Can using a lower voltage power supply ruin an FM transmitter?

instead of a continuity check, it would be advisable to check resistance between BNC shield and center pin. Should be about 50 ohms if your connections are well made and your resistor is correct.

Sorry, I didn't mean continuity. I meant ohms. I'm not sure what it's getting since my multimeter doesn't have the right scale, but I assume it's around 50 ohms because the needle doesn't move all the way when checking both sides of the resister. Or between the center bnc pin and ground.

Nevermind, I rechecked the resistance on the RX10 scale, adjusted the calibration and got a reading of 5. Which I assume is 5 X 10 = 50 ohms.

I hooked it up and it's transmitting a clean signal to my radio. But I'm scared to death of burning this thing up. How will I know if it's getting too hot? Will the connector on the back of the transmitter get hot?

By the way, merry Christmas to everyone.

if your resistance is capable to handle the power discipation then nothing to bother. if you will provide some ventilation you can prevent over heating. connector will not get hot if its in propper contact.

The transmitter case gets luke warm after being on for 10 to 15 minutes. Is this a bad sign? The resistor I'm using is a 50 ohm 10 watt. My transmitter is 5w.

Is there a cooling fan built into the transmitter? I wouldn't expect "lukewarm" to cause any problems. I don't know if you have a warranty that might be violated by opening the case, but if it were me, I would open it and do a "touch test" on the amplifier...not the terminals, just the transistor packaging. Touching the terminals wouldn't be advisable because there are potentially high voltages as well as the RF can cause burns. If you don't feel comfortable with a touch test, if you happen to have a digital thermometer (the kind for cooking turkeys or roasts might be sufficient) that reads in the 100-200 degree range, you might take a reading this way.

All transmitters will cause warming. My amateur radio 144 MHz (2 meter) Handy-Talkie (Icom) puts out 5 watts and after continuous transmission for more than a minute will get quite HOT. It is a pretty small package, about the size of a pack of cigarette, but it isn't designed for continuous duty cycle....but will hold up well despite the heat. Your transmitter is supposed to be designed to stand up to this duty cycle.

As another experiment, if it makes you feel better...I don't see how a 10 minute test with the antenna hooked up would do any harm. Test it under normal conditions with the antenna. Does it get hot to the same degree as with the dummy load?

Luke warm (110 F?) is a good sign.

Ke5frf, thanks again for the response. I appreciate your suggestions about touching the aplifier, etc. What I'll probably do is a test with the regular antenna as you suggested to see if it runs at the same temperature.

However, I think this dummy load is perfectly fine. I've had it going for hours today and it isn't getting even close to hot. I was surprised to find it's still transmitting about 4 or 5 houses down the street. That's with the voltage reduced from 12 to 6.5. I guess that's better than 3 miles.

As far as there being a fan I don't think so because there's no opening in the case for the fan to ventilate.

Perfect. I knew from the get-go that a dummy load would be more than enough to attenuate your transmitter for short-range use. I hope it was a good, cost effective solution.

I wonder, did you follow the PDF I Googled for you to the letter, i.e the same thin film-resistor etc?

I'm not surprised the transmitter didn't come with a fan, at 5 watts a heatsink inside is probably doing the job. However, if I had built this myself a little DC fan would have been included

I did follow the PDF, but used a different brand resistor. It was very cost effective; about 13 bucks for the resister, bnc connector and bnc cable (6ft). It would have cost around $50 if I had bought a dummy load from a shop and that's not including BNC adapters.

One problem I'm having now though is interference. When the transmitter was using it's own antenna and running at 12 volts, harmonics/spurious transmissions weren't a problem. Now, I've noticed it's interfering with digital TV on some channels. I must admit that my TV antenna is close to the transmitter so perhaps that's why. I just hope those 4 to 5 houses in either direction don't have a problem. Otherwise I'll be buying a low pass filter.

Mmm, yes filtering might be a good idea. Also, running a thick wire conductor from the case (metal?) of the radio/and/or BNC shield to a good Earth ground (grounded water pipe or stake) might help with decoupling. If you happen to have a ground rod and a long piece of wire and alligator clips, you may give this a "try".

edit: Another thing I wanted to mention...I don't know what kind of resistor you decided to use, and it may not be altogether important, but I suspect your resistor has a series inductance (and by nature a parallel capacitance too), but the inductance is the primary reactance at 100 MHz. Your supplied antenna is an "open circuit" type of antenna, thus capacitance tends to be the primary reactive component. The dummy load more resembles a "loop" antenna, (thus the inductive quality) Both will radiate, but an inductive antenna (or inductive dummy load) will radiate a stronger magnetic field whereas the capacitive antenna will have a stronger electric field. An ideal resonant circuit will have equal energy in both fields and will radiate "electromagnetically".

Both magnetic and electric fields can cause EMI of course, but stronger magnetic fields, for some reason, tend to couple to other electronic devices and are more susceptable to coupling themselves. I think this is because of the angle of the radiation for magnetic vs electric fields. Electric fields are perpendicular to the axis of the conductor where magnetic fields are more or less parallel (lines of flux with varying densities etc) Because of this, the device receiving electric field interference is only receiving it on a perpendicular axis whereas magnetic flux lines "will find" a parallel conductor in the device and induce currents.

I think this is as good an explanation as I can jog in my mind at the moment for why you are causing EMI.

If we knew just how much inductive reactance is occurring (I'm quite certain it isn't much), we could calculate a value for a parallel capacitor to null it and reduce the magnetic field you're creating. I own an antenna analyzer that does this very thing, it looks at the load reactance and calculates the frequency of resonance and can indicate the degree to which the antenna (or dummy load in this case) is capacitive or reactive.

I just wanted to make an update to this post: Anyone who makes a dummy load for an FM transmitter needs to use a special, non-inductive resistor that can handle those frequencies. The ceramic, wirewound ("cement-powered") one that I had previously bought was causing the transmitter to get warm. I was told using that resistor was the equivelant to transmitting into an open ended coax cable. After making another dummy load with the non-inductive resistor, it doesn't warm up at all and remains cool to the touch. Fortunately, the HLLY transmitter I bought was obviously able to handle the abuse for the long period of time.

Here's a link for where to get one if anyone's interested.

Digi-Key - MP850-50.0-F-ND (Manufacturer - MP850-50.0-1%)