[amsat-bb] 3/4 Wave Vertical Omni

[Bob Bruninga <bruninga@xxxxxxxxxxxxx>]

For the mobile Satellite operator looking for an Omni, I finally did a
real-world antenna plot of a 3/4 wave 435 Mhz WHip over a car roof
to see how well it matches the EZNEC predictions.  The results confirm the
almost 8 dBi gain from about 30 to 80 degrees elevation:

REAL PLOT:    http://web.usna.navy.mil/~bruninga/g3q435.jpg

EZNEC PLOT:   http://web.usna.navy.mil/~bruninga/gainplot.gif

I did this by building a 1575 MHz scale model of both the antenna and a
miniature car roof and connecting them to a GPS receiver.  Then ran the
GPS all  night with my SIGPLOT.BAS program that then plots the GPS signal
strength of all satellites for 12 hours from ALL azimuths and ALL
elevation angles.  THus you get a calibrated signal source over all the
sky and get a REAL plot of the actual antenna pattern.

Tonight I will change the model to do a simple 1/4 wave vertical to see
how well it fills in the low angle stuff below 30 degrees.  BUT I doubt it
will do any good for mobile satellite work, because:

  1) The 1/4 will only have about 5 dBi (3 dB worse than the 3/4)
  2) The satellites will be further away for another 3 to 6 dB loss

Combined, the loss is 6 to 9 dB compared to the 3/4 wave and there is just
not enough signal from the PACSATS to be heard by any system reliably at
that level. 

Probably the only way to fill in the low angle is to go to a high
gain vertical (to get 9 dBi below 30 deg) and switch between the two.
during each pass.  But given the complexity, I just stick with the 3/4
wave (20" vertical) since it is inherrently dual band being an excellent
1/4 wave 2 meter antenna as well for the uplink.  THus, your 2meter 20"
whip in the center of your car roof will serve very well as a mobile
satellite antenna whenever the satellite is above 30 degrees, and most
mobiles cant see much below 30 degrees anyway due to trees.. (back east
anyway...)...

For APRS messaging, though, this is all you need, since the APRS SAT/Igate
will only send your traffic to your mobile when you are within that cone
of coverage, (APRS always knows where everyone is) and conversly, your 2m
mobile uplink has a 18 dB advantage so your outgoing traffic can get into
the satellite at ANY elevation angle...

To see the entire discussion of mobile satellite operation including the
discussion of these antenna plots see:

  http://web.usna.navy.mil/~bruninga/astars.html

Come join us on SUNSAT and any other satellite that may have UI
digipeating enabled...

de WB4APR, Bob


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