I have just finished installing 6 Beverage antennas in the woods north of
my house. Details are below. My question to the group involves measuring the
isolation of these antennas from my transmitting antennas. I am testing by
transmitting on all bands with either 100 or 500 watts and simultaneously
measuring RF levels on the Beverage feedline in the shack. Eventually I want
to set up an SO2R station. To measure the power, I am using a homebrew RF
power meter kitted by Kanga and based on the AD8307 (QST June 2001 - W7ZOI and
W7PUA). This is a 50 ohm instrument and I feed the Beverages with 75 ohm
RG-6. This means a nominal VSWR of 1.5:1 (14 db return loss). I placed an
ICE Model 401 bandstop BCP filter in front of the meter.
The raw results look good with the worst isolation to date being 65 db,
i.e. 160 microwatts while transmitting with 500 watts. I have some more
measurements to finish with the triband yagi aimed in a few different
directions.
How much inaccuracy is there in this method due to the 75/50 ohm mismatch
at such low power levels (16 nanowatts to 160 milliwatts)? Is there an easy
mathematical correction (for a guy who took calculus 39 years ago) for the
impedance mismatch or does it matter?
Beverage antenna details: (map of Beverages is on my QRZ.com page)
My Beverage antenna project is completed and the 6 antennas seem to work well.
They are between 470 and 860 feet long so as to stay on my property and radiate
from a central hub. I used 17g galvanized fence wire 7 feet high held up with
plastic fence insulators nailed to trees. They point to 40, 80, 160, 240, 280
and 340 degrees. I terminate them with Ohmite 470 ohm 2 watt resistors. The
transformers are the usual 6.25:1 (5T,2T) on a BN73-202 core. Four foot ground
rods are driven at the ends of each Beverage. There is 20-30 feet of RG-6 coax
from the transformers to the homebrew switchbox. I use 24v relays (RK1-24V) and
run almost 600 feet of CAT5 for a control line. Unused antennas are not
grounded. The RG6 feedline goes about 30 feet from the switch towards the shack
where the braid is grounded. I put a K9YC choke (8 turns of the RG-6 wound
around a #31 Big Clamp-On) on the shack side of the braid ground and then the
coax runs almost 600 feet back to the shack. I ground the braid again just
outside the shack. In the shack is a homebrew switch box containing a 24v
linear power supply and a 7 position rotary switch. (The 7th position is
labelled for an external switch when I get around to building one similar to
LA4HIA).
Thanks for reading this verbose question. I spared readers the details of
cutting paths for these antennas totalling 4090 feet through woods infested
with an understory of invasive European buckthorn, multiflora rose, and
honeysuckle. Of course it was done in the hottest, most humid and most mosquito
infested part of our late summer.
73, Charlie W2GN
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