[Milsurplus] LO radiation

Sun Mar 27 10:45:39 EST 2005

Hi

I'm afraid that we may not get a lot closer than rough order of 
magnitude calculations. There are simply to many variables. The only 
real point is that it's possible with the gear used in WW2 to have the 
problem. I suspect that with super regenerative receivers the problem 
is a bit worse than with a super heterodyne, but without actual 
measurements on real radios it's hard to say just how much worse.

One thing that would make the situation a lot better would be a 
balanced first mixer. They are pretty common these days, but I have not 
seen them in the typical 1940's HF radio. They certainly existed in 
that era, but did not show up in receiver front ends. I have not seen a 
schematic of the Scott gear so possibly they went that way.

If you ever have to do the isolation thing in the future take a look at 
a cascode (common emitter driving a common base). It can take the 
reverse isolation (S12) up into the > 100 db range at HF. Of course 
your intermod won't be very good. The common base or common gate 
pre-amp is a lot more common way to do the trick.

	Take Care!

		Bob Camp
		KB8TQ

On Mar 27, 2005, at 10:12 AM, Mike Feher wrote:

> Bob -
>
> I am in total agreement with your calculations, on a rough order of
> magnitude basis. As had been mentioned previously we discussed this at
> length years ago and I mentioned then that it is still a problem. In 
> the
> 80's I ran a group at Melpar designing surveillance receivers, mainly 
> for
> aircraft, where the LO re-radiation numbers were specified and almost
> impossible to meet. We went through the link calculations to see if we 
> could
> get away with greater LO re-rad, but, the link calculations showed 
> that we
> could not, given the desired detection distances and anticipated 
> intercept
> receiver characteristics. We used a front end amplifier only to reduce 
> the
> LO from the mixer feeding back to the antenna. Even then a typical 
> amplifier
> only had about a 10 dB of reverse isolation and we needed more and 
> found
> that common base design gave the best performance. These receivers were
> pretty state of the art with fast tuning capability and an IF greatly
> separated from the incoming signal, so the LO was even further 
> suppressed by
> the additional varactor tuned front end filters and the balanced 
> mixer. Yet,
> it was still a problem. In WWII, the IFs were typically of low 
> frequency and
> the front end did not give much suppression of the LO. And, amplifiers 
> do
> feed their output back into the input without oscillation, it is almost
> impossible for them not to. There are well know requirements that must 
> be
> met for oscillation to occur. LO re-rad is a real issue and it has 
> been in
> the past and continues to be today. Back then the intercept receivers 
> were
> worse but the LO re-rad was higher. Today the intercept receivers are
> better, and, the LO re-rad is lower, making it almost the same 
> situation. 73
> - Mike
>
>
> Mike B. Feher, N4FS
> 89 Arnold Blvd.
> Howell, NJ, 07731
> 732-886-5960
>
>
>
> -----Original Message-----
> From: milsurplus-bounces at mailman.qth.net
> [mailto:milsurplus-bounces at mailman.qth.net] On Behalf Of Bob Camp
> Sent: Sunday, March 27, 2005 9:36 AM
> To: Radioman390 at cs.com; Milsurplus
> Cc: J. Forster
> Subject: Re: [Milsurplus] LO radiation
>
> Hi
>
> Tempest compliance is a real issue in some areas. There have been some
> significant documented intelligence exploits of gear that was not
> Tempest compliant. The whole idea behind what has been done is to make
> it theoretically impossible to do a radiation based exploit on a piece
> of certified gear. What does and does not need certification is handled
> by a government agency (with predictable results).
>
> You'll get no argument out of me that the whole thing gets carried
> *much* further than is justified.
>
> 	Take Care
>
> 		Bob Camp
> 		KB8TQ
>
>
>
> On Mar 27, 2005, at 2:41 AM, Radioman390 at cs.com wrote:
>
>> "J. Forster" <jfor at quik.com> 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"
>>>
>> What, may I ask is all the brouhaha about "Tempest compliant"
>> receivers?
>> There's actually a spec for the degree of attenuation of internal
>> signals at the antenna input.
>> And this is still current!
>> Also, look at the shielding in some Racal receivers like the 6775/6778
>> series. Each circuit board is mounted in a mu-metal case.
>>
>> Strangely the 6790 has a single bih "mother board", and the earlier
>> 6772 just had circuit boards mounted in slots. But there was a 6772E
>> which had the same shielding as the 6775/6778.
>>
>> What was all that about?
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