FM Transmitter Problems

[the dude of dudes]

I bought this:

**broken link removed**

My intention is to use it in my work trailer (5x10' cargo trailer, aluminum walls, steel roof, wood floor) to rebroadcast Sirius radio. I work outside, generally within 100' of my trailer and I wear a small MP3 player w/FM radio (keeps the neighbors happier if they cant hear my music 9hrs a day). Problem is I'm only getting a quality signal up to 40' from my trailer. The unit has a built in 55" length of wire for an antenna.

I can see two problems:

1) The portable radio uses the headphone wire as an antenna, probably not the best for what I am doing
2) The setup is inside a metal trailer which is probably interfering with the signal

Is it possible to improve my transmitting antenna's situation? I thought maybe if I wrapped it around a strip of metal and mounted it on the roof vertically. I'd like to know if it would significantly improve the signal before I drill a hole in the roof of the trailer! Is there anything I can do improve the portable radios antenna/headphone wire? The wire moves when I move, and the bulk of it is stuffed inside my coat pocket. I'd look quite silly strapping an antenna to my forehead, wouldn't I. The transmitter is quite small, and only runs on 5v DC, so I know I shouldn't expect much, but the product info claimed a 150ft range .

 

[Sceadwian]

First off you paid too much, you can get similar units for 20 dollars. The metal trailer is obviously going to screw with things, the antenna needs to be well above and away from any possible grounding source, like 10 feet above the trailer on an insulated pole. From within the trailer I'm surprised you get a signal 10 feet away.

 

[RadioRon]

Yeah, I also think the trailer may be getting in the way. This should be pretty easy to check by putting the transmitter up on the roof of the trailer just for a short test. You need to have the transmitter's antenna wire leading away from the metal of the trailer. Try taping the wire to a wooden broomstick and prop it up on the roof. Let us know if that helps.

 

[the dude of dudes]

I'll try it on a stick tomorrow. I did try setting it up in my house, and had similar results with the portable radio. Is it best to have the antenna straight and vertical? How much of a ground separation do I need? Is simply avoiding direct contact with the trailers metal shell okay, or do I need a minimum distance from the base of the antenna to the trailer? 10' above on a insulated pole really isn't going to work (the units antenna is attached, and only 55" long). If this unit isn't going to be sufficient, then I'll probably look at something else and am open to suggestions as to what/where to buy.

 

[RadioRon]

The best way to orient the antenna is in the same direction as the receiving antenna. Only trouble is, your receiving antenna is flopping every which way or is partly tucked into your pocket. This makes it pretty difficult to say which way the transmitting antenna should go, so let's just say it should be vertical. I don't agree that it needs to be a long way from the metal of the trailer. It might be just fine as close as a few inches. The most important thing is that the wire is not parallel to the metal of the trailer, I mean, lying on the metal surface. It is very important that the bare metal wire not touch the metal of the trailer. It will work best if it goes straight up above the center of the trailer's metal roof.

If it is not significantly better after doing this, we'll have to work on a plan B.

 

[the dude of dudes]

The best way to orient the antenna is in the same direction as the receiving antenna. Only trouble is, your receiving antenna is flopping every which way or is partly tucked into your pocket. This makes it pretty difficult to say which way the transmitting antenna should go, so let's just say it should be vertical. I don't agree that it needs to be a long way from the metal of the trailer. It might be just fine as close as a few inches. The most important thing is that the wire is not parallel to the metal of the trailer, I mean, lying on the metal surface. It is very important that the bare metal wire not touch the metal of the trailer. It will work best if it goes straight up above the center of the trailer's metal roof.

If it is not significantly better after doing this, we'll have to work on a plan B.

To remove all doubt about the trailer, I tried it today roughly 80' away from the trailer. I mounted the antenna vertically on a wooden stud on a completely wooden structure, top of antenna was probably 9' off the ground. It only improved slightly. I can actually pick up the transmission ~100' away, but it all depends how I stand and hold the headphone wire. And once I move, I get static.So I think its fair to say that this transmitter is not going to work for what I want to use it for.

So what do you have in mind for plan B??? I've seen other FM transmitters that claim 1 mile range, so maybe thats one option. My 2.4 ghz cordless phone gets pretty good reception at 100', is there another way to do this without using FM?

 

[RadioRon]

I don't have a good idea for plan B, yet. If this transmitter can't deliver the goods, then no FM transmitter that doesn't need a license will do much better. If you felt like experimenting, then perhaps things could be improved by coming up with a better antenna for your receiving unit, but that would be a lot of work I think.

I like the idea of using a different frequency band, but my experience with cordless phones indicates that 900MHz or 2.4 GHz are both going to be only a bit better if at all. It really comes down to the receiving end in these cases too. There are many 2.4GHz audio relay units but they all use low power, not enough to guarantee joy in your application. I looked at some models of wireless headphones made by Acoutic Research and they all claim "up to" 150 feet, which isn't going to do it for you.

