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Thread: FM radio antenna

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    Default FM radio antenna

    I accidently knocked out the FM antenna from my radio receiver, and when I replaced it, it made a loud humming sound along with the radio broadcast. The humming varied as I altered the volume. I thought I had damaged the connection on the antenna, so I bought a new one, which doesn’t work at all.

    Can anyone suggest why the humming would start (I changed nothing else about the radio at all) when I replaced the antenna, and why a new FM antenna (bought from overseas) would not pick up any signal. My radio is about 25 years old, if that makes any difference.



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    Sounds like you did something wrong when replacing the antenna, as a simple replacement should not make any difference.

    Is the hum noise just on FM?

    It might help if you posted some pics of the antenna connection.

    I've moved the thread to the Electronics section, as it relates to a repair.

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    I've had the same/similar thing happen around 10 years ago
    I never resolved it
    Not sure if it was a Earthing Loop with the Receiver or what.
    But it did work fine, then i neatened up some cabling behind the Receiver, plugged the FM antenna lead back in and there was a humming.
    Very frustrating
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    Some antennas form part of a tuned circuit with specific impedance, capacitance, etc, so a replacement needs to match the original exactly.

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    Nothing to worry about....It's only humming cos it doesn't know the words.

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    Have you tried putting a 10pf capacitor in series with the antenna?
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    Many thanks for your answers, and so quickly. I purchased another antenna (FM radio and TV) today from an el cheapo shop, and it receives well, but there is the same interference as with the original antenna that I thought I had damaged.

    The interference is only on FM, though i only get very poor AM reception anyway.

    Oceanboy: your response is disheartening in that I may just need to get a new radio receiver.

    Trash: what is involved in putting a 10pf capacitor in series with the antenna? Excuse my ignorance; do I just buy a 10pf capacitor from an electronics shop and connect it between the antenna and receiver?
    Last edited by bristow; 22-11-16 at 04:09 PM.

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    Quote Originally Posted by bristow View Post

    Oceanboy: your response is disheartening in that I may just need to get a new radio receiver.
    I even went to the trouble to run a separate RG6 to a Passive 3 Element FM antenna
    To no avail though, the humming was still present across the entire FM band on the Receiver.
    If u want to go on an expedition get a Land Rover, if u want to come home from an expedition get a Landcruiser!

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    The 10pf capacitor is a decoupling capacitor. Almost all radios of one sort or another use them. I would assume your radio already has one, but the behaviour sounds a bit suspect like it might have been part of the antenna. But for the effort, almost any small capacitor will do the job. Just put it between the radio and the antenna.

    [radio]------||---------============== Antenna
    Capacitor 10pf

    It's a bit of a long shot because there are plenty of other things that might be the problem, but this is one that is cheap and easy to test.
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    Thanks, I'll give it a try.

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    A description of the radio and antenna and connection would be helpful.
    I have never seen a radio that does not have the decoupling capacitor (actually a high pass filter) built in.
    When you say you knocked it, how forcefully was that?
    Could you have pressed something like a pin in the socket that has shorted the capacitor behind it?
    ...or is it a radio with a telescope antenna that you broke off and the mount is bent or the screw is protruding so far that it is touching a track on the board and is now is bypassing the capacitor.
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    Thanks for your further replies. Attached is a pic of the antenna lying on the carpet. It just looks like a piece of wire with a metal end. This antenna came with the radio (Marantz) and I have use it for years, and disconnected it a number of times. Another picture shows me holding the metal end that fitted over the centre pole of the input on the radio. It is slightly bent but still provides sound but with the interference. This input is shown in the third picture, the terminal on the left.

    I tried connecting a 10pf capacitor between antenna and radio; I still received sound, but the interference remained. I dont know if it was a decoupling capacitor though. The guy working in Jaycar where i bought capacitor didnt know either.

    Should I just go and buy a new radio?




    Last edited by bristow; 25-11-16 at 08:18 PM.

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    Hi Bristow,
    Not 100% sure, but you are connecting that antenna to the MW/LW input, any chance that it was wedged in the FM input instead?
    Before buying a new receiver, try to put a wire on the FM socket (center pin) and see if it works, if you have a TV antenna cable handy you can try to connect the receiver to the TV antenna in the roof and see if that fixes your problem.
    If it is the unit I would try to get it repaired rather than buying a new one.
    Good luck

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    Yes you are using the wrong input.
    Your radio is very sensitive to pick up any radio signal at all with the few cm of wiring or copper tracks inside(or crosstalk from the AM antenna) but it is picking up a lot of interference and very little usable radio signal, hence the noise and hum. It is probably picking up harmonics from the rectifier diodes in the power supply.

