, the feeding point of the dipole.

First of all, the central hole of the insulator shall be enlarged with a 4mm tip:

Then, 2 holes of 4mm diameter shall be made on the sides of the PVC box. The caliper helped me for calculating the exact position. The, the two half dipoles can be pushed inside the insulator, passing through the box walls:

4. Now, two holes of 2,5mm diameter shall be made in the aluminum, using the insulator as guide. Don’t push too much the drill, otherwise the insulator will be ruined!

Now, the two metal lugs can be fixed on the dipole halves by two 2,9x9mm screws:

5. Now le’ts install the N socket connector to the other short side of the box. The connector has two flat notches in the threaded part in order to avoid rotations, so I suggest to drill a 11mm hole first, than work with patience and files (flat and round) as long as the connector fits inside the hole.

6. Now it’s the time to build the RF choke. Let’s drill two 3mm holes in the 16mm PVC tube and do at least 5 windings of coax cable. In my case, I did 6 windings.

Now we have to solder the coax on the dipole lugs and to the N connector. Be patient, because there is not too space inside the box. Nose pliers help in this case. Keep the unshielded part of the cable as short as possible. No matter which conductor you solder on left and right side.

7. Now let’s mark the position of the elements on the boom

This is the final result. The antenna is laying on the table of my Lunchroom. Not too bad 🙂

Since the antenna has an insulating boom, also the supporting tube should be not conductive. I had a 40mm PVC pipe in my garage, so I built this simple 90 degrees support using a small piece of plexiglass, a couple of clips for 20mm tube and a couple of clips for 40mm tube:

Then I started the first tests of my antenna. I put the antenna in my garage (it would be better to work in open space..):

Then I started my measurements with NANOVNA V2 and nanovna-saver software installed on my PC:

The first check, using default design dimensions, showed this situation for 2m and 70cm:

As you can see, it’s not too worth for the beginning, so it means that I did a good job.

For both bands, I placed three markers: red at the beginning of ham band, green at center and blue at the end. The span is greater than the usable bandwidth in order to have a better understanding of the antenna behaviour.

As suggested on the manual, I started to tune the VHF resonance, so I started to cut 2 or 3 mm at time on both ends of VHF radiator. The resonance vary quickly with the lenghts, so be patient and don’t exceed with the cut lenghts!

At the end of the tuning, I got 980mm. The starting design lenghts was 995, so I cut 15mm. The resonance is shown below. It’s not perfectly centered at 145, but the SWR is <1:1.3 across the whole 2m band, so it’s ok for me:

As suggested by Nuxocom, I started to move the Open Sleeve radiator of 70cm for UHF tuning. The open sleeve is the passive element which is very near to the 2m radiator, and it’s a passively coupled radiator excited by the 2m radiator. I didn’t lower the resonance as much as I wanted in this way, so I started to cut the open sleeve (it’s allowed by nuxocom). At the end of tuning, the open sleeve was 320mm. The desing length was 329mm. The final situation is shown below:

The VSWR response is very flat and it’s <1.3:1 anyway. Good!

Now I’m waiting for real tests/comparison in open air, but i have to wait for better weather condition to go on my roof.

Concluding, the kit is highly suggested for OMs that have a minimum of manual skills and like to build antennas, but at the same time don’t have too much time for collecting material and want to start from a well tested configuration in order to have guarantee of success!

The kit is very complete and nothing else is necessary to build the antenna. The quality of the materials is very high and the price is very cheap compared to local shops.

Building instrunctions are also very detailed and easy to understand.

Everyone can customize the antenna and follow different solutions, but the suggestions, in my opinion, are the best way for good and quick results.

The only modification that I would consider, is to replace the steel screw and nut at the center of the dipole insulator (not visible in the previous pictures) with a nylon equivalent, since the screw is very near to the radiator parts (the center gap is only 10mm).

I didn’t have a 4mm nylon screw during the assembly, otherwise I would have changed it.

