FM transmitter causing significant hum in surround components

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Barnys
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FM transmitter causing significant hum in surround components

Post by Barnys » Thu Jun 08, 2017 4:44 am

Hi
I am assembling a project and am having a problem with the FM transmitter.
When I connect the transmitter (power, signal and antenna) all of the other audio output components acquire a significant hum. Also, the transmitters signal (at a receiver) is almost solely the same hum. The hum is present when there is something connected to the transmitter boards outputs, but if I disconnect the antenna and the 50 Ohm board to antenna cable the hum disappears.

I am not sure how much and/or what detail is needed, so here is a general outline:
This is the type of antenna -- http://www.ebay.com/itm/Professional-1W ... Swt5hYbial
This is the type of transmitter board -- http://www.ebay.com/itm/BH1417-200m-0-5 ... Swc-tY1NUm

I have previously used both components without problems, however the current project is laid out on my bench with the antenna mounted on a piece of angle aluminium. Also, with the exception of the transmitter board to antenna coax, all cabling isn’t screened.
I have found that touching the antenna mount with a large piece of aluminium (a pot) almost stops the hum but I have not found a way to remove it completely. I realise adding screened cabling will probably help but I would like to get the system sorted (as quiet as possible) before I add proper cabling.
I know this is a rather brief description so please ask if there is any information I have omitted.

Is this problem simply that there needs to be a better-bigger-more substantial ground plane/plate around the antenna?

Regards.

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Dale H. Cook
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Re: FM transmitter causing significant hum in surround components

Post by Dale H. Cook » Thu Jun 08, 2017 6:32 am

What you probably need are a decent transmitter and a decent antenna. Just because you had gotten previous versions of both to work does not mean that all examples will work.

I presume that you are doing this in some country other than the United States, as this board shows no sign of being acceptable to the FCC for licensed use, and its power output makes it illegal for Part 15 use in the US. The antenna is also illegal for Part 15 use in the US, as Part 15 FM operation prohibits the use of a standard antenna connector [47CFR, Section 15:203].
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Deep Thought
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Re: FM transmitter causing significant hum in surround components

Post by Deep Thought » Thu Jun 08, 2017 2:25 pm

That antenna has no ground plane, so anything around it turns into one and "collects" the RF. If your audio wiring is in the vicinity (1-2 feet) it becomes the ground plane and RF is flowing through it causing the hum. There is no way to fix this other than proper construction techniques.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

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NECRAT
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Re: FM transmitter causing significant hum in surround components

Post by NECRAT » Thu Jun 08, 2017 3:53 pm

Is this for a legal, licensed radio station?
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Barnys
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Re: FM transmitter causing significant hum in surround components

Post by Barnys » Fri Jun 09, 2017 1:58 am

Hi, Thanks for the replies.

This isn’t for a licensed radio station; I am just experimenting in the house.

My previous (much simpler) build has the components inside a metal chassis with the antenna attached to the rear. I used multi core screened cable for the signal and power cables with the screen of each cable collecting at a chassis star ground.
However, my present effort is more complex so I have the PCBs and power supplies laid out on a wooden board to make it easier to organise the wiring and ground point(s).
Deep Thought wrote:
Thu Jun 08, 2017 2:25 pm
That antenna has no ground plane, so anything around it turns into one and "collects" the RF. If your audio wiring is in the vicinity (1-2 feet) it becomes the ground plane and RF is flowing through it causing the hum. There is no way to fix this other than proper construction techniques.
Working from my limited experimentation and some reading I had speculated it may be the lack of a ground plane that is causing the hum. So, in the simplest terms, what would the most salient “proper construction techniques” to start with? I suspect this is an overly general question to what is a complex topic but I am looking more for direction than specific answers.

Regards.

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Dale H. Cook
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Re: FM transmitter causing significant hum in surround components

Post by Dale H. Cook » Fri Jun 09, 2017 4:03 am

Barnys wrote:
Fri Jun 09, 2017 1:58 am
I am just experimenting in the house.
If this is a house in the United States then you are in violation of Part 15 of the FCC Rules and Regulations. This is a venue for people whose work in part is insuring that the Rules and Regulations are obeyed.
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Re: FM transmitter causing significant hum in surround components

Post by NECRAT » Fri Jun 09, 2017 5:25 am

Barnys wrote:
Fri Jun 09, 2017 1:58 am
Hi, Thanks for the replies.

