Has anyone successfully tuned the SOTABeams Picotraps? I’ve got v 1.1 and tried tuning it for 14MHz. The design is a parallel circuit with an SMD capacitor and a toroid-wound coil. The manual says you’re supposed to wind 17 turns to make it resonate on 14MHz.
I have been using the suggested method - plugging a loop of wire to a NanoVNA, passing it through the hole in the toroid core and looking for a dip in SWR.
When I make 17 turns, it resonates on 14.300.
When I make 18 turns, it resonates on 13.900.
Moving turns around (squeezing them, spreading them, …) has no effect.
The dip is quite sharp and neither of the dips reach into the 14.000 point, not in the slightest.
Has anyone managed to tune the Picotraps on 20m please? It looks like the one can’t change the resonance in a finer way.
Sure here it is. It’s curious, however, that moving the turns around the core doesn’t affect the value at all, maybe 1-3 tens of kHz, but it is still too small of a change to move the resonance to around 14.0 where I want it to be.
This is the second time that I am trying, so completely new wire and a new attempt. The interesting thing is that with any way that I wind the turns, the result is very consistent. It is either far too high (17turns on 14.300 MHz) or far too low (18 turns for 13.800 MHz).
Sidenote> But I must also say that I had strugels with the Pico traps. They were on point using the NVA in dip/meter mode but when installed the 40m / 80m trapping did not work at all. But I guess I need to spend more time finetuning
Yes, it’s not soldered yet, only touch contact. (Resoldering and desoldering it every time I change the number if turns is quite frustrating). I know that it makes contact, because when it doesn’t, the SWR dip disappears, when it makes contact, it is there
When I built my 40/30/20 m trapped EFHW in 2017 with T-44-6 toroidal cores, the traps were shrunk using shrink tubing with hot melt adhesive. This caused the resonance frequency to shift downwards by about 2 kHz.
Amazingly, this antenna still works perfectly even after several years of intensive use with the Mountain Toppers; later, links for 17 and 15 m were added.
I have a couple of Pico traps, one for 14MHz and one for 10MHz.
The 14MHz one has 17 turns and resonates at 13.9MHz. The windings are spread our around the core. If I squish them together the frequency drops 150-200kHz. The turns are tightly wrapped around the core. The turns in your picture look to be looser. I don’t know if this has a measurable effect.
Measured using an old MFJ 259B with a single wire passed through the core.
I built a 20/30/40 trapped end-fed using the Sotabeams pico traps. Mine has 17 turns on the 20m trap. It was fairly easy to tune by squeezing/spreading the turns (Rig Expert AA-55). Weatherproofed with a couple of layers of liquid tape which didn’t affect the tuning.
first off, a resonance of 13.9 is just fine. A resonance above the intended frequency not so much.
the trap will likely work better, that is generate a higher resistance, with more inductance and less capacitance. I use 15pf and about 39 turns for 14 mhz.
Every capacitor is a little different, every toroid is a little different, every winder is a little different, so things are not quite cookbook.
As already mentioned, it does not make sense to place the resonance within the band limits.
If you operate the trap at its resonant frequency, the current in the resonant circuit increases extremely and can destroy the capacity.
The measurement without connected antenna wires is also not exact because the wires have an additional capacitance that detunes the resonant circuit towards a lower frequency.
73 Chris
Thanks for all the input guys! Thanks for your comments and pictures too! I did some experiments and went back to 17 turns (just like the recipe says), but I did not use the ceramic capacitor on the board. I used a normal ceramic one with leads, which I threaded through the toroid core. Like that, it resonates exactly on 14.000 MHz.
Glad it’s working. Bear in mind that the supplied capacitor is a high Q device specifically selected for RF use. Your junk box capacitor might not be quite as good.
