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

I have a 5w HLLY fm transmitter that I bought of ebay that goes too far (3 mile radius with antenna in the attic). It has been recommended to lower the voltage in order to reduce the power. I'm curious if this can damage the transmitter?

I am also looking into an attenuator but wanted to see what going from 12v to 9v would do first as far as range is concerned.

Thanks in advance.

What country are you in?
I think 5 watts is way too high and may be illegal in the US for unlicensed transmission. Lowering the voltage will probably cause the radio to not work at all. You have components that are entirely dependent and biased based on set voltages. Your LCD display might even be affected. I doubt lowering the voltage will damage anything. Usually higher voltages are what damage devices. Think about it, do your battery powered devices become damaged when the battery drains?

But lowering the voltage in this case will do no good, and even if it did, a drop of 3 volts say at the amplifier stage would only reduce your power by a percentage. Maybe a watt or two. You wold still be transmitting up to 3 miles. a watt or two doesn't make that much difference at VHF. I can't speak authoritatively about the legal power or antenna for unlicensed commercial FM broadcasting, but I'm certain that it is less than a 1/4 watt to a 1/4 vertical, probably less than 100 milliwatts actually.
If you want a legal, non-interfering transmission...like used on musical Christmas lights or whatever you may...at least legal insofar as not getting FCC complaints, transmit into a 50 ohm dummy load. A 10 watt power resistor should be plenty big enough to handle the power and dissipate the heat without damage. Solder it between shield and center conductor of a short piece of coax that would otherwise go to an antenna. I am an amateur radio operator, I have a neighbor who lives 1/8th a mile away who is a ham. We have QSOs(half duplex tranmissions) through 50 ohm dummy loads, only transmitting milliwatts of power with 9dB signals at the receiving end.

You don't need to purchase an attenuator. A 50 Ohm resistor will do plenty of you are just planning on a simple neighborhood transmission.

Thanks for the advice. I'm in the US. I think the FCC regulations are distance specific rather than power specific.

The reason I wanted to use an attenuator was because I don't want to have to solder nor do I want to have to cut the existing coax cable it came with.

Alcyone said:

Thanks for the advice. I'm in the US. I think the FCC regulations are distance specific rather than power specific.

The reason I wanted to use an attenuator was because I don't want to have to solder nor do I want to have to cut the existing coax cable it came with.

Click to expand...

No, FCC regulations are always power specific. Actually, they are "effective radiated power" specific which includes both transmitter and antenna including feedline radiation. I do not know the exact power for commercial band unlicensed broadcasting but I have no doubt that there *is* a power limitation and no device will be FCC permissive if that is exceeded. Trust me, I know this much. I have an FCC license and rules like this are universal. In fact, commercial FM band would likely be tighter than most under the Part 15 rules.

Let me post a link to help you. Be a minute.

https://www.electro-tech-online.com/custompdfs/2009/12/oet63rev-1.pdf

I posted this on another thread a few days ago. Same info applies here:
https://www.electro-tech-online.com/threads/fcc-regulations.101055/#post822134

Looks like the effective radiated field will be no more than 500 microVolts per meter at 3 meters elevation of the antenna feedpoint for periodic transmissions. If I read correctly a wideband FM transmission would be 250 uV/m. A dummy load resistor might very well be borderline at 5 watts output as far as effective radiated power goes.

You will not have to cut your coax. You can get a short 50 ohm piece of coax with an RF connector already soldered on at radioshack. Be sure the transmitter wants 50 Ohms, it will indicate the antenna impedance on the back. Some cases it might want 75 ohm coax. In that case you would need a 75 ohm resistor for the dummy load.

If you are uncomfortable with soldering, twist wire connectors or crimp connectors could be used to secure the resistor to the coax without much trouble. You could make a very efficient attenuator yourself for less than $15 that will actually keep you legal, or at least legal enough to not worry about anything.

Here's a little drawing to help you out.

