[AMRadio] Local oscillators and TVI


Phil Galasso k2pg at worldnet.att.net
Sun Mar 5 20:59:41 EST 2006


Another source of TVI from local oscillators is the original version of the
Heathkit SB-110. The heterodyne oscillator operates around 42 MHz when the
50.0-50.5 MHz segment of the 6 meter band is selected. I used to use this
rig when I lived near New York City and never had any problems other than
some audio rectification RFI into a neighbor's stereo. When I moved to South
Jersey in 1992, I discovered that this rig would put a herringbone on
channel 6, even in receive mode. A strong second harmonic of the oscillator
at 84 MHz (right in the middle of channel 6!) was being radiated by my
antenna. The TVI extended over a considerable distance, as I walked down the
road with a small portable TV set tuned to channel 6 while the SB-110 was
turned on and receiving. My house in South Jersey is within the Grade B
contour of WPVI-TV, the ABC outlet on channel 6 in Philadelphia.

Heath later added a low pass filter after the heterodyne oscillator and made
a few other changes to tame this rig, renaming it the SB-110A. But it was
discontinued after a few years.

Phil K2PG




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