I have an intermittent RFI issue that comes from a subdivision about a 1/4
mile away. To combat the noise I have been trying to use my ANC4 and I
built a sense antenna out of an 80m hamstick that I put 20 feet in the air
yesterday for a trial to cancel the noise on 80m (cheap and easy
experiment). The location of the sense antenna is at the corner of my
lot as close to the subdivision that I can get. The design is the
hamstick as a vertical element and two 102 inch whips as elevated radials.
102 inch whips would not be an effective counterpoise unless one or both:
1.) The whips were loaded to resonance with a high Q coil, at which point it
would be a single frequency counterpoise and still require a good feedline
choke
2.) The feedline had a much high common mode impedance than the common point
impedance of the whips
Without the above, the coax shield is mostly the antenna...not the Hamstick.
You would have been much better off with a ground rod.
The noise is s9 on my 1/4 wave 80m vertical and 160m inverted L. This is
a multi band vertical with wires for 40m, 80m & 160m. I also have other
antennas such as a 160m 2wl long loop and a trapped dipole. What is
interesting is that the noise is significantly less on the loop and the
dipole. I attribute this to the noise being vertically polarized based
on my research on the internet. Also this sense antenna is roughly 150ft
away from the vertical antenna.
That actual reason for that is the earth acts like a short circuit and
attenuates any horizontally polarized ground wave.
The low horizontal antennas also have very poor ground wave response.
This combines to make horizontally polarized antennas less sensitive to
distant ground wave noise.
The issue is the level of the sense antenna noise is significantly less
than the vertical and I am not able to find a null point that makes a
difference. I can however use the loop antenna and I have enough signal
with the hamstick experiment to get a null as the received level of noise
on the loop is significantly less. So I believe I need more receive gain
on the sense antenna.
So with the above in mind, is there a low cost pre-amp that I could use on
the sense antenna to boost the signal? Or what other ideas are there for
a sense antenna. I don’t want to put up another 1/4 wave antenna for
sensing. I have read where folks suggest putting 50 ft leg dipole at 5
ft above the ground for 80m & 40m but I think the noise is vertically
polarized and this wont do much but I could be wrong and have been wrong
before.
I would build a ground mounted vertical sense antenna with amplifier, use a
ground rod as a ground, and decouple the coax near the sense antenna. A
simple J310 source follower amplifier on a 102 inch whip would have a ton
of signal level.
73 Tom
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