July 2018 – Engineering Radio

A tale of five signals

I am currently finishing an interesting project involving putting up two translators on a diplexed AM tower which also holds a mobile phone/data tenant as well. All-in-all, this seems to be a very efficient use of vertical real estate.

The AM stations are WMML and WENU in Glens Falls, NY. The AM stations are diplexed using a Phasetek diplexor/ATU.

Diagram showing WENU/WMML tower with W250CC/W245DA antenna installed

Diplexor diagram, WENU/WMML Glens Falls, NY

The translators are W250CC and W245DA which are using a NICOM BKG-77/2 two bay 3/4 wave spaced antenna mounted at 53 meters AGL. The translators use a Shively 2640-04/2 filter/diplexor which is a broadband input port in addition to the translator input ports. Since these translator signals are only 1 MHz apart, the higher-power Shively filter was installed because it has better rejection characteristics. The broadband input port allows the NICOM antenna to be used as a backup for any of the three FM stations; WKBE 107.1, WNYQ 101.7, or WFFG 100.3. Two transmitter sites for those stations are mountaintop locations which are very difficult to get to in the wintertime. Having a backup site available takes some of the pressure off during storms or other emergencies.

Shively 2640 -04/2 filter for W250CC and W245DA

The NICOM FM antenna was mounted on the tower when W250CC went on the air in October 2016. When it was installed, the base impedances for both AM stations were measured. For some reason, WENU 1410 KHz seems to be more sensitive to any changes on the tower, thus the WENU ATU needed a slight touch-up. When working on diplexed AM systems, it is also important to make sure that both trap filters, which are parallel resonant LC circuits, are tuned for maximum rejection of the other signal. During this particular installation, nothing was added to the tower and no change in the base impedance for either station was noted.

Shively Filter, connected to transmitters and antenna

As a condition of the construction permit, measurement of spurious emissions of all stations sharing the common antenna needed to be completed to ensure compliance with FCC 73.317(b) and 73.317(d). I made careful measurements of the potential intermod products between the two translator frequencies. This measurement was completed with my TTI PSA6005 spectrum analyzer.

The primary concern here is mixing products between the two transmitters. Both transmitters are BW TXT-600 with low pass filters before the output connector. There are three frequencies of interest;

(F1 – F2) + F1 or (97.9 MHz – 96.9 MHz ) + 97.9 MHz = 98.9 MHz
F2 – (F1 – F2) or 96.9 MHz – (97.9 MHz – 96.9 MHz) = 95.9 MHz
F2 + F1 or 97.9 MHz + 96.9 MHz = 194.8 MHz

That, plus harmonic measurements out to seven or eight harmonics of the fundamental frequency should be enough to demonstrate compliance with FCC out-of-band emissions standards. Being that this site has LTE carriers, it is very important to measure the harmonics in those bands. Mobile data systems often use receiver pre-amps, which can amplify harmonics from the FM band and make them look out of compliance. Having a base set of readings to fall back on is always the best course in case the “out of tolerance” condition gets reported to the FCC.

Measurements on these frequencies must meet the emissions standards outlined in FCC 73.317 (d), which states:

Any emission appearing on a frequency removed from the carrier by more than 600 kHz must be attenuated at least 43 + 10 Log¹⁰ (Power, in watts) dB below the level of the unmodulated carrier, or 80 dB, whichever is the lesser attenuation.

Harmonic frequencies to be measured:

Harmonics for 96.9 MHz fundamental	Harmonics for 97.9 MHz fundamental	Comments
193.8	195.8
290.7	293.7
387.6	391.6
484.5	489.5
581.4	587.4
678.3*	685.3*	US LTE Band 71
775.2*	783.2*	US LTE Band 5
872.1*	881.1*	US LTE Band 5
969.0	979.0

*Frequencies that fall within the mobile data LTE bands. Traces were recorded and saved for these frequencies.

All of that information, once compiled is attached to the FCC form 350-FM, which, once filed grants Program Test Authority.

BW TXT-600 V2 translator transmitters under test and measurement

GatesAir FLX-40 one year in

I was at the WEBE transmitter site recently and took the time to look over the transmitter we installed last year:

GatesAir FLX-40 transmitter, WEBE Bridgeport, CT

Overall, I would say that this transmitter has been very reliable. We had to install a UPS for the exciter and HD Radio exporter, but that is not a big deal. During the first power outage, the exciter went dark first. It took longer for the transmitter controller board to lose power, in the interim the controller turned the transmitter power all the way up. When the generator came online 10 seconds later, the transmitter returned to operation at 41.5 KW. This, in turn, caused one of the other field engineers to freak out and nearly lose his mind (stay away from the brown acid, FYI).

I installed the UPS a few days later.

WEBE TPO 35.3 KW with HD Radio carriers on

The transmitter power output is 35.3 KW, which is getting into the semi-serious range. The reflected power goes up when it gets warm out and goes down in colder weather. Over the winter, it was running about 50 watts. Even at 138 watts, that represents 0.004% reflected power. The TPO forward goes to the 6 bay, 1/2 wave spaced antenna side mounted, 470 feet (143 meters) AGL. The station covers pretty well.

