tech stuff – Page 31 – Engineering Radio

AM Radio Improvement Plan

There has been lots of hand wringing and ink spilled regarding the sorry state of affairs in the senior service. AM is plagued with problems; interference, poor bandwidth, etc. To that end, the NAB has launched studies and initiatives and hired all sorts of pricey consultants to consult with. Here is my own AM improvement plan and it is rather simple:

Clean up the transmitter site.
Get rid of AM HD radio.
Variable IF bandwidth receivers.
Improve Programming.

How many of us have seen AM transmitter site dumps? Deferred maintenance, malfunctioning directional arrays, trees growing up on the ground system, flooded buildings and ATU’s, rusty towers, transmitters not a full power, ground system deteriorated or missing all together, just to list a few problems. Many AM transmitter sites are technical disasters. Think that these things have no bearing on the AM station’s signal? Think again.

differed maintenance, AM transmitter site — Differed maintenance, AM transmitter site (there is a tower in there somewhere)

Trees growing around the tower base can attenuate the signal by 30%. A comment from a well known engineering firm:

…Recently XXX field engineers had occasion to measure an AM station at XXXX kHz before and after removing vegetation in the vicinity. The station had a quarter-wave tower. The base area had grown up in brush and hardwood trees to a height of perhaps 30 feet (9m) and this extended from near the base across the entire ground system. After clearing (cutting, no ground system disturbance), the signal measured at some 16 locations on four radials went up a uniform amount of about 15% or 1.2 DB. That’s about a 30% increase in radiated power…

That is an inexpensive power boost and they didn’t even have to file with the FCC! A 1 – 2 dB power gain is pretty nice and can mean the difference between a listenable signal and static. How many times have I heard the lament that AM band is full of noise and not listenable. Certainly, there are major challenges in the urban listening environment. Putting forth a better signal will overcome some of this electrical noise.

There is a reason why engineering standards were developed for the physical plant; they work.

There is no cure for the noise that AM HD Radio puts out into the adjacent channels. This self interference benefits none, not even the station transmitting AM HD Radio. This dubious technology has proved itself a non-starter and should be discontinued. For smaller station owners, the cost of implementing AM HD Radio is prohibitive. Licensing of a proprietary modulation scheme, new transmitting equipment, specialized exciters plus any needed bandwidth improvements to AM antenna arrays can easily exceed $100,000.00. Unfortunately, it is often the small AM radio operators that are making a good showing, and serving their community of license and making money. These are the very stations that are hurt the most by adjacent channel AM HD Radio interference.

Receiver design over the last twenty to thirty years has been the greater issue with perceived low AM broadcast quality. AM receivers have an average bandwidth of just 3-4 KHz, which is slightly better than telephone quality. AM broadcasting has gotten a bad wrap because of this and there are many comments about how AM is “inferior quality” to FM. With a quality older receiver, AM can sound very good. Of course, the receiver manufactures all point adjacent channel interference as their rational for reducing IF bandwidth. Why not leave it in the hands of the user? The GE Superradio had this feature with a “wide” and “narrow” setting for AM reception. They worked remarkably well. A receiver could also be designed to automatically increase IF bandwidth at higher received signal strengths.

Finally, as the saying goes; Garbage in, Garbage out (GIGO). This holds true for many things including radio programming. Expecting that mediocre satellite syndicated news talk will garner great ratings and huge revenues is silly. For years and years, station owners have put minimal effort into AM radio and expected big returns. It is not working. AM stations that go against that trend; those with unique formats (Gasp! Music, on AM?), local content, and community oriented programming can and do succeed. They are fighting an up hill battle in both directions. With all of the business pressures from larger broadcast groups, interference issues and negative viewpoint on the viability of the AM band, one wonders how long they can last.

Restarting a Harris HT35 FM transmitter

This transmitter was retuned from 107.9 to 92.9 and put back into service. Retuning an HT35 transmitter is no small matter, there are 32 pages of retune instructions. This unit is now in service as the main transmitter for WEZF, Burlington, VT.

The transmitter power output is 22,000 watts into a four bay, three around panel antenna, which gives it an ERP of 46,000 Watts at a height of 824 meters (2,703 feet) above average terrain. The tower is at the summit of Mt. Mansfield, which is 1,340 meters (4,395 feet) above sea level.

This is the Mt. Mansfield FM transmitter room. There are two TV stations in this building as well.

Final frames are of the WVPS Nautel NV-40 transmitter.

The unitless coefficient of Zorch

Zorch is a term used to describe an over voltage or over current condition that usually leads to catastrophic failure, e.g. the power supply was zorched by lightning. There is also a quality to radio signals that defy and exceed theoretical definitions for service contours or power density. That is quality defined as:

Zorch (adj): The ability of an RF signal to be received in unlikely locations; outside of predicted service contour, in steel structures, underground facilities, tunnels, etc.

