Yep, a Cassegrain antenna would work.
John WA4WDL
--------------------------------------------------
From: "Bill Janssen" <bi...@ieee.org>
Sent: Friday, October 08, 2010 11:52 AM
To: "Discussion of precise time and frequency measurement"
<time-nuts@febo.com>
Subject: Re: [time-nuts] GPS backup for the stationary time and
frequencyuser
Magnus Danielson wrote:
On 10/08/2010 03:35 AM, jmfranke wrote:
When I said the feed would work, I was meaning it would work if LHC.
The illustrations and text imply you could just place a normal GPS
receiver at the feed location, but the polarization would be wrong.
Which was what I reacted on...
I am by no means a practical antenna expert, and the EM-theory is a bit
fuzzy on the edges, but I do distinctly recall that signal is RHC and
reflections becomes LHC so an antenna with RHC orientation will provide
some first-degree damping of the LHC reflections. For this antenna setup
the intended RHC signal is reflected and should become LHC... just as the
interference... so it relies on the antenna gain of the dish to
out-perform the other reflections for the half-space receiver that a
normal GPS antenna is. The choke ring for a dish head has a distinct
different pattern (forming an inner cone rather than flat space).
So, a normal antenna would kind of work since the antenna gain would
overcome the poor LHC supression of a simple RHC antenna... yay.
If an LHC antenna was used instead... now we are talking.
Cheers,
Magnus
So a dish reflector and a sub reflector and the GPS receiver at the dish
would work? What is that
configuration called? I can't remember at this early hour.
Bill K7NOM
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