[Milsurplus] LO radiation

[J. Forster]

WA5CAB at cs.com wrote:

> John.
>
> In a message dated 3/28/2005 9:15:06 PM Central Standard Time, jfor at quik.com
> writes:
>
>> Is there no preselector (cavity?) before the mixer? What do they do about
>> images. If
>> not, the reason may be that they are for airborne use. DFing an aircraft is
>> less
>> useful than a ship, because they don't stay in one place nearly as long.
>> Also, since
>> they are designed to detect the much higher EIRP of a radar set, they could
>> well do
>> so from far beyond an range a LO might be detectable.
>
> Sorry.  Yes, there are tunable tuned circuits across the antenna input, as
> there are across the grids of the RF amps in those with RF amps.  By directly
> I meant without benefit of RF amp in between the antenna circuit and the
> mixer.

There are plenty of sets w/o RF amps, but until the direct conversion sets, I'm
not aware of anything w/o some kind of preselection.

Basically, my point is that the same filter that rejects images will also reduce
the LO getting to the antenna significantly. An amplifier increases the
isolation, but the filter attenuates too. Note that to be effective in image
rejection, the BW of the filter must be less than the IF frequency, because the
difference between the received frequency and its image is twice the IF
frequency.

> Sensitivity is of course down but then they were designed for passive ECM
> operations against German airborne intercept and ground fire control radars.
> And Jap stuff too, except that they didn't have much.
>
>> BTW, my comment quoted below is really about HF and below.
>
> OK.  I suspected that later when you or someone mentioned up-conversion.
>
> Robert Downs - Houston
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