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R: phase shift with coax




----- Original Message -----
From: Stan Vandiver <Stan@Vandiver.com>
To: <amsat-bb@AMSAT.Org>
Sent: Sunday, July 08, 2001 10:00 PM
Subject: [amsat-bb] phase shift with coax


> Hi all,
>
> This is probably a no-brainer, but I will ask anyway. I'm needing to
> make a new phasing harness for a Cushcraft A144-20T to generate
> circular polarization.  I have 2 of the 3 pieces that are original, and
> they seem to be somewhat shorter than I expect (about 10-5/8").
>
> Is the "free space" formula for 1/4 wavelength (246 * velocity factor /
> freq in MHz) = length in feet) the correct way to calculate the
> lengths for these pieces? Do I need to factor in a little length to
> account for the barrel connector between the parallel phasing line
> and the delay line? Or account for the "T" connector that splits to
> the horizontal and vertical sections of this beam?
>
> The connectors might explain why the Cushcraft pieces seem a
> little short.  But also I do not know the velocity factor of the coax
> that they used, so I'm kinda confused on how to replace them. I'm
> thinking of replacing all of them, but maybe the better choice would
> be to just make one to replace the missing piece (which is one of
> the RG-59 75-ohm legs)?
>
> Will there be any good way for me to judge the effectiveness of the
> circular pattern.... or is SWR the only measurement I can make?
>
> Your comments are welcome!
>
> 73 de Stan/W4SV

Hi  Stan,

The A144-20T phasing harness is made by two coax lines of  RG59  Zo=75 ohm
and one coax line made with RG58 Zo=50 ohm

Each  of this line is electrically 1/4 wavelenght  long considering in it
the lenght of the antenna connector,the barrel  connector and the
"T" splitter connector.

The purpose of the 75 ohm 1/4 wavelenght line is to transform the 50 ohm of
each antenna to 100 ohm at the other end to be paralleled in to the "T"
connector to get again 50 ohm  for  the RX/TX , 50 ohm transmission line.

The purpose of the  50 ohm 1/4 wavelenght line is to add a delay of
 90°  to one antenna in order to generate circular polarization.

I have in my hand all of this tree lines and they are made with coax  RG59
and RG58 with a velocity factor of  0,66 so that the electrical lenght of
each of this line is 0,66 time the free space wavelenght for 144 MHz

Back to numbars:

Free space wavelenght = 300/144 = 2,083333 meters
Electrical wavelenght = 2,083333 x 0,66 = 1,375 meters
1/4 electrical wavelenght = 1,375 / 4 = 0,34375 meters or 34,375 cm
or 13,533 inch

As you realize this coax lines uses crimped PL connectors and i  see that
your 10-5/8" measurement refers only to the lengh of the outside black
jacket of the cable that you see between the crimped  ends  of the PL
connector and not to the entire line leght considering in it  both lenghts
of the connector itself .

If you connect one end of the 75 ohm line to the barrel and the other end to
the "T" and  now you measure the lenght  from the middle of the barrel and
 the middle of the "T",than you will get exacly 34,375 cm or 13,533 inches
wich is the lenght you need for  1/4 electrical wavelenght  at  144 MHz with
coax having 0,66 velocity factor.

I have made some improvement to this antenna and its phasing harness.

1) Remove the original aluminum connector from the gamma match bracket
    and use N connectors

2) Use RG59 and RG58 but with N connectors N barrel and  type N "T"
splitters

This modification is little bit costly but prevent water to enter in to the
phasing harness mostly from the non standard female aluminum PL
connector wich  in addition  generate aluminum oxide.

In order to see how much your polarization is circular and to measure its
circularity in dB the only way is to receive your carrier using  a 144 MHz
dipole placed several wavelenght away from your antenna and in to the
far-field zone.

The dipole should be directly connected to a microwattmeter like the
HP 435A wich is directly calibrated in dB

Rotating the dipole by 360° it is possible to get a good indication  of  how
much  your A144-20T radiates circularly or not.

I use 4 of this A144-20T in an array in wich all 40 vertical elements are
connected in phase.
The other 40 horizontal elements are connected in phase as well.

Than i have a gear switch antenna mounted made with 4 coax relays to change
polarization from the shak  from Horizontal to Vertical to RHCP or LHCP

I use this array since the beginning of OSCAR 10 and i am very happy with it
because the aluminum by wich the antennas are made is  a very good alloy.

My home is 100 meters from the beach at 50 meters from the see level  in the
sunny but windy bay of  Napoli and the array is still here.

I hope this help

Best 73 de i8CVS Domenico

>
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