All you have to do is strip back the outside black,grey, or white insulation from the outside about two inches. Then, unravel the braid wire down to the edge of your cut insulator. You will expose the center insulator, which is usually rigid white or clear nylon plastic looking material. cut this back about an inch, leaving the solid (usually) or twisted copper center conductor about an inch exposed.

Make sure the braid nor center conductor have a chance of touching and shorting out. you might hotglue them to a piece of plexiglass or tie wrap them in place to a piece of wood. Something nonconductive. Use the twist-locks or crimp connectors as I have illustrated to install the dummy resistor.

I urge you to take precautions like this and not haphazardly transmit an illegal signal. The commercial fm band in particular is NOT a good place to do something illegal. Too many people with radios tuned to that band.

Google is our friend.

I do not think your particular HLLY transmitter is FCC compliant for Part 15 unlicensed transmission. You should read and understand the following link to an HLLY device that IS FCC approved:

**broken link removed**

Transmitter is 1/2 a watt, which is a little more than my "guess"...but the supplied antenna is probably inefficient to help attenuate the signal a bit.

The webpage I linked translates the 250uV/m rule to basically say you are limited to a RADIUS of about 200 feet to be compliant.

If you do not attenuate the 5 watts with a dummy load I gaurantee you will be out of compliance, and honestly even WITH a dummy load you will probably be illegal...though I wouldn't think the FCC would do more than give you a warning if noticed. The dummy load at least shows forethought in TRYING to be compliant.

I sure hope my efforts to help you were not in vain. I'll make the dummy load myself (at cost) and mail it to you to help you stay legal. Sometimes I wonder if some folks really understand the consequences of illegal transmissions. Spurious emissions, harmonics, etc.

Thanks for the response ke5frf. My transmitter is a very basic, cheap little box. It doesn't say whether it wants 50 ohm or 75 ohm coax.

Before I made this post I ordered this attenuator.

**broken link removed**

Do you think your method of using a resistor will work better?

I would be comfortable trying your instructions, but wanted to know how much you'd want to mail me your dummy load. I'd be happy to pay you for the parts and most importantly your efforts. I can send you a paypal payment.

this HLLy transmitter is having a internal 5V regulator i remember for PLL and modulator. so reducing the voltage will not effect your display or etc and for sure you will get reduced out put.

no need to wurry you wil not get it damaged, i have tried many, also the height of your antenna is a big factor for the range. just lower it and your range will be less. also you can use resistive dividers so that your power will be attennuated.

for your information this transmitters are matched with 50 ohm coaxial and antenna.

mbarazeen said:

this HLLy transmitter is having a internal 5V regulator i remember for PLL and modulator. so reducing the voltage will not effect your display or etc and for sure you will get reduced out put.

no need to wurry you wil not get it damaged, i have tried many, also the height of your antenna is a big factor for the range. just lower it and your range will be less. also you can use resistive dividers so that your power will be attennuated.

for your information this transmitters are matched with 50 ohm coaxial and antenna.

Click to expand...

He will NOT, I repeat NOT get an adequately reduced output to remain legal in the United States by reducing the voltage. He must reduce his power output from 5 watts down to at least .5 watts. Going from 12 volts to 9 volts will not be enough.

The attenuator in question might not work adequately. It says it is rated for (1 watt?) on that link. Having the model number would help.

However, FYI, 10 dB attenuation for a 5 watt input will be .5 watts. So a 10dB attenuator would probably keep you legal. You just need to make sure it is able to handle 5 watts without letting the smoke out.

FWIW, the attenuators this company sells seem to be pretty inexpensive. If you can locate a 10 watt model (always give yourself some headroom) that is AT LEAST 10 dB I would think this would be as good a method as any. I really had no idea you could get one that cheap, that's why I recommended making the dummy load. But if you can locate a suitable one this cheap I say go for it.

I understand that the attenuator might not be adequate to reduce the power, but I'll test it and post back. I wonder if lowering the voltage and using an attenuator at the same time will have a better result.

