Topband: Omni VII performance on 160m

[Michael Tope]

----- Original Message ----- 
From: "Richard (Rick) Karlquist" <richard at karlquist.com>
To: <Topband at contesting.com>
Sent: Wednesday, July 04, 2007 7:51 PM
Subject: Re: Topband: Omni VII performance on 160m


> The concept of IP3 is that you can describe nonlinearity
> in a single number.  This presupposes that IP3 is independent
> of excitation level below some threshold, which is typically
> something like 10 dB below 1 dB compression.  I have measured
> a number of devices that simply don't have a defined IP3
> value, since I measure a different IP3 for every excitation level.
> 
> I am wondering if anyone has run into this on ham radio receiver
> measurements?
> 
> Rick N6RK

I suspect this is what is going on with the IP3 numbers in the 
ARRL review. Although you can make IP3 arbitrarily high by 
reducing sensitivity, in the case of the Omni 7 vs the K2, I don't 
think that is what's going on. The MDS numbers for the two 
receivers are comparable (with and without the pre-amp), so 
I suspect the reason for the large discprancy in the IP3 numbers 
is due to fact that they are using the "S5" method whereby
they raise the tone levels until the 3rd order product reads "S5"
on the meter, and then they take these tone levels and 
mathematically extrapolate to IP3 based on a perfect cube law
transfer characteristic. This would result in a number that 
could be used for apples-to-apples rig comparisons (assuming
MDS numbers were comparable) if all receiver followed a 
perfect cube law. As Rick's experience shows, however, they 
may in fact not follow a cube law, in which case the IP3 numbers 
end up being pretty meaningless (or at a minimum not useable 
for straightforward comparisons). 

73, Mike W4EF..................................