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Re: Ground plane & AO-27



   In a message dated 9/29/98 9:57:33 PM Eastern Daylight Time, jeffj@scott.net
   writes:

   > will a ground plane mag mount type antenna be of any use at all 
   >  in working the satellite or will I just end up frustrated due to the fact
   >  th[at] I won't be able to achieve the proper polarization?

   From my experience I predict that you will most frequently experience the
   latter result. Depending on how much power you run, you might overcome the
   improper polarization on the uplink, but receiving is hopeless except for the
   brief period during the pass when your antenna happens to be correctly
   polarized.  That won't last more than a minute or two.

I disagree strongly, and have 25 states/provinces confirmed on AO-27 using
either a quarterwave or gain omni.  The former was on the rain gutter of
a van, hardly the optimal place for a groundplane; a mag mount might well
work better.  However, i'm probably not doing what you think.  Here's an 
earlier posting about that and why it works.

   This may seem counter-intuitive, but try a 2 meter quarterwave tuned for
   decent SWR for in the 70cm satellite sub-band.  That shouldn't be hard, 
   since 146*3 = 438.  Now, as many folks now, a 2m J-pole or quarterwave 
   doesn't work very well at 70cm for repeater or other terrestrial use.  
   That's because there's a slight loss at the third harmonic due to the
   pattern vertically, e.g. most of the energy is skywards.  But that's 
   just what you want for 70cm LEO work!  There's an annoying overhead
   null for AO-27, but otherwise, it worked well for me. I operated AO-27
   mobile for a year using an FT-5100 without a preamp, and worked approx.
   25 states/provinces (which included about 5 contacts with a gain omni 
   on low elevation passes).  And as a added bonus, no diplexer was needed
   with the homebrew 2M quarterwave.  But you need to have a decent ear
   for AO-27 and/or be away from urban RF clutter.  The PACSATs should be
   easier, as i understand that the downlink is much stronger.

So rather than having a minute or two where it does work, it's rather that
there's a minute or two where it fade while it's overhead, and it does not 
work as well after that.  (But contacts are still possible.)  And i still
deal with the "overhead fade" even with an antenna similar to an Arrow.

Your local conditions will have alot to do with how well you do.  I had
to struggle for six months until i finally figured out that there was a
strong ATV station broadcasting 24 hrs/days across the Bay from me and
that i was winning when operating from the El Cerrito hills because that
put me in the shadow of the ATV station.  A large radio site nearby can
also cause such problem, but at least in that case, filtering will help.
In a quiet RF area with a sensitive receiver, you'll probably win.  In
such an area, i've worked AO-27 with a long rubber duck unsing a chain-
link fence as a reflector, but i don't recommend that unless you're already
an experienced satellite operator (and even then it's a real challenge).

With regards to polarization:

   The problem with this on AO-27 is that for the first half of the pass, it's
   vertically polarized (and i haven't been able to establish a particular
   polarization after that, although it seems to tend towards horizontal).
   So a fixed elevation horizontally polarized antenna won't work adequately
   on AO-27...

But a vertically polarized antenna should work just fine where you are, if
there isn't too much RF noise and your receiver is sensitive enough.  (But
if there's an ATV station nearby at 433.1 MHz, forget it!)  But do get a
pre-amp, and don't make terrestrial assumptions.  Or just try it, but make 
sure you can at least copy callsigns before transmitting, or you'll just be
causing interference.  Over Northern American, it is very rare that AO-27 
is quiet.
		    73's and good luck!

			-- KD6PAG  ("Networking Old-Timer, RF newbie")
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