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Medium Frequency ATU design

This is a topic I have covered before, but it is worth doing it again for future reference.  The previous post covered downgrading an AM transmission facilities for WGHQ, Kingston, NY.

This is part II of that process.

WGHQ transmitter site, towers 1 and 2 removed

WGHQ transmitter site, towers 1 and 2 removed

The old towers have been cut up and put in a scrap metal dumpster. They are off to China to be melted down and made into a submarine or a missile or a tank or something useful like that.

Towers scrapped

Towers scrapped

The directional array had a three towers in a straight line with a common point impedance of 60 Ohms.  Dropping two towers greatly changed the electrical characteristics of the remaining tower, therefore the existing ATU needed a bit of reworking to match the 50 Ohm transmitter output.

First step, correct a few deficiencies left over from the old array.

Vise grip tower feed

Vise grip tower feed

This vise grip RF connection has to go. The problem is where the tower erectors attempted to solder the copper tubing.  That tower base plate is pretty big and I would wager they didn’t use enough heat to make the solder connection.  They were probably working in the winter time, thus the “temporary” fix.  This tower was put up in 1993, so that temporary fix lasted 23 years.

I removed the offending tool and soldered the connection to another part of the tower with silver solder.  The smaller cross bar made a good connection point.

RF feed correctly connected to the tower

RF feed correctly connected to the tower

After soldering, I cleaned up and sprayed some grey primer on it to prevent rust forming where I scraped the paint off.

Next, I made an impedance measurement:

WGHQ tower base impedance measurement

WGHQ 920 KHz tower base impedance measurement

That junk on the upper part of the graph is coming from WHVW on 950 KHz. The tower itself looks pretty good, 77.6 Ohms resistance with 130 Ohms inductive reactance.  Since this is not a part of a directional antenna system, the ATU design is pretty straight forward.  Given that WHVW on 950 KHz is located 10.41 miles away, a low pass filter design is optimum.  A basic low pass filter T network has inductive input and output legs with a capacitive shunt leg to ground.

T network diagram

T network diagram

Each leg is used to match the 50 Ohm transmission line impedance (R1) to the 77.6 Ohm tower impedance (R2) and cancel out the 130 Ohms of inductive reactance.  This is a vector impedance problem, much like a vector force problem in physics.   Some basic arithmetic is required (always include the units):

X1, X2, X3 = √(Zin x Zout)

X1, X2, X3 = √(50Ω x 77.6Ω) or X = 62.28Ω

The value of inductance or capacitance for each leg is calculated using the basic inductance or capacitance formulas:

L (μH) = XL / 2πf(MHz)

And

C (μF) = 1 / 2πf(MHz) XC

Thus the input leg, or X1 = 62.28Ω / (6.28 x 0.92 MHz) or 10.78 μH

The Shunt leg, or X2 = 1 / (6.28 x 0.92 MHz x 62.28Ω) or .0028 μF

The output leg is a little different.  The tower has 130 Ohms of inductive reactance that needs to be cancelled out with a capacitor.  Rather than cancel out all of the inductive reactance, then add an inductive output leg, the tower reactance can be used as part of the tuning circuit.  The design calls for 62.28 Ohms inductive reactance, so 130Ω – 62.28Ω = 67.27Ω, which is the value needed to be cancelled by a capacitor:

Output leg, or X3 = 1 / (6.28 x 0.92 MHz x 67.27Ω) or .0025 μF

A little Ohm’s law is used to calculate the base current for both the day and night time operations.

Ohm's law pie chart calculator

Ohm’s law pie chart calculator

Thus the daytime base current is I = √(P/R) or I = √(1000 W/77.6Ω) or 3.58 Amps.

Night time base current is I = √(38 W/77.6Ω) or 0.70 Amps

Current handling requirements:

Base current is calculated to be 3.6 Amps at 1,000 Watts carrier power.  Allowing for 125% peak positive modulation makes it 5.7 Amps.  Having safety factor of two or 11.4 Amps output leg and 14 Amps input leg.

Voltages: 353 maximum input voltage, 439 output.

Thus, 20 amp, 10 KV parts should work well.

