Satgen519   Birdies and Artifacts  by GM4IHJ  (BID SGEN519) 1999-03-06

As satellite technology takes us to higher and higher frequencies, the
radio amateur is forced to adopt a strategy of addon coverters, and
home brew antennas. Noting that not much else is available at a
reasonable price. Unfortunately, as the situation attending an attempt
to track the LEO mobile phone satellites reported below , shows. This
strategy encounters reception problems, caused by many false signals.

Even expensive commercial equipment available for frequencies below
2GHz suffers these afflictions. How do they arise and how can their
nuisance be minimised ? Initial work tracking Iridium downlinks in the
band 1616 to 1626.5 MHz using an ICOM R7000 receiver revealed over 100
false signals in this band. Some entered directly into the receiver at
the antenna but were so strong that they were not eliminated by
filtering, or when in the receiver first mixer combined with spurious
frequencies to produce a signal in the receiver pass band. Some
signals were present on say 1626.0 MHz and were also present on 626.0
MHz indicating a problem in the circuit whereby the higher receive
frequencies are obtained by switching to a dial reading + 1GHz
setting. So these phoneys are identifiable by dudicious use of the +
1GHz switch.

Other false signals can be identified by switching the antenna to
dummy load , where the phoneys are still audible. Some are  generated
in the receiver itself, as can be proved simply by screening the
antenna with metal. Even the fact that a signal is present night and
day gives away the fact that it is certainly not a satellite, while
the reception of a descending doppler shift generally reveals a
satellite . Although that test should if possible be contiued for at
least 20 minutes in order to eliminate the  drifting signals
which come from neighbourhood usage of computer games consoles of
almost zero radio cleanliness.

Taking another approach, it is often the case that birdies appear in
strings. Which ever method is used for 2.5 GHz reception, Drake
converter to Yaesu 9600 , or Drake to ICOM R7000 , the same flocks of
birdies appear . One group spaced 56 KHz apart occupy a band 380 KHz
wide , whilst another group spaced at 11 KHz intervals , spread across
120 KHz. Both these groups were found to be entering the system after
the converter, and were coming from a group of commercial masts 2
miles and nearly 800 ft above the IHJ station, Clearly both Iridium
and Globalstar will have to contend with some of these problems .

Meanwhile at IHJ, the first task is to produce a frequency list
tabling all these birdies , so that they will not be mistaken for the
real thing. Then next a series of procedures for identifying them , +
1 GHz off, antenna dummy load, converter dc off, etc  , will be tested
whilst long term plans are made to try different antenna
configurations Eg the monitoring of Iridium is aimed at checking User
loading , not reading their messages, so constant tracking is not
essential, which allows the deployment of very narrow beam antennas.
Equally some downlink sections have almost no phoneys on them , so
perhaps they will be the ones to be monitored regularly.