The practice for part 15 AM has advanced beyond the 30 to 50 percent efficiency commonly cited as being typical.
For the benefit of providing some real operating numbers I dusted off my lab notes for the high efficiency AM transmitter I built last year. This transmitter incorporates an internal 5 pole lowpass filter and an RF matching transformer so there are losses internal to the transmitter thus any measurement of the transmitter efficiency needs to include these losses. The antenna is a base coil loaded 3 meter pipe. The DC input power was measured by measuring the voltage across the final transistor and the current into the final stage. The input power is the product. The output power to the antenna (Pa) was measured by measuring the RMS voltage at the coil feed point (Va) and the RF current (Ia) at the coil feed point. The coil was adjusted to give a phase angle of 0 degrees between the voltage and the current which indicates resonance.
The data are:
VCE = 2.37 VDC
IC = 42.0 mADC
DC PIN = 99.5 mW
Va = 1.50 Vrms
Ia = 57.6 mArms
Pa = 86.4 mW
Overall Efficiency = Pa/(DC PIN) = 87%
Any report of efficiency needs to state how the measurement was done. For this transmitter the only convenient output power measurement with a radiating antenna is the voltage at the transmitter output connection to the loading coil to ground multiplied by the RF current fed to the coil multiplied by the cosine of the angle between the voltage and current. This does not account for loss in the loading coil or other external losses but it does account for losses internal to the transmitter and is a statement of the power output of the transmitter delivered to the antenna system.
A similar measurement could be made with the AMT-5000 by measuring the voltage and current at the feed point of the internal loading coil when connected to a radiating antenna and adjusted to resonance.
I am not inviting (though I wouldn't object to) a comparison between my tx. and the AMT-5000 but rather am illustrating how a measurement could be made to compare different transmitters. If making comparisons of efficiency it is necessary that the output power measurements be made at the same point in the antenna system circuit.
Unfortunately, the measurement I made and reported does not include the losses in the loading coil and ground system so this information does not predict actual signal strength performance which can be compared to other transmitters due to the differences in the antenna systems which may be used. Knowing the current into the coil which is the same as that in the radiating element and assuming a radiation resistance for the radiating element may provide such a clue to performance but I have not pursued this.
Neil
I agree Neil, without including everything in the path such as coil losses, filter losses, ground conductivity factors, environment factors, the numbers mean nothing really.
Problem with doing review tests is that no two locations are the same, so even with an elaborate lab setup or field setup, the results will not be the same at another location in the typical setup situations that exist. This is why Bill over at HB does several setup tests beginning with worst case scenario there could be encountered, then progress from there to better setups.
The one size (test) does not fit all.
At most that could be hoped for is a ball park set of figures that everyone will have to realize may not be obtainable from their particular setup and location.
Gotta include everything man if you want real results.
RFB
Hi Neil
I remember we use to talk and yes you are right. I was not getting
to the loading coil or antenna part yet. I was just trying to get
to the point where we knew what the transmitter input power verses
output power.
Then yes you have the 5 pole filter which will have losses in it due to
different coils being used as well the quality of the capacitors.
Since cheap disk caps could be used or high quality silver mica caps could
be used. Anyways losses are in that circuit alone. Then comes the design
of the matching circuit built in most of these high cost transmitters like
the Rnge master or Procaster. Now the 5000 has it matching network for
the antenna.
You can have losses in the Q of the coil if you use a Air coil verses an tolroid
type coil. Then yes a base loaded antenna is going to have loses in the coil
due to stray capacitances. All this has to be taken into account. Then
loacation on how well the soil grounds your station. There use to be a
trick in the old days.If you could not get out far enough just take your
garden hose and soak the ground wet all around you antenna and this would
even increase your range a little bit.
Cb'ers did it all the time to gain range.
I knew them to soak their entire yard with a sprinklers just to get
more range.
It amazing what people would do to talk further.
SKW40
I may have tapped into a different column than intended, I think there's an older AMT5000 Review thread, but this will do.
Actually the AMT5000 is working fine, this has to do with the STL (Studio Transmitter Link).
A Panasonic FM tuner was brought out of storage after many years to serve as the audio receiver for the AMT5000 audio input, and three times now I have found that it drifted off of 107.1mHz and I was broadcasting FM inter-station garble at 1680kHz.
So I got down to where the tuner is, near the floor, and used a flash-light to illuminate the all black front controls.
"Oh look," I thought. "The tuner has an AFC."
So I switched the AFC on and things have been fine ever since.
The statement "Everything has been fine ever since" did not let enough "ever since" go by, because everything is no longer fine.
The FM radio remained stable so long as it was left turned on, but at sign off the power strip turns off the transmitter and FM tuner.
When the FM tuner was turned on in the morning the AFC locked onto the nearby strong station at 106.5, and I had to re-tune to 107.1.
It's time to consider using the Sony WCS-990 900mHz Wireless Mic system modified for use as a STL.
It would be better if you used an FM receiver with digital tuning instead of linear "rotary dial" air cap tuning. Wally world has dozens of cheap digital tuned FM radios you could use for that FM STL receiver...and then everything will be fine no matter what time of day it is. Just make sure that radio will retain it's last tuned frequency on power off.
Thing to remember is that even when powered off by the front panel switch, there is still power running through the receiver's CPU and station memory/last tuned memory circuits to retain this information. Simply powering off by a power strip's power switch will remove all power from the memory circuits and thus may cause the receiver to loose it's last tuned frequency and station memory.
Besides that, it is not a good idea to be powering on and off devices from a power strip's power switch. That surges the heck out of everything plugged into it, as well as any other loads tied to that circuit. Not good for electronics to do that.
RFB
Ever since being here at part15.us the members have shared much valuable advice, take for example RFB just advising against powering equipment from a switchable power strip. Tonight I took that advice and unplugged the AMT5000 rather than closing the power strip, thereby leaving the FM tuner turned on for now.
Do smart people ever run out of advice to share?
Or, do less smart people run out of advice to search for....