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R: ao40




----- Original Message -----
From: Jerry <jerco@pi.be>
To: <amsat-bb@AMSAT.Org>
Sent: Tuesday, June 25, 2002 5:08 PM
Subject: [amsat-bb] ao40


> Hello
>
> Anyone knows how much pwr you need for a good 23cm uplink on AO-40?I use
> TS790E with 10W out of trx but I use 21.5m(70.54ft) of aircom+,so loss is
> about 3.3dB,but antennas is 4*32 hor stack.My dwnlink sig is not very
> understandable.
> I don't know what it could be,maybe not enough pwr?
> That should be strange.
> The ant's are homemade DL6WU but drivers are from 23cm Flexayagi.
> maybe there are some dipoles with reversed baluns inside,but I cannot see
> it..
> Maybe it would be better to make my dipoles myself.
>
> Any sugg?
>
> 73's,Jerry.
> ON4CJQ
>
>
Hi Jerry ON4CJO,

If already have a BIRD 43 in your hands connect the instrument to the
output connector of your FT790E and measure the forward power Wf applied
to your transmission line.

Now connect the instrment at end of your transmission line where it is
connected to your 4 port power divider  in the array and measure again the
forward power Wfa reaching the antennas.

10 log     ( Wf / Wfa )
           10

is the real attenuation of your transmission line in dB

Now rotate by 180 deg the measuring head on  BIRD 43 and read the reflected
power Wra at the antenna connector ( not in the shak )

The BIRD manual has a diagram to directly read the SWR from the above two
mearurements of  Wfa and Wra but if you are not familiar yet with it compute
your SWR from the above power measurements of  forward and reflected power
at the array antenna connector.

First calculate the reflection coefficient RHO at the antenna

RHO = sqr ( Wra / Wfa )

Now compute the SWR with the following formula:

SWR = ( 1 + RHO ) / ( 1 - RHO )

Having made this calculation you  already have many informations on your
system but a  low transmission line attenuation and a good SWR are not a
full indication of the array performance.

For example the SWR can be a good 1,2  but the array is not working because
the four  dipoles and baluns are not correctly phased.

If your dipoles ( drivers ) comes from Flexayagi there should be obviously
a way to connect in phase 4 of this original Flexa antennas.

You must discover on the dipoles an existing identification mark for the
inner conductor and for braid of  the coax cable to be connected to it.

On my Tonna 23 element yagi  1270 MHz  for example the folded dipole are
made with copper wire and they comes  with a preset lenght of coax cable
directly soldered to it but the coax cable is incapsulated in to a black
boby of plastic material.

At first it seams to be impossible to identify the side to wich the inner
conductor and the braid  where soldered to the dipole.

Carefully looking to the plastic block shows that manufacturer identified
the braid side with a small but well visible cilindrical plastic
protuberance over it.

Now the only necessary to get the array in phase is to orient all 4 dipoles
with the above protuberance on the same side on the array,no matter if they
are all right or left but all must be oriented to the same direction.

Someting like this should be marked on Flexayagi dipoles.

If the antennas are correctly phased and your transmission line attenuation
is really 3,3 dB or 2,13 time in power ratio you apply 10/2,13= 4,7 watt to
the array and if the SWR at the antenna is a good  1,2 for example you
should not expect to receive a strong signal back uplinking on the L1 band.

We have good examples of  acceptable dowlink signals on AO40 using 20 watt
directly applied to the array connector of a 4 x 23 element Tonna yagi
horizontally polarized antennas and this is the case of iK3VZS for offpoints
belove 15 degrees and my self for example.

The gain of a  single Tonna  23 element yagi for 1270 MHz is 18 dBi and so
for an array of 4 by 23 you should expect a maximum gain of  22 dBi and not
24 dBi as in  theory.

Compare this data with that of your 4 x 23 elements DL6WU equipped with
Flexayagi dipoles and make your calculations.

I hope this helps a bit

73" de i8CVS Domenico

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