FM transmitter in a electrical box

[rkhanso]

For all the Radio guys out there:

I have one of the Vastelec FM-02 transmitters and wanted to permanently mount it in a metal electrical box for shielding. It's not so easy to mount the transmitter board flush with the box and have the holes line up for the power, antenna and audio and make it still look OK.
http://www.rogerhanson.com/images/100_3882.jpg
Is that bend on the pigtail going to cause me problems? The pigtail antenna connection is a little long for mounting it inside the box, but that's what I'd like to do. I know that the 3-loops on the coax by the antenna acts like a filter to keep the signal going back into the transmitter, but I was wondering if something like this short bend would cause any similar effect and if it should be avoided.

The goal was to mount the transmitter completely inside the box and mount a new audio and power jack on the box - as a short extension, making it look a little prettier. It'll look better once the outside of the box is powder coated, too.

[n1ist]

If that's RG316, the minimum bend radius is 15mm, so you should be fine.
/mike

[rkhanso]

Thanks for the OK on that.

I went ahead and put it in the box, though not 100% complete yet. Hooked it up to a boom-box and tried transmitting. I used the same transmitter last year and was never happy with the range and quality of sound (due to range issues, I think).

I still get the same disappointing results. Last year I used 93.1 MHz and only got about 75-100' of good quality sound - but I had a very long cable between the transmitter and antenna - about 50' or so of cable (RG6).

Somewhere I read that higher frequencies may work better so I made a new antenna following these instructions (http://www.doityourselfchristmas.com/wiki/images/a/a7/How_to_make_a_dipole_antenna.pdf) (same as last year's antenna) at 105.1MHz with the same disappointing results. Each leg of the dipole is 22 11/16" long and followed the instructions to the letter, but added the coiled coax balun as seen in the picture.

I cut the coax length down to about 25'. I'm using RG6 (all I had around the house). I found on the interwebs that to make the coiled balun, to mark 27" lines on the coax and then match up the lines to make a 3-loop circle zip tied together. I've also tried bumping the frequency of the transmitter up/down by 200KHz increments to see if the antenna may just be outta tune a little but don't get much difference in range/quality of sound. There's a fair amount of hiss and the radio in the house sometimes fades in/out of stereo.

I'm at a loss as what to do to improve the signal. Is the transmitter adjustable somehow - or am I just asking for trouble messing around with any screw-type adjustments on the transmitter.

http://www.rogerhanson.com/images/100_3883.jpg
http://www.rogerhanson.com/images/100_3884.jpg

[chilloutdocdoc]

[QUOTE=rkhanso;127007]Thanks for the OK on that.

I went ahead and put it in the box, though not 100% complete yet. Hooked it up to a boom-box and tried transmitting. I used the same transmitter last year and was never happy with the range and quality of sound (due to range issues, I think).

I still get the same disappointing results. Last year I used 93.1 MHz and only got about 75-100' of good quality sound - but I had a very long cable between the transmitter and antenna - about 50' or so of cable (RG6).

Somewhere I read that higher frequencies may work better so I made a new antenna following these instructions (http://www.doityourselfchristmas.com/wiki/images/a/a7/How_to_make_a_dipole_antenna.pdf) (same as last year's antenna) at 105.1MHz with the same disappointing results. Each leg of the dipole is 22 11/16" long and followed the instructions to the letter, but added the coiled coax balun as seen in the picture.

I cut the coax length down to about 25'. I'm using RG6 (all I had around the house). I found on the interwebs that to make the coiled balun, to mark 27" lines on the coax and then match up the lines to make a 3-loop circle zip tied together. I've also tried bumping the frequency of the transmitter up/down by 200KHz increments to see if the antenna may just be outta tune a little but don't get much difference in range/quality of sound. There's a fair amount of hiss and the radio in the house sometimes fades in/out of stereo.

I'm at a loss as what to do to improve the signal. Is the transmitter adjustable somehow - or am I just asking for trouble messing around with any screw-type adjustments on the transmitter.

Two questions from my experience (with another Tx) how are you powering it? What computer is playing the audio? Have you tried another audio device? Another computer? Try something that runs completely on battery (ex MP3 player) Laptop WITHOUT Ac charger... It could be a ground loop, or a poor filtered power supply, two issues i've had with my Tx

[N7XG]

You may want to consider using 50 ohm cable. The RG6 you are using is rated at 75 ohms. See if you can locate some RG8 or RG58

[rkhanso]

Two questions from my experience (with another Tx) how are you powering it? What computer is playing the audio? Have you tried another audio device? Another computer? Try something that runs completely on battery (ex MP3 player) Laptop WITHOUT Ac charger... It could be a ground loop, or a poor filtered power supply, two issues i've had with my Tx
I'm powering it with a 12v DC wall wart.
Last year, I used the same PC that ran Vixen to broadcast audio.
I tried it today with a boom-box, not a PC.
I'll get one of the kids' MP3 players and try it to see if there's a change....but I don't think the hiss is coming from the audio, but from a weak FM signal.

