Normally I would agree, John, but Earl was talking about
signals that were down in the noise. At these levels one
might be hard pressed to distinquish 1:1 versus 1:3
behavior. In this case a hi-pass would be a better choice.
In fact, a T-network tuner makes a pretty good hi-pass
(I can barely receive normally strong AM stations below
1 MHz when my T-network is in-line with my antenna
and tuned for 160).
73 de Mike, W4EF.................
----- Original Message -----
From: "John Kaufmann" <jkaufmann@alum.mit.edu>
To: <topband@contesting.com>
Sent: Tuesday, July 01, 2003 5:55 PM
Subject: RE: Topband: A/B testing K6se etc.
>
>
> > If there is actually no 3rd harmonic being radiated by the BC station, I
> > assume that you're saying that the fundamental (at 610 kHz) is causing
> > the receivers under test to somehow generate a pseudo-3rd harmonic.
>
> It seems to me there is a simple test that can be performed to determine
if
> a third harmonic is being generated internally (in the receiver) or
> externally (at the BC transmitter). It just requires a step attenuator
> inserted in series with the antenna. Since most receivers have built-in
> attenuators, they are probably adequate, as long as they are ahead of the
> first active stage in the radio.
>
> The main point is that an internally generated spurious third harmonic
> should respond nonlinearly to changes in input signal strength. An
> third-order nonlinearity that is producing a third harmonic response
should
> theoretically cause the third harmonic generation to drop 3 dB for every 1
> dB of input attenuation. If 10 dB of attenuation is inserted, then the
> third harmonic should drop 30 dB if the receiver is generating the
harmonic
> itself. If the harmonic is being produced externally, then 10 dB of
> attenuation will result in a linear 10 dB drop in the harmonic. It should
> be easy to distinguish whether the harmonic has dropped 10 dB or 30 dB.
>
> 73, John W1FV
>
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