I hope this could help next OMs that want to build a twinband yagi and would consider a kit.

73′

Alfredo IZ7BOJ.

Refurbishment of an old TONNA F9FT Antenna — 9 January 2021

Refurbishment of an old TONNA F9FT Antenna

In the early 2000’s I bought an F9FT 21 elements antenna. I used it during the golden age of packet radio and I enjoyed the beautiful performances of this yagi for establishing links across long distances. The datasheet and manual can be found here below.

When I moved out of my city for work, I uninstalled the antenna from the roof and I stored it without any maintenance. Recently, I needed this antenna for experimenting a link with New Packet Radio. I recovered the antenna from my garage and I found it in very bad conditions. Consider that the antenna was installed very near to the coast, so the sea salt increased the corrosion.

As you can see, the aluminum parts were full of oxide and the feeding point of the folded dipole was completely broken. I didn’t use more than 25Watts, so I think the plastic was broken due to atmospheric and thermal stress. The inox screws are still perfect, but the clamps are rusty.

I have been tempted to buy a new antenna, but the OM should be patient, should save money and love repairing so… let’s repair!

I started from the more boring job: cleaning the aluminum parts. I decided to avoid acids and use only WD-40 and an inox sponge.

It took about 2 hours but the results are good.

Then, I cleaned all the isolators with a brush and Chanteclair. I also cleaned the inox screws with WD-40:

Now it’s the turn of clamps. I used the circular metallic brush mounted on the drill, and a bench vise to hold the pieces during the operations and work in safety. Here below the pieces before the cleaning

And after the cure:

The final step is the painting with spray Zinc. I used the following, a good quality one:

The clamps looks like new after painting. I preferred to pur 2 coats of paint for a better results.

The last but very important operation is the restoration of the feeding point. I found the complete and original TONNA dipole as replacement part, but it’s not very cheap (about 30$ + shipment expenses). So I decided to build it by myself. I decided to use an ABS 10cmx10cm electric box, an SO-239 flange connector (I didn’t have N type in my drawers, but it would have been better…) and two brass electric terminals for contacting the aluminum elements, which are not weldable. The pictures below explains better my idea.

I installed and tested my antenna and it works perfectly. The ROS is 1.2:1 at 431 MHz. The resonance frequency is slightly shifted down compared to the design (432MHz), but the bandwidth is still good.

Here below some pictures of the installation.

73’s

Alfredo IZ7BOJ

APRS spam alarm by telegram notifications — 8 December 2020

APRS spam alarm by telegram notifications

In this article I described how all the aprs stations of my region are monitored on a dashaboard and how all the sysops are informed about downtimes of the Igate/digipeaters.

Here below, I’m going to describe another step for the local aprs situation awareness. Sometimes the sysops see stations transmitting a lot of unuseful beacons with excessive rate, both fixed or mobile ones. Wrong beacon intervals can congest the monitor frequency and increase packet collisions, so this issue is something we’re contiuously fighting woth. When one or more TX I-gates are part of the network, particular care must be taken for the smartphone apps like aprsdroid which sends a lot of beacon on the internet (APRS-IS) and then transmitted in RF by I-gates. More and more popular platform like MMDVM, PI-Star, DMR and C4FM contribute to the 144.800 congestion if they are not properly managed or filtered.

Most of the times, if the abnormal beacons are permanent, local sysops shall get in touch with these operators and explain why they’re doing spam and how to fix it, because they normally don’know very well how the APRS works and they don’t know how their packets are transmitted.

In order to identify as soon as possible these situations, I started to think to an automatic message system, based on telegram and similar to the vitality messages described in this article.

The requisites of the oftware are:

  1. Read logs of my aprx server
  2. Periodically check for spam stations in the log
  3. The spam rate shall be adjustable
  4. The check period shall be adjustable
  5. The notifications shall be sent by telegram

The solution was a simple python script based on telegram-send. The script is available on my github repository.

