[amsat-bb] Effects of Strong Uplink Signals on AO-40 Beacon and Passband

[Charles Suckling <charles.suckling@xxxxxxxxxxxx>]

Effects of Strong Uplink Signals on AO-40 Beacon and Passband
===============================================

Introduction
-----------------

There has been some correspondence recently on the AMSAT-BB concerning the 
effects of strong uplink signals on other users of A0-40.
In response to this, on 5 April from 1000z for about 45 minutes DJ1KM, 
G3WDG and I8CVS made some tests to determine exactly what effects strong U 
and L uplink signals have on both the beacon and passband signals. Passband 
activity was fairly quiet at the time, and there were no other "very 
strong" signals active. We hope that these tests did not disturb other 
stations too much!

L-Band Uplink effects on Beacon and U-Band generated passband signals
-----------------------------------------------------------------------------------------------------------

The first test was to see how the beacon level was affected by a strong 
uplink signal on L-Band. DJ1KM used his panoramic receiver to monitor the 
beacon, and G3WDG provided the L-Band signal in the form of a steady 
carrier. With the uplink power set to provide a downlink signal equal to or 
slightly stronger than the beacon, DJ1KM saw the beacon level drop by 
approximately 3dB when the uplink carrier was on. This drop was also 
observed by I8CVS.

The second test looked at the effect of the L-Band uplink on a U-Band 
generated passband signal. I8CVS transmitted on 435 a steady carrier at a 
level to put his downlink signal approximately 10dB below the beacon. DJ1KM 
and I8CVS both observed the level of this passband signal to drop when the 
L-Band uplink as mentioned before was on, again by about 3dB.

U-Band Uplink effects on Beacon and U-Band / L-Band generated passband signals
-------------------------------------------------------------------------------------------------------------------------

The third test repeated the above experiments, this time using a U-Band 
uplink provided by I8CVS.

Again a carrier was used, which did not seem to attract the attention of 
Leila and so was not nulled out. The results were similar to those obtained 
with the L-Band uplink, ie a very strong U-Band uplink can suppress both 
the beacon and other U-Band generated passband signals. The amount of 
suppression of both the beacon and other U-band generated passband signals 
seemed greater than for the L-mode uplink (up to 10dB was observed briefly 
at one point).

The last test involved looking at the effect of a very strong U-Band uplink 
on a passband signal coming from an L-Band uplink. In this case, the U-Band 
uplink did NOT affect the level of the passband signal.

I8CVS also transmitted a cw test transmission with deliberate high power. 
For most of the transmission Leila ignored the signal. SSB was then tried, 
and Leila locked on very fast and nulled the signal.

Conclusions
------------------

With a U-Band and L-Band uplinks, excessive erp will suppress both other 
U-Band generated passband signals and the beacon. The effects are greater 
for U-Band, justifying the existence of the Leila system for that uplink.

With U-Band uplink, there are probably two causes for the suppression - agc 
action of the U receiver depressing the levels of other mode U users 
(larger effect) , and power sharing effects in the transponder (smaller 
effect). It is not clear to us why L-Band generated passband signals are 
not affected by U-Band uplink level.

With L-Band uplink, the suppression probably comes only from power sharing 
effects.

Recommendations
---------------------------

The use of excessive erp on U-Band and L-Band uplinks reduces the level of 
the beacon and Mode U uplinked passband signals.

If stations operate with only enough uplink erp to generate passband 
signals which are not stronger than 10dB below the level of the beacon, 
then suppression effects caused by one individual user will be very small.

Stations using L-Band should be especially vigilant not to use excessive 
uplink erp - there is no Leila to remind them of their antisocial 
behaviour! Means of adjusting and monitoring transmit power are desirable 
since the erp requirement is a strong function of squint, and what may be 
just OK at 20 degrees will certainly be excessive at 5 degrees.

Stations using cw on U-Band should also take special care, since Leila may 
not be as quick of reminding them that they are too strong, as it does with 
ssb.


73 de G3WDG, DJ1KM and I8CVS

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