All Digital AM?

I have been reading, with interest, the saga of HD Radio on the AM (AKA Medium Wave) band. First question; if it goes all digital, will we still call it AM? Of course, there are other questions and concerns:

  • The proprietary nature of HD Radio, AKA MA3 or NRSC-5D as they are now calling it, is problematic. Xperi, the latest patent owner, currently (their word) has agreed to waive licensing fees for AM station owners who install their system. Is this a limited time deal for early adopters or in-perpetuity for all stations?
  • The NRSC-5D tests on WWFD, Fredrick, Maryland are hopeful, but as I pointed out before, it is one station with a well functioning antenna system. Many AM antenna systems are defective either in design or due to deterioration. Is the FCC going to start policing the AM band again to cure these self inflicted wounds?
  • Of course, the NAB wants zero oversight on the entire adventure. Under their proposal, small ownership AM stations would have a difficult time remediating interference issues from all digital co-channel stations be eliminating any required notification period, as proposed by the SBE.
  • The NAB also wants to nix a 1 Hz carrier frequency requirement, which would help with both the analog and digital interference issue, saying it would be too expensive. I disagree. In this day of universal GPS time keeping, it would be easy to implement this on all modern transmitters, especially if they were already installing an HD Radio exciter.
  • Denis Jackson’s Radio World Article states that reliable coverage can be had out to 0.1 mV/m. This seems very, very optimistic given that ambient electrical noise (non-broadcast related) on the AM band is at very high levels and still climbing. Further, once the all digital conversion starts, more and more co-channel digital interference will happen, likely cutting down that contour to a great extent. It works now, but may not work later. These types of statements seem naive or perhaps disingenuous. Again, WWFD is one digital signal in a vast ocean of analog carriers.

While I am skeptical of some of the statements made in various articles and comments before the FCC, I do believe that converting the Medium Frequency band to all digital will have benefits. The BBC DRM tests carried out in 2007 (The Plymouth DRM long term trial) show that digital on MF can work. DRM has been implemented in various countries with good results.

Getting rid of the hybrid IBOC/Analog is a step in the right direction.

My concerns are the small owners who are still making a go of it on AM. Those guys still doing community radio and serving the public interest. If they choose to wait, are they going to get buried under a digital dog pile and then have to pay the full license fee later? Something like that might be the end for them.

HD Radio in and of itself is not the panacea for the AM band. Other things have to happen to make it work right. The SBE speaks extensively about ambient noise on the MF band. They are entirely correct. In addition, there are many, many AM stations that do not have compliant antenna systems. There are stations operating a DA-2 system full time on the night pattern. There are stations operating a DA-2 full time on the daytime pattern and power. There are stations that are supposed to turn off at night, which stay on 24/7. There are stations not reducing power to night time levels. The list goes on. Simply putting digital carriers on everything will not reduce the station-to-station interference, especially at night.

I am cautiously hopeful that the FCC will look into the ambient noise problem, which simply cannot be over emphasized. They would also need to re-invigorating the Enforcement Bureau. Since they closed down most of their field offices, it has been kind of a free for all out here.

More AM work

I have been working on an AM station lately.   WBNR signed on in 1959 and follows the now familiar AM trajectory; after making bank in the 60’s, 70’s and 80’s, revenue declined, maintenance deferred, yada, yada, yada…

After a stint with a news talk format, the station changed to “Real Country,” a few years ago.  WAT! Music on the AM? Actually, it is doing quite well. The perception is that AM sounds terrible and nobody listens to it.  The stock AM radio in my Subaru (made by Pioneer) sounds pretty good on AM.  I have noticed that when I first tune a station in, it sounds narrow banded, slightly better than a telephone.  However after a second or two, the bandwidth opens up and it can sound quite good.  I have also heard this station playing at several local businesses.  When we turn it off to do maintenance, the phone starts ringing.  Clearly, somebody is listening…

This station is part of a three station simulcast.  The AM station to the north got rid of its directional antenna and added an FM translator a few years ago.  That has made a big difference.  Thus a translator was acquired for this station as well.

The translator was held up by an informal objection filed by Prometheus, Et. Al.  as part of a blanket filing against all new translator licenses by the LPFM advocate.  In any case, the Construction Permit has been on hand for a while, so the owner felt it was time to move forward with building out the new FM signal.

Hoisting the Shively 6812 antenna

Installing the single bay Shively 6812 antenna on the side of one of the night time towers triggered some other things.  A bit of the deferred maintenance was addressed;  new stockade fences around all the towers replaced the original fences put up in 1988. Those original fences were falling down.

4 Tower antenna system, WBNR, Beacon, NY
4 Tower antenna system, WBNR, Beacon, NY

The antenna system for WBNR is actually quite elegant, perhaps even beautiful.  A simple two tower system for the daytime array and a separate two tower system for the night time array.  The night time towers are top loaded, adding about 30.7 degrees in electrical height.

The far tower
The far tower

The CP for the translator required some extra steps because of the mounting on the night tower of the AM array.  Before and after impedance measurements need to be taken on the tower in question.  Another requirement of the CP, a set of before and after monitor points need to be taken.

WBNR tower, with translator antenna side mounted at 390 feet AGL
WBNR tower, with translator antenna side mounted at 381 feet (116 Meters) AGL

While I was measuring the base impedance, I decided measure all the towers instead of just the night time tower that has the translator antenna mounted on it.  This is a good point of reference if any problems arise in the future.  Often, this information can be found in the technical paperwork from the original license application.  Those files can be a treasure trove of information.  Unfortunately, it appears that a good portion of the original paper work is missing.