Alcyone said:

I understand that the attenuator might not be adequate to reduce the power, but I'll test it and post back. I wonder if lowering the voltage and using an attenuator at the same time will have a better result.

Click to expand...

I am not privy to a schematic. Mbarazeen claims that the circuits can handle a 3 volt drop. I certainly would expect the digital circuits, if their are any, to have a 5 or 3 volt regulator dedicated to them.

You won't damage anything. It is worth a try if you have an external power supply capable of supplying the current needed for all the circuits. I'm not sure if the power reduction will be linear...and I also fear that critical components will not be biased correctly. Even if they are, the most attenuation you can expect might be 50%...just a rough estimate. That would still be 2.5 watts. If the attenuator is rated for 1 watt continuous power, you will be exceeding it considerably. This will almost certainly damage it from heat. POWER is what occurs when one form of energy is changed to another in the form of WORK. The unit of measure for this is the WATT. Most transducers are not 100% efficient, meaning that the electrical energy that is being transformed into RF energy will experience LOSS in the form of heat. If 75% of the electrical power is converted to RF, then 25% has to be converted into heat. 25% of 1 watt is 250 milliwatts. 25% of 5 watts is 1.25 watts. Likewise, 25% of 2.5 watts is about 620 milliwatts. So, if you manage to reduce the amplifier output by half, and if the antenna and amplifier are 75% efficient, which isn't an unreasonable figure, you already have 620 milliwatts of power in the form of heat. That may not seem like much, but if the surface area of the dissipating components is small, that heat will be very concentrated.

The attenuator can probably handle more than a watt continuous. Perhaps a watt and a half. They often understate their abilities to prevent high failure rates. That still leaves you with a watt of power in excess. It won't last long before overheating and burning up.

The trouble here is, will this happen with you knowing it? Well, it will probably happen pretty quickly. Within 5 or 10 minutes. I suspect you'll be there observing through an FM radio. Be aware of a sudden, sharp decrease in signal strength. You will only have a few moments before the amplifier stage itself cooks with an open antenna circuit.

Then again, perhaps the website wasn't sure of the power abilities of this model. They did say (1 watt?) with a question mark.

I'm not going to tell you not to try this, I'm just offering justified warnings.

One thing you should do, is not drive the transmitter too hard with your audio input. I imagine it will be a CD player or computer? The audio output? Well, keep the volume fairly low. No need to blast it. Your carrier power will be unaffected by keeping the volume low, and this is the bulk of the continuous power. But your peak power output will be less if it isn't driven very hard. It might buy you a LITTLE wiggle room.

One other suggestion, and this is probably futile as well but here it is. If you heatsink the amplifier you might increase the power it is able to handle by a margin. Encasing it in a form fitted aluminum tube with fins, with silicon heat sink compound between the attenuator and casing might be a good start. A fan with good forced circulation would be another plus. And if you really felt bold, a copper coiled tube wound around the attenuator carrying refrigerated antifreeze would be an excellent heat exchanger. If you happened to have a water pump, a bucket, and dry ice, you could circulate the antifreeze through a second coil in a loop, passing the antifreeze through both coils and placing the second coil inside the bucket of dry ice. Effectively a radiator or heat exchanger. A lot of time, effort, expense, and labor. Better to get a proper attenuator....

And the dummy load is still out there as a possibility. I could build it for you, and will if need be, but this is a very easy project that you should be able to do on your own. I have faith.

Well crap, the attenuator I bought probably is only rated as handling 1 watt. The last thing I want to do is burn up the transmitter. I'd rather avoid external cooling/fans if possible.

If I go the resister route, will I still risk burning up the transmitter? is there a particulare type of resister I need other than one that is 50 ohms?

I've been trying to reduce the power with a universal power supply, but Radio Shack doesn't sell the adaptor tip I need to plug into the transmitter. I've tried ones that look the same as far as thickness but they don't work. i think it has to do with the internal diameter of the tip.

