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KB2VXA > HDTV 14.10.11 16:34l 30 Lines 1348 Bytes #999 (0) @ WW
BID : 8182_VK6ZRT
Read: GUEST
Subj: Re: G0TEZ > polarization
Path: IZ3LSV<I0OJJ<VE3UIL<VK6ZRT
Sent: 111014/1531Z @:VK6ZRT.#BUN.#WA.AUS.OC #:8182 [Boyanup] $:8182_VK6ZRT
From: KB2VXA@VK6ZRT.#BUN.#WA.AUS.OC
To : HDTV@WW
Hi Ian and all,
I know that -3dB is half what you started with but still, when you're
spitting megawatts who cares? Look at it this way, for every bay you add
to an antenna array you need a power divider (-3dB again) but when all
those bays add up the ERP is many, many times the transmitter power.
The greater concern however is cross polarization, at VHF it's around
-20dB and increases as frequency goes up. In microwaves it's common to
send two cross polarized signals on the same frequency along the same
path and separate them perfectly at the far end. That reminds me of a
1.25M repeater in New York City using a 4 bay folded dipole array and a
quirky little problem setting it up. It started out fine but when they
spaced the antennas around the mast to give a circular pattern the signal
dropped WAY off puzzling the heck out of them. Then it dawned on me the
phasing arrangement and how the power dividers connected, suddenly the
mistake became clear. Turns out the bottom two bays were out of phase
with the upper two, I guess you know all about phase cancellation and
reinforcement. Just turn the bottom two over, problem solved.
73 de Warren
Station powered by JCP&L atomic energy, operator powered by natural gas.
Message timed by NIST: 15:21 on 2011-Oct-14 GMT
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