Not sure where to get any RG58 - all I can find is RG6. Maybe I'll stop at the surplus electronics shop downtown this week and see if they have any.

[sparky588]

I just bought one of these. i had a tune cast that wa modified and the sound quality wasn't to good. i was told not to use the 75ohm cable by the manafacturer. they said it would cause distortion and noise. they said to use the 50ohm cable. can any one tell me the antena jack part number you use on this?

Thanks

Joe

[rkhanso]

can any one tell me the antena jack part number you use on this?

Thanks

JoeI think it depends on the model or version of Vastelec transmitter you have. I think the older models had a different connector for the antenna. Mine has a RP-SMA (Reverse polarity SMA or R-SMA) adapter and it was difficult to find one. Last year, I just soldered the coax to the transmitter PCB. Later, I found a pigtail connector on eBay and that's what I use now. Here's what I use: LINK to eBAY (http://cgi.ebay.com/BNC-female-nut-bulkhead-RP-SMA-male-RG316-pigtail-/280460426430?pt=LH_DefaultDomain_0&hash=item414cbe7cbe)

[IdunBenhad]

Hi:
Note that the feed impedance of a dipole antenna is approximately 72 ohms, not 50 ohms. That would indicate that RG59 cable should be used. The RG6 used for TV usually does not have a good braid on the outside and may be very lossy.

In normal use, if a dipole is designed and built correctly, using either cable, RG58 or RG59 would not make any difference. A higher Standing Wave Ratio (SWR) will result from the RG58, but only slightly.

Having said that, it could be the PVC housing on the antenna is changing the feed impedance. I have read that (on this forum) that the PVC changes the velocity factor of the antenna elements and in fact, the lengths calculated for that particular dipole are incorrect.

Now, I do not know for sure that the velocity factor changes as much as the writer said, around 20% if I remember correctly, but if it does in fact change it that much, then the calculated lengths are way off, making the antenna resonant at some other frequency.

You might try removing the antenna from the PVC and just hang it up in open air and see if the signal strength changes.

This may be theoretical only, but make sure the ground leg of the antenna is down. That is the leg connected to the braid of the coax.

I believe this is the antenna designed by Dirknerkle and he has had good luck with it.

[ctmal]

Here's a link to some RG-58.

http://www.radioshack.com/product/index.jsp?productId=2103483&tab=summary

[cessnapilot]

Monoprice has RG-58 with BNC connectors on both ends.

http://www.monoprice.com/products/subdepartment.asp?c_id=102&cp_id=10221

[chilloutdocdoc]

I'm powering it with a 12v DC wall wart.
Last year, I used the same PC that ran Vixen to broadcast audio.
I tried it today with a boom-box, not a PC.
I'll get one of the kids' MP3 players and try it to see if there's a change....but I don't think the hiss is coming from the audio, but from a weak FM signal.

Not sure where to get any RG58 - all I can find is RG6. Maybe I'll stop at the surplus electronics shop downtown this week and see if they have any.

12V DC wall-wart might be your problem. I tried various wall-warts and had a very bad hum, and hiss, and even some crackle... Try using a PC PSU. It might help some, unless of course you have a well-regulated DC Wall-Wart.

[ukewarrior]

I can guarantee that you will have HUM with a wall wart.
It can be eliminated with the use of a ferrite bead around the power cable leading to transmitter.

[kychristmas]

I can guarantee that you will have HUM with a wall wart.
It can be eliminated with the use of a ferrite bead around the power cable leading to transmitter.

I will second that. I found that out for myself last year with the same Transmitter. I ended up putting it in an old DVD Player and used the Power supply that was already in there. Once I did that, I was pretty happy with it.

To test, just use an old PC power suppy temporarily and see if the noise goes away. I would be willing to bet that it does.

Kelly

[LabRat]

I will second that. I found that out for myself last year with the same Transmitter. I ended up putting it in an old DVD Player and used the Power supply that was already in there. Once I did that, I was pretty happy with it.

To test, just use an old PC power suppy temporarily and see if the noise goes away. I would be willing to bet that it does.

Kelly

Test with a battery.. :)

A good source for ferrite beads is an old VGA cable..

[Jrd]

Test with a battery.. :)

A good source for ferrite beads is an old VGA cable..

