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[linrad] Out of band QRM. Worst case ?
I am going to make a front end for 144 MHz that will fit
the WSE converters and Linrad.
There will be two RF amplifiers, one preamplifier for
mounting in the tower and I will probably not make them
since such amplifiers are available from many sources.
The noise figure of RX144 will be about 11 dB and there
is a need for a second RF amplifier with high IP3.
The idea is to have a tower mounted preamp with a
gain (including cable losses) of about 15 dB followed
by a high level amplifier with a noise figure of
about 2 dB that will add about 10 dB gain.
A good preamplifier may have an output IP3 of +25dBm.
The input IP3 of the RX144 is about +20 dBm but if
both signals fall within the passband of the RX70, the
input IP3 is only +15 dBm.
After adding the second RF amplifier, the IP3 mis-match
between a +25 dB IP3 preamplifier and the Linrad system
amounts to 20 dB for in-band signals.
The in-band input IP3 (at the antenna) will be -10 dBm
while compression will start at something like -20 dBm.
The second RF amplifier could incorporate a filter that
converts in-band interference to out-of-band interference.
Are there any signals within 10 MHz from the 2 m band that
could produce more than -30 dBm so they would be near blocking
or is there any risk for a signal pair that could produce
third order intermodulation on 144 MHz ? Two equally strong
in-band signals at -50 dBm would give IM3 at -130 dBm which
would be well audible.
As far as I know there is nothing here in SM but I do know
there are pagers just below 144 MHz in the UK. It will be
fairly easy to design filters that make performance limited
by the mast mounted preamplifier for signals that are more
than 4 MHz outside the band edges if one wants full sensitivity
from 144 to 146 MHz. Likewise it would be fairly easy to make
the system limited by the preamplifier for signals below 142.5
or above 146 MHz if one wants full sensitivity from 144 to
144.5 MHz only.
My question is: Is there a real need for better than this?
Leif / SM5BSZ