[amsat-bb] Re: Which Mobile Mag Mount?

Thu Sep 15 18:14:59 PDT 2011

Ted,

Regarding antenna orientation for handhelds, see the picture
associated with item #6 at this web page:
http://www.amsat.org/amsat-new/echo/EchoHT.php

Check this video out, too, for inspiration:
http://www.youtube.com/watch?v=FCFWrXzdgeM

When using an HT whip, the trick I use is to avoid any sudden movement
or adjustment of the antenna once you have a decent copy of the
downlink.  Even then, it can be challenging as you will need to track
the bird.  I own some of the commonly recommended antennas such as the
Pryme AL800.   I don't own the Diamond you referenced but I have a
similar Comet SMA24 that is useable, albeit not ideal, for AO-51.  My
experience has been that the "cat whisker" style whips tend to have
too much movement.  I might try to make a video to demonstrate this
effect.

Even with a stock rubber dummy load, I can hear many passes of AO-51
and some overhead AO-27 passes.

K6LCS' web page has some good recommendations:
http://web.me.com/clintbradford/Work-Sat/Antennas.html

I recommend the AL800 and similar.  I personally am most pleased with
cost and function of the Smiley 270A, which at $22 is a real bargain.
After some passes with this antenna used, I have listened to the
recording and been unable to distinguish my HT performance from that
of a full-bore home satellite station.  Again, there are limitations
but it is a lot of fun to try!

73
Clayton
W5PFG

On Thu, Sep 15, 2011 at 6:40 PM, Ted <k7trkradio at charter.net> wrote:
> Bob, thanks for the great data.
>
> Question: do you think your data/techniques have any application to using a
> dual band HT with a long whip (e.g. Diamond SRH77CA - 15.5"long)
>
> In other words would you be better off just holding the HT vertical and
> fairly stationary during the pass (adjusting Doppler of course) or moving
> the darn thing all over until you capture the bird. I realize it is not
> duplex, etc. but it does seem to work on a strong pass. I just can't figure
> out the best way to hold the radio
>
> Thanks for any help
>
> 73, Ted, K7TRK
>
> -----Original Message-----
> From: amsat-bb-bounces at amsat.org [mailto:amsat-bb-bounces at amsat.org] On
> Behalf Of Bob Bruninga
> Sent: Thursday, September 15, 2011 8:52 AM
> To: 'AMSAT BB'
> Subject: [amsat-bb] Re: Which Mobile Mag Mount?
>
> Everyone's comments are correct and valid within their context.  But the
> original question was optimizing for the casual mobile operator in motion.
> So here are some additional considerations...
>
>> Second, any "vertical" antenna...will have a [null overhead]
>> So, think about it: A GREAT pass of 90 degrees goes overhead
>> your vertical is valueless during the middle of that great pass.
>
> True, but when we put some actual numbers on that, the loss is quite
> insignificant.  Using AO51, only one pass every 5 days goes above 82
> degrees, and the total duration above 82 degrees is 24 seconds, out of all
> in view times.  So we agree, the antenna is valueless, but for under 1% of
> the time.  The tradeoff is that it is 3 dB better most of the rest of the
> time...
>
> The measured elevation profile of a 3/4 wave vertical (a 19.5" whip on the
> 435 downlink) is shown 80% down the page http://aprs.org/astars.html.  The
> overhead null is only 10 dB down at 82 degrees... BUT the satellite is 10 dB
> closer at that point, so you still hear it almost the same as when it came
> above 25 degrees.  So you get full sky coverage above 25 degrees > 99% of
> the time with the vertical.
>
>> Ideally - in a car - ... stopping and parking and getting a
>> vertical perpendicular to the orbit of the satellite would
>> give best results.
>> Then maybe a Larsen mag mount... And bend - er, I mean,
>> "re-form" that whip right at the base so that it is about
>> 20" bent.  you'll be more successful with the FM birds!
>
> Yes, good idea, but now there is some directionality and so the car needs to
> remain pointed towards the satellite during the pass.  Otherwise there is
> still the null, it is just in a different part of the sky.
>
>> 1. If the satellite is circular polarization the 19.5 " whip looses 3 dB
> of gain.
>
> Yes, but most people have observed a polarization shift during overhead
> passes.  And in that case, the loss of a circularized antenna is much worse
> than 3 dB... usually a complete fade.  SO I agree, a circular antenna gets 3
> dB better half the time, and much worse the other half the time.  I prefer
> the simplicity of the whip that does not give up more than 3dB no matter
> what the polarization is.  Mounting a quadrifilar on the roof of the mobile
> is also problematic.
>
>> 2. The common Quadrifilar Antenna is 1/2 wave 1/2 turn.
>> The 1 wavelength, 1 turn Quadrifilar Antenna has its
>> highest gain near or at the horizon depending on the
>> length to diameter ratio.
>
> But it is still only a 3 dB gain antenna out there, and so it is not going
> to hear the satellite down on the  horizon anyway because the satellite is
> 10 dB farther away and usually blocked for the mobile.  So having gain on
> the horizon for a mobile omni antenna is wasted.  It either is not enough or
> it creats additional nulls higher up.  Better to move that gain higher up
> but smooth where the satellite is closer and then have good contacts, than
> waste gain where it is already insufficient or problematic.
>
> Just different perspectives, but the devil is in the details...
>
> Bob, Wb4APR
>
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