The script performs the following operations:
1) open aprx rf log file and takes the last ‘cycletime’ time interval;
2) for every stations heard, counts the number of beacons transmitted (excluding digipeated packets);
3) for every stations heard calculates the mean tx rate as: heard beacons / interval time;
4) if the rate is > ratelimit, the script sends telegram notification.

Note: the scripts calculates a mean beacon rate over the interval. Sometimes it can underestimates the rate, but I wanted to intercept only serious spam situations.

Installation and configuration

Dependencies: before using this script, intall and configure the library “telegram-send ( https://pypi.org/project/telegram-send/ ) and configure the destination of the notifications (single user, group or channel).

sudo pip3 install telegram-send

telegram-send –configure if you want to send to your account
telegram-send –configure-group to send to a group
telegram-send –configure-channel to send to a channel.

In the first part of the script, the following parameters shall be edited:

interface: radio interface (CALLSIGN) declared in aprx.conf
logfile: aprx rf log file (typically ‘/var/log/aprx/aprx-rf.log’)
telegram_conf_file: telegram_send config file (typically ‘/home/pi/.config/telegram-send.conf’)
cycletime: cycletime in seconds
ratelimit: max frames in a cycletime

The script should be executed every ‘cycletime’ by crontab. Here below, a typical crontab config for 10 minutes interval:

*/10 * * * * sudo /home/pi/Applications/aprs_spam_check/aprs_spam_check.py

Note: This script may have bugs and it’s written without all the best programming rules. But it works for me.

Here below, an example of spam notification is reported:

The script can be easily adapted to work with other logs, like direwolf or similar.

Any comment/suggestion will be appreciated.

73’s

Alfredo IZ7BOJ

Descrizione del mio setup di stazione mobile — 30 November 2020

Descrizione del mio setup di stazione mobile

Viaggiando molto, mi son trovato spesso a operare in mobile, prevalentemente in V/UHF.
Non volendo “violentare” la mia auto con la perforazione della cappotta e installazione di un’antenna fissa, ho sempre utilizzato antenne a base magnetica.
Se da un lato questo tipo di antenna è pratico da installare e lascia la macchina esteticamente “pulita” quando non è montata, di contro presenta lo svantaggio del passaggio del cavo, che rimane pendente nell’abitacolo ed è di intralcio soprattutto quando non si è soli in auto.
Volendo trovare una soluzione più gradevole e meno precaria, mi son messo a pensare come poter sistemare le cose in maniera un po’ più professionale.
I requisiti della mia stazione mobile sono questi:

  1. Utilizzo di antenna a base magnetica;
  2. Passaggio dei cavi quanto meno possibile penzolante e a vista;
  3. Utilizzo del FT2-D e posizionamento stabile sul cruscotto, con display a vista;
  4. Installazione di un amplificatore lineare V/UHF attivabile tramite switch e installato non a vista;
  5. Ascolto dell’audio in ricezione tramite impianto radio dell’auto;
  6. Sistema “vivavoce” in trasmissione.

Di seguito faccio vedere punto per punto quali soluzione ho adottato.

1) La base magnetica è di diametro 20cm, quindi stabile e sicura anche oltre i 100Km/h. L’antenna che ho sempre utilizzato è la diamond NR770R:

Il cavo è stato tagliato a misura giusto per entrare nell’abitacolo e all’estremità è stato installato un connettore SMA maschio. Più avanti faccio vedere quale soluzione ho adottato per arrivare alla radio.

2) Come si vede in figura, per limitare il penzolamento dei cavi, un coassiale a bassa perdita è stato infilato nel percorso tratteggiato blu, dalla radio fino al montante lato guidatore. In quel punto un connettore SMA femmina rimane esposto, e questo purtroppo è inevitabile.

