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R: Help with L1 uplink on AO-40

----- Original Message -----
From: Joe Steinmetz <joes@moon-bounce.com>
To: <amsat-bb@AMSAT.Org>
Sent: Sunday, January 05, 2003 7:32 AM
Subject: [amsat-bb] Help with L1 uplink on AO-40

> Hello,
> I am debugging my ground station for AO-40 and was wondering if someone
> might have been using the L1 uplink tonight could tell me how well it was
> working? I was just barely able to hear myself on the downlink, however I
> could hear the beacon and many others well above my signal level. I am
> assuming that my receive setup is OK since I could hear myself (in the
> noise) and others (well above the noise) and the beacon (very strong).
> At the time of my experiment (1/5/03 05:00 - 06:00 UTC) the squint was
> 12 degrees (assuming I set Nova up correctly).
> I am thinking that I have a serious uplink problem. I have two
> (RHCP) phased for approx 17dBic of gain, with about 60 feet of LMR-400
> ultra-flex and a SSB 80watt amp feeding the system. I am loosing 3.4dB in
> the cable run and according to my directional wattmeter I am getting 2
> reflected. Could something be wrong in the phasing that wouldn't show up
> a reflected energy? I am thinking of swapping the phased helix out and
> replacing it with a 22 element yagi just to help isolate the problem.
> I only plan on running enough power to get my signal 8dB below the beacon,
> but right now, I am way below that.
> How much power are others using for their L band uplinks? And what antenna
> (and gain) are you using?
> Any thoughts?
> Joe
> k6sat

Hi Joe K6SAT,

If  your directional wattmeter ( i suppose a BIRD 43 ) is connected at the
output of your 80 watt RF linear amplifier and if the loss of your line is
3.4 dB = 2.19 time in power,than the incident power reaching your antenna
is 80/2.19= 36.5 watt

Now, if the reflected power that you read in the shak is 2 watt it means
that the reflected power has been attenuated as well by 2.19 time in the
coax cable flowing from the antenna back to the wattmeter so that the actual
reflected power at the antenna connector is 2 x 2.19 = 4.38 watt

The reflection coefficient (RHO) at the antenna is SQR ( 4.38/36.5)= 0.35

The SWR at the antenna is than ( 1 + RHO ) / ( 1- RHO ) or

SWR = (1+ 0.35) / ( 1-0.35) = 2 :1

SWR of 2 is no good but no terrible but in your evaluations rememer that
along with all phasing problems using two helix you are running only
36.5 watt with a SWR of 2 in a coax cable with a loss of  3.4 dB with a
antenna without to know exactly  the actual gain.

I hope this help a bit.

73" de i8CVS Domenico

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