[CQ-Contest] Re: 2-Radio Interference

[Tom Rauch]

Hi Mike,

> Yes, I agree wholeheartily that an S-meter shouldn't be relied
> on for direct measurements in dB unless you have a Kenwood TS-870 
> or something similar where the  meter is known to be accurate. I was going
> to throw in that caution, but I was in bit of a hurry the other day. Shame
> on me, I should know better. 

My pet peeve is the misplaced notion that S meters are 6 dB per S 
unit, and/or that they are reliable. 

There actually is no 6 dB standard. Collins and Drake shot for 5 
dB, as does the FT1000 and most other rigs. All of them are horrid 
across the scale of the meter. 

Even with an 870, I'd verify the meter.

> pretty good approximation. This mathematical relationship between 3rd
> order IMD products and intercept point is described in just about any text
> on RF design (for instance see Hayward's book "Introduction to Radio
> Frequency Design" pp219-232). I have also seen these rules of thumb
> described in CATV technical tutorials (there is more emphasis on 2nd order
> distortion in the CATV world), but unfortunately I got rid of some of that
> stuff before I acquired enough sense to know it was worth keeping. 


But as you and I know, these "rules" only apply when the device is 
not saturated, and the stages are all linear. What I'm after is some 
evidence that it works in real-world situations. My bet is that is 
does not.

Anyway, who cares? All we want to know is if the problem is in the 
receiver, not whether the distortion is harmonic or intermodulation 
based. 

I never gave this a moment's thought, except now I'm trying to undo 
damage done to a guy in Florida who is convinced he has receiver 
overload. Someone (not you) told him to use the same method and 
never warned him to check the meter. Now he is asking me what 
rig to get and I'm convinced his rig wasn't ever the problem.

Now I wonder where that idea is published in amateur literature.  
       
> What I should have said to Martin was that if the interference goes 
> down in level more than a known in-band signal of the same level (from an
> outboard signal generator for instance) when the attenuator is inserted,
> then chances are good that the interference is caused by a distortion
> product occuring in the receiver.  If on the other hand the transmitter is
> generating the distortion (e.g. the products are already present ahead of
> the attenuator), then interference will drop in level the same amount as
> an in-band signal of the same level. 

Yep, that's about perfect. It's so good I'll use that without crediting 
you. (just kidding, but it is a real good way to say it)
73, Tom W8JI
w8ji at contesting.com


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