Modulation Monitor Ordered for Testing

timinbovey wrote:

I have ordered the AMM-SD1 Series AM Modulation Monitor from http://www.radioassociates.com/ /a>.   I will be prepared to test it compared to a freshly calibrated and certified Belar Mod Monitor that I use at the transmitter of the commercial AM I work for.  I will be able to test/compare in three ways -- direct readings from the mod monitor output on a new 5,000 watt transmitter, readings from part 15 transmitter received off a loop sampling antenna near the transmitting antenna, and modulation tests from various stations including my Part 15 utilizing the IF of a receiver.

Congratulations.  But just to note that the modulation percentages produced by an AM transmitter are most accurately measured using the forward (incident) power sample of a directional coupler connected to the output port of that transmitter -- no matter if that is a 5 kW licensed AM broadcast transmitter or a legal/illegal Part 15 AM transmit system.

Amplitude modulation measurements made using radiated fields will include the effects of the r-f bandwidths of the transmit and receive antenna systems, the propagation environment, and interfering signals -- any/all of which can lead to inaccuracies in those measurements.

The equipment/software being tried by Tim will be of great interest as it certainly looks attractive in its publicity.

Even given limitations described by Rich(F), I believe that Tim is creative and will report the strengths and weaknesses of his test experience, perhaps finding uses not so-far considered.

Looking forward to the report.

Well clearly I'm not setup to do involved scientific testing. But I do have access to several broadcast stations and their transmitters and related support equipment. I will be able to intall this software defined mod monitor directly to said transmitters and compare results directly to their installed monitors.  Clearly we'll be relying in my eyesight and interpretatiion as to the readings, however all devices have "lights" to indicate peaks, etc and they should all be indicating pretty much synchronized.  But clearly if the SD unit is completely out of the ballpark it will be obvious.

As for readings off the air through reception, again, I will be able to set up this unit any distance from the transmitters I desire and be able to compare it's indications with those at the transmitters themselves on their own installed commercial monitors. Granted this will be with the help of an assistant at the transmitter, so human time lag will be involved in the compairisons but still we should be able to determine if the SD unit is at least acceptable.  In this case my "assistant" will be a BE with over 40 years of experience in fulltime broadcast engineering who has built dozens of stations and is well respected in the industry.  He too is curious about this system. He recently retired and asked me not to give out his info as he doesn't want to field questions. But I've worked with him for over two decades. 

One advantage for me is in a relatively rural area there is little interference, and few stations.  Off the air sampling will be done in a near field setting, literally with the receiving antenna within inches of my Part 15 antenna, I trust any outside interference will be minimal, and we can work equally close if necessary to the broadcast stations. 

Additionally I will be able to tune in say, 20 miles away, monitor modulation, while speaking with an angineer in the transmitter building who is watching the hardwired mod monitor at the transmitter.  If he's showing 110% peaks, and I'm showing 90%, we know there's an issue. 

Sure, the margin of error for the tests my well be a coupe percentage points of modulation but I believe for the most part that would be accurate enough for our purposes.  Having been through many FCC inspections, real and "mock" over the years, they don't care how you keep track of modulation, only that you know you're not overmodulating.  For part 15 it doesn't even matter, except that it's nice to know so you know where you're at and are using what you can to get the best results, signal, sound and coverage. 

Certainly there are factors that can color the results, but I'm really just looking for evidence that this unit, when compared to traditional equipment appears to be accurate and useful. 

As I am also a ham I will be able to play with this in my ham station as well, where I currently use a 'scope to keep track of things.  So, should prove interesting at least. 

Tim in Bovey

i have the AMM-HF1 the predesseor to the AMM-SD1 but i still need the off air pickup unit to even attempt to use it with my procaster but i now have stuff to think about from rich's statement above about off air modulation. as rich well knows it's kind of hard to pull direct readings off a part 15 legal transmitter it would be nice if a manufactuer (Chez? Hamilton?) would put a modulation monitor direct tap off of the tx to use a directly connected modulation monitor as an option.