This isn’t for a licensed radio station; I am just experimenting in the house.

My previous (much simpler) build has the components inside a metal chassis with the antenna attached to the rear. I used multi core screened cable for the signal and power cables with the screen of each cable collecting at a chassis star ground.
However, my present effort is more complex so I have the PCBs and power supplies laid out on a wooden board to make it easier to organise the wiring and ground point(s).
Deep Thought wrote:
Thu Jun 08, 2017 2:25 pm
That antenna has no ground plane, so anything around it turns into one and "collects" the RF. If your audio wiring is in the vicinity (1-2 feet) it becomes the ground plane and RF is flowing through it causing the hum. There is no way to fix this other than proper construction techniques.
Working from my limited experimentation and some reading I had speculated it may be the lack of a ground plane that is causing the hum. So, in the simplest terms, what would the most salient “proper construction techniques” to start with? I suspect this is an overly general question to what is a complex topic but I am looking more for direction than specific answers.

Regards.
Unfortunately, the transmitter you're using isn't Part 15 legal. The issue you're having is you're introducing too much RF into your home's environment.
If you operate this transmitter, you're violating the FCC rules, and could be subject to some severe fines. You should look into getting a legal (non ebay) part 15 transmitter, as that will help your situation.
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Deep Thought
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Re: FM transmitter causing significant hum in surround components

Post by Deep Thought » Fri Jun 09, 2017 9:45 am

I would suggest you buy a 50 ohm dummy load and operate this transmitter into that instead of an antenna. Enough signal will still "leak out" for you to receive it in your home, and you can experiment with whatever you are doing without causing interference for a mile radius.

This isn't the 1970s when there were many open channels to play in. Today, you are almost certainly going to cause interference to some FM station (or stations...those Chinese transmitters are a hot mess) or even high VHF TV. If you get really lucky you'll have spurs in the aviation band immediately above the US FM broadcast band causing interference to aircraft communications which will bring US Marshals to your door in short order.

And with that, I'm done here.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

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Re: FM transmitter causing significant hum in surround components

Post by COMMENG » Fri Jun 09, 2017 10:36 am

Barnys wrote:
Thu Jun 08, 2017 4:44 am
Hi
I am assembling a project and am having a problem with the FM transmitter.
When I connect the transmitter (power, signal and antenna) all of the other audio output components acquire a significant hum. Also, the transmitters signal (at a receiver) is almost solely the same hum. The hum is present when there is something connected to the transmitter boards outputs, but if I disconnect the antenna and the 50 Ohm board to antenna cable the hum disappears...

Regards.
In the board's current configuration this board is Illegal for use in the US, and I think a number of people have stated that.

The reason the board is illegal for use here is Q2 which ups the power to the antenna port, and the lack of filtering to guard against interference to other, out-of-band services. This type of board is a pure hash maker, regardless of the advertising hype.

I built one of these for my grandsons' using just the chip (purely legal with 20 mW) and the prescribed components from here:

http://pdf1.alldatasheet.com/datasheet- ... 1417F.html


Start over. Build this on a double sided copper board and place inside another metal enclosure using the suggested components from the data sheet.

Included is a power supply which uses filtering in order to keep the RF out of the power supply and audio. As noted, the same filtering arrangement can be used to filter both audio channels. The audio channel's filtering should also be inside the enclosure with the chip and its components.

And as other's have stated, you're going to be hard pressed to find an open channel.


COMMENG
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NECRAT
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Re: FM transmitter causing significant hum in surround components

Post by NECRAT » Fri Jun 09, 2017 2:20 pm

OK the point's been made, and the question has been answered really.

Just so you know, the folks here are only telling you this, so you don't get yourself in trouble. (Expensive legal trouble at that).

No one wants to see that happen.

So the question was answered and advice was given.
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