Squeaking Noise From a Modified FM Radio

You got spurs... that jingle jangle jingle.

God knows what you did to the FM receiver (and he is not telling). So, I can't say precisely, but you probably detuned it to the point where the off-frequency rejection is piss-poor by now.

Any transmitter will transmit on a fundamental frequency and to a much lessor extent on frequencies above and bellow the tuned frequency. How much depends on how well the transmitter circuit is designed and executed.

You also get spurs or spikes of amplitude at intervals either side of the tuned frequency. These generally decrease rapidly the further they get from the fundamental transmit frequency. Chances are you are picking up a spur.

What causes this?

The modifications you made causes it.

Maybe some type of feedbaclk loop.

It sounds like your modified FM radio is suffering from instability or "taking off". The effect you are getting is simliar to very old style superregenerative radios that whistle until "locked" onto a carrier signal.

The reason you pick up the signal on two places on the dial is probably due to the local-oscillator "sum and difference" signals and the lack of "image" rejection in your modified front end tuning.

The IF is probably 10.7MHz so you pick up the signals on the dial about 21.4MHz apart.

Google up superhetrodyne receivers to get an idea of what is going on. You probably know the theory or you would not have got this far....

Is that a "squeaking" or "squealing" noise. If you would call it a squeal, then it is probably what we call feedback.

With regard to the "narrowness" or "the need to move the dial with extreme precision" for your receiver tuner to be able to pick up the transmitter: one thing that you an check on both transmitter and receiver is the bandwidth rating. My guess is that they are of varying bandwidths because one is hi-fidelity and has a wider bandwidth and the other is narrowbanded for simple communications.

You can be picking up a harmonic of the IF (intermediate frequency) which may cause your receiver to pick up the signal on several frequencies. Another possibility is your transmitter can be emitting a spurious frequency outside its intended frequency.

Well with almost 90 kilohertz of different carrier frequencies your transmitting radio is capable of selecting multiple channels of audio modulation signals. Any idea yet which part of your FM radio has been modified? My hunch is that the proximity of your transmitting radio to your mysteriously contorted receiver has saturated some amplifier stage in your receiver so that non-linear distortions are creeping through your bandpass filter. Now depending on how your amplifier functions, you can be getting odd or even harmonic distortions that get in band for the demodulating part of the receiver.

When I do get its signal, it is quiet

What do you mean? How do you know you have received the signal?

I believe that he is saying that there is a crisp silence meaning that he can tell that the radio is on frequency (like the squelch broken on a CB Radio) with no audio modulation from the transmitter.

Well maybe. Even if OP doesn't learn anything about radio he may as well learn how to communicate his observations and pose questions to garner insight.

Here is some more sound recorded from the radio. While it was recording, I was not touching anything. This is definitely some kind of data radio that this is picking up.

Here is another sound recording:https://skydrive.live.com/redir.aspx?cid=408514b6808a1247&resid=408514B6808A1247!141&parid=root

You are right. It sounds like an FFSK data burst.

Well, his transmitter is right in the 70 cm HAM band (420 to 450 MHz). So, there may be another repeater or HAM operating at a frequency and proximity close enough for the receiver to pick ip.

I think you have an oscillation in you receiver that is causing the squeals. When you tune in the transmitter precisely, it's power overcomes the internal oscillation, and the squeal dies down. The internal oscillation can mix with the transmitter frequency and tune in any number of signals outside the normal band. Tuning the receiver will change the frequency of the oscillations. Get another receiver and see if it can pick up the modified one to prove this is the case. If you have an oscilloscope, try measuring some internal signals.

Here are some more strange signals that I have managed to pick up. They are a bit above the 108MHz band and are pretty strong. Do you think that these are satellites that I am getting? There is a weather satellite called NOAA 19 that broadcasts on 137.100 that sends images but I cant seem to pick it up.

Here is the link to the mp3 file that I recorded.

https://skydrive.live.com/redir.aspx?cid=408514b6808a1247&resid=408514B6808A1247!142&parid=root

Maybe it is time to buy a scanner. ;-)

Or maybe at least a frequency generator.

I was thinking of getting a

Vero UV-X4 2m/70cm Handheld Radio

Have you got your Ham license yet?

Why would I need a license for a handheld radio?

In order to legally transmit from it.

Apart from the big brother aspect, obtaining a Ham radio operator's licence requires examination which means you will have to study and understand the theory as well as the legalities of illuminating the ether.

Could be a good way to garner an understanding of what is obviously interesting you at the moment. You will meet folk that could guide you in a hands on environment too.

Right now your questions and observations are all over the shop and I've no idea what you are doing or trying to do.

It may be a good alternative to going to school.

I have 4 handheld radios that have a range of 2 miles and I don't have a licence for those.

They are low powered units that operate in unlicensed bands on limited channels.

If you start fiddling with the tuning on those and interfering with licensed services, men in black suits will take an uncomfortable interest in you and your quest for knowledge.

If you are lucky they will only confiscate your toys and soldering iron. If a licensee complains (or you inadvertently cause loss or harm) you will be looking for safe ways to pick up soap.

I was mainly wanting a high quality radio that could pick up the satellites and transmit to my other 4 handheld radios and that is where the UV-X4 dual band would come in handy. I am not interested in trolling air traffic or boats.

