Reducing Transmitter Range

[Bob Musil]

I have a transmitter that I purchased a few years ago on eBay, from a Chinese manufacturer.  It has no volume control, just a switch for changing the frequency.  I do not know how much power it is putting out, but I have been told that it could possibly be in excess of FCC regulations.  Is there something that I can put between the computer and the transmitter, or between the transmitter and the antenna, that will let me adjust the transmission distance?

 

Thanks,

Bob

[viennaxmas]

Yes - it is called an attenuator. It reduces the RF signal. It does get put inline with the antenna line. Search the forum for attenuator and MPJA and you will find a link...

[Bob Musil]

Thanks. I will check it out.

[james campbell]

how far can you transmit with it? as long as your not transmitting over a large area or transmitting over a radio station in your area you should be fine. I know that many have transmitters that are not fcc approved,but do just fine with them. now thats not to say that you may issues this is only my opinion and i'm not a man in a black van

[Bob Musil]

Probably best if I not give a hard number, just in case you do have a black van or helicopter.

 

I need to scale it back quite a bit, but I have a problem.  I live on several acres in a rural area, and if  I strictly conform to what I understand the FCC limit to be, the signal can not be heard from the road.  What I am trying to do is limit the signal to only a mile or so, where it will be heard by about 30 houses total.  Every year I test to make sure the frequency I want to use has nothing on it in this area.  I also check this site http://www.radio-locator.com/cgi-bin/vacant as a backup check.

 

I am going to see if I can get an attenuator with an adjustable power level.  Hopefully I can find one at a reasonable cost.

Edited May 6, 2013 by Bob Musil

[james campbell]

I have used that website as well. it is not very accurate,you can use it as reference but you will still need to test each frequency out to make sure

[Greg Young]

Attenuators can be easily made with resistors that are readily available for the power ranges of FM transmitters.

Assuming you are using a 1 watt or less transmitter, you can go with a series of small 2 - 5 watt (or larger resistors), depending on how many are needed to reduce your output to an acceptable level.

Start with lower levels of resistance (ie 5  or so ohms each), put in one at a time, and check the range.

[Bob Musil]

Unfortunately, there are no markings to indicate power level, and my electronics skills are rudimentary at best.  I think it is either 1 or 5 watts.

[Greg Young]

What do the instructions/literature that came with the unit say?

If none, and you think it's 5 watts, you are correct , you are way over the legal limit, and will risk issues, especially as some of the Chinese units are dirty, transmitting harmonics that can interfere with other vhf communication (air, police, fire, etc).

If you think it's 5 watts, then use 10 watt resistors.

If you can't easily find 10 watt resistors, wiring 2 of the 5 watt for example in parallel will increase power handling capacity (to 10 watts plus or minus), but will cut their resistance (in half), so I would suggest starting with 10 ohms in that case.

Whatever you decide I would strongly urge you to not transmit without an attenuator if you even suspect this might output 5 watts.

[Max-Paul]

Greg, how would you wire these resistors? And what value would you use? REMEMBER you want to keep the impedance at the correct value or you will have a high SWR that can cause the finial transistor to fry it's self. So far I have not seen anything that addresses these issues.

 

Also if this is a 5 watt transmitter and you want to drop it back to say 100 mW. How many dbs would that be. Attenuators are rated in db so you have to know how many db you want to reduce the signal.

Edited May 15, 2013 by Max-Paul

[Greg Young]

Max-Paul - essentially what you are doing with attenuators is using inductors (resistors in this case) in series to reduce the antenna's radiated output. The rf output will be dissipated as heat within the inductors (resistors) hence my suggestion to use a higher value (10 watt if using a 5 watt output).

Obviously if you use a 50 or 75 ohm total value (depending on the transmitter being used) you would wind up creating a dummy load, with almost no output (outside of that radiated by the actual inductors).

Buy using a  lower value, although not matched impedance wise, the rf signal will go forward, with minimal being reflected back, as long as the final radiating element is matched to the output frequency. If you use a higher value (higher than 50/75), the signal will get reflected back before going forward to the final radiating elemnt and you will have a high SWR.

If he starts off low, he can reduce the range of his transmitter without risking high SWR as long as the total series resistance doesn't exceed 50/75 ohms, which it shouldn't as he will see a decrease in range of signal well before he creates a 50/75 ohm dummy load.

Obviously a commercial attenuator saves this experimentation, but it comes at a cost..

Edited May 16, 2013 by Greg Young

[Bob Musil]

What do the instructions/literature that came with the unit say?

If none, and you think it's 5 watts, you are correct , you are way over the legal limit, and will risk issues, especially as some of the Chinese units are dirty, transmitting harmonics that can interfere with other vhf communication (air, police, fire, etc).

If you think it's 5 watts, then use 10 watt resistors.

If you can't easily find 10 watt resistors, wiring 2 of the 5 watt for example in parallel will increase power handling capacity (to 10 watts plus or minus), but will cut their resistance (in half), so I would suggest starting with 10 ohms in that case.

Whatever you decide I would strongly urge you to not transmit without an attenuator if you even suspect this might output 5 watts.

 

Unfortunately, there are no markings on the unit indicating, well, anything.  On the front it says FM transmitter, and has + and - buttons for tuning.  On the back it has inputs for DC12V (power) and antenna.  That is the total marking on the unit.  If there was any paperwork/manual, it has been lost to the ages.

 

When you talk about dummy loads and SWR, I have no idea what you are referring to.  Help!

[viennaxmas]

http://www.microwaves101.com/encyclopedia/resistive_splitters.cfm

Here are some really good instructions on how to build splitters while

Maintaining the impedance. A splitter can be used to reduce output power by terminating one side in a 50 ohm resistor.

There are also a few more links for further reading.

[james campbell]

Unfortunately, there are no markings on the unit indicating, well, anything.  On the front it says FM transmitter, and has + and - buttons for tuning.  On the back it has inputs for DC12V (power) and antenna.  That is the total marking on the unit.  If there was any paperwork/manual, it has been lost to the ages.

 

When you talk about dummy loads and SWR, I have no idea what you are referring to.  Help!

how about a picture of your transmitter,maybe someone here has one or has seen one.

[rwertz]

Would a resistive T-pad attenuator work?

http://www.raltron.com/cust/tools/t_attenuator.asp

[Max-Paul]

My only advise to you is this. I asked Greg for more precise information. As you can see, I didn't get it. BTW to drop from 5 watts down to 100 mW you would need X db attenuator. Lets do the math and you will have to take some of this on faith. 5W to 500mW is 10 db. And 3db will half it. So 3 more db would drop from 500 to 250, another 3 db would drop it to 125. So adding it all up we are talking about 16 db. if you add one more db to make it 17 db it would be 1/3 of a half 125 / 2 = 62.5 / 3 = almost 21. So 125 - 21 = 104 mW. OK so now we know that if this transmitter is 5W and our target is 100mW. Then you would want a 17db attenuator. NOT saying that this is what you want, cause I really do not know if it is a 5 or 1 Watt transmitter.

 

As for inductors versus resistors. A resistor is a resistor, but an inductor has reactance. Meaning a inductor acts differently at different frequencies. I would not think that you would find an inductor in an attenuator. I do not know Greg's education in electronics. But I suspect that he was not a radio repairman for the military. Nor holds an Amateur radio Lic.

 

Yes over the years I have misspoke. Once when I was animate that the output impedance of non-video transmitters being 50 ohms and video being 75 ohms. This had been the standard for many years. Seems that rules of the past are being broken and I am finding that some low power transmitters are either or both 50 and 75 ohms impedance.