Using a thicker wire, or multiple wires in a drop, always helps. Last single
thin wire I used was around 1970. :-)
----- Original Message -----
From: Pete Smith N4ZR
To: Tom W8JI
Cc: topband@contesting.com
Sent: Friday, January 25, 2013 8:30 AM
Subject: Re: Topband: alternative to vacuum variables
Thanks, Tom. I thought I recalled this advice from an earlier reflector
post, but couldn't find it. Thanks also to everyone else who 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
capacitors from 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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Topband Reflector
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