3) Per l’installazione stabile del FT2-D, ho usato una staffa metallica che si aggrappa alle alette delle bocchette di areazione dell’auto. E’una staffa un po’ particolare che acquistai online un po’ anni fa, ma di cui non riesco a trovare i riferimenti. Mi son sempre trovato molto bene e all’occorrenza si può togliere facilmente, perchè si regge con delle clips elastiche. Allo stesso tempo è stabile e ha sempre retto senza problemi un Kenwood TH-D72 e ora un FT-2D.

4) Il lineare che ho scelto è il seguente.

Ha il vantaggio di avere la selezione automatica della banda V/UHF tramite sensing a radiofrequeza, quindi l’ho installato lato passeggero come dalle immagini, già in posizione ON. L’accensione remota verrà fatta vedere successivamente

5) Per ascoltare l’audio tramite le casse della macchina, ho portato l’uscita audio del FT-2D all’ingresso ausiliario sul frontale della mia autoradio. La qualità ovviamente è ottima se comparata allo speaker della radio, che col rumore dell’auto diventa quasi inutilizzabile

6) Il sistema vivavoce è stata la parte più complessa del sistema. L’attivazione del PTT tramite VOX, sebbene sia disponibile sulla radio, non mi piace molto, anche perchè col rumore dell’auto è soggetta ad attivazioni non volute e la regolazione è delicata. Per avere le mani libere e non incorrere in sanzione (il codice della strada VIETA di guidare con microfoni in mano!) l’unica soluzione era uno switch a levetta di tipo “bistabile”, quindi senza ritorno automatico della posizione, installato quanto più possibile al volante, in modo da togliere la mano destra dal volante solo per un istante. La capsula microfonica ho pensato di installarla vicino al parasole lato guidatore, vicino al montante, in modo che la capsula fosse poco visibile ma allo stesso tempo riuscisse a raccogliere bene la voce. Nelle foto successive, ci sono i particolari della capsula.

Poi ho pensato che la capsula potesse essere utile anche per il vivavoce bluetooth che uso per il cellulare. Ho notato che la capsula microfonica dell’autoradio è di pessima qualità e raccoglie molto rumore e scricchiolii, e l’interlocutore sente molto male. Quindi ho pensato di condividere la stessa capsula microfonica tra vivavoce del FT-2D e vivavoce dell’autoradio, semplicemente aggiungendo uno switch che selezionasse l’uno o l’altro (l’utilizzo in contemporanea è praticamente impossibile).
Di seguto trovate lo schema generale che chiarisce meglio le interconnessioni del sistema.

Per quanto riguarda gli switch di accensione del lineare, del PTT e della selezione del MIC IN, ho installato degli switch in delle scatoline di plastica nera all’interno di un portaoggetti alto 1DIN. La foto di sotto chiarisce i dettagli.

Per agevolare l’attacco e lo stacco dei connettori SMA, che sono molto scomodi da avvitare e anche fragili, ho acquistato questo adattatore: Spectrum 8001-SM21-02 (datasheet qui sotto)

che permette di far diventare l’SMA come un Jack, grazie alle sue alette elastiche interne. Non è proprio a buon mercato, ma è di una comodità incredibile. Lo uso da anni e non dà ancora segni di cedimento, si regge ancora stabilmente. Questo connettore l’ho utilizzato sia sulla radio che all’attacco del coassiale sul montante.

In questo video, si vede quanto è veloce l’attacco e lo stacco:

Di seguito, infine un video che evidenzia come basti meno di un minuto per montare antenna e radio e iniziare a trasmettere:

Sperando di aver dato qualche spunto utile a qualche altro OM, saluto tutti gli OM “mobili” come me 🙂

73′

Alfredo IZ7BOJ

Allarme acustico di sovraccarico elettrico per uso domestico — 21 November 2020

Allarme acustico di sovraccarico elettrico per uso domestico

Chi non ha vissuto l’antipatico black-out dovuto al sovraccarico delle utenze elettriche domestiche? Con l’aumentare degli elettrodomestici di uso quotidiano, i 3Kilowatt del contratto standard sono sempre più insufficienti.

