Hi Andy,
> I've been watching this topic with interest. I can't add anything from
> a detailed technical perspective to the discussion, but I can say that
> perhaps it's not so un-common. During one contest I had major problems
> with a 7 MHz transmission generating a large signal at exactly f/2. It
> was fully locked to the fundamental, but a bit raspy in nature with
> wide sidebands. It was power related in a non-linear fashion, and
> disappeared at low powers.
That is the typical sound and description of a parasitic that is
phase locked at half of the operating frequency. It is somewhat
common in some systems that have shortfalls in design or design
errors, and sometimes mistaken for a subharmonic.
It is NOT an F/2 subharmonic problem, it is a spurious oscillation.
> I never tracked it down since it had gone away when I next came to use
> the station. Gear was a pair of FT1000MPs, pair of ICE switched
> bandpass filters, an LK-800A on the 40m station (and a Ten Tec Titan
> in line on the 80m station). The was however really strong coupling
> between the 402CD and the 80m loop hanging beneath it with, prior to
> the 80m bandpass filter, severals 10s on W of 40m RF indicated on the
> Titans meter in Rx. In reality it was less than that since the SWR was
> high on the 80m loop on 40, but substantial levels of coupling anyhow.
One of the ways something like this happens is because most
amplifiers use only low-pass networks on both input and output. The
only attenuation below the operating frequency comes from mismatch
losses.
I've heard several 160-meter stations during contests with spurious
emissions. I've had that problem myself.
One 160 station with an ETO amplifier has this problem quite often
ONLY when he uses an antenna tuner. The ETO has an inherent
unintentional feedback circuit through the chokes on the PIN diodes,
and when he uses a high-pass T network tuner the amplifier "unloads"
the output. With the output unloaded feedback path loss through the
chokes is less than the gain of the stage, and the PA oscillates
sporadically below the operating frequency. It causes broad clicks on
his signal, and some new frequencies.
A similar thing occurred under the right situation with Ameritron QSK
boards. A very low frequency low-pass filter had to be installed in
the diode supply lines to prevent unintentional oscillation below the
desired frequency when the switch was used with high-gain tubes like
8877's and 3CX800's. With the right (wrong) load impedance at LOW
frequencies, such as half the TX frequency, there would be a low-
level oscillation. It would even phase-lock to the main signal at
times under the right conditions.
The problem I had in my own station related to a situation similar to
yours, when I was on a small lot with close spaced antennas. The
signal level from my transmitter was sufficient to override the
diodes used to switch filters and RF stages in one of my
transceivers, and it allowed the transceiver to become a big
unintentional oscillator and mixer. It was, of course, level
sensitive.
Another problem I had occurred while testing an ALS-600 on the air.
In this case, a spurious F/2 signal appeared at certain power levels.
As I would change antenna directions it would change. After spending
hours trying to stabilize the ALS-600, I used my head. I swapped
amplifiers to another totally different type and the problem
remained. It turned out the Beverage antennas I had were picking up
the radiated signal and feeding it back to the external antenna port
of the transceiver I was using. When the level of that signal
exceeded the holdoff voltage of the diodes in the receiver section,
the antennas would couple into the shared tuned circuits between the
transmitter and receiver and turn that transmitter on as an
unintentional amplified feedback system.
While I not saying you can not have a spurious signal, I've NEVER
been able to intentionally generate a f/2 **subharmonic** with
conventional non-linear components. There have been times when I
**thought** it was happening, but
every time investigation revealed I actually had an unintentional
oscillator or transmitter someplace.
Subharmonic is probably one of the worse and most misleading terms to
be commonly used.73, Tom W8JI
W8JI@contesting.com
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