[amsat-bb] Re: Polarity questions
kl7uw at acsalaska.net
Fri Sep 19 22:42:54 PDT 2008
If I may try to interpret what Clint wrote:
If a yagi has 15 or 20 dB more gain than needed to receive a decent
signal, then one can "throw away" some of that gain to cross
polarization loss and still have enough gain to adequately receive a
signal. Sort like saying if you have a 150-foot dish for receiving
AO-51 it don't matter!
But it "does matter" about polarization as far as maximizing
signal. A yagi is not different than any other linear antenna in this respect.
Now I will have a 22-element CP 2m yagi and 42-element CP 70cm yagi
for satellites. This is obvious overkill for working the Leo
sats. It is the old standard for mode-B sats like AO-10/13. With
this pair you had really good signals with those old satellites. If
and when we get another Heo, they will be appropriate for mode-UV,
LU, US, etc. (of course this will require 1268 and 2400 antennas for
half the link).
On the HF bands polarization is generally less important because the
ionosphere messes with polarity when the signal is
refracted. Perhaps this is where that saying originates. If so, be
cautious about generalizing and applying to VHF/UHF/and above! Get a
copy of the ARRL Handbook or Antenna Book and read it. The
information is in them. There are even better antenna texts like
Jasik and Kraus if you like calculus.
73 Ed - KL7UW
At 03:23 PM 9/19/2008, n3tl at bellsouth.net wrote:
>" ... and the polarization of the Yagi makes no difference in
>quality of TX/RX signal."
>I know I haven't been working the satellites nearly as long as Clint
>and many others - and with all due respect, my friend - I have to
>disagree with the above statement. I routinely work passes during
>which I swing my Arrow through, and sometimes beyond, 90 degrees to
>maintain signal quality. There also have been times when a quick
>twist of the wrist has made the difference between making a contact
>and getting a new grid square ... and not.
>I don't-at-all disagree with the concept that working AO-27, AO-51
>and SO-50 isn't terribly difficult with a handheld station. Frankly,
>that has really (and pleasantly) surprised me. However, I do believe
>that adjusting polarity when hand-holding the Arrow provides
>improved performance during many passes.
>73 to all,
>-------------- Original message from Clint Bradford
><clintbrad4d at earthlink.net>: --------------
> > >>...20-dB, assuming the signal is pure circular
> > polarization...linear, then -3 dB...difference between RHCP and LHCP
> > is 20 dB...difference between Horizontal and Vertical Linear is 20
> > dB...difference between (RHCP or LHCP) and (Horizontal or Vertical) is
> > 3 dB...the VHF/UHF Manual handbook says 20 -30dB...in the real world,
> > expect around 20~25dB loss from being completely cross-
> > polarized...Between linear and circular, expect about 3dB loss...
> > Ahh, thank goodness for the engineers...
> > Just know that to get started working the FM satellites, you do not
> > need to spend much to make successful contacts...Working AO-51 from
> > Southern California (which some claim is not a "real world") at a Watt
> > or so with an HT and Arrow Antenna is a breeze, and the polarization
> > of the Yagi makes no difference in quality of TX/RX signal.
> > And as you refine your satellite comms requirements, you can spend
> > more money!
> > Clint Bradford, K6LCS
> > 909-241-7666
> > _______________________________________________
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