The designed schematic for the ATU:

WGHQ ATU Schematic diagram

WGHQ ATU Schematic diagram

Putting it all together.

Since the tower looks fairly broad at 920 KHz, we are going to attempt a nice broadband ATU to match it.  This station is currently programmed with a classic country format, and I have to tell you; those old Conway Twitty, Merle Haggard, Patsy Cline, et al., songs sound pretty good on the old AM radio.  The Subaru stock radio has HD, which also has a nice broad IF section, thus allowing all those lovely mid-high range frequencies through.

This is the existing ATU, which I believe was built by Collins in 1960:

WGHQ Tower 3 ATU

Existing WGHQ T network ATU

The ATU building is a little rough, but the ATU itself is in remarkable shape for being 56 years old.  The input leg inductor is in the center and will be reused as is. The large Jennings vacuum capacitor at the bottom is a part of the shut leg.  Its value is 2000 pF at 15 KV.  The top vacuum capacitor is series output cap, its value is 1000 pF at 15 KV.  The basic plan is to move the upper cap down in parallel with the bottom cap.  The shut leg inductor will be kept in place to tune out any access capacity.  For the output leg, I have a 2500 pF mica cap and a 10-100 pF variable cap connected in parallel.  The inductor on the output leg will be removed.

After some re-work on the ATU components, I tuned everything up.  The easiest way to do this is to disconnect the legs, measure them individually and adjust them for the desired reactance, which in this case is 62.28 ohms or thereabouts.  The output leg was measured with the tower connected since the tower reactance is a part of the tuning circuit.  The input leg was right about 10 μH.  The shunt leg turned out to be about 0.002 μF.  This is often the case, theoretical values are slightly different than field values due to stray capacitance and inductance in the connecting straps, etc.

This is the load, as measured at the output terminals on the transmitter:

WGHQ tower load as measured at the transmitter output terminals

WGHQ tower load as measured at the transmitter output terminals

Slightly asymmetric on 910 KHz, but overall pretty good. There is a fair amount of phase rotation in the transmission line due to the length from transmitter to tower (855 feet, 260.6 meter), which works out to be 0.93 wave length allowing for the 86% velocity factor of the transmission line.

Time to pack up and go home.

Downgrading an AM radio station

WGHQ in Kingston, NY has been downgraded from a 5KW DA-1 to a 1KW non-DA system.  This was done because two of the three towers in the directional antenna array dated from 1960, were in very rough condition and needed to be replaced.  The remaining tower (furthest from the transmitter building) had been replaced in 1994, is in good condition and is being kept as the non-directional radiator.

Here are a few pictures:

WGHQ 3 tower directional antenna array, Port Ewen, NY

WGHQ 3 tower directional antenna array, Port Ewen, NY

More deferred maintenance

More deferred maintenance

RF and tower light feed disconnected from tower base

RF and tower light feed disconnected from tower base

Second tower base vegetation not as bad, tower disconnected

Second tower base vegetation not as bad, tower disconnected

WGHQ transmitter and original Collins phasing cabinet

WGHQ transmitter and original Collins phasing cabinet

First tower video (sorry, I appear to have no idea what I am doing with the camera):

Second tower video, this one is better:

Towers on the ground:

I made measurements on the third tower and constructed a temporary ATU with parts on hand to get the station back on the air. They are now running 1 KW day, 38 watts night, as per their CP. I will be going back up to finish the job once the brush has been removed from around the existing tower and the ATU building has been repaired.  The coverage with 1 KW is not bad, actually:

Predicted coverage map from FCC website

Predicted coverage map from FCC website

The translator is on its way.

The inglorious task of AM antenna array maintenance

AM radio stations are rough customers. They frequently operate on the margins, both in terms of ratings and revenue. Their transmitter plants are complex and very often have been on a reduced maintenance schedule for years, sometimes decades.  Those of us that understand the operation of AM transmitter plants and all their quirky behaviours are getting older.  I myself, feel less inclined to drop everything and run off to the AM transmitter site when things go awry.  Seldom are such efforts rewarded, much less acknowledged.  Station owners are also finding that their previous demands are unrealistic.  For example, time was that any work that takes the station off the air had to be done after midnight.  These days, I can tell you, I will not be working at your radio station after midnight.  You can find somebody else to do that work.

