Topband: alternative to vacuum variables

[Pete Smith N4ZR]

Thanks, Tom.  I thought I recalled this advice from an earlier reflector 
post, but couldn't find it.  Thanks also to everyone elsewho replied, 
and the many interesting ideas

My single 97-foot tower has two tri-band yagis and a 2-element shortened 
40m yagi at the top, as well as an 80-meter lazy-vee array deployed all 
around it.  When I set up the shunt feed years ago, I did a lot of 
climbing around on it, trying to find a 50-ohm point.  Only after not 
being able to find one did I install the shunt feed at an arbitrary 
point (about 50 feet, and yes, it's a single wire) and configure an 
Omega match with a couple of 3000v 300pf variable capacitorsfrom Henry 
Radio.

I know it is a very sub-optimum system (not enough radials either), but 
I have it set for a 1:1 SWR at 1825 and it satisfies my occasional needs 
for a 160-meter transmitting antenna.

See you all in CQ 160.

73, Pete N4ZR
Check out the Reverse Beacon Network at
http://reversebeacon.net,
blog at reversebeacon.blogspot.com.
For spots, please go to your favorite
ARC V6 or VE7CC DX cluster node.

On 1/24/2013 9:59 PM, Tom W8JI wrote:
>> One of the reasons I have mot gone to high power on 160 is the cost 
>> of capacitors for my omega match - at least $300.  Anyone know of any 
>> workable alternative?  I remember someone writing about using 
>> coiled-up RG-8, RG-213 or maybe Teflon coax.  Where can I find more 
>> information?
>
> Coax isn't the best for two reasons:
>
> 1.) Transmission line effects increase voltage at the open end. This 
> aggravates arcing issues
> 2.) They are lossy
>
> Here's something to think about, Pete. The voltage across a capacitor 
> is I^2 * Xc
>
> So if you have a shunt feed and use a long, thin, wire....you not only 
> decrease bandwidth and efficiency, you also increase the voltage. A 
> thin shunt feed wire requires higher Xc (lower capacitance), and that 
> makes voltage increase and BW narrow.
>
> Use a wire cage and tap the shunt wire down at a 50 ohm point, and 
> voltage goes way down. Do it the lazy way and use an omega match, and 
> you give up bandwidth and need higher voltage parts.
>
> 73 Tom
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