It brings to mind the saying, “antennas are not amplifiers and amplifiers are not antennas.”

During the earlier stages of FM broadcasting, there was a notion that costs could be reduced by increasing antenna gain and reducing transmitter size. While theoretically, ERP (Effective Radiated Power) is ERP, broadcasters soon learned that high gain antenna, low TPO (Transmitter Power Output) installations lacked building penetration and had other reception issues. Realizing that there is a trade off between antenna bays, transmitter power output especially in difficult reception areas, a great debate occurred and continues on what the optimal system is. The answer is, it depends on the receiving environment.

Where this technical detail can be really important is with lower powered FM stations; Class A and LPFMs to be exact. They are already battling against bigger stations that have tens or even hundreds of times more power. Certainly an LP-100 station has it’s work cut out for it. The choice of antenna is perhaps one of the most important technical decisions to be made. Choosing the right balance of antenna type, antenna gain, antenna height and transmitter power output can greatly influence reception reliability and thus coverage area.

A good study of this quality can be had by looking at various LPFM installations:

Station ERP (watts)	Antenna Type	Antenna Gain (power)	TPO (watts)*	Coefficient of Zorch
100	1 bay vertical	0.92	127	0.1
100	1 bay circular	0.46	253	0.4
100	2 bay vertical full	1.98	58	0.15
100	2 bay vertical half	1.40	83	0.2
100	2 bay circular full	0.99	118	0.5
100	2 bay circular half	0.70	166	0.7
100	3 bay circular full	1.52	77	0.46
100	3 bay circular half	1.01	115	0.52

*Includes 100 feet of 1/2 inch foam transmission line, Andrew LDF4-50A, loss of 0.661 dB at 100 MHz, or 0.859 power gain.

Stations should try to get the transmitting antenna as high up as permitted without reducing ERP. In other words, the FCC allows 100 watts ERP with 98 feet Height Above Average Terrain (HAAT) radiation center in their current LPFM rules. Being lower in height will reduce the coverage area. Going over 98 feet HAAT will cause the station’s power to be reduced, which will lower the coefficient of zorch accordingly. Therefore, getting as close to 98 feet HAAT, which is different than 98 feet above ground level in many places, will net the best performance.

If a singular polarization (horizontal or vertical) is desired, vertical polarization should be chosen, as most mobile reception is by a vertical whip antenna. For best reception performance, a circularly polarized antenna will work best, as receiver antenna orientation will not effect the signal reception. A circularly polarized antenna has better building penetration and multi-path characteristics. The FM broadcast circularly polarized antenna in not a true circularly polarized antenna, it is actually unpolarized.

The use of a multi-bay antenna has the effect of focusing the RF radiation outward, perpendicular to the element stack, thus limiting the radiation directly up or down from the antenna. This is more pronounced with one half wave spaced antennas, which may be an environmental consideration in heavily populated areas.

Thus, the best coefficient of zorch for an LPFM station would be a circularly polarized, 1/2 wave spaced, 2 bay antenna. This antenna would have some gain over a single bay antenna, take up less room on a tower than a full wave spaced antenna, offer good RF protection performance for the general public living and working under the antenna, reduce wasted upward radiation and offer good building penetration for the ERP. It would require a slightly larger transmitter and more electricity, but that trade off is well worth the effort.

Details

I found this small, yet very important detail on a DB-37 connector attached to the back of a Nautel V-1 transmitter:

DB-37 connector for Nautel V-1 transmitter

The black wire is the ground wire and the orange wire is the remote RF off command. A closer view:

DB 37 connector from Nautel V-1 transmitter

The transmitter had been shutting down unexpectedly since it was installed. When these shutdowns occurred, there was no overload, no fault, no power interruption or other indication of a problem. When the RF on command was issued, the transmitter would turn back on and run with normal readings until it shut down again. It was a bit of a mystery; the transmitter was removed from its mountain top home and hauled back to the shop to be repaired. It was connected to all sorts of test equipment and studied intently for many days. Still, the problem could not be replicated in the shop.

Then the transmitter was hauled back to its mountain top transmitter site and re-installed. It ran well for about a month and then started going off again. This time somebody looked at the event log and noticed that a “Remote RF off command” was being issued at the same time the transmitter would shut down. Ahhh, the missing bit of critical data. That prompted me to take apart the DB-37 connector used for the remote control interface. The problem was obvious as soon as I removed the hood.

Sometimes the most valuable piece of test gear is the venerable Mk I, Mod 0, EYEBALL.

I unsoldered the ground lead and put some heat shrink over the connection to the DB-37. Hopefully, that will take care of it.