measuring a transmitter output power

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measuring a transmitter output power

Is anyone familiar with "direct" and "indirect" power readings of AM transmitters, specifically for the SSTRAN transmitters?  I'm curious of how to do this and what equipment is recommended.  Presumably use a volt/ohm meter.

I'm not an engineer, so keep it simple if you can help.

I am interested in keeping a log much like the FCC would require of a broadcast station and need to figure out how to read the power and calibrate equipment.

Voluntary Consusltant Signing On

This will be a very interesting thread about professional style transmitter management.

I have a few things to contribute which are waiting in file folders, but I need to stall because of over-booked schedule.

So, keep this thread alive for a few months!

Carl Blare

Determination of Transmitter Output Power

Is anyone familiar with "direct" and "indirect" power readings of AM transmitters ...

The direct method uses an accurate r-f wattmeter to measure the r-f power dissipated in a test (dummy) load of a known impedance.  The wattmeter must be designed for the frequency, and the impedance of the output load in use.

The indirect method uses an efficiency factor, F, which is determined by dividing the accurately known r-f output power measured as stated in the paragraph above by the accurately known d-c input power at the output electrode of the final r-f amplifier of the transmitter.  The d-c input power multiplied by factor F is the r-f output power of that transmitter, for those test/operating conditions.

Issues here for Part 15 AM systems are:

-  Their load impedance is almost never known, and unlikely to equal the value of the test load and power meter used, and

-  The instrumentation needed for accurate, direct r-f power measurement is relatively expensive

What if I can borrow the

What if I can borrow the instrument to read the direct RF? Presumably I'm in business on that point then.

So, Rich, if you wanted to simulate a broadcast station log, to the best of your ability, what information would you record?  I examined a file at the FCC called "EB18AM09_2009" in PDF. It's for AM station and their logs.  I'm thinking I'd like to try and qualify for the Certified Broadcast Technologist with the SBE.  The SBE will want some kind of submission to consider certification.  If I'm keeping a simulated log like an AM station I would think it would help.  I haven't yet gotten a straight answer from SBE on what qualifies.





SBE Certification requirements are posted at:


My guess is you'd have to pass their exam.  I've been certified member for several years. Either pass the exam, or have a First Phone or Amateur Extra with a record of service in the industry for the specified number of years.

As the FCC no longer even requires transmitter logs, what you include on them depends on your circumstances. The commercial AM I work for now logs output power in watts, antenna current, and reflected power.  Then of course all the varying parameters required for the night pattern after sunset and sunrise (we're directional at night) which is way beyond the scope of any Part 15.  We used to log plate current, plate voltage, and antenna current -- but with no plate with the new solid state transmitter, we do things the "modern" way.  LOL.

For my Part 15 I keep tabs on a set of field intentsity readings taken at the same points around town every so often. My advantage is that as a broadcast engineer in real life I have the necessary equipment to do so.

You might consider buying or building a simple, uncalibrated field strength meter that will give a decent reading near your antenna.  Note the reading, and check it every so often to see if there's any significant variation in received strength from time to time. That's about the best you can do. Any sort of actual output readings (with equipment available so far) will need to be connected in line with your antenna, using up some of the power available for transmitting, and again would be inacurrate as th odds of getting a setup that would work with the frequency and impedence of your antenna load is not likely.


About Reply 4

So, Rich, if you wanted to simulate a broadcast station log, to the best of your ability, what information would you record?

Tim (TIB) made a good post to give more insight into this subject.


Something from the past

A year or two back it seems somebody

had the exact points inside an AMT-3000

where one could take voltage and/or (???)

current measurements.  Then these 

measurements were plugged into a math

formula (?) or equation (?) to get input 

power.  Then some efficiency factor

(50 percent or whatever) is used to figure

the output.  Not quite what we want here, but

it's a little more info, I guess.

It's a lot of fun (I think) to travel around

within the Part 15 station's coverage area

and take strength readings of any type

and observe how they all change relative

to each other.  You will see some weird things.

Brooce, Part 15, Hartford




Measuring the AMT-3000 Final Input Power

Congrats for getting on the air.

Here's a link to a post I put up years ago about measuring the DC final input power for the SSTRAN AMT-3000 transmitter.



Neil, that's the post I remember.

But 11 years ago?  Somebody else might

have reposted it more recently?

Where DOES the time go?


...Eleven YEARS?


Would this DIY work for us?

Did a little Google snooping and found this.

It says something about a 50 ohm load but I don't know if that means this too is unusable.





The designer of the aforementioned replied to me and said it would be of some use for a transmitter power reading but not an antenna.

Is it antennas or transmitters I'm usually hearing about wanting to read on




Measuring Power Delivered to Antenna in Part15 AM System

Here's a link to a post I made which describes how I tuned and measured the power delivered to my Part 15 AM antenna system.  It does not require a known or 50 ohm antenna load.



Thanks, Neil.  I'll be using

Thanks, Neil.  I'll be using this.  Been pre-occupied with travel, etc since June.  I hope to build this antenna before the end of the year.





Hi...I always use an

Hi...I always use an oscilloscope with a 50 Ohms input.You measure the peak to peak voltage.You can calculate the power with the formula (Vpp X Vpp) / ( 8 X RL).Where RL is the input impedance aka 50 Ohms.


Power Measurement

Hello, Jard, and welcome aboard.

You are correct in how to measure power delivered to a 50 ohm input scope.  The situation is that almost never is the antenna impedance for a Part15 AM system antenna anywhere near 50 ohms.  The method I described in the link involves measuring the voltage, current, and phase angle of the signal applied to the antenna.  The power is Vrms x Irms x cos(angle).



What about the output jack on

What about the output jack on the back of the Talking House 5.0 that is used for an ATU?  that output can be 50-75 Ohms as I was told.  So if you make your own ATU you would essentially have a 50 ohm antenna system No?

