[Milsurplus] LO radiation

[J. Forster]

Bob Camp wrote:

> Hi
>
> The original post was based on either a super regenerative radio or
> something like the early IFF sets. They had an acorn tube diode in the
> front end and had no amp in front of it. Without a lot of research all
> I can say is that I *think* I have seen low cost HF sets from the
> 1930's that had no RF amp in front of the mixer.

Quite possibly, but there has to be some selectivity, whether active or passive,
in front of the mixer or you have huge image problems. Just look at any band
with a spectrum analyzer w/o a preselecting YIG.

> Tuned RF stages would help, but on a radio with only a 455 KC IF it's
> not clear that significant attenuation would occur when the radio was
> tuned to the 10 MHz range. The loaded Q of the front end stages
> normally is kept a bit on the low side to keep the Qu/Ql ratio high. If
> you don't then the loss goes up and sensitivity suffers. We are talking
> about a fairly simple radio here and not something like a R-390 ....

IMO, the selection on the input has to knock images down 40 dB or more, so it'll
knock the LO down likely 20 dB or more.

> Reverse isolation of the RF stage(s) is still going to be a question.
> Certainly there are modern front end amplifiers with isolation in the
> 10 db range. There are also a number of common designs that get you
> into > 60 db of isolation. Most of the typical tube front ends are
> closer to the 10 db designs than the 60 db designs.

No argument.

> Net result, you would get at least another 10 db on a radio with an RF
> stage. Without measurements it's not real clear how much past 20 db
> that would get you.

Can we agree it's likely more than 10 or 15 dB, possibly more? My main point is
that it is not 0 dB.

> Another issue that's open to question is the assumption of a specific
> LO power level. There is easily another 10 db or so there that could be
> questioned. Measurement would tell us in which direction the 10 db
> should be (+ or -).

In tube stuff, drive dB levels may not be the real issue, since they are very
far from 50 Ohm systems. 1 V RMS at 100K is way below 0 dBm.

> Again, all we are trying to figure out is weather this *could* have
> been possible with gear on a merchant ship not if it ever happened.

Not quite. I think the objective is more to figure out if a merchant ship with
typical radio gear could be detected by an Axis sub, and if an Allied plane
could detect an Axis sub by DFing the LO emissions of their receivers. And if it
can be done, would it have been at a range to be useful from a tactical point of
view.

> Can this be done? My opinion is that it could from the standpoint of
> radiation levels. I can see no way you could come up with a *practical*
> operational shipboard (let alone submarine) DF system based on this at
> HF though.
>
>         Take Care!
>
>                 Bob Camp
>                 KB8TQ