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.
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!
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 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:
http://www.sbe.org/sections/CBT.php
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.
TIB
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.
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
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
But 11 years ago? Somebody else might
have reposted it more recently?
Where DOES the time go?
Brooce
...Eleven YEARS?
Did a little Google snooping and found this.
http://www.cromwell-intl.com/radio/power-meter.html
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 Part15.us?
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.
http://www.part15.us/forum/part15-forums/general-discussion/rf-current-transformer-antenna-tuning
Neil
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 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.
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).
Neil