[amsat-bb] APRS Satgate Antenna page (More work needed)

Robert Bruninga bruninga at usna.edu
Sun Jun 28 21:13:50 UTC 2015

Well, someone with time to burn might want to check all these numbers.
Today I am getting inconsistent results and probably because to make a 9'
ground plane at 2m with good segment sizes, I need more than the 500 point
limit in EZNEC.

Also, to cover the USA with omni satgates with good gain above 25 degrees
will need almost 9 times as many satgates compared to the number with full
AZ/EL OSCAR arrays.  But then the omni satgates are 10? Times simpler...
(just your HT while  you are not actually using it)...

At this point we need to simply see what a 1/4 wave over a large ground
plane can actually HEAR.  It is easy to just run it for days and see how
many PSAT packets are received per 24 hours.  And then compare antennas
based on that number.  If a 1/4 wave can hear down to 25 degrees, then there
is no need for the higher 3/4 wave gain at higher angles.  Anyway, a great
opportunity for study.

-----Original Message-----
The gain of the 3/4 wave vertical is a real winner over the basic 1/4 wave
vertical for OMNI Satgates with no moving parts optimized for high elevation
gain above 30 deg.  Low elevations are simply covered by more SATgates....
New:  If we can get the angle down to 25 degrees then still need 9 times
more stations)

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 vertical 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.

Ill eventually put all this on the Omni SATGATE page:

Summary:  The 3/4 wave vertical makes it so you CAN hear 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!

THE BIG QUESTION though is What is the lowest elevation angle at which an
OMNI can hear PSAT?


-----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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