Thus, today, we took this particular AM station off the air from Noon until 3 pm to diagnose and repair a problem with the four tower daytime array.  Once again, this involved a shift in common point impedance and a drastic change in one tower’s current ratios.

Antenna Tuning Unit, mice have made a mess

Antenna Tuning Unit, mice have made a mess

In all fairness to the current owner, this ATU reflects years of neglect. At some point, mice made a home in here and created a mess. The ATU smells of mouse shit, piss and mothballs.  It is full of mouse droppings, grass seeds and fur.  All of the ATUs in this array are in similar condition.

Paper wasp, inside ATU

Paper wasp, inside ATU

It was warm enough that the wasps were active, if not a little bit lethargic.

Broken stand off insulators in ATU

Broken stand off insulators in ATU

This coil is being held up by the tubing that connects it to other components. When the ATU was built, no nylon or cork bushings were used between the insulators and the wall of the ATU they were mounted on. Heat cycling eventually did all of the insulators in.

Catwalk to the other towers

Catwalk to the other towers

Catwalks to the other towers. At least the swamp grass has been cut this year, it is only four feet tall instead of ten.

Tower base

Tower base

The tower bases are all elevated above the theoretical maximum water level. The ATUs are also up on stands with platforms build for maintenance access.

ATU Work "platform"

ATU Work “platform”

I cannot even blame the current owner, who has spend considerable money to make repairs and upgrades to this site. It is very difficult and very expensive to catch up with deferred maintenance. Sadly, most AM stations we encounter have similar or worse problems.

I think it is too late to save many of these AM stations.  The technical issues, lack of revenue, perceived poor quality, lack of good programming are all taking their toll.  At this point, the hole is so deep there is no hope of ever getting out.  The FCC’s faux interest in “revitalization” followed by two years of stony indifference seems to be a final, cruel joke.

Installation Check Lists

On the subject of project management; often times, we need to keep track of the small details that can derail a project, blow the budget and upset schedules. A quick check list can help to identify things that might not have been planned for. I developed a checklist mentality in the military. There, we had checklists for everything. Simple day to day things like disposing of garbage over the side, or pumping the CHT (sewage) tank to complex evolutions like entering or leaving port all had a checklist.  On the aforementioned CHT tank; the Coast Guard cutter I was on had a vacuum flush system to conserve water.  Emptying the CHT tank involved a complex set of valve openings and closings to rout compressed air into the vacuum tank and literally blow the sewage overboard.  Anyone can see the danger in such a design.  Failure to follow the exact procedure resulted in raw sewage blowing out of the nearest toilets, which were unfortunately (or perhaps humorously) in the lower level officer’s staterooms.

But I digress.

I have made a series of outlines for different project types.  These can be used as general guidelines for project planning and management.  Of course, each project is different, but these are flexible enough that they can be adapted on a project by project basis.

These are for general use, and should be adapted for your own purposes.  Don’t forget to document and label all the wire runs, etc.

Also, do not forget the transmitter site maintenance checklists: FM transmitter site maintenance list, AM transmitter site maintenance list. I have used these reliably at many different sites since I committed them to writing in late 1999.

Copper Thieves

If you are the type of person that drives around to transmitter sites and steals things; fuck you. You have no idea the problems you are causing to get a few extra dollars worth of scrap copper.

Missing copper ground buss bar

Missing copper ground buss bar

I have a feeling that most of these copper thefts can be attributed to out of town tower contractors removing old cellular equipment from towers.  Notice, only the buss bar and copper ground wire is missing.  They did not try to cut the transmission lines.  In other words, they seemed to know what they were doing.  I have noticed around here that a when a particular contractor, employed by an unnamed large company that rhymes with glint, would work at a site, things would be missing afterwards.

Perhaps it is just a coincidence. I have never been able to catch anyone pinching things. However, if this is you, and I catch you, you can rest assured that I will block you in with my car, then walk down the road and call the police.