Nel mio caso, la posizione dell’interruttore magnetotermico del nostro distributore di energia non è interno al palazzo, ma è fuori in giardino, nel cortile interno del condominio, quindi in caso di intervento della protezione, anche se si è in ciabatte e tenuta domestica, anche se fuori piove o fa freddo…mi tocca scendere a riarmare l’interruttore, con tutte le maledizioni del caso.

Col ripetersi di questa incombenza, mi son messo a pensare a un possibile rimedio. I requisiti principali dovevano essere:

  1. possibilità di monitorare la corrente assorbita a monte di tutto l’impianto domestico
  2. possibilità di impostare la soglia d’allarme a 3KWatt
  3. allarme acustico

Quest’ultimo punto per me era fondamentale, perchè quasi sempre chi provoca il sovraccarico è ignaro di cosa sta accadendo nel resto della casa (tipico esempio, moglie che accende in forno in cucina, io che accendo il phon nel bagno), quindi era necessario un buzzer che suonasse per avvisare chi era in casa e quindi permettergli di spegnere l’utenza che aveva provocato il sovraccarico stesso.

Mi son messo alla ricerca di un allarme del genere, ma non ho trovato nulla che soddisfasse tutti e 3 i requisiti.

Tuttavia, sui vari Aliexpress, ebay, ecc.. , ho trovato un interessante monitor di tensione e corrente con soglia d’allarme a circa una decina di euro, con la limitazione che l’allarme era solo visivo, ossia la retroilluminazione del display inizia a lampeggiare. Poco utile, se chi è in casa non sta lì a guardare il display. Non disperando, ne ho comprato uno per provare ad aggiungere un allarme acustico.

Ho comprato il modello GSOT5600-100, con le seguenti caratteristiche:

  • monitor dei parametri elettrici: tensione istantanea, corrente istantanea, potenza istantanea, KWh totali consumati, )
  • Funzione di allarme di sovraccarico con soglia regolabile e retroilluminazione lampeggiante
  • Tensione di funzionamento: 80-260Vac
  • Max potenza: 100Ampere / 22KWatt
  • Frequenza di lavoro: 45-65Hz
  • Precisione di misura: Classe 1
  • LCD a cristalli liquidi con retroilluminazione

Questi dispositivi sono disponibili con due tipi di sensing di corrente:

  1. sensore a “clamp” toroidale in cui va fatto passare uno dei due fili della monofase domestica.
  2. shunt di corrente

Tra i due tipi, ho scelto primo, anche se leggermente più costoso, perchè non mi piaceva affatto l’idea di far passare 3Kwatt attraverso la mia “cineseria”. Dal punto di vista della sicurezza, è sicuramente meglio avere la clamp galvanicamente isolata.

Aspetto circa 3 settimane prima che l’accrocchio arrivi dalla Cina e subito lo provo, collegandolo alla rete e facendo qualche test con dei carichi. Dalle prime prove, la precisione non mi sembra male, e il display mi appare subito chiaro e intuitivo. Non male insomma.

Provo anche a impostare una soglia d’allarme a 1KW e a far scattare l’allarme accendendo una stufetta. Il display inizia a lampeggiare di blu come atteso. Perfetto!

Penso che se il display lampeggia, ci deve essere per forza un punto del circuito in cui prelevare questa tensione di comando dei led e riutilizzarla per far suonare un cicalino.

Quindi parto subito ad aprire la scatoletta di plastica e a ispezionare il circuito. noto che il PCB ha due lunghe strisce di contatti passanti che portano tensione al display, dalla faccia opposta del lato componenti.

Faccio scattare l’allarme e inizio a misurare con un multimetro tutti i pin e … in pochi minuti becco subito quello che porta la tensione ai led della retroilluminazione! Su tester vedo una simpatica 5Volts che va e viene allo stesso ritmo del display. OK, sento di essere già a metà dell’opera!

