Topband: "Grounds" for a Beverge
jmwooods at yahoo.com
Tue Jan 18 07:00:37 PST 2011
----- Original Message ----
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.
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.
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