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Re: [SPAM] Re: 435 offset yagi phasing



----- Original Message -----
From: "Andrew Rich" <vk4tec@people.net.au>
To: "jonny 290" <jonny290@gmail.com>; <amsat-bb@amsat.org>
Sent: Saturday, December 29, 2007 3:25 AM
Subject: [amsat-bb] Re: [SPAM] Re: 435 offset yagi phasing


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

Hi Andrew, VK4TEC

Because only a 1/4 electrical wave or any odd numbar of 1/4
electrical wave of 75 ohm transmission line make the impedance
transformation from the 50 ohm impedance of the antenna to
112.5 ohm as follows:

112.5 = 75^2 / 50

> I have made it so now, and added a pre-amp as well
>
> Now we sit and wait for a satellite
>
>
> --------------------------------------------------------------------------
--
> Andrew Rich VK4TEC

73" de

i8CVS Domenico

>
> -----Original Message-----
> From: amsat-bb-bounces@amsat.org [mailto:amsat-bb-bounces@amsat.org]On
> Behalf Of jonny 290
> Sent: Saturday, 29 December 2007 11:29 AM
> To: amsat-bb@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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