I want to build a wire dipole using these SOTAbeam traps:
I like these at the tuning looks much easier than say a coax trap, I definitely can get access to equipment to tune these.
What I don’t know is how much shortening of length these give?
Will it be the length of the wire in the coils or more complicated than that?
I can’t decide what bands I want to make the dipole for until I know really, but mostly likely might be 20m and 15m. not sure if with these I can squeeze 17m in between?
what would be the best process to go about making say a 3 band antenna (2 trap pairs)?
I’m assuming I couldn’t work a band at a time like I would if I was making a linked dipole, and there may be some interactivity?
Ultimately I will probably end up with a link dipole, but I want to start with some traps.
I will still be doing a link dipole. I do want to do a trapped one too. I do like experimenting with antennas. This trap malarky is a whole new world to me, more used to yagis, gamma and delta matches, stacking etc
That link, while useful didn’t really tell me enough. Everywhere on the net is instruction on how to build ‘that’ 4 or 5 band antenna with detailed instructions for that particular build.
I have since got this, which I think fills in enough blanks to get stuck in.
I think tuning the traps 300 kHz lower than band edge is weird. It says it is to avoid trap losses. that sounds like the wrong core and wrong type of capacitors being used. The traps will look like capacitors anywhere inside the band and so the 40 m section will be “seen” by the 20 m dipole.
I have always set the inner dipole on the desired frequency +/- 25 kHz on 20 m and preset the traps to that frequency. Then add a tail on each side and trim back for resonance on the lower frequency. How long should the tail be? Depends on the traps. I’d start with about 80% of the un-trapped length. Better to finish up with a pile of cut pieces of wire than a dipole with soldered extension wires.
At one stage I thought of using a coil with a self resonance above the band but to get isolation between wires you need a high impedance. Think 10 k ohms plus. 1,000 or 2,000 ohms just doesn’t cut the mustard. Run an antenna simulation program like EZNEC and see what happens without leaving the shack.
Expect only about 200 kHz bandwidth on 20 m. Depends on the Q of the traps. An ATU will allow full use of the band.
I agree with you about the desirability of modelling antennas. It is on that basis that the advice about trap resonant frequencies was developed by Tom W8JI.
I also agree with you about sections of the antenna “seeing” each other. This does make the antenna construction more challenging. For the easiest build, traps resonant at the operating frequency are the way to go.
Hi Richard,
Thanks for the reply and the link. Very interesting. The first section showed lower losses away from the resonant frequency but my reading of the SWR graph was that at the lower loss point the SWR exceeded 3:1. However this did not seem to be the case for the later examples. I need to read it more thoroughly - I’m sure it will make more sense to me then. It’s on my To Do list for today.
Certainly the circulating currents and peak voltages are greatest at resonance in a trap and hence losses are greatest here. A loss approaching 1 dB in a trap is tolerable IMO but if you can save 0.5 dB or so easily then of course you would go for it. So long as the impedance is significantly higher than the end impedance of the dipole you will get isolation.
I wonder if the resistances quoted for the traps in the table are measured or calculated. They are a bit higher than I would have expected. None of my traps are quite that good. Maybe I should buy some from ???.
Did you ever have any success with this Steve? I’m deliberating over some multiband (2-band or 3-band) groundplane/vertical antennas that don’t need an ATU. Got a 10m/6m one that works well, thinking now about a 17m/15m/12m and possibly a 20m-30m. How did you get on?
I’m on the verge of building the traps. I have had my finger in too many radio pie projects but I am picking off some one by one. I have recently sorted out a quick mount for the 10m pole at home (needed for my 20/40m trapped dipole) so next up is the dipole.
I have pondered over the out of band/in band trap resonance, and I am going to go for the in band resonance easy option, because I like easy, and also because the G3 guy who designed the hexbeam and the non folded cobweb reckons the loss difference is negligible and that’s good enough for me.
I’ll deffo make an update when done. I probably would have built the traps this week if I wasn’t rough as heck with man flu.