Progressive Rock (Album Rock, Deep Tracks), Classic Rock

More Power for Hobby Broadcasters

ATU Power

Good question.  I have no experience with the Talking House or the ATU but I do have experience with antenna tuners in other applications.

Generally, the purpose of an antenna tuner is to present an acceptable load to the transmitter.  In ham radio this is accomplished by tuning to provide the minimum SWR measured between the antenna tuner and the transmitter.  Used this way the antenna tuner does nothing regarding the match between the transmission line and the antenna.  Also, minimum SWR in this case does not necessarily mean the transmitter sees a 50 ohm load, it only means that the reflected power is minimum.  But this IS NOT HOW THE TH ATU WORKS.

The TH ATU, according to a schematic which I reviewed, tunes the antenna to resonance.  The meter indicates the relative voltage at the base of the radiator element and tuning is to peak this voltage.  This does not mean that the transmission line or the transmitter is presented with a 50 ohm load.  For example, in my antenna system the peak radiator voltage happens when the system is resonant and presents about a 30 ohm load to the transmitter.

In none of the situations which I have presented is the load seen by the transmitter assured to be 50 ohms.  It may be but it may not be 50 ohms.  Therefore, assuming that the power measured with a 50 ohm load is not likely to be the same as the power delivered to an antenna system.

The proper way to measure power delivered to an antenna is to measure the RF current to the antenna and if the antenna Z is accurately know the power can be calculated.  Another method is to use a directional wattmeter (not a SWR meter) which can measure the forward and reflected power.  The power can also be measured by measuring the voltage, current, and phase angle at the antenna feed point or at the input to the transmission line if one is used.



AMT3000 power

Rich, your AMT3000 posts have been most useful. While waiting for delivery of an AMT3000 kit I'm simulating the AMT3000 RF output stage in LTSpice. The aim is to optimize the circuit for a 50 ohm load. 

It's an interesting circuit that runs 100 mW DC under all conditions. But, the efficiency isn't the best. R18, the 820 ohm collector resistor eats up a lot of the power and I suspect it's there to provide a load no matter what. Removing R18, and using a 900-to-50 ohm L-network the transmitter outputs 26 mW carrier and 40 mW at 100% modulation. 


My interests are antennas, transmitters, studio equipment, and setting up a useful Part 15 AM radio station. 

SSTRAN Output Power

Interesting results from your simulation.  If you install the L network and make make measurements, please let us know what you find.  I found that simulations using LTSPICE are fairly good at predicting performance of RF circuits at BCB frequencies.

Any resistance in the final output circuit will waste power so you are on the rignt path by removing this resistor. 

I am interested in your choice of 50 ohms as the impedance to transform to 900 ohms.



I chose 50 ohms because that

I chose 50 ohms because that's what I have instrumentation for and so that the modified AMT3000 is not set up for one particular antenna. Antenna adjustment will be performed with an MFJ-259B. Any antenna I build will have a feedpoint impedance lower than 50 ohms and transforming that to 50 ohms only takes a small feedpoint shunt inductor.

The 900 ohm collector load allows the PA transistor to swing half of the available 12 volt negative swing with an unmodulated carrier. With carrier it swings down to 9 volts and with 100% modulation it swings to 3 volts. 

The PA is so inefficient that when set to run 300 mW DC input the RF output of 65 mW is what an efficient 100 mW class-C stage would run. So, I'm looking at changing the PA to run 100 mW DC input with high efficiency. Tripling the RF output power gives the station 3X the coverage area. 



My interests are antennas, transmitters, studio equipment, and setting up a useful Part 15 AM radio station. 

Good Plan

It appears that the final can run class C but I am not sure.  Your experiments will yield interesting and probably useful results.



I have the new AMT3000 RF

I have the new AMT3000 RF output stage and the audio modulator working nicely in SPICE. The RF stage is operating class-B with an efficiency of 67%. It's simple (much simpler than what's already in the AMT3000) and can be grafted into the exisiting AMT3000 PCB. The new circuit does not have automatic DC power control and must be set manually. So, 67 mW RF out vs 23 mW (in simulation) for the stock AMT3000. 

I think I'll build the kit stock and have the new RF output stage on perf board. I can then explore both circuits and modify as needed. The next project might be to build a class-E stage with close to 100% efficiency. 

My interests are antennas, transmitters, studio equipment, and setting up a useful Part 15 AM radio station. 

Wouldn't Class D be easier to

Wouldn't Class D be easier to build and adjust?

Druid Hills Radio AM-1710- Dade City, FL. Unlicensed operation authorized by the Part 15 Department of the FCC and our Resident Hobby Agent.  

I was looking for a one

I was looking for a one transistor amp and that would be class E. I'm not liking what I'm seeing with the class-E amp in simulation as the fast collector slew rate is causing high current spikes into the L-network capacitor. On the other hand, class-E is attractive because of the high efficiency. It could output 40% more power than the class-B amp. 

If I understand it, a class-D amp is a push-pull amp using an output transformer and I'll explore that at a later time. 

The class-B amp is nice in that it's tame and looks hard to break. 

My interests are antennas, transmitters, studio equipment, and setting up a useful Part 15 AM radio station. 

High Efficiency


Though not applicable to the SSTRAN you are working with you might be interested in these posts and the threads which contain them.  They report on a design which produced an 86% efficiency.  The design now uses a PLL instead of a crystal and has been on the air here for several years feeding a base coil loaded antenna.  Range observed with a car radio is one mile good copy and 1.3 miles recognizable signal.






thanks for the links and I'll play with your transmitter schematic in SPICE.

My interests are antennas, transmitters, studio equipment, and setting up a useful Part 15 AM radio station. 

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