[amsat-bb] Re: Good mobile antennas to use
bruninga at usna.edu
Thu Sep 15 11:20:25 PDT 2011
> Thanks for the plots Bob (http://aprs.org/astars.html 85% down the page).
Remember, ALL plots are for 70cm performance, even though some of them are
made using "2m" antennas to show how they perform as a dual band satellite
> The 2m 1/4 wave appears to do well on 70cm above
> 30 degrees, and fairly poor below that elevation.
> The 70cm 1/4 wave appears to suffer from no real
> nulls except at the very high elevations. It is
> about 5 DB down from the 2m 1/4 wave (70cm 3/4),
> but is also much more usable on the below 30 degree passes.
> So, would a dualband that is 1/4 wave on each band
> cover things pretty well? It appears like it would.
I agree. That would be the ideal coverage. But it depends on the
definition of "cover" and "pretty well". Yes, you will have COVERAGE, but
can you hear anything "pretty well" at 2400 miles and 6 dB farther away at
that low elevation angle? Much less even see the horizon in a mobile. I
personally dont think so.
Sometimes we focus too much on the "antenna coverage" and ignore the
GEOMETRY of a pass which has a full 10 dB difference between the horizon and
overhead. Please look at the scale drawing of the geometery of a LEO pass
shown on http://aprs.org/rotator1.html.
It all depends on what an individual considers "Good enough" and what is the
minimum signal to hear anything at all. If we assume that 3 dB of gain on
the horizon is good enough to hear a LEO satellite at the horizon, (I do
not) then the best combination is to start with a 5/8 wave 70cm vertical.
At 15 degree elevation, switch to a 1/4 wave 70cm and with that combination
and with the up to 6 to 10 dB improvement as the satellite gets closer, then
you have good no-null coverage from the horizon to overhead.. But remember,
to do this now you have two UHF omni antennas, and a diplexer plus a
separate VHF uplink antenna (again, maybe 2 to avoid nulls) and you have to
know where the satellite is.
But, if you believe that there is no LEO currently on the air, that you can
hear on the horizon with only 3 to 4 dBi gain, (like I do) then, write off
that area of coverage since you wont hear it anyway and take the +5dB
performance imnprovement of the 3/4 wave vertical (19.5" whip) which is dual
band and optimized for the higher elevation and gives no nulls and does give
strong signals (but only for 1/3rd of all passes).
I'll be the first to admit that I do not have enough actual experience to
know what is the minimum gain needed to actually hear the leo at the greater
distances (lower angles) on an omni. Looking at the gain plots, as you say,
if you can actually hear a LEO at say 15 degrees on a 1/4 wave whip, then it
is the best antenna because it will be useable lower and the poorer
performance higher up will be made up by the reduced range.
On Thu, Sep 15, 2011 at 12:07 PM, Bob Bruninga <bruninga at usna.edu> wrote:
I donno. The only 1/4 19.5" antenna I ever used I just drilled into the
roof above the dome light and installed an NMO mount and simple whip. For
conversion of an existing mount, if it is a 5/8's wave vertical (most
mobiles are) then there is a matching network in the base.
The little tiny micro whips are ideal, but most of them now are being made
as "dual band" which then destroys their 7 dBi gain above the horizon.
No easy answers here.
From: Ng, Peter [mailto:Peter.Ng at bccdc.ca]
Sent: Thursday, September 15, 2011 12:47 PM
To: 'Bob Bruninga '; AMSAT-BB
Subject: RE: [amsat-bb] Re: Good mobile antennas to use
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.
This is off topic and didn't want to start something on the list... :), but
can I convert my current magmount gain antenna simply by replacing the whip
or is the "gain" stuff in the magmount itself? If so, would any thin steel
rod do? I'd really like to give this a try!
73's Peter VE7NGP
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