Ora devo solo capire se quella tensione può alimentare un buzzer. Con una lente d’ingrandimento seguo la pista del circuito stampato e vedo che va a finire su un transistor formato SOT23.

Potrebbe reggere l’assorbimento di un cicalino, ma non voglio rischiare e decido di usare quella tensione solo come comando e montare un transistor più robusto su una millefori. La tensione di comando del cicalino vorrei prenderla in un punto a monte dell’alimentazione del PCB, in modo da essere sufficientemente robusta. Con un po’ di intuito, mi metto a cercare le capacità più grosse del circuito, che di solito servono a “spianare” la tensione di alimentazione, e subito trovo una bella 12Volts su un condensatore. Decido che sarà lei ad alimentare il cicalino.

Su un pezzettino di millefori di circa 2cmx2cm monto un 2N222 con una resistenza di 6,8K sulla base. Collego il comando di base al punto del circuito individuato precedentemente e la tensione sul collettore alla 12Volts del condensatore. L’emettitore del transistor al cicalino, come da schema seguente:

Di seguito un’immagine della millefori adagiata sul retro del misuratore di corrente. I fili entrano nel misuratore dopo aver fatto una piccola feritoia nell’involucro di plastica, col taglierino.

Provo il tutto e funziona al primo colpo! Ora non mi resta che inscatolare tutto in una scatolina a tenuta stagna. Posso scegliere anche una soluzione non troppo elegante esteticamente, visto che il tutto sarà installato accanto al quadro elettrico di casa, nello sgabuzzino. Opto per una scatola Gewiss 10cmx10cm opportunamente sagomata. Prevedo anche uno switch per disattivare l’allarme acustico (meglio non svegliare i bimbi quando dormono! Preferisco scendere in giardino a riattivare l’interruttore piuttosto che svegliarli 😛 )

Con una punta da muro da 6mm, inclinando opportunamente il trapano, faccio un foro in modo da portare il toroide di misura nel quadro, come da figura sottostante:

Di seguito alcune altre foto di dettaglio.

E il video dell’allarme in funzione: https://youtu.be/nUMwFz7L6UQ

Sperando che possa essere di ispirazione a qualche altro smanettone, saluto tutti.

Alfredo

A simple Web interface for APRX — 2 August 2019

A simple Web interface for APRX

I use Aprx from many years and I think it’s one of the most complete and flexible APRS Software available. Nevertheless, every time that a sysop wants to check his aprx node, he has to check the log and try to analyze it. Even simple operations, like check if a certain callsign has been heard by the node, could be cumbersome.

In case of monitoring or troubleshooting, I usually download the log file and then import it in excel and then analyze it by filters and other useful functions.

Before Aprx, I used for many years the aprsd daemon, and it implemented a simple but very useful web interface with simple reports. While I was remembering that interface, I thought that a web interface could be useful for aprx too, and I started to google for this kind of software, hoping to find a plug&play solution.
I was very surprised to find something. I tried just a pair of interfaces, then I choosed “aprx simple webstat“, written by Peter, SQ8VPS.

This is a simple statistics and information generator for APRX software, which allows to monitor load, frames, stations details, statistics from selected time window and more.

I quickly downloaded and installed it. I got it working after few minutes, because I already had lighttpd server installed on my raspberry, so just did a quick configurazion of the log file path and other few things.

I started to use it and I liked it very much. I contacted Peter by email, and I congrats with him for his nice work. Peter is a young OM from Poland, he still attends school but he’s very clever and has very good programming skills and know-how about aprs.
He was very surprised and happy that someone was using his interface!
We had a nice opinions exchange and he asked me for new ideas for improving the interface.

