Wandering radio

[Radiowanderer]

Hey guys,

So I've been having a "discussion" with my brother over his analogue radio (i assume you'd say it's analogue), the KNSTAR K-258 which will quite often wander through different frequencies and often settles on a station. Now he seems to think somehow, someone is manipulating the radio remotely. I say maybe it's just a shitty radio and we don't know enough about how it actually works to say whether it could do this on its own.

MY question is therefore, do "analogue" radios sometimes randomly wander through the frequencies or is someone doing it to mess with us?! 

Much appreciated,
Cheers.

[cdev]

Yes, its very common.

Its almost certainly temperature drift as the radio warms up or cools down. Older "valve" (tube) radios did this a lot. Newer solid state radios less, and more modern, digitally tuned radios even less.

Another cause could be voltage variations.

There is a chance that a voltage regulator could be faulty.

Depending on what kind of radio it is, if the drift is a lot more than one would expect it may need a part - like a voltage regulator, to be replaced.

Its manufacturer's web page is here.
http://www.knstarcn.com/radios/pocket-radio-fm-88-108mhz-530-1600khz-k-258/

Looking at the picture, my gut feeling is its a very inexpensive radio, likely built to meet some price point.
He should accept that stability may not be its strong point.

[Radiowanderer]

Thanks a lot for the prompt reply.

My next question would be would there be anyway to manipulate the radio remotely without having tampered it or bugged it?

This might seem like silly questions but he can get quite worked up about it.

[Brumby]

Firstly - a little technicality:  All radio is analogue.  The information transmitted over it can be analogue or digital.  This unit receives analogue information transmitted by two different types of modulation - Amplitude Modulation and Frequency Modulation (So, guess where the names come from)

Not that this matters at all to your question.

This particular unit is just a basic AM/FM receiver.  The only controls over its operation will be the knobs and switches on the unit itself.  Changes in tuning can come from a number of causes.  Thermal drift and voltage fluctuations are two.  You can also have humidity affecting tuning, especially if it's high enough to cause condensation on the PCB or components.  You don't need to see droplets of water for this to be a problem - an invisible film can do it.  If there is dust on the PCB, the effects could be made much worse.  Another possible issue can come from the tuning mechanism.  With various mechanisms to translate the rotary motion of the control to the linear motion of the dial, there may be a little stiffness in some parts that may have a bit of reverse pressure on the tuning capacitor so it might be possible for it to back off the station.  I've noticed this in some cheap radios in my time and I tend to overshoot a tuning point and back it up a little with these.

If you are a fair way away from a transmitter and the signal strength is low, you can also get some weird things happening from atmospheric effects.  Short wave radio is well known for ionospheric propagation - but those aren't the only frequencies that can bounce around the atmosphere.  While it is somewhat unlikely for you get this on such a radio, it is not impossible and reception will not be stable.


The bottom line is quite simple - NO, nobody will be manipulating his radio.

[Brumby]

Having said all that - let's go with a conspiracy theory answer.......   

If someone knew the frequency his radio was tuned to, they could set up a transmitter to transmit a rock solid signal .... and then send their own sound track - which they could make to sound like anything they wanted!

Problems with this are:
 - Every other radio within range tuned to that frequency would also pick it up.
 - He would just have to change the station to get away from it.

[vk6zgo]

His radio is faulty!

Radios of all types may pretty much be left on a frequency, & not drift much at all.
If you are listening to an SSB signal & have nicely "tweaked" it in so the speech sounds normal, it may well drift a bit over a reasonably short  time & need some adjustment, but "drifting all over the place" is not the hallmark of a properly designed radio in normal operation.

Many FM Broadcast radios have "automatic frequency controls" to keep them on frequency, but this is more a "nicety" to ensure that the stereo pilot is resolved correctly to ensure decoding works.

In an ordinary mono FM radio, any drift would have minimal effect------ after all, the channel width is " as broad as a bull's bum" at a couple of hundred kHz wide.

If, however, the AFC itself is faulty, that is a possible cause of your problem, as it may be over correcting wildly.

If you are referring to Medium Wave AM broadcasting, drift should be of even less consequence.

Many thousands of such radios from the tube era sat on shelves in Workshops, tuned to the same station for decades.
They might have drifted enough to sound a bit "muffled" after years, but nothing on the scale you are referring to.

Of course, you could tell your brother to switch "scanning" off! 

[vk6zgo]

But wait--- there's more!

The quoted behaviour is very like that which occurs if the Phase Locked Loop of a radio using such circuitry goes out of lock.

I have experienced something very similar with a transmitter, where the PIC normally received a command, told the PLL to go to the required frequency, & all was well.
The plug on the cable between the two failed, & the PLL happily went to a incorrect "default" frequency!

With "old style" radios, the tuning is pretty much determined by the physical constants of the tuned circuits used in their Local oscillator, RF input tuning & IFs.
Small changes can &  do occur with heating, but they are very small compared to the component values.

[Brumby]

His radio is faulty!

Oh, yeah.  That too.

[cdev]

Thanks a lot for the prompt reply.

My next question would be would there be anyway to manipulate the radio remotely without having tampered it or bugged it?

This might seem like silly questions but he can get quite worked up about it.

Not with analog radio but with digital radio it would be entirely possible to send different target programming to different groups of people or whatever they want. Just like with the web.

However, its highly unlikely that a digital radio could or would send any information back.

Until we have 5g, adding some low cost back channel for bidirectional communication or data exfiltration would likely triple or quadruple its cost.

Also it would likely create sounds, similar to those caused by a cell phone which might become audible under certain conditions. (inadvertent rectification by other electronics)

[Radiowanderer]

Hi guys, i'm just popping back in to say thanks for the help on this bizarre question. I did read the replies but didn't have time to respond.

Merry Christmas a Happy New Year to you all.

[Brumby]

No worries.  We like bizarre.

And thank you for touching base.  It's always nice to see an acknowledgement.  There are many people who give their time freely to offer assistance and share their knowledge (more than I do) and even the simplest show of appreciation is warmly received.