[amsat-bb] Re: Req. help with making a Coax Hybrid
domenico.i8cvs at tin.it
Sat Nov 18 08:55:07 PST 2006
----- Original Message -----
From: "Joop & Tineke Verdoes" <joop.verdoes at nameplanet.com>
To: "'i8cvs'" <domenico.i8cvs at tin.it>; "'AMSAT-BB'" <amsat-bb at amsat.org>
Sent: Saturday, November 18, 2006 10:35 AM
Subject: RE: [amsat-bb] Req. help with making a Coax Hybrid
> 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
> Joop, PA1JAV
Hi Joop, PA1JAV
At one side of the 75 ohm 1/4 wavelenght coax you have soldered the
100 ohm termination and at the other end of the line the coax is directly
soldered to the back of a male-chassis-N connector with the front side
of it screwed on to the female N connector of your MFJ-269
In this conditions it is hard to say how much the length of a male-chassis-
N connector is part of the 1/4 electrical wavelength line and how much
of this connector is part of the instrument.
The velocity factor of your TV 75 ohm coax cable is in the range of
about 0.75 and for 145 MHz the electrical lenght of a 1/4 lambda is 388 mm
while for 166 MHz it is 339 mm so that the difference in the electrical
lenghts is 49 mm wich is greater than the lenght of your male-chassis-N
Anyway the coax of your 1/4 lambda line is soldered to the central pin of
the female-N connector and the braid of the coax is twisted before to be
soldered to the flange and this introduces inductance so that when you
unsolder the coax from the N connector the resonant frequency of your
1/4 lambda line rises a bit.
To prevent uncertainty in the measurement of velocity factor when building
hybrid rings with coax cable I normally use PTFE 75 ohm coax RG-302 for
145 MHz and semirigid UT141-75 for 435 MHz because the velocity factor
for teflon is constant at 0.694 so that without using connectors you can cut
the pieces with the PTFE right flat to the braid or right flat to the
outside copper tube of the semirigid cable leaving out only 1 mm of central
conductor protruding off the teflon at both ends of the line without
problems with teflon material soldering the coax at high temperature.
When cutting the 1/4 and 3/4 wavelenght pieces the golden rule is "When in
doubt,cut it a little bit short".Do not allow extra bits for what you may
think may be part of the line lenght -you are almost invariably wrong but
using RG-302 or UT141-75 the velocity factor is invariably 0.694 and
without the need to make extra measurements to find the velocity factor
you can cut the electrical pieces with great accuracy.
In addition you don't mention the type of 100 ohm terminations used
at one side of the coax but at 145 and 435 MHz Allen Bradley carbon
resistors should be used for your test because they are less inductive
than spiralized resistors.
A better solution is to use a SMD 100 ohm chip resistor directly soldered
between the central conductor and the braid or two 100 ohm in parallel if
a 50 ohm termination is required.
If only conventional spiralized carbon resistors are available than wrap
up the body of resistor with a thin foil of copper recovered from the braid
of a coax cable and then solder the foil only at one terminal of resistor so
that the capacity between the foil and resistor will partially cancel out
the inductive reactance of the carbon spiral.
I hope this helps
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