Troubles at the Tower

3 tower AM directional array
3 tower AM directional array

Troubles at the AM tower; I don’t know why, it won’t switch power.
Over the phone I can tell, the program director’s day is not going very well.
Press the “day” button but there is no kerchunk, the directional coupler shows the load is junk.
Out into the big field I go to find the problem quickly and fix it just so.
The wind is cold, the snow is deep, I think of the contract terms I must keep.
Reaching the tuning house, take out the keys, lock, do not be frozen, please.
Once inside, there I find, no big surprise, the mice have been a working this pre-sunrise.
A nest they have build in a most inconvenient place, in the back of the phasor wiring chase.
Oh, the wires they have chewed, the circuit’s destroyed, all for the lack of mousetraps deployed.
As I reach in to clean out the mess, the smell of mouse makes me gag, I confess.
The fuses are blown, the contactor is jammed, perhaps, if I am lucky, I can move it by hand.
A large screw driver strategically employed, I pry up slowly, further damage to avoid.
The bar thunks up, the contacts engage, the transmitter is ready to apply amperage.
Call on the cell phone, tell them its fixed, stand back and watch the base current meter, transfixed.
Then; Up it goes! Wonderful radio frequency current flows!
I clean up, lock the door, lock the gate, carrying bad news the owner will hate.
The damage is grave, the repair bill is steep, if a good relationship with the FCC you desire to keep.
Business is off, the accounts are low, is this really necessary, he wants to know.
The terms of the license are your obligation to keep, getting caught out of tolerance will not be cheap.
Looking forlorn, the owner says in disgust, it is only the AM, but fix it if you must.
Happy as a lark, with a song in my heart, I dig though the manual and order the part.
Time to go home, eat breakfast, brush teeth, take a shower. I have another client to see before the noon hour.

40 amp RF contactor
40 amp RF contactor

Dedicated to all those who have been there, done that and the breed of RF men and broadcast engineers who are slowly fading away.

At a crossroads

This is a situation that is and will be playing out over and over throughout the country as the decay advances. W*** signed on the air in March 1963. I believe this is the original tower:

W??? tower
W*** tower

As you can clearly see from this picture, this tower has several problems. Aside from the loose guy wires, the rust and general structural decay, it is bent in several places.  Currently, the forces are in equilibrium, but for how long, no one knows.  It is certainly not safe to climb.  At 144 feet, it is no longer required to be marked or lit, thus, over the years, the paint peeled, the weep holes filed up, the guy wires rusted and loosened, which leaves us with the situation today.

At the transmitter building, there are other issues with the basement flooding, mold, etc.  Truth be told, this station makes no money on it’s own.  It would cost several tens of thousands of dollars to fix all these issues, and for what; a high end of the broadcast band class D AM station which has not shown up in the ratings for fifteen years.  Once upon a time, it was a surviving, perhaps not thriving, local radio station. Those times have long since past.

The question is; what to do with it.  Sign it off and surrender the license?  Fix all the problems and continue to broadcast?  Donate it?  If so, who would take it?  Or, more likely, wait until the tower collapses and deal with it then.

I’d imagine that there are many others just like it dotting the country.  On the whole, the AM broadcasters that are viable would be better off if this dead wood was cut away and discarded.

Tower lighting transformers, isolation chokes, etc

Series excited AM towers require some way to get standard AC across the base insulator to the tower lights, if tower lights are required.  While many new AM towers do not have base insulators, through the use of a folded unipole, it is still a very popular design and has several technical advantages.

There are two methods for getting 60 Hz AC from zero RF potential to an excited tower:

  1. Tower lighting choke
  2. Austin Ring transformer
LBA Group TC-300 tower lighting choke
LBA Group TC-300 tower lighting choke courtesy LBA Group, Inc

When to use which depends on the tower and the RF potential on the base of the tower. For towers that are under 140 electrical degrees (RF) and carrier power levels up to 100 KW, a lighting choke works well. They are simple and less expensive than an isolation transformer.  They can be installed inside the ATU cabinet or placed in their own weather proof enclosure as required.  Tower lighting chokes will add series impedance to the base of the tower and needs to be compensated for by adding capacitance to the circuit.  This will become more pronounced at the lower end of the band, where, if one is not careful, RF from the tower can be coupled to the transmitter building’s AC mains, which is very undesirable.

