I have a .5-watt Mobile Black Box Eclipse-4000 FM transmitter (a rebranded Chinese HLLY unit). I use it mostly for transmitting Pandora.com to the various FM receivers on my property (on 107.1) from my laptop. The transmitter works great in all respects, except for an annoying hum that's only discernible between songs or during quiet portions of songs. I'm pretty sure that I'm experiencing a grounding problem: if I grasp near the top of the unit's 6.5" rubber-ducky antenna with my thumb and index finger, the hum disappears completely. The antenna is attached to the transmitter via a BNC connector.
As an attempted fix, I've attached a piece of stranded copper wire (jacketed except for the bared ends) to the antenna and then to an electrical outlet ground box. This decreased the hum somewhat, but not enough. Any suggestions? Thanks.
I scanned the related Library items before I posted my inquiry, but I didn't find anything that addressed my problem directly.
I feel confident in saying that my transmitter hum problem is caused by some sort of inadequate grounding because I can COMPLETELY cure it by pinching the top of the antenna between my fingers. My body is very obviously grounding the transmitter and eliminating the hum. Of course, that's not a practical solution. And attaching a ground wire to the antenna by wrapping it around the antenna a couple of times and securing it with a zip tie, then attaching the other end of the wire to a grounded power outlet box, reduces but doesn't eliminate the hum. I think I'm on the right track.
I thought I'd tap the experience of my fellow part15.us members before I proceeded with further experimentation.
vermonter,
I had a hum problem with my Ramsey FM 25 which is lower RF power than your unit. My solution was to enclose it in a metal box. This is not necessarily applicable to yours but it indicates that the hum with mine originated with the RF getting back into the transmitter circuit from the antenna.
Your observation about the hum vanishing when you touch your antenna or "ground it" as you described might well be due to the detuning of the output circuit which results in lower RF power being radiated and a change in the pattern lowering the RF getting back into the transmitter rather than just your grounding it.
Others have also mentioned that nearby electronics, especially cell phone chargers or wall warts can cause hum.
If your transmitter is already in a metal case, then look around in the vicinity for wall wart type chargers and nearby power cords. Also, get some clamp on ferrite filters (RS has them) and put one on the power cord and on each of the audio leads to the transmitter.
You could also get a short BNC extension cable and move the antenna away from the transmitter.
Many times it takes a bit of trial and error to nail this kind of problem and it may also be that there is more than one cause. This is frustrating because it may take more than one solution.
Neil
Making sure it is in a metal box and grounding that box is a very good suggestion.
Thanks, Neil and Greg_E.
My transmitter is in a nice black-anodized aluminum enclosure: http://www.mobileblackbox.com/content/view/42/76/. But, it looks like RF was getting back into the box and causing my hum: Using a two-foot BNC extension cable as Neil suggested, I now have my rubber antenna segregated from the transmitter and attached to the back of my desk with a small spring clamp. No hum at all!
So much for my "grounding" notion.
I've noticed that some laptops hum when connected to external amplifiers, while others don't. When I set up sound equipment for public events where a laptop will be used as an audio source, I always take along a ground loop eliminator such as a 1:1 transformer. Connect it inline between the laptop and the amp, or in your case, the laptop and the transmitter. I'm thinking that will do the trick.
Here is an example: http://www.amazon.com/Xitel-GLI1-S1-Ground-Loop-Isolator/dp/B0019K4UJC/ref=dp_cp_ob_e_title_0