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Re: LF: RE: 80 meter butternut conversion

from [Andy Talbot] [Permanent Link][Original]
To: [email protected]
Subject: Re: LF: RE: 80 meter butternut conversion
From: Andy Talbot <[email protected]>
Date: Thu, 9 Feb 2012 16:12:35 +0000
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I have vertical of 7m height, with a top hat of 12m length making a T antenna.  The capacity hat is made of three wires spaced by 200mm each.  Total capacitance is 220pF and tunes with ~ 5mH.
 
Ground resistance  is around 90 ohms when dry increasing to 110 ohms when raining (and yes that is correct that way round).   Ground system is about 18 stakes randomly placed around the garden, all cross tied with buried 2.5mm copper wire.  One run passing directly under the entire capacity hat.
 
Andy


 
On 9 February 2012 15:46, Rik Strobbe <[email protected]> wrote:
Hello Bob,
 
as the antenna itself is resonant well above 136kHz it will have a very large capacitive component at 136kHz.
Assuming it is a HF2V the antenna is a 10m vertical contruction, so using the 6pF/m rules the antenna capacitance will be about 60pF. This means a reactance of about 20kOhm (at 136kHz) and a coil of 23mH will be needed to bring the antenna to resonance at 136kHz.
Although this is possible, such a large coil will be rather lossy (100 Ohm at a Q of 200) and will cause a very poor efficiency. It will also cause very high antenna voltages during transmission, with possible arcing problems.
I believe that Andy (G4JNT) is using a small antenna with similar dimensions on 136kHz you he maybe best to give you a "how-to".
 
73, Rik  ON7YD - OR7T
 
 
Van: [email protected] [[email protected]] namens [email protected] [[email protected]]
Verzonden: woensdag 8 februari 2012 19:13
Aan: [email protected]
Onderwerp: LF: 80 meter butternut conversion

hello
I have a 80m butternut antenna how can I adapt this to resonate at 136 khz.
 
regards bob MM0RJR
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