Tower lighting chokes generally consist of three separate windings, one for the beacon, one for the side lights and one for neutral.  Their inductance is typically in the 800-1000 µH at 1 MHz region.  They can be stacked to increase their peak voltage handling capacity:

LBA Group tower strobe light choke
LBA Group tower strobe light choke courtesy LBA Group, Inc

Peak voltage is determined by the base impedance and carrier power + modulation.  On any AM station these days, a 150% peak modulation figure should be used (125% modulation allowed by FCC rules plus a 25% safety factor).  For example, station B has a base impedance of 50Ω (typical 90° guyed tower) and a carrier power of 50 KW.  The peak modulation power will be 600 KW.  Thus, the peak voltage will be Epeak = √Ppeak x R, or Epeak = √600,000 watts x 50 ohms or 5,477 volts.   With higher base impedances, the base current goes down but the base voltage goes up.  A typical 140° tower will have a base impedance of 760Ω.  Thus the peak base voltage for a 50KW carrier power modulating at 150% will be 21,354 volts.  This is the worst case scenario, as few installations are designed that way and every tower impedance is different than the theoretical self impedances given.

For towers over 140 electrical degrees, it is better to use an isolation transformer because of the RF peak voltage/peak current conditions at the base of towers that are electrically tall.  The ring transformer design minimizes stray inductance or capacitance at the base of such towers.  Austin Insulators (previously Austin Decca) makes a variety of tower base ring isolation transformers.  These having varying input and output voltages.

Diagram of typical Austin Ring transformer
Diagram of typical Austin Ring transformer courtesy Austin Insulators, Inc

I have seen these at many locations over the years. They are rugged and add only a small bit of capacitive reactance to the base of a typical tower.  They also completely isolate the building AC mains from the tower.  For very high power installations, Austin has the A-9600, which was designed for the Navy VLF transmitter towers where base peak RF voltages can run 200,000 volts or more:

Austin A-9600 oil filled isolation transformer
Austin A-9600 oil filled isolation transformer courtesy Austin Insulator, Inc

Voltage drop is another consideration in tower lighting design. Long runs from the transmitter building to the tower should be on heavy gauge wire and at 230 volts if possible.  FAA Circular AC 150/5345-43F “Specification for obstruction lighting equipment” advises that the input voltages for incandescent lighting systems vary by not more than ±3%.   Additional tower lighting and painting information can be found in FAA Circular AC 70-7460-1K.

What is “Phasing” as it relates to radio?

Occasional reader Jeffery asks a good question, which I will attempt to answer here in simple terms. Phasing, when used with antennas, refers to the relationship that two or more radiating elements share with the waveform being transmitted.  It is used to create an RF radiation pattern by adding energy to the wave front in one direction by taking energy away from the wave front in another direction.

Phasing is often described as +/- X number of degrees from a reference point.  Graphically, it would look like this:

One wavelength with +/- 180 degrees notated
One wavelength with +/- 180 degrees notated

The reference point can be changed to any point on the wave form, in radio applications it is usually oriented around +/- 180 degrees.  If the reference point is a single tower or element then this would be the end of the story. Add a second tower to this system and it would look something like this:

Double wave form
Double wave form

In this picture we have two waves being radiated from two separate elements. These elements are spaced 100 degrees apart and tower #2 is phased to +90 degrees.  RF generator is coupled to both towers via a power divider, the reference tower (tower #1) is feed with 57% of the power that tower #2 is being feed.  Thus, the ratio of power to the respective towers would be 57:42.  Thus, if tower one had a power reading of 1.00, tower two would be 0.74.  The towers are on a north/south line with the reference tower bearing 180° from tower #2.  In the area of subtraction, the wave forms from each tower cancel each other out to some radiating less power toward the south; in the area of addition, the wave forms sum to create a more powerful waveform, radiating more power towards the north.

Resulting pattern (WKIP, Poughkeepsie, NY):

WKIP 1450 Poughkeepsie, NY pattern plot
WKIP 1450 Poughkeepsie, NY pattern plot

This is a typical two tower array, however, there are two slight differences; the reference tower is 215 degrees tall, tower two is 90 degrees tall. This is yet another use of “degrees” to relate electrical length or separations. The second, more notable distinction is that this array is Directional daytime, non-directional night time, which is the opposite of most AM stations in this country.

Electrical height can also be described as a function of wave length, e.g. 1/4 wave, 1/2 wave, etc.  Most AM towers in this country are 1/4 wave length, which equates to 90 degrees.  Often, higher powered stations, and some low powered stations put up towers near 1/2 wave length due to the better ground wave performance of those towers.  At lower dial positions, a 1/2 wave tower becomes an expensive proposition due to the height required.

In theory, an unlimited number of towers can be used to create a pattern by introducing nulls (areas of subtraction) and lobes (areas of addition).  In practice, the highest number of towers I’ve ever heard being used in an AM directional array is twelve; KFXR 1190, Dallas, TX.  There may be others, too.

An excellent resource for AM directional antenna technical information is Jack Layton’s Directional Antennas Made Simple, which is out of print but available from various sources.