Metering an AM Transmitter

radio8z's picture

My station uses an outdoor AM transmitter which has a "control point" provided by a simple box with two switches and a LED. The box transfers DC power to the outdoor transmitter from the wall wart source by means of one switch and the LED indicates power on. The other switch is an audio mute switch.

The original oscillator in this transmitter used a rather poor grade crystal so it was necessary to use a crystal oven to keep in on frequency as the temperature changed. I wanted some indication of the heater cycling so I connected an analog DC milliammeter in series with the transmitter power lead at the control box. With this I could tell the duty cycle of the heater and monitor it for correct operation.

The crystal oscillator has been replaced with a PLL and the heater is no longer used but I left the meter in the circuit to indicate the total current going to the transmitter.

Some time ago (years perhaps) someone, I believe it was Carl Blare, posted the idea of building a metering panel to connect to transmitters. The proposed parameters to meter were more than just the gross current but I am limited to this because there are only three conductors going to the transmitter (power, audio, and ground).

Over time I have grown accustomed to glancing at the meter and noting the current is 107 mA. But yesterday I noticed the pointer wasn't where it usually is and was indicating a current of 95 mA. The signal sounded fine but something had changed. The change was that the measured field strength was down due to snow on the loading coil. It turns out that there is a slight but definite relationship between this current and the field strength due to the decreased power output into the antenna system as it was detuned by the snow. Today, the snow is gone, the current is back to 107 mA, and the signal strength is normal.

Monitoring the transmitter total DC input current can be a simple way to tell if something has changed even if what has changed is not immediately known. This current reading will alert to many problems which could develop. If for no other purpose the meter indicates the power is on so maybe I don't need the LED anymore, but it looks nice so I'll keep it.


Good Idea

Meters are very handy, remote meters are even handier. I like the way you think. By the way Neil, what do you broadcast on your AM station. I remember the FM is or was Classical music is it the same for the AM?

 Barry of Blue Bucket Radio 1620 AM  - - WQYY 664

Meter Adding

The metering of gross current is a keeper. Count that as meter no. 1 in what could amount to many more meters as they are added to the system.

The simple second meter could be a volt meter to measure the voltage line, and if it's let's say nominally 12VDC the power supply ought to be adjustable from 3V to 15V so you could tweak for exactly 12V on the meter.

Fun starts to pick up as meters are added.

Carl Blare

Program Content

Barry, I don't yet maintain a reliable air schedule but what I do is broadcast audio which I am in the mood to listen to at the time. Carl's LPH is one, misc. music is another, and sometimes science programs which I am viewing on the net.

In other words, I have no program director.

I usually try to have something airing during the middle to late afternoon for the sake of anybody who might tune in finding something to hear at about the same time day to day.

My only known, but very loyal FM listener, enjoys the 24/7 availability of classical music. If this isn't working I hear about it but usually it is because her radio batteries are dead.


I always wondered why the

I always wondered why the typical loading coil designs don't include some kind of "roof" over the base-loading coil to reduce snow & rain? Seems like a simple plastic cone over the coil that gives you a drip edge.

A meter is a great idea. I think i'll rig one up.

Coil Roof

Mine has a "roof" (Photo) but it has a flat top. I put it together last summer and wasn't thinking about the snow. Am a bit wiser now and a cone would be better. But since it is mounted on the ground it is easy to brush the snow off as it is.

The metering idea is not new to me and is used quite a bit as a means of monitoring system performance in many applications. Once an operator gets used to viewing the analog pointer being at a particular spot it is easy to notice when things have changed. The current is kind of a catch all for seeing changes in this application.


Meter No. 3

At one time I kept a meter on the AC house power.

House power goes by many names... 110, 120, 119, etc.

And in fact electric power does vary over time and the meter drifts a little bit up and down.

It would be interesting to observe how changes in AC power transfer over to the smaller voltage being sent to the transmitter.

Carl Blare

Line Voltage Variations

Line voltage, as you mention, is expected to vary with time. How it affects a particular transmitter would depend on the transmitter. None of the wall warts I use are regulated but the voltage is regulated internal to the transmitters so unless the voltage drops quite a bit I expect no problems.

Some of the kit transmitters do not internally regulate the voltage so it could be a problem. I don't know how or if regulation is done in the "certified" units.

As an example of the utility of monitoring, during my professional escapades I came across a situation where monitoring equipment in an ICU was periodically failing to make accurate measurements. I monitored the line voltage continuously for two weeks with a strip chart recorder and found the line voltage varied widely, especially when an elevator was in operation. The variation exceeded the specifications for the equipment. We fixed the power circuits in the hospital and published the observation in a medical journal. As a result, new standards were established for medical monitoring equipment regarding line voltage tolerance.

A Meter Question

As I have expressed, it is my interpretation that the "final RF stage" in 15.219 is in fact the antenna, not the output RF amplifier.

