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!