[amsat-bb] 3/4 Wave Vertical Omni

[Bob Bruninga <bruninga@xxxxxxxxxxxxx>]

Last week I posted the results of a 435 MHz 3/4 wave vertical whip
satellite antenna (2m 20" whip) on a scale model car roof.  Last night I 
re-ran the test using a standard 1/4 wave vertical so I could get a
benchmark for comparison.

The 1/4 wave did have a better pattern at low angles down to about 10
deg, but the 3/4 outperformed it everywhere above 27 deg by 3 to 4 dB or
more.  Since the satellite is closer there too, that is why this
antenna does well for mobile satellite reception.  AND it is dual band
for the 2m uplink as well.  See the updated plot (side-by-side) on the WEB
page below.

On Wed, 12 Jul 2000, Bob Bruninga wrote:

> 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:
> 
> See near bottom of:   http://web.usna.navy.mil/~bruninga/astars.html
> 
> 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 many 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 omni 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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> 

APRSdos REPLY/COMMENT:

Reply mail addr:   wb4apr@amsat.org   
US mail address:   115 old Farm Ct, Glen Burnie, MD 21060
See DAYTON97 HISTORY:    http://web.usna.navy.mil/~bruninga/dayton.html
See Maryland APRS LIVE:  http://web.usna.navy.mil/~bruninga/aprs.html
See GPS on ANY radio:    http://www.tapr.org/tapr/html/mic-e.html

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