Topband: A couple of Beverage questions

Greg - ZL3IX zl3ix at inet.net.nz
Mon Dec 14 00:55:01 PST 2009


Hi all,

I have recently constructed a 2-wire Beverage at my remote Rx site.  
It's performance is fair, but the F/B ratio in the forward direction 
leaves a little to be desired, so I have been doing a couple of 
impedance and insertion loss measurements.

The differential mode characteristic impedance is as expected, and the 
VSWR is highly dependent on a differential termination resistor placed 
across the far end of the line, also as expected.

The common mode characteristic impedance is also more or less as 
expected, but does not seem to be so dependent on the far end 
termination, leading me to believe that there is considerable loss in 
this mode.  First question - what should I expect the common mode loss 
to be for a 2 w/l Bev?  Eznec gives the average gain as -17 dBi, leading 
me to believe that the radiation "loss" is only 2% of the input power.  
Of the remaining 98%, how much is really dissipated in the far end 
termination, and how much as transmission loss along the line?  Eznec is 
not consistent with this, giving the load loss as 35 dB.  In my view the 
load loss cannot exceed the negative gain.

Second question.  The Beverage runs along the top of a wire fence, and 
is possibly only 30 cm from the fence wire in some places.  This was 
done for practical reasons.  I know that the ideal would be to use 
independent supports, but that is not possible for this antenna.  How 
far above the fence wire should the Beverage wire be, to avoid stray 
coupling, added transmission loss etc?

I must say that the signals I receive from the forward direction are 
very similar to those on another Beverage in a different direction, 
which does have its own supports, so I would not expect the transmission 
attenuation of the two to be very much different.

73, Greg, ZL3IX


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