radio reception oddity

I have an armband digital FM radio, for running. The earphone cord is the antenna.

Reception is very sensitive to antenna orientation, unsurprisingly. (a problem as I circle a track) It's not the signal strength so much, but the noise level varies greatly.

Anyhow, here's my question: the reception is also highly sensitive to the radio orientation, independent of the antenna. The noise again varies a lot, as I rotate the package.

Can anyone explain this, in terms of circuitry or physics? Is it normal, a product design flaw, or defect in this particular unit?

-- Rich

Reply to
RichD
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This is normal in areas where noise is high and station signal strength is low. I presume when you say "it's not signal strength so much" what you mean is that it's not the music loudness varies so much rather than it's that you are actually looking at a signal strength display on the radio. If that is true then what is actually happening is that the signal strength IS actually varying a LOT, and as it drops down low the signal to noise ratio of the radio drops and you hear lots of noise. The music volume doesn't change much be cause with FM the volume depends on frequency shifts not signal strength.

This sort of thing is common because at FM frequencies wavelengths are relatively short (a few feet) and this causes lots interference which is not meaning "noise" but where one wave cancels another creating a dreaded "dead zone"! Buildings, stadiums, even terrain will do this. Plus FM antennas are rather directional (people building the radio probably didn't give a damn about this) which makes the problems even worse, Usually the answer is to listen to a stronger station which won't drop out as much, even if you hate the music they play.

That's the basic story.

Reply to
Benj

Reply to
Benj

I assume you've tried different stations with different signal levels?

No, it's not normal.

Bingo. That's most likely the reason. Bet you have one of those cheap $5 no-name units...

Dave.

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Reply to
David L. Jones

"RichD"

** The signal level is varying greatly - that is how FM behaves.
** Must be a bunch of signal bein gcapacitively coupled into the radio by you body being in such close contact with it - the body makes a nice FM antenna.

This "body" signal may well conflict in phase with that arriving from the ear cord antenna and its level changes with postition and orientation on your arm.

All normal behaviour for RF at that frequency.

Ignore that David Jones - he is a digital geek total moron.

..... Phil

Reply to
Phil Allison

FM uses limiting in the IF. That keeps the volume level, at any received power level, but the signal to noise ratio drops along with the received power level, which is typically specified in microvolts for full quieting at the antenna input.

There is nothing odd about this, at all.

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Reply to
Michael A. Terrell

I cannot be definitive without examining the particular situation. I would have to blame the equivalent of laser speckle for your problem until it is proven otherwise. This phenomenon has analogs in multi-path effects, acoustic room resonances, radar scintillation, star scintillation, short wave fading, and the like. Much of this is missed because pf the capture properties of FM.

Reflection and scattering cause a fine structure to the electromagnetic field. Shining a red laser off of a rough wall causes a three dimensional speckle pattern around the light spot on the wall. The same thing happens throughout the entire electromagnetic spectrum albeit on a scale proportional to the wavelength of the radiation. This effect limits the quality of cell phone reception in the canyons of cities with skyscrapers where there can be plenty of signal strength, but the signal gets garbled anyway.

The distribution of signal strength follows a log normal distribution. This means that there is a good chance that the signal drops close to zero locally in the presence of large average signal.

If you look at laser speckle on a wall, you will see some very dark spots in the pattern even though the average illumination is quite high. I have personally taken a HeNe laser, placed it shining it up against a wall to make a small spot. When viewing it from a short distance, it is easy to see the speckle pattern. As you go farther away from the spot, easier for your eye's pupil to fall in a dark spot in the random antenna pattern generated by the laser spot with random phase across it. I was able to move my head and eye so that the spot was completely dark and not to be seen. Slight head motions made it possible to see a relatively brightly illuminated laser spot. When you scale up to FM wavelength, the signal drops out at such nulls and all you hear is noise.

Bill

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Reply to
Salmon Egg
[Snip huge barf of crap]

Radio fading is laser speckle? :-)

Bill, that was the most scientifically and intellectually opaque pseudo-explanation for a common radio phenomenon I've ever heard. Let us all PRAY that you have nothing to do with the technical education of young people! God help us all.

idiot!

Reply to
Benj

The OP asked "Is it normal, a product design flaw, or defect in this particular unit?" It is generally not normal behaviour in a properly designed armband type radio. So if it's as bad as the OP is making out then odds are he's got a cheap badly designed radio. A better designed one will likely fix the coupling problem at least. I don't get the problem even on the weakest signals on my Creative MP3 player or Nokia phone for example.

But yeah, most likely his problem is capacitive coupling and phasing as you say. Doesn't mean he has to put up with it.

And if his signals are that marginal in the area(s) he uses it, there is always the MP3 option, along with podcasting of any particular radio programs he might like to listen to.

Dave.

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Reply to
David L. Jones

Coming from you, I consider it a compliment.

Bill

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Reply to
Salmon Egg

Actually I would agree with Bill, since the underlying principle is exactly the same ! Only the physical scale in terms of the length of the wave would be larger at 90 - 100 Mhz.

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Reply to
Baron

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