Topband: alternative to vacuum variables

[Tom W8JI]

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 at 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
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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