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[aprssig] What a diffeence an antenna makes

Robert Bruninga bruninga at usna.edu
Mon Oct 31 17:26:57 UTC 2005


Ah, but how do you feed a ½ wave vertical center
fed on the roof of a car?  J pole does the same
thing, but has the advantage of being fed at
the bottom...  Bob

>>> dave at emv.co.uk 10/31/05 12:22 PM >>>
A 1/4 wave on a car still has a fairly high angle of radiation, with a
null (supposedly) directly above.

A 1/2 wave dipole (centre fed, mounted with it's elements vertical) will
have a near horizontal radiation pattern, (sort of doughnut shaped) also
with a null above.  But, will put a better signal to the horizon,
usually.

The choice of antenna somewhat depends on if you want to hit the bird as
it skims the horizon, or get a good signal as it passes near overhead.

In practice, both a 1/4 and 1/2 wave antenna will work of course, also
the Turnstile type will send considerable RF towards the horizon, but
predominantly horizontally polarized, as opposed to vertical for whip
types.

Horses for courses etc...

For sheer ruggedness, a 1/4 wave whip can't be beat, so long as you have
a good RF ground plane to drive it against. (Sea water is not so good at
VHF as it is at HF)  The 1/2 wave is good if you cant use a ground
plane, but is less rugged.

The Turnstile probably will do the best job with satellites, but is the
least rugged, and most difficult to implement of the three types
mentioned so far.

Nearly time to quit here.

Dave B.  G0WBX.

> -----Original Message-----
> From: aprssig-bounces at lists.tapr.org 
> [mailto:aprssig-bounces at lists.tapr.org] On Behalf Of Curt, WE7U
> Sent: Monday, October 31, 2005 5:06 PM
> To: TAPR APRS Mailing List
> Subject: RE: [aprssig] What a diffeence an antenna makes
> 
> On Mon, 31 Oct 2005, Dave Baxter wrote:
> 
> > A "Turnstile" or crossed dipoles?
> 
> Yea, as I understand it you want a higher radiation angle for 
> this kind of satellite than you get with typical home-style 
> ground-plane style antenna, which was why Bob B. was 
> suggesting a while back that you digi from your house through 
> your D700A mobile to hit the sats.
> A 1/4 wave whip over a suitable ground-plane is a reasonable 
> compromise.
> 
> A turnstile might be ideal if you can physically fit it on there.
> If not, shouldn't a 1/2 wave positioned vertically perform 
> slightly better than a 1/4 wave antenna on a car?
> 
> --
> Curt, WE7U.   APRS Client Comparisons: http://www.eskimo.com/~archer 
> "Lotto:    A tax on people who are bad at math." -- unknown
> "Windows:  Microsoft's tax on computer illiterates." -- WE7U 
> "The world DOES revolve around me:  I picked the coordinate system!"

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