[Milsurplus] LO radiation

[mstangelo at comcast.net]

Maybe some members could test the theories and fire up their TBX and other
receivers and see if the LO radiation is detectable.

73,

 Mike N2MS


> 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 
> > 
> > 
> > 
> 
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