what's that squeal noise

[wenzeslaus]

a lot of shortwave radios make a squeal noise as you approach a station. the squeal goes down to 0 hz on the station, then as you leave the station it goes back up to a high pitched squeal before it fades away entirely as you continue scanning. anyone who has ever used a shortwave radio knows what I'm talking about.

what causes that?

I have a Grundig FR-200 and around 7 MHz on shortwave, it squeals loudly as you approach a station, and if you put your hand close to the antenna it varies the pitch. it's obvious there is capacitance between your hand and the antenna, so when you move your hand, the C varies, making the frequency vary, and you get the same effect as if moving the tuner. according to the equation f=1/(2*pi*(L*C)^2)

why does this radio do that, when other radios don't? is a capacitor damaged or out of adjustment? antenna improperly connected? or what?

then there's my Grundig Yacht Boy 205, which doesn't squeal at all. why not?

??

 

[Token]

I would say that what you are describing is classic for if you had the radio in SSB mode, and that changing pitch tone is the signal beating against the BFO, but your radio does not have SSB mode, so it can't be that.

Sionce it does not have a BFO or SSB, there must be something else that is happening that is functionally simillar to that. At a guess there is a strong signal or image of a signal close to the IF of the radio. As you tune across the band that signal is beating against carrier of the signal you are shifting (the signal you are tuning to) and you are hearing the hetrodyne of the two signals beating against each other.

Your other radio does not do the same thing because either it has a different IF, or it is more immune to the image or the strong signal.

T!

 

[wenzeslaus]

that might be it, because I soldered a length of wire to the antenna (inside) so I could take off the back without yanking on the wire going from the antenna to the PCB. maybe the wire is laying on top of a microchip somewhere. I could test that by taking off the back, make sure the wire isn't against the PCB, and see if it still does it.

 

[wenzeslaus]

that was it. I took the back off, turned on the power and listened through the headphones. whenever I moved the antenna wire near the variable capacitor (sticking out of the PCB like a little tower) it made it squeal. same for the wire going to an LED (not on) so apparently any wire near the VC makes it do that.

moved all the wires away from the VC and put it back together. still does it but greatly reduced, no more than any other radio.

the YB-205 must be engineered to not have any such capacitance from wires or circuits near the VC.

 

[jwt873]

why does this radio do that, when other radios don't? is a capacitor damaged or out of adjustment? antenna improperly connected? or what?

then there's my Grundig Yacht Boy 205, which doesn't squeal at all. why not?

I've had many radios over the last 50 years. I've never had one squeal as I tuned in stations. (Unless, as Token mentions above, it had a BFO that was turned on or the radio was in CW/SSB mode.)

I'm going to guess that you're hearing one of the gain stages breaking into oscillation. There might have been some damage (from dropping?) or the radio was poorly designed or it could be out of alignment.

You can get feedback if there is any coupling between the input and output of an amplifier stage such as the IF amp. (Or the RF amp if the radio has one). This could be caused by unwanted capacitance or inductance.

The 'feedback' would be to some extent similar to what you sometimes hear when the speakers in a public address system are too close to the microphone and the output gets back in to the input.

 

[wenzeslaus]

the 'squeal' isn't feedback like you hear from a microphone and amplifier. it's a squeal or whine noise.

 

[Boombox]

If it sounds like a slight whoosh noise or extremely slight whistle-type noise it could be the sound of the ceramic filter in your radio. I have a couple radios with tight ceramic filters (and analog dial tuning) and you can hear a slight whoosh noise when you tune through the channel.

Another possibility is that you are hearing heterodynes from adjacent channels. This would only happen if your radio has a wide bandwidth setting (6 khz will sometimes do it on my Tecsun), or if there is a strong station 10 khz away (on the MW band) or 5 kHz away (in SW).

It sounds like your Yacht Boy has tighter selectivity than your Grundig FR200. Wide bandwidth + two strong stations next to each other (especially on SW) = whistle sounds from the signals interfering with each other at the edges of each signal.