Now I wish I didn't trash a bunch of dead monitors in the past without harvesting parts from them first...

[rkhanso]

Is the length of the coax an issue? I did check the monoprice website and they have a 50' length with BNC's on the ends for $13 with shipping included. I really only need about 35' from the transmitter to the antenna. Will the extra 15' affect the signal enough to justify making my own cable?

I will also heed the advice on the ferrite bead on the power cable. Thanks.

[rkhanso]

I did get some RG58 and hooked it up. The range is no better than when I used the RG6. If I get 75' away from the antenna, the stereo light on the radio in the car starts to flicker. The sound may be a little better quality now (I did add a ferrite thingy on the power supply, but not sure if that gave the best improvement or if it was changing the cable). It's the range that I'd like to improve. I did get a 50' piece of RG58 and am using the entire length - if that's a problem I could take around 10-15' off of it.

At least I'm no worse off than last year.
Anyone have any more suggestions?

[dirknerkle]

I think you might be better off by locating the transmitter about 3 feet from the antenna (very short cable) and running a longer audio cable to the transmitter. 35 feet of coax is eating up a lot of your power, and with these low-power transmitters, you really don't have the power to burn like that. Just IMO...

[Jrd]

And if you can't find an 1/8th" 50ft stereo cable or it is too expensive you can do what I did and make two adapters so that you can use a standard Cat-5 cable as an interconnect.
I took a jack like this:
http://rsk.imageg.net/graphics/product_images/pRS1C-2265992w345.jpg (http://www.radioshack.com/product/index.jsp?productId=2103452&numProdsPerPage=60&filterName=Category&retainProdsInSession=1)
Three strands of Cat-5 and an RJ45/8P8C jack:
http://rsk.imageg.net/graphics/product_images/pRS1C-2264715w345.jpg
(similar to what I used but not the same, this one at RS is way to expensive)

Locate the pins carrying the audio signal (left, right and ground) on the jack and solder on 3 pieces of wire, I found 6 inches to be an ok length.
Next attach the wires to your RJ45 jack, if using a wall jack like I did then use your punch down tool but you could certainly use a PCB mount jack as well in which case you would have to solder the wires. Whichever way you choose make sure that you build both adapters the same way other wise your audio may be backwards on the other end.

I can post pics tomorrow.
EDIT: Pics in this (http://doityourselfchristmas.com/forums/showpost.php?p=127987&postcount=22) post.

I experience no loss in sound quality or volume with headphones over a 50ft piece of Cat-5.

[rkhanso]

I'll try the cat5 option since I have quite a bit of it. When recording video, I have experienced long audio runs on unbalanced cables to microphones causing noise in the audio, so I'll likely avoid the 50' audio cable. Maybe I'll try a weatherproof box with the transmitter mounted in it on the same pole that the antenna is mounted on.

[Jrd]

Here are the pictures of what I did, they aren't very good because I had bad lighting but you can see the idea.

[scootchu]

I just noticed something. You have your BNC mounted to your metal box. I just got done using a aluminum DVD enclosure with the same transmitter and mounted the BNC to the box. It was grounded via the outside of the BNC mount. I had some hum and some static. I removed the BNC from the mounting hole, taped it up and then connected my antenna and it got a lot more quiet. I don't know about my range, but it's about 300-400' feet with the transmitter and antenna in the basement.

[rkhanso]

Today I cut the coax length down between the transmitter and antenna, to about 15-20 feet. It works a lot better now - and that was with the antenna on the ground -and using RG6 coax.

Once I figure out a mounting method to get the transmitter up near the antenna, waterproof and everything, I'll switch back to RG58 coax.

I'm thinking about making this a permanent mount up on my pole barn - way up by the peak. Mount a bracket on the outside of the barn and run PVC up vertical above the roof and all the metal of the pole barn, run the coax down through the PVC, through the siding and mount the transmitter inside where it's dry. The antenna will end up being about 25 feet up in the air and should be a big improvement (hopefully).

Now, gotta get the audio worked out like Jrd suggests and I should be good to go.

[Jrd]

Sounds good, (no pun intended) If you want any more info on my method just ask.

[billyb27]

so where can i get a pig tail from

[IdunBenhad]

Hi:
Scootchu, you should not have to remove the BNC from the metal box. In fact, it should be left there. The metal box provides shielding to the transmitter.

You have a ground loop somewhere. I found in one instance that I had to isolate the third wire ground on the power supply AC cable to stop hum. I used a 3 wire to 2 wire adapter. This was a converted PC power supply. Another way is to ground the shield on your audio cable at one end only, usually at the source.