[Date Prev][Date Next][Thread Prev][Thread Next] - [Date Index][Thread Index][Author Index]

Re: Have Distortion Problems-Can't Work FO-29

Hi Domenico,

You are, of course, correct that the 3rd harmonic is well outside the 
satellite downlink passband.  But never the less, I was fooled several times 
while making the transition from the Easy Sat (RS-10) to FO-20.  At least, 
fooled by something.  The characteristic was a really bad audio, no doppler, 
and (the little clue) reception after the satellite had set below the 

If it wasn't the 3rd harmonic, or something related to it, I can't think of 
what else it could be.  My rig is a Yaesu FT-736R, and at the time I did not 
have a preamp.  I think I did have a Mirage 2m power amp - the 2m uplink 
antenna wasn't very good, and I had 60' of RG-8 between the rig and antenna.

Maybe related, but I also had desense problems with AO-40 mode US, whenever 
the satellite went behind some nearby trees.  Seemed like something from the 
uplink was bouning back and messing with the receive side.  Maybe the 
problem is trees?

Gerg  KO6TH

Hi Greg, KO6TH

The 3rd harmonic of an FO-29 uplink fall between 146.000 x 3=438.000 MHz
and 145.900 x 3 = 437.700 MHz  wich is about 2 MHz far away from the
downlink between 435.800 and 435.900 MHz and so it is difficult
to confuse the 3rd harmonic with the real translated satellite signal.
However the 3rd harmonic can be very strong causing gain compression
of the 70 cm receiver or intermodulation products wich can appear as ghost
signals in to the downlink particularly when more than one signal is present
in to the passband.
At first make a check without FO-29 in range and while transmitting LSB
between 145.900 to 146.000 MHz tune the RX between 437.700 and
438.000 MHz with the intent to find the presence of the 3rd harmonic.
Since the generation of 3rd harmonic is by multiplication and not conversion
you realize that moving 100 kHz in 2 meters you have moved 300 kHz
with the 3rd harmonic in 70 cm
In addition you are transmitting LSB in 2 meters with a 3 kHz BW and by
multiplication x 3 the 3rd harmonic is still LSB but about 10 kHz wide and
since your receiver in 70 cm is set USB you will receive the 3rd harmonic
with a lot of distortion because of the above mentioned reason all added
Having verified the existence of the above 3rd harmonic wich is normal now
tune your receiver between 435.800 to 435.900 MHz while transmitting in
the uplink and verify if you spot some ghost signal produced by your self.
If the downlink is clear set your 2 meters TX in LSB and your 70 cm RX in
USB for normal traffic with FO-29
Waiting for the next pass of the bird you have the time to make a QSO in
SSB with a local friend and ask to him to check if the modulation of your TX
is clear or distorted.
If your LSB modulation is locally clear than you have a chance to be
succesful via FO-29 but a final test is necessary when the satellite is in
range with a numbar of users because in this situation your hown translated
signal can intermodulate with the others in to the passband and can generate
noise like spits while you speak particularly if the IP3 of your RX is
low,the gain in front of your mixer is excessive and the signals in to the
passband are strong.
If this is the case than the spits desappear as you move the antenna from
the satellite or after the satellite LOS
More simply KD8SY was probably hearing himself distorted because he was
using USB instead of LSB for the uplink.
Curious to know your comment.

73" de

i8CVS Domenico
Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
Not an AMSAT member? Join now to support the amateur satellite program!
To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org