[HBR] Re: Walt's transmitter project

[Bill Higgs]

Just a thought on getting rid of the first image . . .

With a fixed image about 10% away, you may want to consider trying an LC shunt trap at the image frequency. This is somewhat akin to the old trick of using a shunt crystal to help remove any leftover carrier back in the days when balanced modulators might only give you 30dB or so carrier rejection. I don't know if you could use a modified IF can for this (the Command set IF transformers have a nasty reputation for being difficult to modify) but a BFO coil might work. You should be able to try it with the toroids, though I don't know how well this might work with the higher impedances found in tube circuits. If you can get your hands on an old "loopstick", these things have pretty good Q and will let you use a fixed cap, saving some space and possibly expense. Some of the designs from the late 50s used these (modified) as IF transformers or antenna coils.

A trap of reasonable Q at 10% off might cost you a couple of dB at the wanted freq, but should knock the image down an additional 25dB or so.

Just an idea . . . strange what happens when you are awake at midnight because you ate too much pizza for lunch.

-Bill, NT4C