Subject: RE: 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: http://aprs.org/aprs-satellite-igate-antennas.html
This shows the very common misunderstandings of the term "ground plane". (As a MATCHING system with 1/4 wave radials, or as a true Ground reflector (infinite)
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 _______________________________________________ aprssig mailing list [email protected] http://www.tapr.org/mailman/listinfo/aprssig