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Polarization
of man-made noise
By Michael Tope,W4EF
Topband Antenna mail list, October
30, 2000
I have been observing an interesting phenomenon at our club station.
We have a full size 160 meter dipole at 90 feet, which is for the
most part horizontal (there is a slight downward tilt in the tips
of the element halves of 10 to 20 feet over the 133 foot span).
On 80 meters we have two inverted vee dipoles - one at 90' for cut
for CW, and one at 70' cut for SSB. The elements halves of both
of these dipoles droop at roughly 45 degrees to the horizontal.
I have noticed that when compared to the inverted vee's for receiving
on 80 meters, the 160 meter dipole produces an SNR which is one
the order of 5 to 10 dB better than either of the inverted vees.
My first explanation for this phenomenon was that the additional
directivity afforded by the two half-waves in phase was responsible
for the improved SNR. But considering the fact that the antennas
are located on a college campus smack dab in the middle of an urban
jungle, and that the drastic SNR improvement doesn't appear to be
a strong function of incoming azimuth, it seemed unlikely too me
that directivity was the only factor at play.
Then it occured to me that it might be polarization.
The 160 meter dipole is almost purely horizontally polarized whereas
the inverted vee's surely have a stronger response to vertical polarization.
I had always wondered why it was said in radio handbooks
that man made noise is vertically polarized, until the dim lightbulb
in my head finally went off the other day. It isn't that man made
objects produce only vertically polarized noise, its that true ground
wave propagation (surface wave solution) only supports vertical
polarization.
If I took our same antennas out to Farmer Brown's Field back in
Smallville, USA where noise was dominated by skywave sources, there
probably wouldn't be much difference between them (back in small
town Ohio my 40 meter inverted-vee was a great receive antenna on
160 meters).
But in the urban jungle where the noise floor is dominated by numerous
local sources from multiple azimuth angles, the horizontal antenna
seems to have the advantage.
Assuming this theory has some validity, does the standard
strategy for top- band receiving antennas of seeking azimuth directivity
apply in the urban jungle? After all, most of the popular receiving
antennas for 160 meters (bev- erage, EWE, pennant, probe array)
are vertically polarized. Has anyone played with phasing low horizontal
antennas for a combination of azimuth directivity and ground wave
noise rejection?
By Tom Rauch,W8JI
Topband Antenna mail list, October
31, 2000
You hit the nail on the head Mike.
The reasons local noise sources are predominately vertically polarized
are because horizontally polarized signals are radiated and received
at high angles by most "antennas" (intentional or accidental antennas)
because of their low height. Besides that, any horizontal content
of the signal is attenuated more than vertical components as it
propagates along earth.
Bottom line is nothing makes it along the earth on groundwave any
distance on low frequencies because the earth "shorts out" any horizontal
electric fields, and of course when the time-varying electric field
goes to zero so does the accompanying magnetic field. You can't
have one without the other.
The problem with low antennas that are truly horizontally
polarized is they only radiate at higher angles, and that almost
always isn't good for DX. The Beverage is an exception because its
length allows it to respond to the attenuation-caused-tilt of the
vertical wave as it propagates along the earth in line with the
wire.
The EWE and other related antennas (flags, "K9AY" loops,etc) have
considerable high angle response from the horizontal wires...but
other than that the pattern is the same as two short phased verticals.
These antennas become somewhat vertically dominant because the earth
reinforces the sensitivity to vertical signals while the earth attenuates
horizontal signals except those at high angles. The better the earth
the better they work, and if the earth is perfect they work almost
like two short phased verticals.
A low horizontal antenna will "fight the earth effects" and
lose sensitivity except at higher angles, and you would have a tough
time phasing the antenna to get directivity. You'd wind up with
something with very poor sensitivity focused up at a very high angle
and a broad response in many directions, not a very good antenna.
If you could get it up in the air some distance (like 1/4 wl or
more) it would likely work OK.
Nearfield noise is a roll of the dice. You never know what
will work because you never really know what is going on. Sometimes
verticals are better, sometimes horizontals. Sometimes small loops
(which are magnetic dominant within ~1/10 wl and electric field
dominate beyond that distance) help, and at other times short verticals
(which are electric field dominant closer than ~1/10 wl and magnetic
field dominant beyond that distance) help in nearfield noise.
All you can do is try a bunch of stuff when the environment around
the antenna is noisy. What works for you might not work at all for
the next guy.
By Gary Breed, K9AY
Topband Antenna mail list, October
31, 2000
When I lived in the suburbs of Denver a few years ago, I could not
get any help in S/N using shielded loops or EWEs. I even decided
to search the neighborhood for noise sources, thinking that removing
the worst few would help. There were so many noise sources that
this effort was useless. Once when we had an ice storm power outage,
I hooked up the rig to a battery and listened a while -- it was
amazing, nothing but a little distant storm static on 160 and 80...then
the power came back on.
My "best" receiving antenna in that environment was my transmit
antenna, with lots and lots of attenuation to get the noise floor
below S-1. Apparently, the TX antenna made the "S" in S/N as big
as possible, since I couldn't reduce the "N".
Here in Georgia, I'm in a quiet semi-rural location, but development
(and its noise) is rapidly moving my way. |
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