Re: [amsat-bb] continuing education

["Edward R. Cole" <al7eb@xxxxxxxxxxxxx>]

>From: Jerry Pixton <jpixton@shentel.net>
>Hi all,
>
>thanks for the notes on metal in middle of helix. Think I will continue 
>along those lines. Makes the mechanical part easier.
>
>My question for todays "continuing education" session  ---
>
>When we compute the G/T performance metric, do we use the full gain of the 
>antenna?  That is, we don't subtract 3 dB because of polarity mismatch 
>(circular to linear). Seems to me if we measure the G/T metric using the 
>sun then we are getting the whole gain.
>
>The 3 dB loss only comes in when we think about a particular signal we want 
>to receive. Correct????? Or am I confused?

Jerry,

When you calculate G/T from sun measurements [vs cold sky] you input the
solar flux, as precisely measured by an institutional radio observatory,
into the calculations.  If they use linear polarization then your
measurement will compare directly without a 3 dB correction factor.  If
they made their observation using circular polarization then your
mesurement will be -3 dB if you are linear.  Its safe to assume they use
circular polarization since the sun radiates in many polarizations {and
probably is randomly polarized}.  Also at VHF and UHF frequencies the
ionosphere twists the signal via the Faraday Effect.  At 2.4G Faraday is
negligible.  OK?

Ed

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