But how can the point between the loading coil and antenna be metered for 100mW accuracy?

Carl Blare

Power Metering

The power delivered to an antenna system can be measured before the loading coil or tuning network by several methods. The one I use is to measure the current by means of a RF transformer, measure the voltage present at the input to the network, measure the phase shift between V and I, and calculate the power as P=VIcos(angle).

This is not the power which is canonically considered the "final input power" in AM transmitters. A stage in electronic systems has been traditionally considered one in which a signal controls applied energy from an external source to produce an output. This view would require the inclusion of an active device such as a transistor or tube.

I know of no references to procedures which describe measuring the final stage input power of an AM transmitter at the antenna.

For FM transmitters it is very common to specify the power output of the transmitter with a given load or to also include the loss of the line and the antenna gain and express the power radiated as effective radiated power, but in this case there is no need to identify the final stage.


Why I Think So

In previous postings regarding the Talking House being FCC Certified to drive an outdoor ATU/antenna by lengthy coax was dependent upon naming the antenna as the Final RF Stage, thus not counting the coax as part of the "transmission line".

I continue to believe that the notion of an active stage being the only definition of an RF stage is an old mis-interpretation being carried forward.

But let's just go to theoretical discussion.....

How would one determine 100mW into the antenna?

Carl Blare

Antenna Power

To measure the power delivered into an antenna element could be done by measuring the current at the feed end of the antenna and calculating the power as P = I*I*Rr where Rr is the antenna radiation resistance which is found by a theoretical calculation which is based on the antenna physical properties and the wavelength.

There are other methods utilizing directional couplers, network analyzers, and impedance bridges to name a few.


Final RF Stage

Part 15.219 states "The total input power to the final radio frequency stage (exclusive of filament or heater power) shall not exceed 100 milliwatts".

Going back to the days of tube transmitters, it was always standard practice in amateur radio to define input power as the product of DC voltage and DC current from the power supply to the final RF amplifier stage, excluding filament power. The final stage consisted of one or more tubes connected in parallel or push-pull feeding an antenna matching network. The phrase "exclusive of filament or heater power" was also part of the amateur radio definition. In years gone by, the FCC rules for amateur radio placed limits on maximum power based on input power measured as above. The amateur rules have since been changed to define power limits based on output power to the antenna. Part 15.219 still uses the old definition.

If it weren't for the very apparent anomaly of the TH, there rally wouldn't be any confusion. Previous postings of RMS power measured at the coax connector feeding the coax and ATU exceed 100 mW RMS RF power by a wide margin, up to 200 mW or more. Working backwards from the coax connector to the final RF stage, the implication is that the input power to the final RF stage greatly exceeds 100 mW. For example, if the final RF stage efficiency is 75%, then the input power is 200 mW / .75 = 267 mW.

Assuming FCC certification was done correctly, there are some possible explanations:

1. The TH was certified under part 15.209 (field strength limit). The FS limit was met due to very high losses in the path which includes losses in the long coax and losses in the ATU transformer and coils.

2. The TH was certified under part 15.219 (100 mW limit). The input power to the final RF stage was under 100 mW in the certified unit, but was increased in post-certification production units.

3. Somehow the certification lab was convinced that the long-standing standard definition of the "final RF stage" was in fact referenced to the ATU. This doesn't seem plausible in light of the 200 mW measurements at the coax output connector on the transmitter. The loss in the coax from the TH to the ATU would need to be at least 50% to get 100 mW at the ATU end.

There is something fishy about all three scenarios.

So, as usual, the certification of TH + ATU continues to be a puzzle and a ripe target for controversy. In fact, no other certified Part 15 transmitter has ever included a coax connection to the antenna, nor produced a non-adjustable "input power to final RF stage" higher than 100 mW (according to the long-standing definition).

The TH + ATU is what it is. It's certification is a puzzle, but the bottom line is that it's range is less than or equal to the other certified transmitters. Its input power and total losses somehow work out to produce a range roughly equivalent to the other 100 mW certified transmitters. In other words, the apparent violations of the power and feed line length rules are moot.

Elevating the ATU is another story. It is equivalent to elevating one of the other certified transmitters.

Antenna Power

Neil, I'm sure you know this already, but your equation should be P = I*I*{Rr+Rg) where Rr is the antenna radiation resistance and Rg is the antenna ground loss resistance. Since Rg is very much greater than Rr, most of the transmitter output power is lost in Rg and the radiated power is reduced accordingly.

Rr can indeed be calculated very accurately from published formulas, but Rg is not easily calculated. It depends on ground conductivity and the effect of the ground connection (ground rod, ground radial layout and dimensions). The Rg contribution of the small number of short radials typically used with Part 15 antennas cannot be reliably calculated.

Rg would need to be measured by some means to obtain a value to plug into the power equation.


Yes, Phil, it is true about the losses but I took Carl's question to be the power into the antenna itself. Thus the use of the term antenna element.

