[CQ-Contest] so2r

Dick Frey k4xu at arrl.net
Thu Dec 21 23:03:31 EST 2000


With a radio like an FT100, you need to consider a third cause of
interference which is more likely than the two you mention.

Phase noise and high transmitted noise floor.  The smaller the radio, the
more likely it is using all PLL oscillators to generate the carriers inside
the rig.  These are not very clean signals. Each has a "noise tent"
associated with it. This is clearly shown in the ARRL reviews as a
transmitted noise floor.  It will be stronger on the low side of the signal
because of the output LPF. The transmitted noise floor can be as much as -80
dB below peak output.  If you transmit 100W or 50dBm, the tx noise is -30dBm
or S9 +40.  It takes a lot of filtering and antenna separation to get that
down below -100dBm. Using multi-band antennas doesn't help the situation

The same thing happens in reverse when receiving on the same rig.  It's then
called reciprocal mixing.

The smaller or cheaper the radio, the more likely this problem will be. Have
you heard some radios in the contests lately that sound like a kazoo? Not a
clean tone but kinda fuzzy?  This is the result of driving a big amplifier
with a radio with a lot of phase noise. On some particularly bad examples,
if they are really loud, you can hear their noise-modulated signal calling
CQ all over the band. Yes, it's legal, the FCC spec is -40dB, but it sure is
not best commercial practice.

As more of the semi-little pistols do SO2R, we see more second- and
third-tier radios in contests. Ya pay for what you get. The guys who
successfully run full power SO2R have very clean rigs to start with and use
filters and stubs to take out the (relatively) little noise that's left.


>On most bands, I have S-9 or higher broadband hash on the second radio =
when transmitting on the first (there are a few band combinations where =
there's almost no noise at all, and a couple where there is full-on =
rectification of the transmitted signal). Running a FT-920 and FT-100, =
with a tribander on the roof and a vertical in the back yard, probably =
25 ft from the tribander (not much of a contest station, I know - but =
it's what I have...)=20

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