[Milsurplus] LO radiation

[Bob Camp]

Hi

Ok, here's a stab at it. I'm assuming a single diode mixer. A common 
setup in early IFF receivers. Simply to keep things straight the local 
oscillator radiating radio is called the transmitter.

TX LO power					+3 dbm
TX isolation LO to antenna		-3 db
TX antenna gain				+0 dbi

ERP from the TX				0 dbm

RX MDS (1 uv)					-107 dbm
RX antenna gain				+0 dbi
Minimum S/N for null			+27 db

Best guess at the RX			- 80 dbm

Maximum path loss for intercept 	80 db

In free space the greatest distance you have to worry about is roughly 
1,000 wavelengths. On a curved earth and real air the distance would be 
  less. You *could* go quite a ways at fairly low frequencies. Just 
plugging in 7 MHz (40 meters) gives you 40 KM (24 miles). That's far 
enough to make it a useful technique.

All of the numbers are subject to challenge. I confess to making them 
work out to nice round results. Since it's all in db, if you have 
better numbers feel free to plug them in. They probably make sense for 
a HF radio setup. For a VHF or UHF system you might add another 20 or 
30 db (mostly antenna gain). Another 20 db will extend the range by 
10X. That would give you about a 12 mile range at 150 MHz.

Bottom line - it's *theoreticaly* possible to do this. Doing it on a 
*practical* basis is a whole other issue. Simply deciding that an 
unmodulated carrier is a signal of interest sounds pretty hard to do. 
Any radio I have seen from the WWII era has plenty of unmodulated 
carriers running around it. The only documentation I have ever seen on 
this kind of thing refers to LO radiation from radar warning receivers. 
In that case the sub was the "TX" not he "RX".

Is it something worth worrying about? - yes, an enemy *could* use the 
technique. Did it cost us a bunch of lives? - not as far as we know.

	Take Care!

		Bob Camp
		KB8TQ









On Mar 26, 2005, at 11:58 PM, J. Forster wrote:

> Hue Miller wrote:
>
>> Yes, but.....Scott Co. boasted of their "safe" receivers in numerous 
>> magazine
>> ads. I think they really believed it. I do NOT think this was some 
>> kind of
>> "disinformation campaign"
>
> Perhaps, but advertising has never been known for truthfulness. Look 
> at some of the
> ads from the 30's... they sold 'drip pans' for your grid leak 
> resistors. Cigarettes
> were advertised as healthy... Smoke stacks pouring out black clouds 
> were touted as
> signs of 'industry' rather than polluters.
>
> It has not ended today. Just listen carefully to ads on TV. The 
> wording is
> purposefully deceptive. Autos are advertised as best in their 
> class.... often a class
> of 1.  BS then and BS now.
>
> As to disinformation campaigns, governments have always done it. Some 
> were (and are)
> more skillful than others. Others are just brute force liars.
>
> I challenge someone to actually do the link calculation, using typical 
> LO levels,
> isolation from the antenna, path loss, and receiver noise floor. This 
> is an issue
> that can be analyzed.
>
> Here is the outline:
>
> LO Power
> -  isolation to antenna
> + antenna gain
> - path loss
> + receiver antenna gain
>
> Gives you the power at the DF Rx input terminal.
>
> BTW, most WW II allied DF systems use nulls, rather than peaks, 
> because the null is
> sharper and phase sensitive, so DFing weak signals is not all that 
> easy.
>
> -John
>
>
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