Does it make sense from the definition of Rr? That would be the total radiated power from the antenna divided by I^2. Usually the total radiated power is calculated from antenna theory for a given I so it is not actually measured. It appears this would exclude losses in the ground R.

Another way to view this is to use the equation P=I^2(Rr+Rg)=I^2Rr + I^2Rg where the first term is the power in the antenna and the second is the power in the ground.

In order for the power to get to the ground to be lost then it has to be radiated and we are back to the calculation using the total radiated power/I^2 which is Rr. The power radiated to and lost in the ground reduces the field strength.

This is interesting but is probably moot because in a practical sense your inclusion of the ground loss affects the field strength and needs to be included since this is what we try to predict.


Make a Meter Do It

Thank you PhilB for the history behind 15.219 and for explaining the interesting alternate way of defining output power in the amateur rules.

And also to you Radio8Z for a formula for determining power at the input to the antenna.

Now we build on what has been said by wondering how to transfer the task of reading power to remote meters.

While we're at it, we want a meter at the input to the Final RF Amplifier, a meter at the out put of the transmitter which will tell us the efficiency difference compared to input, and a power meter after the loading coil at the input to the antenna which will describe coil loss.

Three meters that reveal the small power being sapped away as it passes down the line.

Carl Blare

Meters Everywhere

As I read my last post I am not sure I answered anything useful so don't get hung up on it. My musings were with an isolated long wire antenna out in space somewhere and Phil's mention of having to include the ground and other losses would apply practically here on earth.

If one is interested in measuring efficiency there are two methods I used. First is on the bench with a dummy load where the DC input voltage and current to the final stage (the output transistor) are measured and the dummy load power is measured by taking the RMS voltage across it and using Pout=V^2/R. EFF= Pout/PIN.

For the other method, I measure PIN the same way but the output power is now the power to the antenna system which may be what Carl was asking about earlier and which I had answered about using the current transformer and voltage and phase angle.

About all to add is that practically if these measurements are to be done remotely, there would need to be some decoupling of the power input signals so the lines don't load the circuit or radiate. Measuring the antenna system power input would require similar decoupling and conversion of the signals to DC lest these lines radiate.

At least for my transmitter, the efficiency doesn't change much even if the load changes so there isn't much need to remotely monitor this.


Real Power in RF (a-c) Circuits

Measuring the d-c input power applied to the final r-f amplifier of any transmitter can lead to its r-f output power, as long as the d-c input to r-f output conversion efficiency of that final r-f stage is known -- AND the r-f current and r-f voltage present at the output load terminals of the transmitter are exactly in phase.

If that r-f current and r-f voltage are not exactly in phase, then the real (non-reactive) power at that point is equal to the voltage x the current x the cosine of the phase angle by which the current leads or lags the voltage.

The cosine of zero degrees is unity (1).

More Meters

The desire to add metering to part 15 transmitters is probably a reflection of having worked with professional radio transmitters which are loaded with big prominent meters.

As a 3rd Class Radiotelephone Licensed Operator the job was to jot down those meter readings every half hour on a transmitter log, and I think some settings had to be tweaked if they drifted too far outside of normal.

But memory no longer knows what all those meters actually revealed about transmitter performance, and I never collected a Professional Transmitter Manual, which would contain useful information about what I'm questioning.

Be all that as it may I will continue the inquiry into adding permanent metering to small transmitters, for the bell and whistle aspect.

Carl Blare

Meter What?

Remember the old sci-fi films where meters were everywhere? It didn't matter what was being metered as long as there were lots of them plus a Jacobs Ladder thrown in. It all looked cool.

Carl's remarks prompted me to recall the meters on a tube FM power amplifier we used for a ham 2 meter repeater. Four meters were on the top of the front panel and indicated plate V, plate I, grid I, and RF output line Forward/Reverse relative power. Though not required, I would log the readings every week or so. When the tube started going "flat" it was evident by the meter readings.

I just noticed the DC current to the AM transmitter is up a bit and so is the signal strength. We had about six tenths of an inch of rain yesterday and the soil is wet.


Even More Meters

The LPB2-20 Carrier Current Transmitter has a dual-purpose meter, that is, a single meter that can be switched between two different functions.

In one setting it is a VU meter for setting audio input level, and the other way it's a power meter between 0 and 20-Watts.

Of course such a transmitter is designed to be always fed into a constant 50-ohm impedance, so the accuracy of the power meter is designed around that.

In a somewhat similar way the AMT3000 has a meter port, where a DC meter can be added, but since impedance loads to a part 15 transmitter vary based on antenna design, it is a "peaking" meter which shows the best match between transmitter output and antenna.

We hear tell that some models of the Rangemaster have a meter for accurately setting for 100mW into the final RF stage.

Based on these few examples, we note that useful utility meters can be added to these tiny transmitters.

Carl Blare

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