Need AM RF Amp

I assume this is for Part 15 use.

This would be an interesting project. Looking at the specs for the monitor shows it requires 5 to 10 Vrms into a 1000 ohm load. This is 25 to 100 mW so an amplifier would be needed for Part 15 use.

AM amplifiers can get complicated due to the need for linearity and probably tuning. Maybe you could investigate using an untuned linear operational amplifier circuit instead. Possibly the signal at the antenna output of the transmitter could be amplified to give the correct input to the monitor.

The signal at the antenna output for a typical Part 15 AM transmitter will be around 20 to 100 volts rms typically (after the loading coil). This is plenty of voltage but the impedance is very high and any loading will disrupt the output signal so a high input impedance amplifier would be needed. The signal before the loading coil will be around .5 to 2 Vrms so some voltage amplification will be needed but the impedance at this point is low and easier to tap without disruption than at the output of the coil.

Without knowing more details this information is just to help you plan an approach.

Neil

Thanks, Neil.  I wish I had the electronics know-how to undertake a project like that and report back to everyone.  But I'm afraid my expertise ends with plugging things in and hooking up a couple of wires!

Anyway, if anyone knows of an RF amp that a fellow Part 15-er might purchase, please keep me in mind?

What you *really* need is a mod monitor that operates off the air signal so no direct connection is necessary.  It's easy and doesn't even use a smidgen of your power!  There are several.  I bought a brand new Inovonics this past year.  I spent more than any sane person would do to monitor a Part 15 station, but I can also use it to monitor the commercial stations I work for AND their competition, since it's frequency agile.  Just punch up the station you want to check on. 

But there are older Inovonics units for the task, as well as some others that escape my memory at the moment. 

TIB

i'm very sorry to be the bearer of really bad news but i have been down this road with trying to make belar stuff monitor a part 15 am or fm off air.

my results have been less than satisfactory. you best bet is to find and old inovonics 520 agile off air AM mod monitor they can be had for about $500.00 shipped if you look real hard and be patient.

even with the RFA off air amp it really doesn't do very well for 100mW and a 3 meter base loaded antenna, there is just not enough RF drive. on top of that it is rock bound and belar charges $600.00 currently to recrystal and calibrate.

there is the REA with the off air module and tim in bovey can comment more on that as i never had the off air amp from REA i did have the first gen mod monitor and for carrier current am it worked great but the off air amp from what i could acsertain is simply a near field off air pickup.

i locked in on the inovonics simply because of the agile nature of the hobby and needing the ability to change frequencies at a moments notice.

for part 15 fm there is the inovonics 530 fm off air monitor. it has a full reading sensitivity of 250uV so if the receiving whip is placed within 10ft of a legal part 15 fm it will decode and give proper readings and again is a agile monitor.

For those on a budget:

AM: Use an inexpensive O-Scope. Lots available and lots of info on the web.

FM: Find on Ebay and other places, Heathkit Deviation Monitor.  Not great but better than guessing. They actually have 75 uSec settings.  I used one for a LPFM and the results were amazing close to the real deal.

An oscilloscope with a frequency response flat from 20 Hz to ~ 15 kHz connected to the audio output of an FM receiver can be used to set the deviation of a Part 15 FM transmitter, approximately.

Just tune the FM receiver to one or more commercial FM stations, and note the peak amplitude(s) of their demodulated audio seen on the oscilloscope display.

Then tune that FM receiver to your Part 15 FM station, and set your deviation (modulation level) not to exceed that of commercial FM stations.

Will the presence of sub-carriers on the licensed FM station used as the "example" cause audio deviation to be some percentage less than from stations without subcarriers?

Rich, my apologies.  My original post said I was looking to fire up an FMM-3A.  I meant AMM-3A.  So I'm looking for a way to measure a totally different kind of modulation than my post led you to believe.  Any thoughts you might have would be appreciated.  Thanks!

And... thanks, Tim, kc, and Dade City for some valuable guidance!  I'll be poking around on the Interweb for recommended solutions.

Rich:  That is a good way to do it assuming the FM station is "functionally compliant."

Functional compliance is almost always better than perceived compliance.

🙂

One should take a sample of a few stations so set a realistic base-line I would advise.

Yes. and don't change the volume control on the receiver when "calibrating" with licensed stations, and adjusting the modulation level of your own.

Listen to a commercial radio station and turn the volume control to the point where you can just barely hear it.  Then adjust your transmitter to the same level. This procedure works on both AM and FM.  However a scope is best.