[amsat-bb] Re: Good mobile antennas to use

[Art McBride]

Just to be fair, I have two comments on the Quadrifilar Antenna.
1. If the satellite is circular polarization the 19.5 " whip looses 3 dB of
gain.
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. 

Art,
KC6UQH

-----Original Message-----
From: amsat-bb-bounces at amsat.org [mailto:amsat-bb-bounces at amsat.org] On
Behalf Of Bob Bruninga 
Sent: Wednesday, September 14, 2011 7:31 PM
To: AMSAT-BB
Subject: [amsat-bb] Re: Good mobile antennas to use

>> The problem is hearing the downlink.  That is 
>> why the 19.5" whip is so good... and has 
>> almost 7dBi gain above 30 degrees up to over 70.
>> So it beats a quadrifilar easily...

> As an example, my last four AO-51 passes were 
> 36, 15, 26, and 27 degrees -max- elevation. 
> Would you prefer gain at low to mid elevations, 
> or above 30 degrees to work this satellite today?

We need to be careful to not bring pears and pommegranets into the apples
and oranges comparisons...

Were those 15 to 27 degree passes done with a Quadrifiliar on the downlink?
We need to keep comparisons to apples and apples or oranges to oranges.. not
all the other possibilities.

> A 1/4 wave will work, sometimes, but it is far 
> from ideal for anything but occasional downlink 
> access.

True, It limits operation to passes above say about 25 degrees, and that
does limit you to 2 or 3 passes per day, but at least they will be GOOD
passes that you can hear well with smooth performance across the sky above
that while mobile and with no pointing cappability. (except for a very rare
direct overhead pass above 80 degrees once in a blue moon) which lasts for
less than a minute..

> As Clayton pointed out, it's easy to spot the 
> guys on crappy antennas, as they are the ones 
> missing calls, or transmitting on top of QSOs, 
> because they mistake a loss of signal as a 
> break in the action.

What Clayton referred to were not simple 1/4 wave verticals.  He was
referring to standard "mobile" antennas that have gain on the horizon and
*guaranteed* nulls then between about 10 to 40 degrees (the primary
satellite operating range) or so plus the small null overhead.  By
mentioning the fades, he was clearly referring to these standard gain
antennas.  By definition, a gain mobile antenna has multiple nulls between
the horizon and overhead thus really playing havoc with satellitie contacts.
And that is precisely why a standard mobile GAIN antenna is well known to be
*no-good* for satellite work.

But this is not what a 19.5" whip does.  The 1/4 (3/4) wave 19.5" whip does
not have gain on the horizon (so it is rarely used for terrestrial mobile)
but its pattern is ideal for satellite work on high passes.  It does NOT
then have a null in its pattern that causes the "crappy" contacts, and it
does have plenty of gain above about 25 degrees.... and it is a smooth
pattern.... not like the multi lobes of a standard mobile gain antenna.

> PS My satellite grid addiction started in 2000 
> with a DR-605 Alinco and a 1/4 wave magmount, ...
> Since then I've operated mobile from probably 
> 100+ grids on the FM and SSB birds both. 
> It's addictive.

Yes, I think we are actually agreeing.  A 1/4 wave 19.5" vertical works
well.  But a common mobile gain antenna *does not*.  There is a big
difference.

Thanks
Bob, Wb4APR
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