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Re: [TowerTalk] Was "43ft Vertical Feeding

To: jimlux <>
Subject: Re: [TowerTalk] Was "43ft Vertical Feeding
From: Dennis OConnor <>
Date: Mon, 24 Nov 2008 09:30:35 -0800 (PST)
List-post: <">>
HI Jim, thanks for the comments...
Yes, the relays will handle the voltage and current at 1.5kw... The ones I use 
are ~ $2.50... I buy typically 2 dozen at a time for the price break.. 
You have to remember that the voltages are not that high at the base of a 43 
foot vertical - and they are low on the 50 ohm coax side and not much more 
at the antenna feed point, 1200-1500 volts maybe at worst case...
I build tuners, for converting 50 ohm coax to 600 ohm open wire line and the 
voltages there are considerably more, and the relays work..  My tuners handle 
1500 watts without blinking... I have 7 out in the field (woods actually) at 
the moment and #8 is on the bench...
We make our own condensors (motorized in my case) from aluminium sheets; and 
0.200" spacing handles 1500 watts in the tank circuit feeding 600 ohm line with 
unknown (but high) impedences and reactances..  A conservative estimate is that 
it will handle 5KW minimum, though I have no way to test that... The dielectric 
breakdown for air is 3000v per millimeter... In a 1500 watt antenna system you 
are not going to see those kind of voltages... A Tesla coil is a another story..
I have one I just whipped up to convert an 80 meter base fed vertical into a 
halfwave base fed vertical for 40 for CQWW - so I needed a parallel tuned tank 
circuit with a tap for the coax feed... ten feet of copper tubing and 2 pieces 
of aluminium from the scrap box to make a 71 pf condenser and I was in 
business... Took all of an hour, with lots of coffee time and lolly gagging 
around... I gave this one 0.300" spacing because the voltages across the 
parallel circuit will be high at 1500 watts... No signs of arcing so far and 
this is in a plastic box laying on the ground - and has a half inch of snow on 
it as I type this......
Be aware that building a single L-match for one band, as I described, is a 
trivial matter... And taken one band at a time, and later integrating it into 
an enclosure with relays, etc., is not daunting when taken one step at a time - 
over time...
Basically, your analysis says you don't want to build... That is fine - your 
denny / k8do


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