Re: NEC-LIST: RFI/EMI Policies at Universities and Research Labs?

I am a professional technologist and have specialised over the last 10
years in resolving RFI/ EMI and intermode problems on radio high sites
and in urban environments.

In my experience it would be suicide to allow mobile radio systems
within 2-3 miles of any radio research establishment. 10 miles would
be preferable.

The problem is not so much the actual transmission itself on its
assigned frequency, but rather the wide band noise generated by high
power mobile radio systems. This noise can spread from as low as 30
MHz right up to the cut-off frequency of low pass filters which could
be 900 MHz + for SMR systems. Cavity filters and combining systems do
help to reduce the wide band noise.

The other problem is transmitter intermodulation which can occur up to
9th order and need not necessarily be generated in the transmitters
themselves, but can be generated in any metalic junction within a few
hundred yards of the transmitters. Telephone distribution boxes where
crimped or badly made connections start to corrode are a common source
of intermod mixing. Another source is in CATV and off air TV cable
amplifiers or even antenna connetors. This intermod and noise can
then radiate at fairly high levels for several miles. I have tracked a
re-radiated intermod from a television amplifier at a distance of over
10 miles.

Remember that FCC regs require all spurs and harmonics to be -70dB
with respect to a 100 Watt erp transmitter. If the transmitter
radiates 100 Watts erp (typical) (+20dBW) and spurs are at say -80dB
this still represents an absolute level of -60dBW or -30dBm. Assume a
path loss of 40 - 50 dB (100 yards horizontal separation) then the
signal arriving at the receiver is -80dBm. If your receiver has a
sensitivity of -120dBm then you still have over 40dB excess signal to
block any signals you are trying to receive. Even working indoors on a
test site, 40dB is a lot of attenuation.

I believe if this policy is permitted, you can forget about any space
research and even low level signal work on campus. Fight to keep them
out.

Chris Turner, Pr Tech (Eng)
ZS6GM / G4HKP
Received on Sun Mar 07 1999 - 19:58:18 EST