[NLRS] shack local noise issue - 2M - help?

[Dr. Gerald N. Johnson, electrical engineer]

It doesn't take instability of the RF stage to cause the noise to change
with termination impedance. The modern VHF and up RF stage gives its
best NF when mismatched. That's more pronounced with FETs, especially
GaAsFETs. That makes for one of the large uncertainties in measuring NF
and is why the best noise generator heads include 15 or 20 dB
attenuators, so the noise output of the stage being tested changes far
less from impedance change than from noise source.

That being said, any given hunk of RS coax or cheap coax with minimal
shield IS an antenna and IS a noise source. The loss component of the
coax (even good coax) is a resistor at room temperature, warmer than the
cold sky.

BNC connectors are not used much for serious test equipment because the
shield connections are rarely aligned the same way twice and that
changes their leakage. N or TNC leak less though they use essentially
the same shield connection, the screw rings align them more solidly and
consistently.

Watch out for 75 ohm BNC connectors, their center pin is small and may
or may not touch the fingers of the 50 ohm jack. Sometimes they are
clearly labeled and sometimes not.

When the computers and TNC radiate so much (Paccom are quieter at VHF
than MFJ) noise it may be necessary to go to double shielded coax in the
shack or heliax jumpers and to minimize leaky connectors by making the
system standard be type N or TNC away from the radio.

73, Jerry, K0CQ
-- 
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.