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Author Topic: How to measure a DIGITAL radios output power  (Read 2960 times)

Posts: 25

« on: December 31, 2012, 10:37:00 AM »

Hi. I am looking to find out what equipment I would need to measure the output power of DIGITAL Transmitters in the amateur 2m band (say, of an icom D star radio) and 800 Mhz band of a commercial Fleetnet radio. I have Bird 43 meters, and a  Diamond SX-1100 meter. It is a simple thing to measure FM for example, but digital modulation??? I could really use some education and help here! Thanks in advance and 73 de Jack VE4AOD.

Posts: 2409

« Reply #1 on: December 31, 2012, 12:35:42 PM »

If we talk about D-Star it is basically the same as FM. Actually it is GMSK (Gaussian Minimum Shift Keying), which is similar to FSK. There are also AM methods like Amplitude Shift keying and Quadrature Amplitude Modulation.
You should be fine with the correct heads for the Bird 43 and a dummy load.

73 and Happy New Year

Posts: 1813

« Reply #2 on: December 31, 2012, 01:12:28 PM »

we have had digital transmissions for commercial broadcast for a long time now.

curiously enough, we have not had to dump all of our pre-90s design radios and stuff to get their signals.

even as a thought exercise, that should tell you that out of the tank and into the ether, signals is signals, and if it works in band X with known type equipment, it's the same thing.  if it's the same thing, you check the envelope and decode like you always did for that emission class.  doesn't matter if they have 20 100-watt triodes in parallel, a PWM FET output, or sparks in a bunsen burner flame, that's just how you encode it.

Posts: 4280

« Reply #3 on: December 31, 2012, 01:52:54 PM »

On a vaguely related note... This may not be typical, but until recently I had three local AM BCB stations running iBiquity digital.

They have all reverted back to Ancient Mary. I still hear a few AM Digi signals at night on the skywave, but not very many compared to a year ago. You can tell a digital AM station by tuning across the signal, it's about 30 kHz wide with 'white noise' sidebands top & bottom. I'm glad to see them gone as the band has enough noise on weak signals without some 50 Gallon operation piling on.

Maybe I need to shop the better stores, but I don't think I've ever seen a digital radio in a retail store....  If I wanted one it would be most likely ordered through a web site.
Logged through this world I've wandered I've seen lots of funny men.  Some will rob you with a six-gun, and some with a fountain pen.  - Woody Guthrie         

Posts: 7357

« Reply #4 on: January 01, 2013, 06:43:01 AM »

GMSK isn't a problem, but some of the digital modes need care, such as OFDM. (Orthogonal Frequency Division Multiplex). That uses multiple carriers, each carrying part of the signal: depending on the coding, they can add up to give a high peak power but a much lower average power. Proper coding to avoid that can keep the peak to mean ratio to under.

Posts: 15660

« Reply #5 on: January 01, 2013, 08:04:11 AM »

A lot of the digital equipment still uses a constant carrier with FM modulation,
so a standard power meter will work just as well.

Even with more quirky modes, you still have average and peak power, just as
you do with an SSB signal, and the same meter should still measure them.

If you do get into OFDM (for example, with 802.11a/n WIFI links) then the
peak-to-average power ratio can be 9dB or so, and even higher with the
faster/wider modes such as 802.11ac.  You can also run into measurement
issues because the signal is a series of bursts, and the power (and modulation
type) can vary from one burst to the next.  So you may need use a spectrum
analyzer on zero span if you need to see how the power varies with time, or
correct the measurement from a standard meter for the transmit duty cycle.

But most digital signals in the 2m band can be measured with the same equipment
that you use for an FM signal.


Posts: 25

« Reply #6 on: January 01, 2013, 09:28:51 AM »

Thank you everyone. I appreciate your help! I have been attempting to measure a >800 Mhz Fleetnet signal from a Motorola radio and the new SX1100 meter reads far too high. I will try with another meter and see what happens. Again, my thanKs!

Posts: 2409

« Reply #7 on: January 01, 2013, 10:24:25 AM »

The SX1000 has a 10% tolerance by itself. Only a correct impedance load will result in an accurate measurement. So using a dummy load would be the safest way to exclude any antenna and cable impedance mismatches.
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