Variable power transmitter

Discussion in 'Radio Circuits, Repair & Performance' started by FLDAV76, Dec 6, 2017.

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  1. FLDAV76

    FLDAV76 QRZ Member


    I have a project in mind for a very low power transmitter, on par with your standard car FM transmitter.
    Looking online has yielded many plans for simple am and fm transmitter units, but all of them are configured for one set output power.

    My goal is to create a transmitter with a variable power level such that I can accurately control the distance where the signal can be received.... for example, a strong signal within my house, but not beyond the property lines. I'd like to be able to turn the unit on, then adjust it for the proper signal strength.

    My question is about how such output power control can be obtained.

    I've seen a few designs to variable frequency transmitters, but none that include a section for varying the power output. Can anyone point me in the direction of learning how to have fine control over transmitter power?

    My guess is that the power of the transmitter is controlled by the power supply, and not the transmitter circuit itself... which explains why transmitter designs omit this section. They assume you are feeding the unit with a set power supply.

    It would be very helpful just to have confirmation of the method used to vary the power in a transmitter.
    I've always had trouble wrapping my head around power supplies. I'm more used to thinking of devices "drawing" whatever amps they need provided the voltage is kept at a suitable level... and I suspect that is a poor description of the physics of what is actually happening.

    Many thanks
  2. G3YRO

    G3YRO Ham Member QRZ Page

    It's not that simple !

    In any variable power transmitter you reduce the amount of "Drive" to the Power Amplifier . . . in a similar way to how you reduce the signal from the Pre-amp to the power amp in an Audio Amplifier. Obviously these simple little transmitters you buy don't have that. (but you could add a drive control, or build one that does)

    However . . . the main problem with what you are trying to achieve is that radio signals don't suddenly stop dead !

    To be strong enough inside your property to be received on any suitable radio will mean you will STILL be able to receive a signal further away.

    That aside, with these simple/cheap little FM transmitters, the easiest way to reduce the radiated signal would be to just make the antenna smaller.

    Roger G3YRO
    Last edited: Dec 6, 2017
  3. KA9JLM

    KA9JLM Ham Member QRZ Page

    You can put a variable attenuator on the transmitter output.
  4. FLDAV76

    FLDAV76 QRZ Member

    Thank you both.
    Indeed, I thought I was missing some fundamental concept.

    A few more facts may help move this is the right direction.

    Regarding using an attenuator on the antenna output.
    While I'm sure this would work, as would using a poorly matched antenna (i.e. starting with a proper match and then trimming it down until the impedance mismatch resulted in a desired output power) it's not very desirable as it wastes power.

    Regarding the fact that radio signals don't stop dead.
    You are quite right, and this is actually one reason I want to have a variable power control.
    I'm not entirely sure what field strength will give me good signal in the desired rooms, but not too much and bleed past the property lines some distance from the building.
    So being able to set a level, walk around testing reception, and tweak the level a bit... would be perfect.

    In audio equipment, the mixing console almost always outputs a "line level" signal to another device called a "power amplifier" which then boosts this signal up to whatever very large output is needed to drive the main speaker arrays.

    this might be the piece of the puzzle I'm missing... do "low power" FM transmitters (or am for that matter) send their output (relatively low power) (RF) signal to a separate "power amp" device, which is in turn connected to the radiating antenna?

    I.e. something like this...
    with its RF output connected to something like this...

    Searching for the term "RF power amp" has yielded some info I didn't have before, and I think I'm now on the right track... the bit I haven't found yet is a power amp design that doesn't use set steps of amplification. For my application I need a smooth variability from low power to a higher power.

    Does a power amp have any special needs in order to handle RF frequencies compared to audio frequencies?

    Many thanks
  5. K7JEM

    K7JEM Ham Member QRZ Page

    Are you operating in the standard US FM band? What minimum and maximum RF output are you looking for? There are simple ways to do this, within reason.
  6. G3YRO

    G3YRO Ham Member QRZ Page

    You're making this over-complicated . . .

    If you're just looking to radiate a signal around your house, or even a large building, you won't need more than a few milliwatts ! So you certainly wouldn't want to be using any kind of extra Power Amplifier.

    It's not even worth building, as you can buy something like this so cheap:

    This has adjustable power output, so would do what you require.

    Roger G3YRO
  7. KA9JLM

    KA9JLM Ham Member QRZ Page

    That looks cool. The price is right.

    Transmit Power: 0.01 / 10 / 100 / 200mW
  8. FLDAV76

    FLDAV76 QRZ Member

    As to the band.
    Either commercial FM or commercial AM would do fine, I have no preference. I've heard it said that the AM band is less heavily monitored by the FCC, so sight unseen I'd try that. The main reason for using these bands to to be able to use cheap thrift store receivers (i.e. boomboxes).

    as to min and max rf output.
    I'm not at all certain, the range may be up to, say, 500 feet or so to the perimeter of the property. The area where coverage is truly needed would be much less, perhaps 200 feet. But there are some walls, etc to be overcome. Perhaps a range from 50mw to about 3watts?

    as to the provided example product.
    That unit seems to have stepped power output, and I'm looking for more granularity. A smooth ramp up to exactly the needed power, and not more, to reach my range. Also, I'm interested in this not only for a practical application, but to understand the electronics involved. For about $100 I could buy a devices that does what I want, but I want to know how it works, I'd rather like to build it myself, and though it's hard to believe perhaps... $100 is a bit on the high side for me.

    G3YRO had mentioned building a transmitter that includes a "drive control" which sounded quite like what I am aiming for. Can anyone point me towards the proper terms used in such a design? All the "simple transmitters" I've found are just that, bare bones transmitters that lack a power output control section.

    Many thanks again.
  9. KA9JLM

    KA9JLM Ham Member QRZ Page

    You can not legally transmit using 3 watts on the AM or FM broadcast bands.

    Unless you have a commercial broadcast licenses, That is a no go.
  10. G3YRO

    G3YRO Ham Member QRZ Page

    The audio quality is poor on the AM band, compared to the FM band.

    Also, any simple home-made transmitter will drift in frequency, unless you buy a crystal for the frequency you want.

    In terms of receivers, you can pick up cheap FM radios secondhand just as cheap as AM ones.

    As I explained before, the signals will be very variable around the property, so something with stepped output like the unit I suggested would be ideal.

    You really couldn't build something for that price! (I built a little FM transmitter years ago, to feed the same music to receivers around the house and garden for parties . . . but somebody gave me a cheap commercial unit later that works MUCH better !)

    Roger G3YRO

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