[TowerTalk] Trapped in antenna trap confusion

[Jim Lux]

-----Original Message-----
>From: Gene Smar <ersmar at verizon.net>
>Sent: Jun 19, 2008 6:07 AM
>To: Dan Hearn <dhearn at air-pipe.com>, William Moore <whmoore at sympatico.ca>, towertalk at contesting.com
>Subject: Re: [TowerTalk] Trapped in antenna trap confusion
>
>Bill:
>
>     I agree with Dan about the three dipoles fed from a common coax line.  That was my antenna system as a young Ham back in the '60s.  It's a simple design and is almost guaranteed to work as well as separate dipoles.  I've not tried a trapped system but can only expect weeping and gnashing of teeth until you get the three bands to work compatibly with each other.  
>
>
>73 de
>Gene Smar  AD3F
>

I heartily concur... and, a further piece of advice..

A typical configuration is to cut dipoles for 10,20, and 40 (which also works on 15) and hook them up.  If they are at all close to each other, getting a good match on all bands is a bit of a challenge, unless you have an antenna analyzer that's fairly easy to use.

If you don't currently have an analyzer, I would take the couple hundred bucks and spend it, instead, on an automatic antenna tuner.  Typically, you can get pretty close on the lengths of the wires by just cutting them to length via the usual equations  The tuner will take out the rest of the mistuning, and since the resonance will be reasonably close (generally a bit low, in my experience), the loss due to the tuner will be small. (that is, the antenna might be resonant at 13.9 MHz, instead of 14.2 where you wanted it)

(It also takes care of the inevitable variations as your antenna sways in the wind or trees move near it, the wires move around, etc.)

OTOH, if you can spread the 3 dipoles out (i.e. every 60 degrees), the coupling is quite low, so you could probably prune it to resonance without too much trouble.  When you've got 3 wires a foot apart, though, the interactions are a pain.  After doing it the first couple times using just the rig and SWR, I bought an analyzer.

Jim, W6RMK