AM / FM Band Broadcast Transmitters

I found a number of sources of FM transmitters that clearly would exceed the F.C.C.’s part 15 regulations, and yet don’t appear to be F.C.C. type accepted and approved and therefore not useful to legitimate broadcasters.

For example, this FM Transmitter Station sold by HobbyTron.com is advertised as having a one watt output which unless not connected to an antenna and buried 30 feet underground, or operated into a dummy load, would clearly violate F.C.C. part 15 rules. HobbyTron.com also sells some AM transmitters. These are very low power and from the descriptions might even be legal under Part 15. They claim a maximum range for their AM transmitters of around a quarter of a mile. Years ago I experimented with an AM transmitter, 100mw input to the RF final, and a ten foot British joystick antenna, (at the time Part 15 allowed a ten foot antenna, now they’ve gone metric and it’s 3 meters), and using a Sony Earth Orbiter receiver, I was able to get nearly three miles range on 660 Khz. Granted, this was the range at which it was still intelligible with a very sensitive receiver and a very quiet frequency, not a range that would be useful for the general population. But this experience leads me to believe that their claims are legitimate with respect to their AM transmitters.

On the AM band 1 watt would be ten times the power allowed under Part 15 rules, however, on the FM band much less power is required to travel significant differences so even a few milliwatts may violate F.C.C. rules.

I know when I was in high school I built a little FM power oscillator transmitter using the triode section of a 6U8 tube (which is an itty bitty low powered tube) and with what little power that generated, a few milliwatts, we found we could receive the signal several miles away. One watt, in the absence of other interfering signals, with a decent transmitting antenna located at a reasonable height, and a decent receiver, can go a number of miles.

Then there is this outfit:

FM Transmitters and Broadcasting Equipment

Providing FM transmitters up to 10 Kilowatts. They ship mostly to the US and Canada but will ship outside the continent if the order is substantial, the entity established, and proper arrangements are made. As with HobbyTron, you are required to sign an agreement to operate these only if you have a license for the proper jurisdiction, however, they do not mention their transmitters as being F.C.C. or C.R.F. certified so I am not convinced this is even possible. They do not list prices on their website.

C. Crane Company sells an FM transmitter that they claim is F.C.C. approved. I don’t know exactly what that means, I’ve heard of F.C.C. type acceptance, but at any rate, it’s a very low power unit that probably does fall within Part 15 rules and probably won’t transmit any significant distance.

Then there is Decade which makes FM transmitters as well. With a solid state FM amplifier that they also sell you can get one watt output, which fed into an antenna would almost certainly violate Part 15 rules.

PCS Electronics sells a variety of FM transmitter products, they also sell studio gear (which visually at least looks professional).

Quasar Electronics has FM transmitters of various power levels including one 25 watt unit. They also make a variety of other devices such as CB amplifiers. These guys could really get you filling the F.C.C.’s coffers if their products are used illegally.

Armstrong Transmitters makes an interesting AM transmitting unit, 500 watts and 1000 watts. These use multiphase modulation like the old CCA transmitters but these are solid state. I will have to admit I never much cared for this type of modulation scheme as it tends not to support very high levels of modulation without distortion. With high level modulation, in theory audio output power that is 50% of final input power is all that is needed to achieve 100% modulation, but in the transmitters I built, I tended to make the audio section around 200% of the final input power just to make sure I could modulate the positive peaks real heavily and that the audio section would be not at all stressed and distortion free at normal modulation levels. Using multiphase modulation does eliminate the need for a modulation transformer saving weight and improving frequency response. The downside to this type of modulation scheme is that it invariably leads to some phase modulation as well and because phase modulation introduces sidebands at not only the carrier plus the modulating frequency, but also the carrier plus modulating frequency plus harmonics of the modulating frequency, it results in some harmonic distortion. This can be somewhat compensated for with pre-distortion techniques or negative feedback, however, the phase modulation component remains.

Here are some instructions for a small AM transmitter that you can build. This is a very low power unit, useful for piping audio into an old radio or around the house.

Here are plans for a class E 80 or 160 meter AM transmitter. Class E is otherwise known as pulse-duration modulation and it’s really the best way to do things. It can provide upwards of 90% efficiency, matching or even exceeding multiple phase modulation schemes, but without the inherent distortion and phase modulation products. Note that 160 meters is around 1.8 Mhz, just above the top end of the AM band and 80 meters is around 3.6 Mhz or so, the lower end of the shortwave spectrum. This transmitter would be legal in those bands if you have the appropriate amateur radio license, however, most work in 80 meters is either single side band or CW because the latter use much less bandwidth and cover greater distance for the same amount of power. With SSB, ALL of your power goes into broadcasting intelligent information, with AM, two thirds goes into the carrier and only one third goes into intelligent information and AM produces both upper and lower sidebands using twice the bandwidth of SSB.

Low Power Radio and Broadcast Company makes some 100 mw AM transmitters, which, if operated with the proper antenna, would meet Part 15 requirements, but unlike many low power units these are crystal controlled so you won’t have issues with frequency drift.

Harris makes a 200 Kw solid state AM broadcast transmitter. I’m not sure where in the world 200 Kw would still be legal on the AM broadcast band, to the best of my knowledge the US and Canada both restrict the maximum power for AM broadcasters to 50 Kw. What is interesting about these transmitters is that they claim to use direct digital synthesis of the RF envelope as their source of modulation.

So there are some interesting transmitter sources if you are looking into low power broadcasting, or maybe in the case of the Harris unit you can buy a ship, go into international waters and do a Radio Caroline type operation.

1 thought on AM / FM Band Broadcast Transmitters

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