----- 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