[amsat-bb] Re: Req. help with making a Coax Hybrid
Joop & Tineke Verdoes
joop.verdoes at nameplanet.com
Sat Nov 18 01:35:02 PST 2006
Thanks for your thoughts and thanks for the info and drawings you
provided some months ago, these are what I am using as a reference.
H'm I have thought about the connectors, but
- I solder the 100ohm terminator directly on one side of the coax. No
hidden lenght here.
- I solder the coax directly on the back of a male-chassis-N connector.
So there is hardy any extra length, at least not the amount that would
lower the match by 20-24 MHz (at least I think so!)
- The male-chassis-N connector goes on the MFJ-269, but.....the 75Ohm
part ends at the backside of the mal-chassis-N connector, so from there
on it should all be 50Ohm. That's why I like to think the length inside
the MFJ-269 doesn't matter.
I guess I'll start with pieces that are about 24MHz too long, and see
where it ends up when I connect the 4 pieces and then probably have more
food for thought :-)
Thanks again Domenico
Van: i8cvs [mailto:domenico.i8cvs at tin.it]
Verzonden: zaterdag 18 november 2006 5:49
Aan: pa1jav at amsat.org; AMSAT-BB
Onderwerp: Re: [amsat-bb] Req. help with making a Coax Hybrid
----- 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
> 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
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
> 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
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