Not later than 10 years after the date of enactment of this Act, the paired electromagnetic spectrum bands of 420–440 megahertz and 450–470 megahertz recovered as a result of the report and order required under subsection (c) shall be auctioned off by the Federal Communications Commission through a system of competitive bidding meeting the requirements of section 309 of the Communications Act of 1934.
Will this mean broadcasters be losing their Part 74 Broadcast Auxiliary RPU licenses? Section 74.402(4)(b)(4) lists those frequencies as 450.03125 through 450.950 and 455.03125 through 455.950 MHz in various channel configurations. These frequencies are used mostly for RPU but are also used for TSL systems. This is the NITA frequency allocation chart. The RPU frequencies are shared but I doubt an entity that has paid through the nose for exclusive use of a frequency band would be interested in that. Further, according to Part 97.301(a), the 70 cm Amateur Radio frequencies are from 420 to 450 MHz. That has the Amateur Radio users quite upset, and rightly so. I don’t know if this has filtered down to broadcasters yet, but losing RPU and TSL frequencies would likely be an inconvenience, to say the least.
What gives? Reading through the bill, it seems there would be a multi-part shuffle over several years to move the “first responders” to a nationwide system on the 758-768 and 788-798 MHz frequencies. The then “empty” frequencies would be auctioned off, except some of them aren’t so empty.
Does this mean that all the existing police, fire, and ambulance radios will be phased out in favor of the 700 MHz units? Didn’t they just install a bunch of trunked 800 MHz systems recently? Wasn’t that an expensive boondoggle that still has yet to be sorted out in some locations? Ah well, it’s only $2 billion or so taxpayers dollars, which, to fight terrorism, anything goes.
We have received somewhere between 5-6 inches of rain in the last four days. That, coupled with the deep snowpack and the still-frozen ground has led to some flooding. The WLNA antenna array is located along the Peekskill Hollow Creek in northern Westchester County, NY. Back in 1980, it might have seemed like a good idea to locate an AM station in a tidal swamp along the Hudson River. I am sure the land was not that expensive and from an engineering standpoint, having a continually wet, partially brackish ground system may have seemed like a slam dunk.
Unfortunately, the idea never really panned out in the application. First of all, the neighbors had other ideas, fighting the radio station owners all the way to the NY State Supreme Court. Secondly, technically, it never lived up to expectations. The original non-directional antenna on 1430 was a 1/2 wave tower which by all accounts, worked very well. It did not, however, allow for nighttime service, which is why the new sight and array were sought. By the time the system was built, AM was already in steep decline and I doubt the owners ever recouped their investment.
Fast forward to today. All five base insulators are under water and the transmitter is off the air. These are pictures from last Wednesday after the first flood waters receded from the Monday/Tuesday storm. I imagine it looks worse this morning, although I don’t own a boat and won’t be wading out there to look.
Base insulator, tower 2 WLNA array, Peekskill, NY
This is tower two of the daytime antenna array. Clearly, it spent some time underwater. We cleaned off all the debris from all the tower bases. A far worse prospect is the ATU’s:
WLNA tower 1 ATU, Peekskill, NY
This is the Antenna Tuning Unit for tower 1, which is the reference tower for both the day and night arrays. The E.F. Johnson contactor in the bottom of the cabinet was fully submerged for an undetermined amount of time. The bottom of the unit is covered in fine silt. The high water mark is visible on the right side of the aluminum cabinet.
The contactor is going to need to be replaced, or at least rebuilt. The ATU cabinet will need to be washed out. There are two other ATUs that suffered the same fate.
WLNA antenna array, towers 3 and 5
This is the end of the catwalk next to the Peekskill Hollow Creek looking west towards the Hudson River. The water level reached the bottom of the catwalks and had receded about 4 feet when this picture was taken.
WLNA antenna array, tower 5, Peekskill, NY
Lookup east, upstream at tower 5.
WLNA antenna array looking north, Peekskill, NY
This is the antenna array looking north, with my back facing the creek. Tower one is the center tower, tower two is on the right and tower four is on the left. The daytime array consists of towers 1, 2, and 3 bearing 300 degrees. The night time array consists of towers 1, 4, and 5 bearing 335 degrees, so the array makes a big X in the swamp. More from the FCC database.
It is going to take a lot of work to clean out all these ATUs and repair the damage. Clean water is at least 1000 feet away. My question is; why bother? Once upon a time, this station was viable, well thought of in the community, etc. Now, I doubt anyone knows it is off the air. The current ownership over the last thirteen years did, what I’d like to call, a controlled flight into the ground. Axing staff, cutting maintenance, and generally neglecting the station. Why not take it dark for a while and figure out what to do with it? Likely somebody would buy it, even if for the land it sits on. Anyway, the grind continues…
Amplitude Modulation (AKA AM) was the first modulation type to impress audio on an RF carrier. Prior to this, information was transmitted via on/off keying of a continuous wave transmitter using Morse code or some equivalent.
There are several methods for generating AM in a transmitter.
1. Low-level modulation. The modulation is developed in the first stage RF section, then amplified by subsequent stages to full power. Simple and easy to implement, especially for mobile transmitters and SSB installations. Disadvantages come from the need for linear amplification through all the stages requiring class A or AB amplifiers and do not reproduce wide band AM well.
Grid Modulated AM transmitter
2. Doherty modulation. William Doherty came up with an ingenious way to use a low-level linear modulator with good to excellent efficiency. Under full carrier, no modulation conditions, the carrier tube is generating the RF carrier, and the peak tube is mostly cut off (very little current). When modulation is applied, the peak tube then begins to conduct, the output of this tube is combined with the output of the carrier tube through a 90° LC network, which is the same as a 1/4 wavelength transmission line. The effect of this is to lower the output impedance, thus allowing the carrier tube to modulate 100 percent.
