Times change.
In recent years vertically polarized man-made electrical noise has been a
growing problem. I was forced to use a dedicated receive antenna to
mitigate severe local noise problems (switching power supplies, plasma TVs
etc.) from my surrounding neighbors. Because the only practical transmit
antenna option was a vertical (little space), I had to give up on it as a
receiving antenna. After some experimentation, I settled on what is
essentially a folded dipole antenna cut for 40m, which is very close to
the ground. It's about 60 ft long and the wire spacing is about 4 ft, with
the lower wire only 2 ft from the earth, with a balun at the feedpoint in
the middle of the higher wire. 50 ohm coax runs from the balun to the
shack, lying on the ground to further minimize noise pickup. It is
strung along a fence on the lot line.
On all bands from 40m to 15m, the difference in signal to local noise
ratio between it and the vertical transmit antenna and this antenna is
considerable, up to 15dB. I suspect it would be a terrible transmit
antenna, but it works fine as a receive antenna. DX signals are often
heard by it much better than on the vertical. I don't have problems with
the transmit signal overloading the receiver input, perhaps because they
are polarized differently. The folded dipole feedpoint is about 50 ft.
from the vertical antenna.
The advantage of the separate receive antenna is that I can use an NCC-1
phasing system box in conjunction with the handy RX ANT In/Out insert
point on my K3. It's configured to use the Tx antenna signal from RX
ANT Out as the noise signal into one of the NCC-1 inputs, with the other
receive antenna connected to the other, and the NCC-1 output connected to
RX ANT In. The difference in vertically polarized local noise pickup
between the antennas lets me get a deep null on any particular noise
source in the surrounding houses. The effect is to allow me to operate
when I'd otherwise have to give up and be content with watching the
man-made noise waveforms dance around on the P3 screen. Using the NCC-1
introduces a directional effect of course, but as long as the signal isn't
arriving at exactly the same azimuth as the local noise it works well.
I think that a lot of folks afflicted with strong local noise are using
small so-called magnetic loop antennas for the same reason.
At times I feel like the Grinch in "How the Grinch Stole Christmas",
muttering "...the noise noise noise!".
73,
Bob NW8L
title, you're going to have to make a case for them even if you have to
stretch a bit.
I remember bolting a 115 VAC coil Dowkey relay on the back of my DX100 for
antenna change over in 1960 or so. It was several years before I had a
transceiver. The idea that separate antennas were the norm until transceivers
came along is nonsense, IMHO of course. Even the publisher of this book,
ARRL, had many QST articles, such as "A Novice T.R. Switch", by Lew McCoy in
the January 1961 issue that popularized T.R. switches. Lew even stated, "It
is always to the amateur's advantage to use the same antenna for both
transmitting and receiving."
In the scheme of things, if my memory of the last 60 years isn't too faulty,
separate RX antennas are a relatively new thing, popularized for the lower
hand bands (40, 80 and 160), where of course they are supposed to have
advantages. Personally, I'm two (SV/A and FR/G) away from top of the Honor
Roll and have 9-band DXCC and I have never used a separate RX antenna. I
guess I'll have to try one someday.
Wes N7WS
On 9/9/2018 5:58 AM, hawley, charles j jr wrote:
The ARRL recently published a book “Receiving Antennas for the Radio
Amateur”. It maintains that “The function of transmitting antennas is to
radiate power efficiently, while the function of receiving antennas is to
present the best signal-to-noise ratio to the receiver”. It maintains that
“using the same antenna for transmitting and receiving roughly coincided
with the advent of the transceiver in the 1950s and 1960s.” And “The
glaring differences in priorities between transmitting and receiving
antennas becomes...well...glaring...when we start looking into the concept
of efficiency.” And “some of the most effective receiving antennas are
abysmally poor performers when efficiency alone is considered”.
It’s an interesting book.
Chuck
KE9UW
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