On 7/31/2014 7:29 PM, Jeff Liebermann wrote:
On Thu, 31 Jul 2014 08:16:29 -0400, Jerry Stuckle
wrote:

Yes, I know how it works - I've held both amateur and commercial
licenses for well over 40 years and worked on almost everything from $40
CB sets to multimillion dollar mainframe computers.

Well let's see... I got my novice license when I was 15, so that means
I've had a ham license (various call signs) for 51 years. My FCC
First Class and now GROL for about 46 years.

I've even done some design in my free time.

My time is not free. I did RF design and ran a 2way shop for about 11
years. I guess that having more years of experience entitles me to
tell you how to run your life.

I suggest you not try to teach those who know
more than you your "facts".

Yep. Quantity is a good substitute for quality. Unfortunately, I
have to agree with most of your comments.


True. Fortunately, all of my experience has been "quality".

The facts a to have the beat frequency, you need two signals within
15kHz or so of each other. Those signals must be mixed, which means at
least one must be non-linear.

Signals are not linear or non-linear, but are usually called
"distorted" if not a clean sine wave. The circuitry through which the
signals pass can be linear or non-linear. Linear circuits do not mix,
unless overloaded. If the mixing circuit is non-linear, you can use
harmonics to create more mix combinations that result in 15KHz
frequency difference.
n*f1 +/- m*f2 = beat_freq
where n and m are integers.


Just because a signal is not a sine wave does not mean it is distorted.
A mix of several different frequencies (i.e. music, voice) produces
something far from resembling a sine wave - but it is not distorted. I
won't get into the math here - but there is a solid foundation.

And if a linear circuit is overloaded, it is no longer linear.
Harmonics may or may not come into play - it all depends on the
characteristics of the circuit. For instance, the harmonics of an FM
band (88-106 MHz) amplifier are nowhere near the audio range. And mix
combinations are even less likely to be anywhere near the audio range.

This will give you a beat frequency in
the audio spectrum.

Yep. However, my hearing is kinda marginal and needs something lower
than about 10-12KHz.


Audio range is generally considered to be 20Hz - 15Khz. But that is a
general range; individual people can have greater or lesser ranges.

Once you have this, you need something to generate a strong enough
magnetic field in the audio frequency range to act as a driver, and
something close enough and made of a magnetic material to vibrate.

True. There has to be an electric to audio transducer somewhere.
However, there are situations where the mechanism is obscure. For
example, when I moved into my house in Ben Lomond in 1973, the 200MHz
radar on nearby Mt Umunhum was running megawatts pointed straight at
me. Every time the rotating dish went by, my hi-fi would produce buzz
out of the speakers at the pulse repetition frequency. That was easy
enough to understand. However, the coils inside my kitchen electric
oven also went twang as the beam went by, which was more difficult to
explain. There had to be a rusty connection or bolt, but I couldn't
find one. Maybe the oven or coils were resonant at 200 Mhz. I never
did figure out how it worked.


Could be - but what was radar doing at 200Mhz? Not only will it
interfere with both business band and amateur frequencies, but the large
beam width would make it pretty useless. And as weather radar, it would
be useless.

I've seen a fair number of spurious emissions over the years (mostly
from VHF/UHF radios). Every one of them has been RF, and none of them
have created audio oscillations.

When I transmit on my VHF HT near my comptah speakers, they buzz
loudly. Does that count?


Whatever trips your trigger.

A few months ago we even had a case right here where a VHF radio in a
county bus had a spur on the input to the local 2 meter ham repeater.
Once again, no indication in the bus this was occurring. The county
found out about it only after the hams contacted them.

The county should have had a new digital or narrow band radio. Was
this a new radio? I usually look at the output on a spectrum analyzer
before letting something go out the door.


This is an 800 Mhz radio, not VHF. Only the spur was on VHF.

But then it appears you're just trolling like you usually do. The
difference is this time I'm not going to fall for it. You can have the
last word.

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Jerry, AI0K

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