Hi All,
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?
Yearning to hear better on the low bands.
Mike, W4EF................................
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