The only other thing that comes to mind at the moment is to use a 2.4GHz wireless audio link to get the signal into the middle of your work area, then pipe the audio from that into your FM transmitter. Since the FM transmitter is much closer to where you are working, the signal would be better. Can't help but think that is kind of kludgy though.

Here's an outfit that sells cheap 2.4GHz audio links, but be warned, their website is the worst that I have ever seen. Their stuff does, however, work OK.
**broken link removed**
You might want to explore their stuff to see if they have anything that will work. Their 2.4GHz stuff, that I have seen, is very directional (narrow beam antennas) so tends to be good mainly for point to point, not broadcast over a wide area.

 

[Hero999]

I'm confused, I thought the transmitter only broadcast on the FM radio band and your phone works at 2.2GHz, are you sure you're not just using the FM radio feature on the phone which works at FM radio frequencies?

 

[the dude of dudes]

I'm confused, I thought the transmitter only broadcast on the FM radio band and your phone works at 2.2GHz, are you sure you're not just using the FM radio feature on the phone which works at FM radio frequencies?

Apologies, I worded it wrong. I was comparing the poor sound quality of the FM signal to a cordless phone, which seems to get great reception at 100', and has a very small antenna. It would be nice if I could use a 2.4ghz transmitter/receiver like the phone uses to listen to my Sirius radio. The link RadioRon posted for a 2.4ghz audio link is a good alternative, but I'm not sure if it has enough range. Is there another frequency I could transmit on that would get better results? The key is receiving the signal on a small portable device.

 

[RadioRon]

It is government regulations that limits the transmitter power of all these different devices, so you tend to get about the same range from similar devices that use a common frequency band since they are all putting out about the maximum that the government allows. So, a lot of 2.4GHz audio link devices will have similar range, and a lot of FM broadcast band transmitters that are "license-free" have similar range.

I guess it may be possible to get an FM transmitting license so that you can use higher power, but don't know what the requirements are. I have a feeling they don't just give them to anyone that asks, since that would soon clutter up the band.

 

[Sceadwian]

You could always use an FRS radio, using an 'external mic' feed for the audio. Just lock the transmit button down. Those get 2-5 mile range?

 

[RadioRon]

FRS will have very poor audio frequency response, kind of like listening to music through your telephone.

 

[the dude of dudes]

Would this do the job for me:

**broken link removed**

If so, is there a canadian supplier of a simlar type?

 

[RadioRon]

If it does what they say it does, then, yes it would work. You would also be breaking a few laws as this unit has no FCC Type Acceptance (assuming you are in the USA) so it would be impossible to license, and if it had FCC Acceptance, you would have to get a broadcasters license. (same goes for Canada, just have to deal with Industry Canada instead of FCC, but the rules are the same). Since its signal would be heard for perhaps up to a mile or so, there is also a stronger chance you might be interfering with someone.

I'm sure there are both Canadian and US suppliers of approved units, but they can be several hundred dollars to buy.

You might want to consider a 1 watt unit. It would be just as illegal, but wouldn't be heard quite so far away.

If you want to be legal, here is some reading material (assuming you are in the USA):

https://www.fcc.gov/mb/audio/lowpwr.html

Where are you located?

 

[the dude of dudes]

I am in southern Alberta.

 

[RadioRon]

In that case, this link may be more appropriate:

http://strategis.ic.gc.ca/epic/site/smt-gst.nsf/vwapj/ric40.PDF/$FILE/ric40.PDF

this link is interesting:
http://www.faqs.org/faqs/radio/broadcasting/low-power-faq/


and here is a link to some manufacturers:
**broken link removed**

 

[the dude of dudes]

Wow they are VERY pricey (~$1000 CAN). Is there a way to amplify the antenna's output on my small transmitter? Maybe to connect it to an RF amplifier circuit?

 

[Sceadwian]

Yes, but it's almost garunteed to be illegal.

 

[musthave]

try this link they have modified a fm transmiter good for 100 ft
**broken link removed**

 

[RadioRon]

The point of all this information is to show that you have these choices:

- find a way to move the transmitter that you have closer to where you are working (some sort of audio relay from the truck). How about running an audio cable from the trailer up to within the work area? Cheap and relatively easy and fairly foolproof. Just annoying. This is legal and cheap.
- go illegal with more transmitter power (either buy something new or modify your unit)
- give up and get some sort of Sirius receiver that you carry with you.
- wear an antenna on your head (I could suggest a design) to extend range another 50 foot or so, but look like a dork.

Personally I think you should consider running an audio cable and putting your transmitter close to where you are working. And don't fall for the scam of brand name audio cables like "Monst--". They are vastly overpriced and unnecessary. It shouldn't cost you more than about $35 for a couple of hundred feet of audio patch cable.