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R: phase shift with coax




----- Original Message -----
From: Vince Fiscus, KB7ADL <vlfiscus@mcn.net>
To: i8cvs <domenico.i8cvs@tin.it>
Cc: <amsat-bb@amsat.org>
Sent: Saturday, July 14, 2001 5:31 PM
Subject: Re: [amsat-bb] phase shift with coax


> At 02:52 AM 7/11/01 +0200, you wrote:
>
> >Dont forget that this type of antenna was produced as well for non
amateur
> >frequencies and the size of your phasing  harness is not for 144 MHz  but
> >for a higher one around 183 MHz
> >
> >73 de i8CVS Domenico
>
>
> When you checked the point of resonance on each of the driven elements on
> your two meter array, what frequency was it? Mine was way above the 2
meter
> band. Did you change the point of resonance on your antenna?
>
> KB7ADL
>
>

Hello Vince,

There are at least 3 different type of  this antennas and i will list only
the lenght of their radiators.

        A147-20T
  Vertical radiator :                         37 7/8"    =  96,20  mm
  Horizontal radiator:                     38,1/2"    =  97,79  mm

        A144-20T
* Vertical/Horizontal radiator      38 1/2      =  97,79 mm

        A144-20TE
  Vertical/Horizontal                           ? ?           97,20 mm

* A144-20T Horizontal and Vertical elements are cut to the same lenght.
Dimensions shown are for 144 MHz ,they will vary with antennas cut to
different frequencies.(This is from the instruction sheet)

My antenna is model 144-20T and all tree 1/4 electrical wavelenght long
lines of phasing harness are 13,38 inches from tip to tip

This is the tuning  procedure recommended by  CUSH CRAFT

The A144-20T uses two separate 52 ohm feed lines.
The A144-20T uses the harness/delay line included in the package.
Before connecting the harness or feed line,tune the individual Reddy Match
driving elements.
( Reddi Match is the arm of the gamma match)
When tuning the antenna at ground level,it can be mounted with the
boom in a straight vertical position with  the reflectors approximately
7/8" from and parallel to the ground.

1) Connect a 52 ohm feed cable to the driven element with a PL-259 fitting.

2) Insert the standing wave ratio bridge in the line between the transmitter
    and the antenna.
    Set the transmitter to your center operating frequency.

3) Adjust the bridge for maximum forward power .Check the standing wave
    ratio with the bridge in the reverse position.
    If the VSWR is high,loosen the tuning strap and adjust it slightly in
    either direction.
    Move away from the antenna and check the reading .
    If the VSWR has increased,move the strap back to its original position
    and adjust in the opposite direction.
    If the VSWR dropped,repeat the adjustment procedure until there is
    little or no reflected power indicated on the bridge.

4) Do not use the phasing harness with model A147-20T

Now my comments follows:

As soon the antennas are adjusted individually for the lowest VSWR
connect to it the phasing harness and be happy for the overall VSWR
even if it is greater than individually obtained.

Do not attemp to adjust the gamma match straps with the phasing  harness
connected to the antennas because changing the lenght of the gamma match
arm changes the phase angle and the delay time between one antenna and the
other one is not anymore 90°

The gamma match system is not a very good device at all to achive circular
polarization because any different in lenght of  one arm changes the phase
angle from the 90° provided by the delay line of the phasing harness.

In addition,inside the aluminum pipe of the gamma arm there is a piece of
coax cable RG213 without the jacket and without the braid.
The inner conductor of this coax cable and its polyethylene insulating
material forms a capacity with the inner sourface of the  pipe of the gamma
match arm.

The lenght of this piece of coax cable has been calculated in order to
get a capacitive reactance -jX at 144 MHz  to cancel out the inductive
reactance +jX  introduced by the lenght of the gamma match arm.

Now if the gamma match straps are individually adjusted at different
distance from the antenna connectors in order to get a low VSWR
this lenght difference introduces as well  a difference in the phase angle
at wich one antenna radiate RF in respect to the other one.

The best approach is to search for an identical  position of  both straps
wich provides an acceptable VSWR and leave it as is.

The final test is to verify if the gamma match system works properly.

The gamma match is intrinsically a device to connect directly an unbanced
feed  line like a coax cable to a dipole without using a baloon.

This system has many limitations and generally the RF current flowing
in one semidipole is different than that flowing in the other semidipole.

To verify if this is the case is very easy:

Supply the antenna with roughly 50 watt and using a small neon bulb like
GE 52 touch the glass envelope to both ends of the same dipole.
If the bulb light up different it means that the RF voltage and as well the
currents at the dipole ends are different.
In this case the antenna radiate a bad lobe and it is best to change with
antennas  using folded dipoles particularly at 435 MHz where the
gamma match is absolutely unsuitable.

With my  array made of 4 x  A144-20T i got an acceptable compromise with
all  the above advantages and disadvantages.

Best 73 de i8CVS Domenico















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