I have been following , with great interest, the thread on feedline
isolation for the pennant and flag type antennas. I am on a 120' x 120'
lot and use 8' diameter shielded, tuned (160m) loops made of 1"
hardline.--thought I would try, on my feedlines, some of the techniques
that have been mentioned.
FWIW, I wound a conventional type transformer, the two windings separated
as much as possible on an FT-114-43 core (using 13 turns on each winding--I
use 75 ohm CATV coax on receiving antennas). With a 75 ohm resistor load,
using my resistive bridge and GDO, it showed about an 8:1 SWR. Using a
borrowed Autek and a 50 ohm load showed about a 10:1 SWR. I could
distinguish no difference in the S-meter readings on the transceiver with
the transformer installed at the loop as compared to its not being
installed--true for signals, noise, and the bad computer birdie (S-7) at
l823.7 kHz.
Then I tried a choke balun made of 6 FB-73-801 beads (three bifilar turns
through each bead--the one that W8JI mentioned in one of his posts using a
binocular core would be a lot neater and easier, but I used what I had).
With each bead having an AL of 3900 mh/1000 turns my calculations show a
reactance of about 2400 ohms for the choke. It didn't make any difference,
either, when installed on the feedline at the antenna.
I really do not have room for a receiving antenna as large as a flag or
pennant type--so I will stay with the loops and try to make sure they are
operating as well as possible. But several of the comments made by Tom in
his posts have made me curious. Statements in two of his posts were:
< That's what the shield over a loop really does, it acts as the antenna
and allows a ground at the exact electrical middle of the loop. The winding
inside can be unbalanced, because the real loop antenna, the shield, is
forced into balance by a common point ground. >
< This is what happens in a small loop, where severe unbalance can be
tolerated without ill effects IF the loop is isolated from earth. The
larger the loop, the more important balance becomes. >
Should I ground the shield of the loop? The loop is slightly loaded with a
small value series resistor to broaden its response and is well matched to
the 75 ohm coax--the lowest part of the loop is about 30 " above the
ground. I sometimes phase two of these loops and the directivity seems to
be fairly good, in spite of my neighbor's truck being parked 15 ' from one
loop--if I open the feedpoint of my 3/16 wavelength inverted L while
receiving. I have been unable to determine any difference between
grounding the loop shield, or leaving it ungrounded--but there haven't been
many weak signals to work with. I really would like to know which, in
principle, is better.
If I go to untuned loops I could use them on more than one band. Is there
anything published in the ham literature on the care and feeding of untuned
loops? Is it enough just to put a good choke balun on the feedline where
it attaches to the untuned loop? Is it practical, or desirable, to match
the very low impedance of the loop to the coax?
Until I can find some rural acreage, I will continue to fight to improve my
receiving S/N in this noisy, electrically poluted environment--every db helps.
Any help and/or suggestions would be greatly appreciated.
"General Knowledge is Remote Knowledge; it is in Particulars that Wisdom
consists & Happiness too."
William Blake
73 Paul W5DM--A ham since 1937
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