[Elecraft] High SWR problem

[Edward Kenworthy]

On 17/8/03 10:30 pm, "David A. Belsley" <belsley@bc.edu> wrote:

>>> Edward:
>>> You don't give a lot of information to go on.  I gather your antenna is
>>> 20 meters long.  Is it end fed or otherwise.  Supposing it is end fed,
>>> does that length include the feed line? or do you have a wire coming
>>> from your K2 that is 20 meters long total?  If the latter, that length
>>> wire is going to present a very high impedance on 40 meters (where it is
>>> a half wave) and above.  Perhaps the impedance is too high for your
>>> tuner to handle.
>>> 
>> 
>> It's end fed (is there anyother way to feed a longwire ?)
> 
> Sure, you can feed a wire anywhere you want.  Doing so is an effective way
> to change the input impedance.

Ah ok.

> 
>> and it's 20m long plus it's fed by some low-loss coax (with a balun).
> 
> What are you doing with the shield side of the coax?  And how long is the
> coax? Assuming the shield is unconnected at the antenna side, and the coax
> is longish relative to the operating frequency, it is going to act somewhat
> like an extension of the length of the antenna. How effectively it is
> acting as coax is problematic, and there will likely be a good deal of
> common mode current flow in the shield as well as the center wire.
>

It's connected to the Balun, obviously it plugs into the back of my ATU
which is then earthed via braided copper to a copper rod.
 
It's about, hmm, 3m long something of that order.

>> 
> 
>> 
>>> If you are unable to get an end fed 20 meter wire tuned, you might try
>>> something that is not resonant with a high impedance at the end.  A 40'
>>> or a 100' wire will often behave nicely.  Or, use the 20 meter wire with
>>> a feed line whose length you can vary.
>>> 
>> OK.
>> 
>>> If you are using an end fed antenna, by the way, be sure to use an
>>> appropriate counterpoise to get maximum effectiveness from the antenna
>>> and maximum comfort in the shack -- you could otherwise end up with a
>>> lot of stray RF in the shack.
>>> 
>> 
>> Counterpoise == rf earth ?
> 
> Well, not really; an RF ground, perhaps, but not an earth since it has
> nothing to do with and no connection to the earth.  A counterpoise is
> anything that presents a very low impedance to the RF at the frequency of
> operation.  This is usually accomplished using a wire that is 1/4 wave
> length long at the op qrg.  Some tuners have an "artificial ground" which
> simply allows the cp to be tuned to achieve this low impedance without
> having to have separate cps for different operating frequencies.


I'm using a 4' copper rod stuck into the ground outside my window as my
counterpoise.

>>> 
>>> 
>>> ----------------------------------
>>> David A. Belsley
>>> Professor of Economics
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>> 
> 
> 
> 
> ----------------------------------
> David A. Belsley
> Professor of Economics
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