1270b.1

  I have recently taken the opportunity to observe electromagnetic
compatibility of the MFJ-1270B as used by several of our NET/ROM nodes.
(some of this may apply to other brands also.) T4e initial esults were not
as good as they should have been.  The question was 'Do the TNC's cause
interference to co-located sensitive VHF receivers ?'  The answer is YES,
depending on the situation.

  In two different sites measured, the noise coming from the TNC was
sufficient to cause 4-7 db worth of desense to a 0.2 uv (12 db SINAD)
receiver (145.01).  There are two types of noise that are that radiate from
the MFJ-1270B.  One type is broadband in nature (probably due to
data/address bus activity and DC-DC converter operation) the other kind is
harmonics from the crystal oscillator and sub-multiples thereof
((4.9152/2) * 59 = 144.998 MHz).  The broadband noise can cause interference
on all frequencies if a sufficiently sensitive receiver in a quiet location
is involved.  The harmonic energy can cause problem on specific
frequencies.

  This interference is detrimental to the weak singals that serveral of our
nodes are required to operate with.  In addition it may cause problem to
the communications of our hosts whenwe share a commercial site.

   It is relatively easy to determine if you have a harmonic problem
because the receiver noise will quiet when the TNC is turned on.  The
broadband noise usually can not be heard unless you observe the noise level
in the presence of a weak signal.  (With the normal antenna connect, inject
a signal that provides 10 db of quieting then turn on the TNC and observe
the change in noise level.  If you have a 0.5 uv receiver or are in an
otherwise noisy location you may not be affected. With a 0.2 uv receiver
you may lose 6 db of quieting.)

continued in 1270b.2