Topband: "Grounds" for a Beverge

[Jeff Woods]

----- Original Message ----
<snip>

         I have often read that the terminating "ground" at the far 
end of a Beverage is not very critical.  However, I believe that we 
should strive for a good "ground" at the feeder end. 

<snip>

Charles,

Directivity of the antenna is strongly affected by the "far end" termination.  
When perfectly matched to the characteristic impedance of the antenna, all the 
signal traveling toward the termination will be absorbed by the termination, 
leaving none to bounce back and make its way into your receiver.  In the real 
world, you'll never get perfect termination to all signals at all frequencies, 
but 10 to 30 dB attenuation is practical and sufficient.

On the feed point side, the match is less critical because a reflection from an 
imperfect match will bounce back towards the (assumed perfect) termination on 
the other end and be absorbed.  Sure, you don't _want_ to send any useful signal 
away from the RX, but in dB terms it's not significant unless you're really 
poorly matched.  Insertion loss due to mismatch  isn't significant until the 
VSWR is above 3:1, so you could use a 50:200 ohm transformer on a single-wire 
terminated beverage with good results.  This assumes a beverage of a decent 
length (600' or more?) such that there's plenty of signal and noise at the RX 
port.  If there's not plenty of signal (short beverage, low height), then the 
loss may contribute to a degradation in the RX system noise floor.  


At my QTH, the noise floor is around S5, even after 800' of cable loss (which 
frankly isn't much in RG-6 at 2 MHz).  The 6 dB resistive splitter used to route 
the signal to a pair of receivers doesn't make a dent in Minimum Discernible 
Signal (MDS) level, so I'm not inclined to worry about minor losses at the 
antenna in this particular installation.  


-Jeff
W0ODS
Iowa