Cleaning up

We removed this old Harris BC5HA transmitter recently:

Harris BC5HA, WROW Albany, NY

Harris BC5HA, WROW Albany, NY

It was installed new in 1974, when the station moved to this site from another one a few miles up the road. It functioned as a main transmitter until the BE AM5E was installed in late 2001. The BE transmitter, other than a power supply issue, has been a solid, reliable unit. Truth be told, the last time the BC5HA ran was in 2006. After that, the unit refused to run, a bad modulation transformer was suspected. It was deemed not worth it to repair, thus, out the door it goes. We ended up giving it to a local contractor who scrapped the metal in lieu of payment for his labor.  The only thing he could not take was the aforementioned modulation transformer, which is full of PCB’s.  That will have to be hauled away by a licensed disposal company.

Broadcast Electronics AM5E, WROW Albany, NY

Broadcast Electronics AM5E, WROW Albany, NY

We may be getting a second hand Nautel transmitter from another station as a backup transmitter.  If that comes to fruition, then a couple of racks can be added to the end of the Phasor/transmitter/transmitter row and the wiring for the remote control and STL can be simplified and neatened up.

GE BT-25 Mystery transmitter site

Update: Thank you, Jim.  The mystery site is KFIG (formerly KFRE) in Fresno, CA.  This is what the transmitter building looks like from the outside today:

KFIG transmitter, circa 2011

KFIG transmitter, circa 2011

In the previously mentioned the rescued file cabinet and along with the WFLY transmitter site construction information, I also found these interesting photographs:

Mystery AM transmitter site

Mystery AM transmitter site, plate 9296

I do not know what transmitter site this is. From the photo, it has a two tower (guyed) directional antenna. It looks to me to be somewhere out west. The transmitter is a General Electric BTA-25 or BT-25, same as the former WPTR and WCKY transmitter. I know this back hallway well:

Mystery AM transmitter site, back hallway of GE BT-25.  Plate 9300

Mystery AM transmitter site, back hallway of GE BT-25. Plate 9300

This is the transformer vault:

Mystery AM transmitter site, transformer vault.  Plate 9297

Mystery AM transmitter site, transformer vault. Plate 9297

Modulation transformer and three pot plate transformer:

BT-25 modulation transformer.  Plate 9299

Mystery AM transmitter site, GE BT-25 modulation transformer. Plate 9299

I would say that these may be promotional photos, because of the spotlessly clean installation and the plate numbers on each print.  Unfortunately, there are no pictures of the front of the transmitter, including the operator console.

Does anybody know where this is?

Bucket Trucks and Landing Craft

Hopefully that title is descriptive enough:

ATT bucket trucks, , mobilized via landing craft to Pleasure Beach

ATT bucket trucks, , mobilized via landing craft to Pleasure Beach

We loaded a couple of ATT bucket trucks on a landing craft and waged an assault on Pleasure Beach.  This is to finalize the repair work from Hurricane Sandy last year.  The other factor is the construction taking place on the Island.  The City of Bridgeport is constructing a park, which involves extensive repairs and renovations to the buildings.  Construction vehicles driving under the old lines have ripped them down several times, thus repairing the lines on the new utility poles was necessary.

ATT truck offloading

ATT truck offloading

ATT truck offloading

ATT truck offloading

ATT is the LEC for the Bridgeport area, something they don’t do in most other parts of the country, from what I am told.

Landing Craft Challenger

Landing Craft Challenger

It took approximately four hours to complete this work and reload the trucks back on the landing craft.  The boat itself looks like a slightly modified LCM (Landing Craft, Mechanized), which were produced from 1943 onward.  This is an LCM-8.

WICC towers

WICC towers

WICC towers almost in line, I was about one second too late with this shot. This would be “down the bore” of the daytime pattern into downtown Bridgeport.

WICC towers

WICC towers

Another shot of the WICC towers. These were designed to hold up a horizontal T top wire antenna strung between the two of them.  At some point in the early thirties, somebody realized that the tower itself could be excited as a vertical radiator and the antenna configuration was changed. Up until the mid 1970’s there was a horizontal wire which supported third wire element hanging between the two towers, making it a three tower directional array.  This was removed and it was then that the current phasor and two tower DA-2 system was installed.

All in a day’s work.

