[Milsurplus] LO radiation

[J. Forster]

Bob Camp wrote:

> Hi
>
> The three db isolation number is typical of a single diode mixer. I can
> dig out some papers if need be. I suspect it's also typical of the
> front end on a regenerative receiver. Better mixers or radios with RF
> stages would have significantly better isolation. Best guess would be
> in the 10 to 20 db range for single diode mixer designs with typical RF
> stages. A double balanced mixer gets you into the 30 db range with no
> RF amp and a matched antenna, less if there are mismatches in the
> system.

IMO, 3 dB is on the low side. AFAIK, receivers where the antenna goes directly
into a mixer are a post WW II development. The first such design I am aware of
is a roughly 1965 Racal design where the RF is mixed with a LO somewhere in the
30 to 70 MHz range, and later down converted again. This design has problems
even with today's high intercept mixers and Racal and Harris receivers often
require pre selectors.

If you rule out the direct up-conversion sets, the input stage of a receiver
would have a tuned circuit of fairly high Q before the mixer, even if there is
no TRF stage. Thus, I'm inclined to think that the LO would be isolated from the
antenna by 20 dB or more.

[snip]

> Again it's not a calculation of what *does* happen, it's only a
> calculation of what *could* happen. Without real data on real radios
> (and antennas - good point) you are never going to get real world data.

It's also unlikely any receiver is going to achieve the noise floor of a
shielded dummy load in actual service.

> If you were going to look at real data I think it you should look both
> at the LO and at harmonics of the LO. The combination of antenna gain
> and match might be better on the second or third harmonic ....

I'm not so sure the harmonics will get past the input selective stage in WW II
gear.-John

>         Take Care!
>
>                 Bob Camp
>                 KB8TQ