    I often just clip a crocodile lead in the centre pin of the round 75Ω FM socket and push the insulating plastic far enough over the clip so the metal part doesn't touch the outside tube of the socket.
    That also give it support. If you clamp two of those alligator cables together you get an 'antenna' that is roughly 75cm long which is a 1/4 wavelength for FM broadcast and quite a good match for that input.
    Rarely ever needed a roof antenna.

    Just in case, these are the leads I am talking about. 1001 uses for them:



    Of course you can try to find a coax plug but you would usually need to solder the wire to the centre pin.
    The leads are connected in seconds and you can quickly play around with the length by adding more or less in series.
    Last edited by Uncle Fester; 25-11-16 at 10:22 PM.
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    This is why I suggested in post to to add some pics.... much easier to understand what's going on.

    Given that you said "Another picture shows me holding the metal end that fitted over the centre pole of the input on the radio" it sounds to me that you are in fact putting in the 75ohm FM Connector on the left of the pic.

    Only the AM antenna connects to the two push connectors on the right.

    Buzzing/humming is very common on AM stations, especially weak signals, but FM usually has hiss with weak signals.

    As suggested, many older-type TV antennas covered the FM band, so you could try that, depending on what type of TV antenna you have, etc, but if it used to work with just the length of wire dangling down the back of the receiver, then it still should.

    The pic of the connector on the end of the wire looks to be a strange shape for pushing over the centre pin.

    Another thing... have you added any LED or flouro lights in your home since the radio worked last?

    There's quite a lot of those that generate radio interference.... just a thought.

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    The new antenna is not working because it is not compatible with your old radio.

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    Thanks for your replies again. In the pictures posted previously, the FM antenna is not shown connected. The wire shown connected to the two push connectors to the right is the AM antenna in place. And under the connectors it says AM. The round connnector on the left (coaxial?) with FM written under it was the one I had the antenna attached to. It fitted around the centre pole.

    The pictures below show how the original and the new FM antennas were connected. (I have removed the AM antenna that was shown in the previous photos). They both get a good signal, but with the same loud hum.

    The connection on the original antenna (which one of the previous photos showed me holding) is bent out of shape from being knocked out of the connector on the radio (more than once) because I had to string the antenna up the wall to get a good reception.

    I did buy two new antenna. Th first one didnt work (obviously not compatible), and second one does work well, except it causes the same humming that the original antenna now does.


    Last edited by bristow; 26-11-16 at 08:27 AM. Reason: Clarification

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    That is just a bit wire in the second picture, I see no space for any matching circuits or even a capacitor and as you said the 10p cap didn't improve anything.
    Something may have happened inside when you knocked it, maybe a dry/cracked solder joint behind the socket and you are also sure it is not humming when you play a CD or something (volume turned up a bit)?
    Have you got(or can lend) a portable FM radio to test if the is some interference in the location where you have the Marantz. Also turn the Marantz on and off while you are testing with the portable to ensure the Marantz is not causing the interference itself.
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    bristow, the second photo appears to show an ordinary wire coming off the centre-post of the fm antenna connector. Is this the 'new' antenna that works, but hums?

    If it is, this could be your hum problem. The fm connector is a 75ohm unbalanced input and needs a balun before a (dipole) antenna, to work properly.

    The easiest way to have a half-way decent fm antenna, is to buy a cheap "rabbit ears" tv antenna, and a female-to-female tv coax adapter. The "socket" in your receiver, is actually a male PAL TV connector. It needs a female-to-female adapter, to fit a standard tv antenna plug.

    If you are in a marginal signal area, an old VHF-only 2 or 3 beam, TV antenna is really good. They're almost impossible to buy now, which is why I have collected all the old ones I have come across. Just whack a new balun on them, and bingo, you've got yourself a high-gain FM external yagi antenna.
    Last edited by Onefella; 26-11-16 at 03:41 PM. Reason: gender issues

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    I have an identical connector on my surround sound Amp/receiver and all the so called FM antenna is just a single insulated (cut) length of wire with a pin on one end which you insert into the centre of the Co-Ax connector.
    Where the pin connects to the wire is pretty flimsy but shouldnt just break.
    Where I am that antenna wouldnt pick up static so I connected a lead from my old VHF with a MHA antenna to that connector but I didnt have a spare PAL connector when I was setting up the receiver so I just bared the end of the coax exposing the inner wire and rolled the braid back.
    I then inserted the inner wire into the centre pin and the pigtail of braid into the one marked EARTH but it worked with or without the braid connected.
    I am wondering if whatever broke the connector has damaged that connector or failing that, by sheer coincidence this hum is caused by an internal component that has only just begun since the antenna was damaged.
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