Later, Continental Electronics and Jim Weldon somewhat modified this system in their 317C series high-power transmitters.
Continental 317B simplified schematic diagram
3. High level or plate modulation. The RF and Audio sections are developed separately within the transmitter, then combined in the final stage of the transmitter. Older systems used a modulation transformer. The advantages are all the amplifiers can be run class C or greater, which reduced electrical consumption and power supply requirements. Much higher power levels are achievable with this design. These transmitters also reproduce wide-band audio much better than low-level modulated units. They are also extremely rugged. Disadvantages are the system requires large audio sections and they take up a greater area and are not as efficient as later modulation methods.
Plate Modulated AM transmitter
4. Ampliphase. A phase-modulated system developed by RCA where the transmitter developed two RF signals in the final, 135 degrees apart. To modulate the signal, the phase relationship between the carriers is varied, more toward 180 degrees would be a negative peak, and more toward 90 degrees a positive peak. These transmitters required less space and were more efficient than traditional plate-modulated transmitters. They required careful setup and tune-up to reduce distortion and somewhat unfairly earned the name “amplifuzz” from some engineers.
RCA BTE 20 ampliphase AM excit
5. PDM or PWM. This is also a high-level modulation scheme but with some slight variations. The carrier power level and modulation levels are set by a PDM encoder card. In Harris transmitters, the PDM frequency was 75 KHz. The carrier is set by the amplitude of the PDM waveform, and the modulation is determined by the duration of the pulse. PDM transmitters require power supply voltages about twice the voltage of a standard high-level plate-modulated transmitter. They also require a damper diode to conduct the B+ voltage to back to the power supply during negative peaks, otherwise, the PA voltage will attempt to rise to infinity. I have found the damper diode to be the weak link in a tube-type PDM transmitter.
Solid state transmitters also use this design with either MOSFETs or BJT, which are then combined in parallel to generate the required output power. This is most often called “Class E” or something similar. In that system, each pair of modulator MOSFETs has its own fast-acting damper diode, usually protected by a fuse.
Harris MWx tube type PDM transmitter
6. Direct Digital Synthesis. This is a patented design from Harris Broadcast used in their DX series transmitters. The incoming audio is sampled at either the carrier frequency or 1/2 the carrier frequency depending on where in the band the station falls. The solid-state PA modules are then switched on and off at the carrier frequency with the audio levels imposed on the carrier information. The explanation is simple, the application is complex:
Harris DX series AM transmitter block diagram
Of all these transmitters, the Harris DX series is the most efficient from a power input (AC) to power output (RF) perspective. There are several methods of reducing electrical use by reducing carrier power levels during lulls in modulation. The Continental 418E and later series transmitters can reduce carriers up to 6 dB using CCM. Harris and Nautel use similar systems on their DX and XL transmitters respectively. The wheatstone corporate blog has an article: Greener AM transmission Methods that details others.
As far as simplicity, serviceability, and rugged design, the high-level plate modulated transmitters cannot be beaten. Many Amateur Radio operators build these units from scratch using old parts, tubes, and other reused equipment readily available, often for free. I have, in fact, donated several 1 KW AM transmitters to ham radio operators over the years.
If I were to design a “transmitter of last resort,” to use in case everything else fails, it would look something like this:
813 Tube type 250 watt transmitter final813 Tube type AM transmitter modulator section
The disadvantage of that design is it requires a 2KV power supply, which has its own set of safety concerns. I might substitute 833s for 813s and use heavier iron in the modulation transformer. That way the transmitter could develop a 500 to 1,000-watt carrier. The great thing about tube transmitters is, given the right output components, they can be tuned into almost any load. They are also easily adaptable for emergency operation into temporary antennas.
This is a map of the AT&T microwave relay system as it was in 1960. It is interesting for several reasons. First of all, before there were communications satellites, this is the way that data was transferred from one location to another. That data would have been digitized and TDM encoded on a T-carrier, then loaded onto a microwave path. TV networks had loops that transversed the country, distributing network video and audio to all the markets in the US. The first transcontinental New York to San Francisco microwave route was established in 1951. Through the fifties and sixties, the network was filled in across the US and Canada.
Radio networks had been using wired TELCO networks for program distribution for years, although they required far less bandwidth than TV. This was during the time when network affiliation was vitally important to a station. Radio networks provided news and other special event programming, as well as some long-form shows which were an important source of information for the listeners. Any network programming prior to 1980 or so would have been carried by this system.
It was not until the use of C and Ku band satellite services that networks could offer multiple channels of programming. Now, entire radio formats could be programmed remotely and beamed into hundreds of stations across the country simultaneously. That would have been far too expensive to implement over TELCO lines, as the line charges were based on the mileage of the circuit.
Bell System microwave relay routes
Click for higher resolution.
This system included thousands of hardened microwave relay sites, each built to exact specifications and fully redundant. At the time, the long-distance telephone system was an integral part of the US defense planning. Sites were spaced 20-40 miles apart, depending on terrain. In congested areas, like the northeast, area mountain tops are dotted with these sites today, mostly empty. Most of these sites went offline in the late 1990s as phone companies switched to fiber optic cables for telephone and data traffic.
American Tower, Inc. purchased most of these sites in bulk from AT&T in the year 2000. Some sites are well positioned for Cellular Telephone, 3G, and 4G wireless data services, plus other things like Media Flow and general use applications like FM broadcast and two-way. Many sites, however, do not meet any specific needs and sit empty. There was a large fire sale by American Tower in 2002 in which they unloaded about 1,900 of these sites as they were redundant.
I wrote a post titled Cold War Relic: ATT long lines site, Kingston,NY detailing one of these sites near me. Keep in mind, there were thousands of these sites throughout the country.
