[aprssig] APRS Satgate Omni Antennas

Robert Bruninga bruninga at usna.edu
Tue Jul 14 22:27:40 EDT 2015

Finally did some PSAT tests with 1/4 wave on car roof and TH-D72 Kenwood

* On a 60 deg pass I heard 25 packets decoded one at 5 bars.
* On 21 degree pass heard 5 packets but none above 1 bar.
* On 40 degree pass  heard 37 packets decoded 5 when they hit 5 bars. Also
got in with 5W.
* on 40 deg pass (in Annapolis) heard a single 5 bar packet and no decode.

NOTICE, the first three were at the beach with no real planning as to
building obstructions.  But apparently the noise open squelch was S0.  But
in Annapolis, after hearing nothing (1 packet) I opened the squelch to see a
full 3 bars of noise.  Remember, the radio ALWAYS shows ZERO S units  until
the squelch actually opens, so the presence of even S9 broadband noise will
show nothing and HEAR nothing unless you open the squelch manually!

The Baghigawatt  area defense radar on 149 MHz in the DC area was operating
at the time.  Again, shows the value of having any SATGATE OMNI antnenna on
the ground well below horizon (and only able to see sky above 30 deg)...

P.S.  On this 19" magmount whip, the 435.350 PSK31 came in FINE!  Remember
this is a 3/4 wave vertical on UHF.  So to pick up the needed 3dB more on
VHF, the next tests should use a 3/4 wave at VHF or about 58 inches...

Much more testing needed.  de WB4aPR, Bob

------------------------  original message ----------------------

Some EZNEC models:
1/4 wave with radials:  -2 at 30 and -6 at 60  ==>  Basically a dipole

3/4 wave with radials:  +2 at 30 and -1 at 60  ==>  4 to 5 dB better, 2 dBi
3/4 wave w 3' gndpln:  +3 at 30 and +1 at 60 ==>  5 to 7 dB better, 3.3 dBi
3/4 wave w 6' gndpln:  +2 at 30 and +3 at 70 ==>  4 to 9 dB better, 5.5 dBi
3/4 wave w 9' gndpln: +3 at 30 and +3 at 65 ==>  5 to 9 dB better,5.3 dBi
3/4 wave perfect gnd: +3 at 30 and +3 at 68 ==>  5 to 9 dB better, 6.9 dBi

Even the 3/4 with just the four 19" radials gives a huge 4 to 5 dB
improvement over a classic ground plane for omni satellite  SATgate work.
Also, this antenna does not need to be high.  Just see sky above 30 deg.  In
fact, is better to be low to reduce terrestrial QRM.

Notice too that the Lilenblatt, eggbeaters, and all other "omni"s that try
to keep their gain down on the horizon, are equally as poor as the 1/4 wave
vertical at these higher elevations.  This is OK for strong satellits like
the ISS with 5 to 10 watts, but abisimal when all that gain on the horizon
is completely wasted when you cannot hear the satellite anyway because it is
6 to 10 dB farther away! The result is these "omni's" give up the 4 to 9 dB
at higher elevations where the satellite is much stronger.

See the Omni SATGATE page:

Summary:  The 3/4 wave vertical should make it so you CAN hear weak LEO
satellites 5 to
9 dB better when they are closer and  does this by giving  up on hearing
them when they are so far away you can't hear them anyway!


-----Original Message-----
From: Robert Bruninga [mailto:bruninga at usna.edu]
Subject: RE: [aprssig] APRS Satgate Antenna page

> One question,what constitutes a large ground plane?
> Is it 12"or 12'? Is it a multiple of the vertical whip?

Good question.  Most people think that four 1/4 wave radials make a ground
plane.  But if you model a 1/4 wave vertical over 1/4 wave radials, all you
get is the exact pattern of a DIPOLE.  The radials are just providing a
"groundplane" to complete the electrical part of the antenna and give a good
match.  They do not affect the pattern at all.

I modeled verticals over 6' or larger ground planes and only found that  you
get the added "reflection" gain when the ground plane starts getting that
big or more.  I wish I had time to use EZNEC to show the added gain
(skyward) versus the size of the ground plane.  And how "radials" (above
actual ground) have nothing to do with the "pattern".

SO, the bigger the better.  It should be worth 2 to 3 dB if you could make
it very large...

This is the page in question:

Bob, WB4aPR

On 6/19/2015 3:57 PM, Robert Bruninga via aprssig wrote:
> Subject: APRS Satgate Antenna page
> Since the ideal APRS Satellite IGate OMNI antenna is exactly the
> opposite of the typical terrestrial IGaate antenna, I prepared the
> following WEB page:
> http://aprs.org/aprs-satellite-igate-antennas.html
> It shows how a vertical ¼ or ¾ wave VHF omni equals the performance of
> a full OSCAR class array (over half the sky) but does it with no
> moving parts.
> It makes up for the weak-signal horizon part of the sky by there
> simply being more omni-IGates.  The APRS-IS cloud with all of its
> IGates is probably one of the largest spatially distributed satellite
> receiver system in the world (?).
> But with people used to the 10W transmitter on the ISS, just a few
> IGates can capture just about every packet from the ISS horizon to
> horizon on a whip.  But with the 14 dB weaker signal from PSAT, we
> need more SatGates to make up for their smaller skyprint.  For the USA
> we need more than a dozen such Omni-SATgates.
> To see the significance of the weaker downlink from PSAT, look at the
> successful IGates on the http://pcsat.aprs.org page compared to the IGates
> that hear the ISS packets on http://ariss.net   Both are listening on
> 145.825 and passing along every packet they hear.  But only TRACKING
> IGates or good vertical gain satgates hear PSAT. And since we want
> these running 24/7/365, we do NOT expect people wearing out motors
> when an omni will do fine (if we have enough).
> The page also shows how every SATgate with a HIGH and terrestrial type
> antenna actually creates a DEADZONE around it, effectively blocking
> any nearby user heaerd direct from appearing on any of the APRS-IS
> cloud satellite web pages.  Hence, Omni-satgate antennas should be low
> to everything surrounding it w hile still seeing the sky above 30 degrees.
> If your TH-D7 HT is just sitting there, not in use 99% of the time,
> then just hook it to a vertical whip and let it be an IGate.  With the
> low antenna it will also probably be safe from all weather lightening too.
> Put
> the UHF side on 435.350 and turn up the speaker.  When PSAT PSK31
> comes into view, you can watch the PSK31 activity as a bonus!
> Bob, WB4APR
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