how far can you go with attenuation

Hi Bob

I have worked more in audio but a little bit with RF, and I would say that you can attenuate all the way down to zero (a so-called "dummy load), or to small fractions of 1-watt.

For you I would start by lowering the power as far as possible with transmitter settings, then go the rest of the way with output attenuation.

Radio transmitters usually output at 50 ohms, and if the output is balanced you'd build an H-pad, if unbalanced a T-pad, made from resistors matched to the power they will absorb. I'd even go better than 5-watts and use 10-watt resistors NON-INDUCTIVE, which aren't too expensive. The pad would be 50 ohms on both sides and have a very specific attenuation in decibels, which I think are the units used for calculating attenuation pads.

There might also be step-down transformers but their design might be very specialized.

Ultimately you'd need to have measurement equipment to know what you're ending up with, but IT CAN BE DONE!

I want to see what other people say!

.

Bob,

I don't want to talk you into or out of anything so let me just present some things to consider.

Carl is on the right track with suggesting building an attenuator. He mentioed a "T" pad but you could also use a "pi" attenuator. They do essentially the same thing.

The legal limit for FM part 15 is expressed as 250 microvolts per meter measured at three meters. We can do a calculation to convert this to a power output. The result is if you are using a dipole antenna the power at the antenna feedpoint to produce the legal limit is approximately 11 nanowatts.

To attenuate 5 watts down to 11 nanowatts requires 86.5 dB attenuation. ( dB = 10log(5/(11E-9)). This can be done but in practice it is very hard to achieve due to RF leakage. Even with the proper attenuator the leakage from the coaxial cable will create a field strength much over the limit. There will probably be significant leakage from the transmitter enclosure as well.

I designed and built a VHF repeater and recall what was involved to keep the transmitter power from getting into the receiver. This is not a trivial task. I had to use mesh gaskets on the receiver enclosure and feed through caps. for all power and signal lines into the enclosure. I used double shielded RG11 for the lines. The isolation did not approach 85 dB.

Certainly, it can be done but I just wanted to comment on what may be involved. Personally, I would pursue another transmitter which may be better suited for part 15 use.

Neil

I think I might attenuate that transmitter by selling it on ebay and buying one that's a bit closer to the power level you want to use 🙂

The FCC NOUOs are loaded with FM operators and the field agents seem to be pretty picky about that metric Neil referred to.

I love experimentation, and I love pushing the envelope, but I don't think I'd love five watts 🙂

I think the leakage from the case alone would blow 250uV/M out of the water.

You could make two people on ebay very happy - the person who bought your unit and the person who sold you your next unit!

DOE! I told you I did more audio than RF. Of course there'd be leakage from a 5-watter that would exceed the tiny power you were trying to achieve. But just for discussion, depending on the circuit, what if the final amplifier were disabled, and perhaps earlier stages, and pick off the signal at an early point in the chain?