Hi ! I’ve started to build some mini-traps with toroid cores, for the 20m and 30m bands.
On what frequency should i tune them respectively ? I’ve read that it should be a bit lower than the band start… But why and how much lower ?
Does anyone have the RADCOM nov 2019 article written by GM3NHQ “Antenna trap design consideration” ? Maybe i can fid an answer there ?
Thanks and 73
Patrick TK5EP
Hi Patrick,
SOTABeams sell traps and also have some useful information on this subject.
73
Roger
Why traps? go full size with jumpers 
I’m from Corsica, so a bit lazy… 
I don’t want to put the pole up and down when on the field !
73 Patrick
I am currently building the SOTABeams Pico traps. I’m going to make one for 30m and one for 20m. The instructions suggest 22 turns for 30m for 10.1 freq, and 17 for 20m for 14 freq. I wound the 30m one first and the SWR drop was too high, so I will need to add a turn. I used the same winding to do the 20m trap, but 17 turns didn’t dip at the right freq, so I will be starting over again. I have an LC meter but don’t know what reading I am looking for since it’s not shown in the SOTABeams instructions.
Dave, AE9Q
I made a set of traps ( Sota Beams ) which worked perfectly electrically ( was hoping to change bands without having to do a walk to change the links )- but even though very light still too heavy to use on a 40/60/80 linked dipole without casuing too much sagging. So back to the links… Paul
Hi Roger and Dave,
I looked on this SOTAbeams document, and indeed is it lacking for details…
Dave, without tuning the trap first, prepare yourself to have countless hours of headaches !
The position of the turns on the coil has a GREAT influence on the trap resonance. Also the provided cap has at best 5%, so you have all the ingredients to wander in the dark !
Here is what i have for my 30m trap using my NanoVNA.
For now, i need to know WHERE to tune it. Some say below the operating frequency, other say above ! 
73 Patrick
OK Paul,
That might be a concern for me as well… My pole is rather thin.
I also hope to reduce the overall length, as on most of our summits, i have very little room to put my antenna.
Maybe i will switch over to a vertical…
73 Patrick
Your timing is excellent… here’s my strategy.
I typically tune them right at the bottom of the band (14.0 and 10.1). If they end up right on the operating frequency, your losses will be slightly greater. If they are too far off, they won’t provide sufficient impedance at the desired frequency and it will de-tune the antenna on that frequency, also resulting in losses.
Adam,
Using one of the hyperlinks in your video, I was able to get the inductance figures I was looking for. https://www.omnicalculator.com/physics/resonant-frequency-LC Thanks!
I previously built one of the QRP Guys 3-band endfed antennas and got it tuned just fine. Unfortunately, when I tried to protect the wound toroid with liquid electrical tape it raised the SWR. Grrrrrrrr…
Dave, AE9Q
The inductance and resulting tuning of the trap is very dependent on the positioning of the windings on the toroids. A small amount of movement makes a big different. I find the epoxy potting helps a lot. Be very careful to avoid moving the windings after your final test and before epoxy application.
Adam
No one has this RADCOM article ?
Thanks
Patrick
You may have trouble if you use the traps being sold by any of several different sources as they don’t produce a high impedance at the design frequency. Suggest you look at < TOROIDS.INFO > plug in the values and see what you get. Impedances of less than perhaps 700 ohms will be difficult to tune.
I have been experimenting with tiny traps since about 2013 and wrote up some thoughts derived from those experiences which Rex, KE6MT, has posted on his blog.
https://www.ke6mt.us/category/ham-radio/portable-gear/
73, Fred KT5X (aka WS0TA)
Take a look at Pacific Antennas and QRpguys - they both sell trap-style dipoles at cheap prices. I’ve built both and they work FB.
GL ~ k6sdw