WROW

WROW 590 KHz, Albany, NY is another one of those successful AM stations.  They have a music format, which I would characterize as a blend between nostalgia and oldies.  They do well in the Albany book and most importantly; make money.  The Wikipedia entry is a little dated, as they have had a music format for over four years now.  The low dial position helps, as I can get the station up near the Canadian border and most of the way down to Poughkeepsie during the daytime pattern operation, which is better than across town WGY, 50,000 watts non-directional.

Here are a few pictures of the transmitter site:

Broadcast Electronics AM5E, WROW, Albany, NY

Broadcast Electronics AM5E, WROW, Albany, NY

WROW main transmitter

WROW transmitter room

WROW transmitter room

WROW transmitter room; main and backup transmitters, phasor, equipment rack, etc.

RCA BTA1AR former backup transmitter

RCA BTA1AR former backup transmitter

The former backup transmitter for WROW-AM. This was moved from the original transmitter site, located a few mile north of the current transmitter site in Glenmont, NY. The current transmitter site was constructed in 1974.

Onan diesel generator

Onan diesel generator

Backup power

WROW antenna array, three tower DA-2

WROW antenna array, three tower DA-2

WROW antenna array. The station transmits with 5,000 watts daytime and 760 watts night time. The towers are slightly tall at 105 electrical degrees. It is hard to do tall towers at the low end of the dial because the towers get very tall. These are 149 meters (488 feet) tall.

It is kind of tragic, in a way

Working on another AM directional station (WGDJ) which was damaged by lightning recently. In this case, the antenna array controller ceased working and one of the towers in the daytime pattern was out tolerance. Before we stared working, I told the owner to have all the vegetation cut down around the towers. This is what we ended up with:

WGDJ catwalk, East Greenbush, NY

WGDJ catwalk, East Greenbush, NY

I can’t really fault them for this, but it does make work more difficult.  That strip of tall green grass; that is the catwalk. The grass itself is called Phragmites, which is tall, tough, reedy stuff that can scratch and cut person unaware. The array is in a low swampy area next to the Hudson River in East Greenbush, NY. Stepping off of the catwalk, one can sometimes find solid ground, or find ones feet six inches under water.

WGDJ tower one ATU cleanout

WGDJ tower one ATU clean out

This is Mike cleaning out the mice and bees nests out of the tower #1 ATU. Notice the can of bee spray in his back pocket.  This was after he was stung in the forehead.

Mouse nest, WGDJ daytime ATU coil

Mouse nest, WGDJ tower #5 daytime ATU coil

This mouse nest, at the attendant dead body in it, was responsible for a -10 degree phase shift in the daytime pattern for that tower. I hate cleaning this stuff out, it is a dirty, nasty job but necessary nonetheless. While doing this work, I wore gloves and a dust mask. The entrance hole where the AC power and control cables come into the bottom of the ATU was plugged up with some steel wool.  There is still a bad capacitor in this ATU for the daytime array, that damage was likely caused by lightning.

At the end of the day, we repaired the antenna array/phasor controller; bad AC transformer and rectifier bridge and several bad logic steering diodes for tower 4 and 5, cleaned out all the vermin nests and isolated the remaining problem with the daytime antenna system.  Parts should be in next week to finalize repairs.

All in all, not a bad day’s worth of work.

Axiom


A pessimist sees the glass as half empty. An optimist sees the glass as half full. The engineer sees the glass as twice the size it needs to be.

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~1st amendment to the United States Constitution

Any society that would give up a little liberty to gain a little security will deserve neither and lose both.
~Benjamin Franklin

The individual has always had to struggle to keep from being overwhelmed by the tribe. To be your own man is hard business. If you try it, you will be lonely often, and sometimes frightened. But no price is too high to pay for the privilege of owning yourself.
~Rudyard Kipling

Everyone has the right to freedom of opinion and expression; this right includes the freedom to hold opinions without interference and to seek, receive and impart information and ideas through any media and regardless of frontiers
~Universal Declaration Of Human Rights, Article 19

...radio was discovered, and not invented, and that these frequencies and principles were always in existence long before man was aware of them. Therefore, no one owns them. They are there as free as sunlight, which is a higher frequency form of the same energy.
~Alan Weiner

Free counters!