[amsat-bb] Re: [SPAM] Re: 435 offset yagi phasing

[Andrew Rich]

I know that, but why does it have to be 1/4 wave ?

I have made it so now, and added a pre-amp as well

Now we sit and wait for a satellite


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Andrew Rich VK4TEC
vk4tec at people.net.au <mailto:vk4tec at people.net.au>
http://www.tech-software.net




-----Original Message-----
From: amsat-bb-bounces at amsat.org [mailto:amsat-bb-bounces at amsat.org]On
Behalf Of jonny 290
Sent: Saturday, 29 December 2007 11:29 AM
To: amsat-bb at amsat.org
Subject: [SPAM] [amsat-bb] Re: 435 offset yagi phasing
Importance: Low


The 1/4 wave sections transform the 50 ohm antenna impedances to 100
ohms at the end of the 75 ohm matching section. If you then use a tee to
parallel the two antennas, the impedance follows the same rules as
paralleled resistances, which means you have a 50 ohm  system at the tee.

You can do it another way, too - just tee the 50 ohm lines and off the
tee run a 1/4 wave section of 37 ohm cable, made with two runs of 75 ohm
coax in parallel. This transforms the 25 ohm impedance at the tee back
up to 50 ohms at the end of the matching section.

I don't know which is superior - you have your options, though.


Andrew Rich wrote:
> Ok I have to ask the question.
>
> With offset yagis, those antennae that introduce circular polarisation by
> means of offsetting the second yagi 90 degress,
> why would you cut your coax to a 1/4 wave length ?
>
> You already have 90 degrees from the displacement of the yagis.
>
> It should not matter what length the 75 ohm coax is ?
>
> Would it not ?
>
>
>
>
> --------------------------------------------------------------------------
--
> Andrew Rich VK4TEC
>
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