WBNR tower #1 Antenna Tuning Unit
WBNR tower #1 Antenna Tuning Unit

The Phasor and ATU’s are a late 80’s Harris product.  They are actually in remarkable shape, all things considered.  All of the RF contactors are Harris HS-4P motor driven units.  They are rated at 30 Amps, RF-RMS.  I don’t think that they are supported by GatesAir.  I have a small stock of spare finger stock and contact bars.  I suppose, if I had to, I could make or adapt parts to repair.

Looking at the base currents and the base current ratios for both the day and night patterns (base current ratios are on the station license), the tower impedance has changed very little over thirty years. That is good news, especially with those 215 degree tall night time towers.

The WBNR license application did contain an overall system diagram showing the Phasor and all the ATU’s.  It did not contain any component ID’s or other information.  I scanned that in, created a vector graphics file and expanded it to a 24 x 36 inch size.  I was able to fit all the component values and other information on the diagram.

Schematic diagram WBNR day/night antenna systems
Schematic diagram WBNR day/night antenna systems

The other issue is the monitor point descriptions.  They include statements such as “Point is marked with yellow and white paint on a tree,” or “In the northeast corner of the Texaco research facility parking lot.”  Those references are long gone and I would prefer to use a set of GPS coordinates.  Using the topographical maps from the proofs, I found each monitor point and then recorded a set of GPS coordinates for each.  In the future, they will be much easier to find.  If anyone is still doing monitor points, I would recommend this method.

Yet another problem; the phasor control system was damaged by lightning. The overly complicated Harris Phasor control card was replaced with something a little more straight forward and reliable.   I designed a simple set of relays, one for daytime and one for nighttime, to change the antenna system over.  The transmitter interlock goes through the relay contacts, so the transmitter PDM is killed while the power changes.  Tally back from each of the towers is handled by a set of relays for each pattern, which is also interlocked with the transmitter.  All of this prevents the RF contactors from switching hot, something that has caused some damage in the past.

W243EM is 100 ERP watts, non-directional with a 1 bay Shively 6812-1R antenna installed at 381 feet (116 Meters) AGL on one of the night time towers.

Transmitter is a BW Broadcast TXT-600.  The power calculation is as follows:

ERP 100 Watts = 50 dBm
 
System gains and losses:
 
Transmission Line loss, 500 feet (152.4 Meters), RFS LCF78-50JA =  -1.75 dB
Isocoupler loss, Kintronic ISO-170-FM = -0.8 dB
Antenna gain, Shively 6812-1R = -3.39dB
 
Total system losses and gains: -5.94 dB
 
TPO: 55.94 dB or 393 Watts
 
With all that work completed, the license application was filed to cover the construction permit.  Once that was accepted by the FCC, program test authority was granted and the transmitter was turned on.  Hopefully, with the translator on the air, the perceptions regarding listeners will change and the station can bill more.
 
I really enjoy working on Medium Frequency antenna systems.  I don’t know why, but antenna systems in general are always fascinating to me.

BE AM tuning network

Occasional reader Scott asked for a picture of the inside of a BE AM output tuning network.  I figured it might be helpful to make a short post about it.

These things are pretty simple; a T network with a capacitive leg to ground.

BE AM Output tuning network
BE AM Output tuning network

This particular unit is for 1230 KHz.  I believe the capacitor is frequency determined and they may also use larger inductors for lower frequencies.

BE AM output tuning network schematic
BE AM output tuning network schematic

The inductors are Kintronic LV-15-20 (15uH 20 amp) and the capacitor is 0.0018 uF CDE 6KV 5.6 amp.

The issue with this particular unit is dirt.  The inductors have round metal plates that roll along the inductor coil to make the variable inductor tap.  Dirt has accumulated on the coil turns and on the inside of the plates.  This, in turn, causes arcing anytime the Tune or Load controls are moved.  A through cleaning should take care of the problem.

More AM retuning work

Working on another old AM station, this one is a simple Class C one tower on 1230 KHz.

Broadcast Electronics AM Output Tuning Network
Broadcast Electronics AM Output Tuning Network

The main problem today was this BE AM output network unit between the BE AM1A and the ATU.  This site has had some dirt difficulties over the years and the internal parts of this tuning unit arc at full power.  I attempted to drive the ATU directly with the transmitter, which was a no-go.

Gates Radio 1 KW AM ATU, circa 1947
Gates Radio 1 KW AM ATU, circa 1947

I took a look at the ATU, which is a pretty standard Gates 1 KW ATU from the late forties or early fifties.  I have seen perhaps dozens of these things.

My first thought was that over the years, likely due to changes in the ground system, the base impedance has shifted away from its licensed values.  However, a quick measurement of the base impedance shows it to be exactly at the licensed value, 17.3 ohms.  The tower is 67 degrees tall, so that impedance value is right in the theoretical norm.

I  measured the input to the ATU, which showed 38 ohms with about 7 ohms capacitive reactance.  I can only surmise that it has always been this way.  The transmitter in use before the BE AM1A was a Harris/Gates Radio BC-1G.  That model transmitter will drive anything including an open transmission line.

Retuned ATU input; 49 ohms resistive, 0 ohms reactance
Retuned ATU input; 49 ohms resistive, 0 ohms reactance

Having the bridge on hand, I decided to retune the ATU for a better match. I put the bridge on the input terminals of the ATU and set it to 50 j0.  Using the remote control, I turned the transmitter off and on while making small adjustments to the output strap on the coil until the resistance was 49 ohms with zero reactance.  I would have gotten it to 50 ohms, but the strap on the output side of the coil would not stretch far enough to reach the proper spot on the coil.

Now the transmitter will run into the ATU directly at full power with about three watts reflected.   The BE AM output matching network unit has been removed for cleaning and repairs.  I will reinstall it once those repairs are completed.