Re: [amsat-bb] Explain the RG62 thing to me, please...

[Hamish Moffatt <hamish@xxxxxxxxxxxx>]

On Sat, Apr 07, 2001 at 07:20:43AM +0100, Howard Long wrote:
> The version you describe apparently assumes a 3/8wl distant reflector, which
> I wasn't.
> 
> I'm not quite sure how the impedances in this example are coming together to
> give a 50 ohm impedance, but hey, if it works, don't fix it!
> 
> Without the reflector I calculate 75 ohms in parallel with 108 ohms, which
> is 44 ohms - not a bad match. With a reflector that's going to drop quite a
> bit.

Well, the theory for the design I used is, as far as I understand it:

Each dipole is 100 ohms individually. (I measured this with the SWR
meter -- the SWR was about right for 100 ohms matched to 50 ohm
coax/transmitter). In parallel, that makes 50 ohms. In this case,
the RG-62's 92 ohms is an approximation to 100 ohms (where it acts
as just a phasing line, not as a matching transformer), rather than
86.6.

As I calculate it, sqrt(92 * 100) = 95.92 ohms, and 95.92 || 100 ohms
= 48.95, which is pretty close to 50. The actual SWR is a bit higher
than this, but I didn't bother to tune the reflector distance since
it's working OK.

The 4/10 lambda separation gives a bit of a drop in gain overhead
(but not a complete null) to increase the gain to the sides; I figured
that if the satellite's overhead, it's lower and louder anyway.


Cheers,

Hamish
-- 
Hamish Moffatt VK3SB <hamish@debian.org> <hamish@cloud.net.au>
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