> Tom, I thought Dave said that the 21 MHz spurious signal tracked his
> fundamental and held a 3F/2 relationship with it all the way across
> the 20 meter band. I don't see how a the 14 MHz fundamental beating
> with a fixed frequency oscillation could do this (Dave was the spur
> tracking really exactly 3*F/2 WRT to the 14 MHz signal??).
A possible clue would be to change frequencies and watch the spurious
frequency change.
> Can a free running oscillator injection-lock to a pump signal near its
> 2nd or 3rd harmonic? If so, perhaps that would explain it? Another
Locks at half the frequency are nearly impossible, because the
positive peak of the oscillating system would have to lock on both
positive and negative excursions of the main signal. True sub-
harmonics in non-linear devices are totally impossible.
One thing that works is to understand the complexity of and the
special devices with special characteristics required to **
intentionally** make the same problem. Considering how we would do it
intentionally, we can guess at how likely it is to occur in an
accidental circuit.
I remember in a local electronics class the teacher heard my 160-
meter signal on a broadcast radio at about 900kHz when I was on about
1800kHz. he used it to demonstrate "sub-harmonics" to the class, and
the question even appeared on an exam.
When I caught wind of it, I had him do a simple test and beat an
oscillator against my alleged "sub-harmonic" while tuning the radio
dial. The pitch of the sub-harmonic changed when he tuned the radio,
which it never would have done if it was a sub-harmonic.
I explained to him it was a receiver deficiency, aggravated by the
455 IF and local oscillator choice. The local oscillator was at 1355
when the receiver was on 900kHz. (455+900=1355) My signal was at
1810. 1810-1355=455
What worked out to be a subharmonic really was a IF image.
The point of all this is we sometimes miss simple in-your-face
frequency relationships. With so many oscillators in the multiple
conversion receivers and transmitters, and other signals in the
outside world, a lot of stuff can be going on.
A virtually impossible scenario is a f/2 unintentional divider
remixing, because sub-harmonics are virtually impossible if not
totally impossible to intentionally generate, let alone have by
accident.
On the other hand accidental mixers are simple and common, easy to
build, and can occur many places in the outside world...but they
require the correct accidental frequencies.
> possibility would be some poorly shielded RF device with a frequency
> divider that is capturing on Dave's nearby TX signal (or one of its
> harmonics). The frequency divider could produce the F/2 component
> which in turn could beat with the 14 MHz signal and to produce the
> 3F/2 component. The fact that the spurious signal is bidirectional in
> its response to Dave's antenna suggests to me that its due to a
> harmonic of his signal that is created either in or before the 20
> meter antenna (I would expect a Yagi to look bidirectional at its
> harmonics, whereas it would be unidirectional at its operating
> frequency).
>
> Interesting problem.
>
> 73 de Mike, W4EF...........................
>
> ----- Original Message -----
> From: "Tom Rauch" <w8ji@contesting.com>
> To: "'Amps'" <amps@contesting.com>; "David Robbins" <k1ttt@arrl.net>
> Sent: Friday, May 17, 2002 6:45 PM Subject: RE: [Amps] al-1500
> parasitic?? or some other problem?
>
>
> >
> >
> > > Well, I can now say for sure that it is not the amp. I have found
> > > that by feeding only the top 6 ele 20m beam and pointing it either
> > > due east or west I can generate the f/2 and 3f/2 products with as
> > > little as 45 watts. The beam heading is not what I was expecting
> > > as it puts both the 40m and 15m towers off the ends of the 20m
> > > elements. The low power needed does really narrow it down to
> > > something related to that top antenna or something very close to
> > > it... especially since I can not generate them with 1500w to a 4
> > > ele 20m beam 45' below it.
> > >
> > > I can also say that its not the guy wires as the 17 year old rusty
> > > ones were replaced yesterday.
> >
> > Hi Dave,
> >
> > I'm positive you are wasting time looking for non-linear joints
> > producing a F/2 or 3F/2 product. Sub-harmonics are impossible to
> > produce by virtue of a non-linear system. . Look for a mixing
> > product with another local oscillator or signal of some sort, or a
> > spurious oscillation in a device with gain.
> > 73, Tom W8JI
> > W8JI@contesting.com
> >
> > _______________________________________________
> > Amps mailing list
> > Amps@contesting.com
> > http://lists.contesting.com/mailman/listinfo/amps
> >
>
>
>
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>
73, Tom W8JI
W8JI@contesting.com
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