I really don't see, that if you're taking the signal off the air, from a loop antenna mounted a couple inches from your transmitting antenna and feeding it right into their pickup unit  that you are going to also get a substantial amount of external noise picked up that will make these modulation readings unuseable. I would expect that stray noise and signal would only show more modulation and wouldn't you be able to turn off your carrier and then read any existing stray modulation/signal that could then be subtracted from your readings? Field strengh readings from right next to the antenna are quite substantial and more than enough to get a good sample. 

There are also commonly used off air monitor units used in the broadcast industry, I can't remember the specific units off hand, one is a Belar and there is another I know of. 

For me, use of this device may prove useful for me in my regular broadcast work, especially if it proves to be accurate with direct connection to mod monitor outputs on commercial transmitters. I will also explore it's use with ham radio (basement full of AM ham transmitters). But if it compares favorable with direct mod monitors off broadcast transmitters it's going to prove useful to me in my work and will also be a tax decudtible expense 🙂  Being able to give a decent but not lab precise indication of part 15 modulation will be useful as well, unless it's just horrible. 

I think that for the most part, the typical part 15 broadcaster has no clue what their modulation level is. Turning it up until it distorts and backing off seems to be the general method.  This may provide guidance.

Tim in Bovey

I did also note the older thread on the AMM-HF1 from several years ago from the same company. It appears this newer model would perform similarly but with a laptop as the display.  It seems many had good luck with this one?

http://www.part15.us/forum/part15-forums/general-discussion/amm-hf1-modulation-monitor

Tim in Bovey

While awaiting results from Tim's Modulation Monitor Tests, I experimented this morning with observing modulation on a Spectrum Analyzer, which in the past I have used mainly for seeing power levels.

I began checking the on-screen waveform of my 1550 kHz signal coming from the AMT3000, with loading coil and resonant Wintenna, because I thought the audio quality sounded slightly distorted on several radios.

I magnified the waveform to its fullest setting, and could see asymetrical distortions of the two peaks that reppresent carrier and modulation, as well as sporadic flashings out of band on nearby frequencies.

The carrier waveform is like a canine tooth with a rounded top, and the amplitude waveform is a second form that takes the same shape when un-modulated, but gets jagged and active when modulated, like lightning following the legs of the carrier peak. The carrier peak wobbles a little bit like a mound of jello, depending on the modulation.

It's probably takes Eastern Asiatic Himalayan meditation to do it exactly, but it sounds better now.

After placing my order I received an email from REA that the wireless "off air" RF pickup units were out of stock, but they expected a fresh batch ready to go in a couple weeks.  In the meantime I was shipped the unit, the direct pickup for use with broadcast transmitters and have downloaded the software and installed it in a new Windows 8 Laptop.

I will be able to test the unit using commercial broadcast transmitters including a vintage 20V-3 (1000 watts), a Harris MW-5, and a brand new Nautel, (both 5,000 watts) all on 1320 KHz, and be able to compare readings on the software defined mod meter directly with those on a just refurbished and calibrated Belar Modulation Monitor.  These will all be in the direct connection operation.

I have made arrangements as well to use the off air pickup unit, tapping the IF of an AM receiver, to directly compare the modulation reading of these transmitters off the air, 5 miles from the transmitters on the REA setup to their actual Belar MM readings at the transmitter site. These will be on 1320 KHz.  I have also setup the same test to compare the readings off a calibrated commercial modulation meter to those read "off the air" from a 10,000 watt station 30 miles away on 650 KHz. 

I will also use the unit in the "off air" mode to receive directly from these stations at the transmitter sites without the AM receiver to compare the modulation readings to the actual commercial mod monitors installed at the sites.

I will then use the off air pickup, without a radio, to check my part 15, and also try the same using an IF tapped radio to see what that method gives me for modulation.  I so far don't have access to a commercial grade mod monitor that will work with my Part 15 to use as a standard for that test, but if it passes all the above scrutiny, I will consider the close field antenna sample, and the radio IF sample of my part 15 to be OK too.