And the fun continues....

Beware of the polarity for those wal-wart universal supplies! (Positive voltage inside/or out and ground inside/out and vice versa) Usually ground is the outer connection but again, make sure. Make sure the supply is rated for the same current (amps) as the one supplied with the transmitter. IE if the supplied power supply is 800 mA, be sure the universal one at LEAST meets this requirment.

And as far as the resistor for a dummy load goes, all you need is one rated for above 5 watts...next step up is 10 watts...and 50 ohms. 50 ohms is not optional. This is like impedance matching a speaker for your stereo. If it says 8 ohms on the stereo, you need 8 ohm speakers. Antenna is the same way. A resonant antenna for 100 MHz will present 50 ohms to the transmitter. A dummy load has to be the same.

Here:
https://www.electro-tech-online.com/custompdfs/2009/12/dl_30w_hf-uhf.pdf

The reason thick film power resistors are used is because they present nominal reactance to an RF circuit. A wirewound resistor can become inductive at certain frequencies and isn't a good idea.

That PDF points to part numbers that should be available for 50 ohm resistors, Check and see. If I were you, I would research the same variety of resistor in higher wattage packaging if available.

The heatsinking he presents would be overkill for a <5 watt application. However, heatsinking to SOME metal piece would be required to dissipate heat. A small aluminum block with holes drilled through it would do the trick to increase surface area. Make sure to not allow the legs to touch metal or each other. Heatshrinking would be a good plan.

Soldering the connector as I mentioned or using twistocks is still an option.

You won't need another power supply here. The resistor will be turning most of the power into heat. It will radiate however. Trust me. Not sure how far you need this to go, but a 100 foot radius is not unlikely.

I'd still consider a step up, 10 watts, in an attenuator. Make sure the dB attenuation is adequate if you go that way.

Ok, so I was able to find a darn tip that worked. It's a type L with 5.0mm external and 2.5 internal diameters. You'd never have guessed that was the right one by looking at it.

Anyway, I was able to run the transmitter on 7.5 volts, but this made almost no difference as far as range from what I can tell. I drove all around my neighborhood and got a clear signal the whole time. It wouldn't even power on at 6 volts

Do you know what 7.5 volts would get me as far as wattage? If it's closer to 1 watt perhaps it'll be safe to use the attenuator I bought?

Thanks for all the help ke5frf.

As I said earlier, I can't be sure that the reduction in power will be linear.

The reason it wouldn't power on at 6 volts is because you were getting close to the minimum some of the circuits needed for operation. I'm sure some of the circuits are 5 volt regulated, and by the time the RF circuits loaded the supply down, the supply couldn't offer the 5 volt regulator enough power to work properly.

I would only venture a guess that 7.5 volts cuts the output power in half, Probably not much lower than that. You might be lucky to get 2 watts out. No way of knowing without an RF wattmeter or a scope. Like I said, if you turn the input volume down as not to drive the amplifier too hard you MIGHT get a slight degree of extra attenuation, at least the peak power will be less.

If you have a fan you could directly cool it with forced air. This will help matters a bit.

Oh, BTW you really won't notice much difference between 2 watts and 5 within your neighborhood. You might have cut your effective radius down to 1/2 to 1 mile instead of 3 miles. The effect on distance is logrhythmic. (I hate that word never can spell it right)

You really have a long way to go my man, and the attenuator or dummy load is the only choice if you don't want half your town picking up your signal.

I understand. I'll assume it's between 2 and 2.5 watts. That being the case, I don't feel comfortable hooking the 1 watt attenuator up. So I'll either have to find one that's adequate or make the dummy load.

My reservations with the dummy load are being able to find the coax ends and being able to crimp them. I've never done those types of connectors before (only cat 5 for network cables).

The reason I don't want to turn the input down is because the level has to be kept really low as it is to avoid overmodulation. Any lower and it would be too quiet.