WEBE Pump station, pump is running 2/3 speed and fans are running at about 1/2 speed

Overall, I would give the liquid cooling system an A grade. The transmitter still dumps a fair amount of heat into the room from the RF combiners and PA power supplies. Most of the heat, however, ends up outdoors. Previously, we had two Bard 5-ton AC units running almost full-time. Now, only one AC unit cycles on and off except for the hottest days of the year. The outside temperature when this picture was taken was 81 degrees F (27.2 C).

Next year, we will have to send a sample of the coolant to be analyzed.

I have had good experiences with the GatesAir FLX/FAX series transmitters. I would recommend this to a friend.

Status of AM revitalization

It has been about five years since the AM revitalization initiative was first proposed by the FCC and about five years since the first rules changes took place. Those rules changes included:

FM translators for AM stations
Allowing stations to use MDCL (Modulation Dependent Carrier Level)
Changing some of the antenna radiation efficiencies requirements
Changing some of the allowable interference towards other stations requirements
Loosening some rules regarding proofs, MOM, nighttime coverage over the city of license, etc

Things that were not addressed:

Receiver quality and technical advances
Ambient noise levels on Medium Frequency (among other) bands
HD Radio or any other digital modulation scheme

Things that were discussed then changed subsequently as a separate initiative:

The main studio rule, which was eliminated for all broadcasting stations

What has been the net effect of these changes? Has any of this revitalized AM radio? The net effect has been approximately more of the same. There have been many stations that have applied for and received licenses for FM translators. Those stations, in most cases that I am aware of, receive some benefit of extra revenue because of this. Stations with carrier power levels of 10-50 KW have taken advantage of MDCL technology to save some money on their electric bill. Nothing wrong with that.

For stations that use a directional antenna, proofs of performance and other DA matters with the FCC have become slightly easier. Medium Frequency (MF) directional antennas are very large, require a lot of land, are expensive to build, license, and maintain. I know of several stations which have downgraded from a class B station with a directional antenna to a class D station with a single tower and greatly reduced nighttime power. Those downgraded stations certainly benefit from an FM translator.

I have heard from more than one AM station owner who says after four years, they are going to “turn in their AM license and just keep the FM.” I am sure that they are not informed regarding translator rules. Perhaps, however, the FCC will allow this in the future; a sort of back-door commercial low-power FM station classification.

The AM band zenith occurred in November of 1991 when there were 4990 licensed AM stations in the United States. As of June 30, 2018, the total stands at 4633. That is a decline of 357 stations. There are currently 90 AM stations listed as silent. That represents a decline of approximately 9 percent or less than 1/2 of one percent per year.

The last number of AM stations actually transmitting HD Radio that I found was approximately 110, which differs from the iBiquity (and FCC) number of 240. The FCC database includes stations that are currently dark or stations that were transmitting HD Radio at one time but have since turned it off. Either way, it is a small percentage of licensed stations. As of this time, AM HD Radio appears to be a non-starter. In other parts of the world, Medium Frequency DRM seems to be doing well. The difference seems to be that the DRM operation is all digital and the digital carriers have a much higher power level than that of the hybrid AM HD Radio being used here.

Of those 4633 standard broadcast stations, approximately 260 belong to iHeart radio, Cumulus owns approximately 120 and Townsquare owns approximately 80. That accounts for 460 stations. The remaining 4000 or so stations currently on the air are owned by medium-sized corporations or individual owners. The reason for the distinction; I have noticed that large corporate owners tend to concentrate resources and effort on those licenses that will make the best return, e.g. FM stations. Of course, there are a few exceptions to that trend, often in major markets.

Of those 4000 or so remaining AM stations, most seem to be treading water. They are making enough money to stay on the air. There are a few AM stations that are doing remarkably well. Those are the ones with primarily local content. The vast majority of AM stations are running some type of syndicated talk. News/talk and sports radio are the two most common formats. Conservative news/talk seems to be the bread and butter. Liberal news talk has been tried, but none have succeeded.

Last May, the Supreme Court overturned the Professional and Amateur Sports Protection Act of 1992. That federal law prevented gambling on outcomes of professional and college sports games. With the overturn of that rule, individual states can now legalize sports betting. It will be interesting to see what states allow legalized sports gambling and whether that has any effect on the various sports radio formats. I can see where individuals and odds makers may want to get good inside information regarding team dynamics and so on. The sports network that can furnish such information may be in a good position to carve out a niche.

Music can and does sound good on AM when it is done correctly. There is a great misconception that AM fidelity is poor. That is not necessarily so. There are a good many AM receivers these days that have much better bandwidth than the previous generation receivers. I am noticing that car radios in particular sound much better. Yes, there are still problems with electrical noise and nighttime interference. There are still technological improvements that can be made for analog AM on the receiver side.

In summary; the revitalization efforts have benefited some AM stations in some areas. The truth is, that many AM stations have been let go for so long that there is no saving them. Other AM stations that are still viable are making a go of it. In nautical terms; there is six feet of water in the hold, the pumps are working and the ship is not sinking… for now.