As i said, I am not wanting to transmit to anything so it can't interfere. Here is the radio that I was wanting. It is FCC approved so how would it be illegal for me to own one? http://www.ebay.co.uk/itm/260943719314?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

It's not illegal to own it, it's just illegal to transmit on certain frequencies allocated by FCC, and other organizations for restricted use.

I agree with AH, you need a scanner, not a transciever.

It is not illegal to own one until you press the transmit button. Then you are breaking the law unless you are licensed.

Just get your ticket.

In the UK, retailers keep records of names and addresses of purchasers of amateur radio equipment, even if the individual has a licence. Eventually, the mismatch between equipment and non-availability of licence will be detected....

And then actioned....

After listening to your link, I think it's not an oscillation, but a broadcast outside the FM band. I'm not familiar to that area. I have a short wave radio, but it's 30MHz and below. Some of the strange noises I have heard there are from hi speed data links and Russian Jamming.

What are the outside broadcast bands for? There is one at 140 - 144 Mhz. Is it an un allocated and un licenced band which you can just use normally?

Start here: Radio spectrum - Wikipedia, the free encyclopedia

Then go here: American Radio Relay League | ARRL -

The only bands you can transmit on are the Part 15 bands with FCC approved equipment for that band at the specific power level (100 mW). Examples would be: Family Radio and CB.

Just because you think a band is unallocated does not give you a right to transmit on it at any power level. The 140 - 144 MHz band is 2m Ham Radio. Transmit on that and they (Hams) will hunt you down like a dog chasing a rabbit. You will be the rabbit. Amateur Radio operators police their own bands with prejudice.

They seem like pretty nasty people, I will just get a reciever instead rather than spending 40 pounds on a transciever.

Wait a minute! I think AH will agree that you would be an very good fit as an amateur radio operator. Hams do a lot of tinkering with equipment to make it different/better and you've already got that part down.

As a ham, you will have people from all over the world to give you advice when you're only getting squeals out of the reciever. Sort of a CR4 for radioheads.

Check this out:Ham license in 10 hours - Fast way to prepare for ham radio exams

Hams are a great lot, as long as you are legal.

When I have the money, I will definitely look into it further because it does seem pretty interesting.

The cost is very cheap. Find a local club and ask them the cost.

What the Amateur Radio people are keen on in the correct construction and operation of equipment in those bands. Any operator that falls short will be encouraged to improve quickly. That isn't nastiness; it's about community behaviour, like anywhere else.

One could say the same thing about CR4, for example.

The PMR 446 equipment is supposed to be self-contained and user-modification-free. That is why it is available to anyone that wants to use the band without needing a licence. Modifying it or re-tuning it exposes one to the full might of radio spectrum licencing legislation enforcement, which varies from country to country.

Citizens' Band radio is available to licensed users, and the licence conditions are not onerous in the UK. Certain technical conditions need to be satisfied, otherwise the station is likely to be closed down, the equipment confiscated and the operator/owner prosecuted.

Amateur radio is available to licensed users on certain frequency bands. The licence application protocol involves satisfying an examiner on radio and electronic theory that the applicant has at least a basic understanding of relevant aspects of this theory and the practical application of it.

Everything else is commercial, and a no-go area for the amateur.

It is highly recommended that the original poster undertakes a theoretical and practical training course on the topic and the local amateur radio club is probably the best place to start.

Why is it that the broadcast radio band is very clear and high quality but anything else outwith that has far lower quality?

Broadcast has greater occupied bandwidth.

Higher signal strengths? After all, no-one wants a broadcast that is faint, hissy and wobbly; they will re-tune elsewhere.

The bandwidth on an AM MF broadcast signal is of the order of +/- 3.5kHz, which is little more than that of a land-line telephone. Which is sort-of-why many users moved to VHF.

The bandwith on a FM VHF stereo broadcast signal is of greater than +/-25kHz, which exceeds the frequency response of the human ear; "Eff emmm - no static at all, no static at all..." (with apologies to Steely Dan).

The bandwidth of an amateur radio CW (morse) signal could be as low as a few tens of Hz.

Now, a lot of it has gone digital...

There's a variety of ways to answer your question. First, the broadcast radio bands are using the modulation technique your radio is designed to demodulate. So a demodulated signal picked up in these bands are clearer because the correct modulation technique is being used. Second, the modulation and transmitting equipment of the professional radio broadcasters must meet regional control standards. Third, the regional controls designate what power level and frequency a station can transmit so that they reach their licensed region without interfering with other broadcasters. This last aspect is why rogue transmitters are swiftly dealt with by the community because a rogue transmission can prevent proper reception. Rogue transmissions in controlled frequency bands is one of many techniques of jamming a radio transmission. Fourth, your radio was originally designed to only receive the signals inside the FM/AM bands. By modifying your radio to receive signals outside the anticipated frequency range, a variety of different signal distortions can be happening inside your radio. In some cases, the distortion itself is why you can "hear" the original signal. There is also a remote possibility that your out of band signal that your picking up is a transmission that is getting reflected off of the ionosphere and its random motions.

You definitely should pursue contacting your local ham community and obtaining some training toward a ham license. This will teach you the vernacular and concepts so you can ask more precise questions and so we can provide better answers. You have the engineer's perspective that cannot be taught. You need the tools which can be taught.

I did do a radio course at air cadets and got certificates for it. We were using a Yaesu base station radio and normal handheld radios.

Do consider embracing the training and practices of the amateur radio community. Everything here points towards it.

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