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!


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 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.


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.


Alfredo IZ7BOJ