I'll post a complete rundown once the work is completed.  Word is the off air pickup should be shipped any way.  For the record, emails to REA are always answered quickly.  So customer service is fine.

Incidentally, yesterday the Minnesota Broadcasters Mock FCC inspection was done at the commercial AM and FM stations I engineer for, and both stations passed close scrutiny in all areas of a typical FCC inspection. My vintage Nems Clarke AM Field Intentisy Meter was right on the money compared to the inspectors $15,000 dollar modern unit at all required directional monitoring points.  Not bad for a tube machine 50 years old!

If you're interested what a commercial AM goes through for inspection the FCC self checklist is here:   http://transition.fcc.gov/eb/bc-chklsts/EB18AM09_2009.pdf

That's a bit outdated, from 2009.  Our inspector had an addendum with about 20 additional points on it, all of which we passed. 

Tim in Bovey

Congratulations Tim for passing the mock FCC inspection with an A+. That certainly will make your station management confident that you are the man for the job.

Tapping the IF of a receiver. There's something worth threading about. That sounds like something worth knowing how to do.

As for the detailed plans you have mapped out for testing the REA Modulation Monitor, you have a very good mind for planning. We can take an example from that.

On top of it all, I think you are having fun doing it all, and that's when life is the best.

Back in my commercial AM radio days the 3 things that would get you gigged for sure from the FCC was, EAS logs, Public Inspection files and tower issues like not identified with FAA info, paint or lighting.

Yes, EAS is still one of the big ones.  And it gets more complicated and less reliable every couple years.  Each State now has their own system and some are much better than others.  Ours stinks. And of course now the system ties into the internet as well as required monitor stations.

The other biggie for us is AM directional monitor points.  I keep a very close eye on those!

Tower lighting is also big, but on the administrative side now is the EEO rules which FAR surpass what any other business has to do.  We have to prove we've gone out of our way to find and hire minorities.  The paperwork is mind boggling.

Transmitter logs, no longer required.  Modulation monitors no longer required. Much deregulation.  But you DO have to be able to demonstrate that you're operating legally.  So we do all the logs just like in the good ol' days.  That always impresses them.  And the fact that there's an engineer on staff who actually knows where the monitor points are and has a meter!  You have to have proof of performance, on frequency, not overmodulating, supressed harmonics, etc. 

Tim in Bovey

i have a nems clarke 120E FIM that was basically a desk queen. taken real good care of.

A good friend of mine built a 6 tower

directional AM station around 1985. 

I helped him a little bit with the testing. 

I was sitting on a lawn chair in an unknown

street, in an unknown neighborhood, reading

an FSM, and reporting the readings back with

a commercial two way radio. 

The day pattern was very different from the

night pattern.  So he was messing with the

phasing controls in the transmitting shack,

while a whole bunch of us were reporting

back the readings.  I thought it was a lot

of fun, but I was not responsible for getting

it right.  It was fun for me though, because

being visually impared, I never got out much. 

(I still don't - even going downtown to pay the

taxes is a treat for me.)

Anyway, we did what we had to do.  But my

friend was always responsible for all of those

monitoring points in weird places, like bad

neighborhoods, the middle of people's back

yards, and so on.  Of course you guys all

know this stuff, but to me - a nonprofessional -

it was facinating, and it still is.  Have the rules

been relaxed?  I thought i heard that somewhere.

Bruce, Monitoring station, CT

Many rules have been relaxed somewhat, or deregulated, but monitoring points are not among them.  Our night pattern was determined back in 1971.  Since then forests have grown, homes have been built, roads have been moved or eliminated, etc.  One of our points is now in the center of a county highway, one is in someone's backyard, one is along a busy four lane highway.  One is now 250 feet from the side of the road into a planted farmers field through an electrified barbed wire fence.  We've since been able to have this one moved to the roadside.  We only have 4 points to monitor, friend has a station with 14 points to monitor!  You must come in under the max field intensity allowed at those points.  Things can cause changes in these points, such as ground conductivity (they tend to go up if it's been raining for a week) and construction, buildings, forest growth, etc. Lots of weird things can happen.

Tim in Bovey