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Re: Antennas Signal Strength



Jim,

Jim Bennett wrote:
> I have dual KLM's (22C & 40CX), Dual Landwear Preamps, 150' feet of
> 9913/flex, and a Yaesu FT-736R.  The birds I am currently trying to work are
> AO-27 and FO-20.  I have no problem working the birds since they are low
> flier, but I am interested in trying to work AO-10, RS-15, etc.  I haven't
> been able to hear them.
150' of 9913 @435 MHz will have around 6.5 dB of loss.  That is
considerable.  20 watts transmitted from the transceiver will be 4.5
watts at the antenna, IF you use constant impedance N connectors,
perfectly installed.  On a similarly reciprocal basis, a received signal
at the antenna will only be 1/4 the signal once it travels the 150' of
9913. The noise figure will increase dramatically, and you probably
won't hear much at all from the birds.  The received signal will
probably be right at the noise level.

Your situation is a classic example of where a mast mounted preamp will
improve your situation.  You didn't intimate to us that the preamps are
on the mast, if they are, fine.  If not, put them there!

I have 50' of 9913 to my antennas.  I use my preamps (mast mounted)
every time I work the birds.  I don't think of them as an overdrive
device, but as full time turbo boost!  :-)

The same situation applies to mode B.  Your 70 cm uplink is severely
degraded because of your coax losses.  You will need a brick amplifier
to work AO10 on a regular basis.  The exception will be perigee passes
when the bird is lower.  

RS15 is a difficult bird to work.  I have designed a 10 meter turnstile
using PVC cross pieces supported by a single PVC support.  I wire tied
copper wire to the frame for the elements.  It looks like a giant
clothesline, 10 feet off the ground.  The trick to RS15 is transmit
gain.  It is also very sensitive to polarization sense.  If you have the
correct polarity, and your antenna is pointed directly at the satellite,
AND you deliver sufficient power to the antenna, you can hear your
downlink.  It is not a popular bird, so you might want to make a sched
with someone first.  I'm willing.  Let me know when you're ready.

> I am not able to hear the birds unless I use my preamps on FO-20.  When the
> birds come over the horizon I can hear it, but no meter reading.  I have
> been told the my antenna needs some work on it, because I should be able the
> work the low fliers without the preamps.
Nope.  You WILL need the preamps.  Coax loss indicates degraded signal
levels.

As I see it, you have a couple options.  Move your array closer to the
shack and continue using 9913.  Scavenge some hard line and connectors
for the 150' run.  Or, compensate for losses with amplifiers in the
shack, and receive preamps at the mast.

In any event, you will need an amplifier for AO10.  

Tell me how much power your rig puts out.  If it is capable of 50 watts,
3.5 dB of loss on your 9913 @ 145 MHz will translate to about 22 watts
delivered to the antenna.  That would be more than adequate for the
LEOs.  If however, your rig is only capable of 10 watts output on 2
meters, you will only deliver 4.5 watts to the antenna.  This will be
marginal much of the time.

Let me know if you need any more help, or I'm sure others will jump in
with more and better information.

73 Jim, and I hope to work you soon!

Mike in Fort Myers, FL


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