[amsat-bb] Re: Req. help with making a Coax Hybrid

i8cvs domenico.i8cvs at tin.it
Fri Nov 17 20:49:10 PST 2006

----- Original Message -----
From: "Joop & Tineke Verdoes" <joop.verdoes at nameplanet.com>
To: <amsat-bb at amsat.org>
Sent: Friday, November 17, 2006 1:06 PM
Subject: [amsat-bb] Req. help with making a Coax Hybrid

> OM's
> I am at the moment attempting to make a 2m and a 70cm Coax hybrid
> divider/combiner.
> I get interesting results.
> I have made 3 1/4 and 1 3/4 lamda pieces of 75 Ohm coax.
> - I included the velocity factor (after measuring this)
> - I tested each by using them as a 1/4 or 3/4 impedance tranformer 100
> Ohm to 50 Ohm, my MFJ 269 sees each of the pieces terminated with 100
> Ohm, nicely as a 50 Ohm thing, right on the frequency I want (146 MHz)

Hi Joop, PA1JAV

When you test the above 75 ohm coax 1/4 and 3/4 electrical wavelenght long
transmission lines as impedance transformers using a MFJ269 you includes
the lenght of a coax connector in the above line.

In this condition with connector included you found a lenght of coax +
connector by wich at 146 MHz the impedance transformation between
50 ohm and 100 ohm occurs because the 75 ohm matching line is actually
1/4 wavelenght long resonant at 146 MHz 

As soon you removes the connectors and solder the 4 piece in to a ring to
form an hybrid combiner/divider than the 1/4 and 3/4 lines becomes shorter
for 146 MHz by the lenght of the above connectors so that the hibrid 
resonates hygher as you reported at 166 MHz.

> I solder the 4 pieces into a ring, terminate with 50 Ohm at 3 ports and
> connect the MFJ at the "TRX" port.
> The thing works (ie shows 50 Ohm with and swr of 1.0).........but at
> 166MHz. At 145Mhz, the swr is 1.2 and the impedance shows some
> reactance.
> Before attempting to make wild guesses which leg of the hybrid I am
> going to cut/extend, would like to ask whether there is someone who can
> point me in the right direction.
> The coax cable is 3c-2v tv 75Ohm coax.
> It appears that the pieces are a bit too short, but this doesn't agree
> with my test of each leg seperately as an impendance transformer.
> Any suggestions?

The pieces are now a bit too short because you have removed the
lenght of the coax connector.

When cutting a resonant lenght of coax cable the lenght of any coax
connector must be included in it because the connector becames
actually part of the resonant line.

> Joop, PA1JAV

73" de

i8CVS Domenico

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