OBSERVATIONS    GM1SXX             28 Sept 2009

Feeling 'Trapped"!

While tidying up all the stuff that was lying around after working on the bathroom sink and cabinet, I thought I'd hang on to a short bit of 40mm sink waste... cause 'it might be useful'

Well, ten minutes after saving it from the bin, I thought up a use for it. I've always wanted to try building traps for aerials and since I'm so space restricted, an inverted L seemed like a way to go. This calls for a 40M trap, so armed with a few inches of pipe and some 'recycled' RG-58, I set about trying to build a coaxial trap.  I suppose the 'proper' name for such a thing might be a "distributed reactance parallel resonant circuit" .  You know the sort of thing.... a bit of pipe wound with some coax and the outer of one end is connected to the inner of the other with the 'free' ends (one inner and one outer, connected to the aerial radiator.

The cable I used was 'Palstar RG-58'.... odd, because it was double lapped!! ... whereas as we ALL know, 'real' mil-spec RG-58 aint. I found some software, measured the capacitance of 2M of co-ax  (divided by two for the pf''s /m), picked the nearest pf/metre figure in the software (palstar RG-58 didn't get a mention:-)  and used that to set about designing said trap.
 

The software I used was http://www.qsl.net/ve6yp/CoaxTrap.html

Calculations done, I cut the length I needed from my 2M piece of co-ax It was about 1.57M. Of course I DID remember to add 4 inches extra for connections at the ends!  I drilled a hole, poked the start of the co-ax through, wound onto the sink waste, drilled another hole at the end,  and overlaid it with three layers of PVC tape to secure the turns. I made a pair of small holes at each end of the pipe and threaded some heavy tinned copper wire through both ends to form connecting loops and soldered the free ends to them.

Crunch time!  I dipped the trap with my aerial analyser.  It is resonant on 7.215Mhz.  Not bad for 20 minutes work. I was aiming for around 7.2Mhz so as you'll expect, I ain't disappointed;-)  It's pretty textbook.

Believe it or not, this is my first attempt at making an aerial trap. being only 15KHz high gives me confidence that the design is repeatable.    Here's a photo of the trap. It would be fine for indoor use like this but for outdoors, it really should be potted.... perhaps with closed cell spray PU foam, for lightness.

Sometimes it all goes just so horribly right :-)  And, no' it wasn't an accident... it was designed. That's what I'm saying and I'm sticking to it

I dipped it with the trap in the middle of the floor, sitting on a wooden chair. Nearest to free-space that I can manage.
Resonance is sharp... definitely a good sign.
 

It would be possible to make this trap suitable for outdoor use by placing it in a larger diameter sealed PVC tube, with bonded-on end-caps. Placing a small bag of silica-gel inside would 'mop-up' any residual atmospheric moisture.  Alternatively, some PU expanding foam could be used to fill the gap between the inner and outer tubes. Professionally made traps are generally 'potted' in epoxy resin.  If you go down this road, expect the epoxy to change the resonant frequency. Some experimentation will be needed.

73 Al.

GM1SXX