Hi all,
maybe we have an avionics specialist here because we are running out of ideas.
I have a Microair 760 installed in my Europa. My mate is ATC-engineer but he
could
not get hold of the following problem yet even when using the most expensive
equipment (I have no clue at all about this strange matter). Of course I also
will get in contact with Microair (I'm afraid they will ask me to ship the
transceiver to Australia for check/repair, beside that I have to search for the
serial number on the installed transceiver, which the previous owner did kindly
not notice in the appropriate field on the manual). I need a 8,33 KHz
transceiver
anyway, but have time until 2018 and I am reluctant to spend the money
now.
Okay - here comes his letter for Microair:
AGC-Trouble at Microair 760-Transceiver
Our Transceiver Microair 760 has a Trouble to receive signals from stations at
ground with low power and great distance.
The squelch (setting at Minimum) opens, the green LED is on, but there is no
audiosignal
to hear in the headsets. Some ATC-stations in Germany are not readable
in the aircraft. The Stations can read our transmission loud and clear, so
it seems to be only a Receiver-problem.
The Transceiver is properly installed in an aircraft Europa XS with Lambda/Half
Copper-strip-Dipol in the Rudderfin, connected with 6 m Cable RG 58/U and
BNC-Coax-Terminator.
BNC-Terminator, Cable and Antenna are checked with Network-Analyzer
Rhode&Schwarz
FSH4, no unexpected reflection on the cable and the antenna is normal. The
Reflection of Antenna was displayed at a distance of 6,5 m, that seems ok.
Then we tested the Microair 760 with an calibrated HP 8648 C Signalgenerator and
RG58-Cble direct in the Aircraft. The AM-modulation was set to 50% with
Frequencies
400 Hz and 1KHz.
These are the Measurement results:
Squelch-setting without signal at minimum, squelch is broken by RF-Signals from
the generator greater than -121 dBm ( 0,22 V ). We had no NF-Voltmeter to
control
the audio-signal, only Headset and subjective ears.
At this very low RF-inputlevel we can hear low noise and no modulation.
Pulling up the RF-Level to -110 dBm, low noise, and weak modulation.
At -100 dBm the noiselevel seems unchanged, the modulation is clear and loud.
By pulling up the RF-Inputlevel to values -90 dBm and -80 dBm, the noiselevel
seems
unchanged, but the audiolevel increases proportional to the RF-Inputlevel.
This effect is observerved up to Inputlevel of -40dBm to -35dBm, than we can
hear
unchanged and constant modulation signal at higher RF-Inputlevels up to -25
dBm.
This effect was measured at all Frequencys in the Airband from 118,00 MHz up to
136,00 MHz in 2 MHz Steps and selected Frequencys, were the Receive-Problems
were reported.
With an Handheld-Transceiver Sportys, the Measurement was done in the same way
to compare the results. This receiver is not so sensitive, his squelch opens at
-110 dBm.
But here we have normal conditions, the noiselevel is high, the modulation is
loud,
but distorted. By increasing the RF-Inputlevel, the noiselevel is decreasing,
modulation-level is unchanged. Proportional to the Inputlevel, the distortion
of modulation decreases, the level is unchanged. This receiver operates normal.
So it seems there is an error in the AGC or in the interference part. The
audiolevel
follows the RF-Inputlevel.
How can we fix this problem, is there an easy way at a local Avionics service,
or must the transceiver shipped to Australia?
Thanks for any ideas!
Roland
PH-ZTI
XS Trigear 914
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