[aprssig] APRS Satgate Antenna page (dB's)
bruninga at usna.edu
Fri Jun 26 15:13:03 CDT 2015
OK, I did a quick look and 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 another SATgate 600 miles away....
We will write it up in detail, but here are the rough figures.
1/4 wave vertical with radials: -2 at 30 and -6 at 60 ==> Basically a
3/4 wave vertical with radials: +2 at 30 and -1 at 60 ==> 4 to 5 dB
better, 2 dBi max
3/4 wave vertical w 6' gndpln: +2 at 30 and +3 at 70 ==> 4 to 9 dB better,
5.5 dBi max
3/4 wave vertical w 9' gndpln: +3 at 30 and +3 at 65 ==> 5 to 9 dB better,
5.3 dBi max
3/4 wave vertical perfect gnd: +3 at 30 and +3 at 68 ==> 5 to 9 dB better,
6.9 dBi max
We are going to do a 3' ground plane too for comparison, but 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.
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!
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:
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:
> 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.
> 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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