[Milsurplus] LO radiation

[Bob Camp]

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.

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 ....

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.

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.

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 -).

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.

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



On Mar 28, 2005, at 2:06 AM, J. Forster wrote:

> 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
>
>
>