Thanks to this renewed enthusiasm, I started to propose the improvements and he started to implement my ideas and releasing new versions.
I always intend hamradio hobby like an opportunity to explore and learn new things, so I decided to take this occasion to learn php.
I already developed something in jsp, but php was a quite new language for me, and it was a challenging activity.
In the meantime, Peter became very busy with school, driving license and other things, so, in the last period, I developed some new features and bug fix by myself and Peter did the “merge” on github.
Hereafter, I breafly explain the main functions of the Software:

  • Polish and English language
  • Possibility to monitor multiple radio interfaces
  • Possibility to show cusom info and custom logo file on the bottom of the page (edit “custom.php” file, which can contain HTML and PHP code)
  • Management of SSnN “State” abbreviations aliases;
  • Counters of:
    – total frames in log
    – frames received by radio
    – frames transmitted on radio
    – frames received from APRS-IS
    – received frames/sec (calculated on last 20 frames)
    – transmitted frames/sec (calculated on last 20 frames)

aprxsw_home1
Figure 1 : homepage – first part with custom.php

 

  • System status table (reads parameters directly from OS):
    – OS version
    – kernel version
    – Aprx version
    – System uptime
    – CPU temperature
    – CPU frequency
  • Aprx config table (read parameters directlty from aprx.conf):
    – Beacon interval
    – APRS-IS server
    – Location coordinates
    – Tx active/not-active
    – Number of radio ports
    – Digipeater active/not-active

 

aprxsw_home2
Figure 2 : Home page – tables view

 

  • Heard station list table (observation period selectable by the user in a drop-down list)
    – Heard Callsign
    – N. of packets heard for callsign
    – Direct link for showing the station on aprs.fi
    – Static/moving indicator
    – Via indicator (digi/direct/digi+direct)
    – Raw packet view for each callsign

aprxsw_stationlist
Figure 3 : home page – station list view


 

  • Station details page:
    – N. of frames totally heard
    – Last position frame heard
    – Station position (lat,long, distance and bearing from your location)
    – Frame comment
    – Frame type (normal/Mic-e)
    – Station symbol
    – Frame digipeating path
    – Transmitting device (device list can be updated by dedicated file. Both new and  legacy Mic-e devices are included)
    – Last status frame heard
    – Status field

aprxsw_details
Figure 4 : station details page


 

  • RAW FRAMES page: log of raw frames received from a specific station

aprxsw_rawframes
Figure 5 : RAW packets page


 

  • Possibility to sort the table in ascending or descending order on each colum.
  • Live monitor page: ajax page for watching ax25 traffic in realtime. Fields are coloured for better readings

aprxsw_live
Figure 6 : AX25 Live traffic page


 

You can see my APRX Simple Webstat interface at work here: http://iz7boj.ampr.org/aprx.html

The github repository with SW installation instruction is here: https://github.com/sq8vps/aprx-simplewebstat

Note1: This is a BETA software. It can contain some bugs and may be written in non-efficient way. Please contact authors if you find any bug.

Note2: Project is free for non-commercial use. You can modify and publish this software, but you have to put an information about original authors.

I hope that aprx users could install and enjoy this interface.

For any further details or questions, don’t hesitate to contact me!

I would like to say thanks to my colleague Peter SQ8VPS, which thought, developed and mantained this beautiful interface.

73’s

Alfreo IZ7BOJ

Python script for Raspberry thermal control and MCP3008 ADC — 5 June 2019

Python script for Raspberry thermal control and MCP3008 ADC

When I built a telemetry system for DMR repeater, I observed high temperatures of the transmitting radio, especially during summer and hot periods of the year.
I realized that a cooling system could be helpful to dissipate the heat produced by Motorola radio.
The acquisition system was already done, so only the actuation system was needed.
I searched on the internet, and I found some scripts used to control a fan installed on the microprocessor of the raspberry Pi.
I took one python script and modified to acquire the temperature of the external sensor connected to the MCP3008.

The original script was here (reads the System CPU temperature) and the customized version is here.

Some notes for the correct script configuration:

  • Script Cycle time must be configured. Default value of 30sec is good for starting;
  • Log file name must be declared.
  • Output pin to be used for fan switch on;
  • Both upper and lower thresholds must be declared, as for every hysterical control;
  • The logging function can be useful during the first tuning period. Once the thresholds have been found, the function can be eliminated;
  • Value, beta and pull-up resistor of NTC must be declared inside the script for a correct temperature conversion. The NTC, in series with the pull-up resistor, must be connected to the the same 5V coming from the Raspberry and powering the MCP3008.

    NTCbias

  • ADC channel connected to the NTC sensor must be specified.
  • Adafruit library for MCP3008 must be installed before running the script.
  • A compensation variable is useful to align the temperature reading to a reference thermometer.

I used two small 12V PC fans, which fits well to the Motorola heatsink:

img_8634.jpg

The Raspberry output pin cannot directly drive the fan, so a small BJT driver is needed. I built one with a 2N2222 NPN transistor and a 1KOhm resistor, 1/4W on the base.
Here below the schematic is reported:

Circuit-using-2N222-NPN-transistor

This is the prototype board of the fan driver:

IMG_8631

The general block diagram is the following:

schema

Once the hardware is ready, we can try to manually switch on pin 25 and see if the fans start, using “GPIO” library from shell:

gpio mode 25 out
gpio write 25 1

The above commands set pin 25 as an output and then sets the pin to a logic 1.

If the fans starts, we can try to configure and launch the script. Remember to give the execution permission (sudo chmod +x MCP3008_raspy_thermostat.py).

In the logfile, we can read the temperature acquired by the NTC and eventually tune the “compensation” variable to align the temperature reading with a reference thermometer.

Then, we can tune the thresholds for best results.

I hope this project could be useful.
73’s
Alfredo IZ7BOJ

SPF5189Z LNA measurements on VNA — 11 April 2019

SPF5189Z LNA measurements on VNA

Since January 2019, I’m sharing a simple satellite receiving station on SATNOGS network, based on a RTL2832 USB SDR dongle and a home-made turnstile antenna.

The RTL2832 Noise Figure is not so good (on the internet I read about 7dB), so one way to increase the receiver performance is to place a LNA as near as possible to the antenna.

I searched on the internet, and I’ve found the LNA4ALL a very good  solution. It also has an internal bias tee in order to be powered directly on the coax cable.

Nevertheless, it’s not very cheap and the shipment expenses are too high, then I decided to try first a SPF1589Z LNA for few euros from ebay, then decide if throw it away or keep it.

foto

It arrived in about three weeks from China, then I wanted to test it on a Vector Network Analyzer, in order to compare the real gain with the datasheet.

Once the LNA has been powered at 5V, the current absorption is 90mA, perfectly aligned with the datasheet range.

Here below, the S21, S11 and S22 plots are reported between 50MHz and 3GHz (the LNA reaches 4GHz but the VNA not).

I palced these 4 markers:

  • 144MHz
  • 435MHz
  • 1GHz
  • 2GHz

S11
Out

S21
Gain 50MHz-3GHz

S22
Output Return Loss 50MHz-3GHz

When I downloaded the datahseet, I saw that the SPF5189Z has two possible configurazion:

  • Optimized matching at 900MHz
  • Optimized matching at 1900MHz

From ebay description and board P/N, it’s not possible to understand the frequency optimization.

Here below, I reported a comparison the datasheet (green line) and the measurement (red line) data, for both configurations. The frequency range of the datasheet is smaller than my measurements, sto the comparison is only on the datasheet frequencies and not wide-band.

900MHz_measured_vs_datasheet1900MHz_measured_vs_datasheet

The gain seems to be 1dB less than the datasheet on both configurations, while the input matching shows better results on 900MHz range.

At the end, the LNA is quite wide-band and is a compromise between the 900MHz and 1900MHz configurations.

In the next days, I will install the LNA near the antenna and I’ll let you know the improvements on the receiver.

Best 73′

Alfredo IZ7BOJ