AM tower – Page 2 – Engineering Radio

The isocoupler and the SX2.5

The second post in the series, “things to do with a truck body tool box.”

We have this client who, several years ago, moved their translator to their AM tower. All is well for a few months, then the much beloved Harris SX2.5 transmitter begins burping. The SX2.5 transmitter was of an age when, apparently, VSWR fold-back circuits were just a gleam in Hilmer Swanson’s eye. The correct description of the sound made over the air during this event would be “motor boating,” because that is what it sounds like. Obviously, very undesirable.

Thus, the isocoupler was removed from the tower, dried out, waterproofed, and replaced. That lasted about six months.

Once again, the isocoupler was removed from the tower, a capacitor was remounted, drain holes and a small vent were added to the top of the unit and it was replaced. That lasted about a year.

I am getting a little tired of this and so is the client. Time to rethink the entire setup.

We had several leftover parts from various AM decommissioning over the last few years which included these nifty sample loop isolation coils:

AM antenna system sample loop isolation coil

Why not repurpose one of these to make an isocoupler for the translator?

Enter; the truck body toolbox. This one is slightly smaller than the last one, measuring 23.5 x 18 x 16 inches (60 x 45 x 40.5 cm).

The isolation coil consists of 35 turns of 3/8 coax on an 11.5 inch diameter form. The coil length is 15 inches. I calculate the length of the coax on the coil to be out to be right around 100 feet using the π x D x (turns) formula. I measured the inductance with my analyzer, which came out to 200 μH. Not to shabby.

Checking length of cable with TDR — Checking the length of cable with TDR

The coax is Cablewave FCC38-50J which has a velocity factor of .81 and the TDR shows it to be 100 feet also.

Coil impedence and reactance — Simple coil impedance and reactance

At 860 KHz, the isolation coil presents 1,200 impedance. I don’t think that will be good enough for that cranky old SX2.5. I decided to make a parallel LC circuit (AKA a tank circuit) to bring up the impedance.

Tank circuit formula:

Where:

F_R = Resonance frequency in Hertz
L = Inductance in Henrys
C = Capacitance in Farads

Given that I have two leftover capacitors, one is a .001 μF and the other is a .0012 μF, those values determine where the coil needs to be tapped. I also wanted to have a good bit of coil in the circuit on the tower side before the capacitor tap to dampen any lightning strikes on the tower. Thus the inductance needs to be about 28 μH.

Using Wheeler’s coil inductance formula:

L= (d² x n²)/(18d+40l)

where:

L = inductance in micro Henrys
d = coil diameter in inches
l = is the coil length in inches
n = is the number of turns

I removed a small portion of the outer jacket on the coil at approximately the 28 μH point (12 turns) then installed a .0012 μF capacitor. I used a small variable capacitor to tune for resonance on the carrier frequency. With this setup, at 860 KHz, there is >47,500 impedance. That goes down to about 16,000 ohms +/- 10 KHz.

That should make things better.

Then I mounted the coil and capacitor in the truck body toolbox. There is a fair amount of stray capacitance from the box itself, which raised the resonant frequency by 5 KHz.

Device Under Test, initial testing of isocoil after fabrication — Device Under Test; initial testing of isocoil after fabrication

Resonance is slightly above the carrier frequency with the permanent fixed .0012 μF capacitor. I think this will change once the unit is connected to the station ground plane. The network analyzer indicated there is too much capacitance in the circuit. Unfortunately, this may be as good as it gets, however, the analyzer shows the impedances are still pretty high:

Frequency (KHz)	Impedance (Ohms)	Deviation from Carrier (KHz)
850	9,950	– 10
855	14,720	– 5
860	28,590	0
865	59,580	+ 5
870	24,780	+ 10

The base impedance of this tower is 34 ohms on the carrier frequency, so the isocoupler should be invisible to the transmitter across the 20 KHz occupied bandwidth of the station.

The FCC38-50J cable has a loss of 1.04 dB per 100 feet at 100 MHz, which is the figure I will use to calculate the insertion loss on the FM translator antenna system.

The old isocoupler is made with RG-214, but likely a somewhat shorter length. RG-214 cable has a loss of 1.9 dB per 100 feet at 100 MHz.

Installation:

Before and after measurements with the network analyzer show a very slight change in the reactance at the tower base. Nothing major and easy enough to tune out with the series output inductor of the ATU.

If I were to do this again, I would simply tap the coil at ten turns from the bottom, measure the inductance and install the proper value capacitor. Since this had to be constructed with the parts on hand, less the truck body toolbox, it because a bit cumbersome to get close to the resonant frequency.

All this got me thinking; there are other possible uses for such a design. Crossing a base insulator with an Ethernet cable always presents some unique problems. I know the WISP forum that I read, they are always talking about how difficult it is to mount an antenna on an AM tower. What if… armored Cat5e or Cat6 cable was used with waterproof RJ-45 jacks? Something like that could carry Ethernet data and DC voltage past the base insulator to a three or four around sectorized access point and an edge switch or router mounted on the tower.

Armoured category cable specifications — Armored category cable specifications

just thinking…

Anyway, it would not be hard to make coils and install capacitors for the right frequency

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This information is from an occasional reader who wished to remain anonymous.

Another AM station surrenders its license, this time from north of the border. CKSL, London, Ontario, Canada is gone for good. The current owner, Bell Media, has determined that it would cost more to repair the deficiencies with the antenna system than economically feasible, especially considering its low ratings. Here is their filing with the CRTC:

Bell Media is the licensee of CKSL-AM 1410, assuming stewardship of the station in 2013 as part of the Astral Media acquisition.

A technical review of the transmitter site was recently completed both by Bell Media and contractors, which has resulted in the determination that the AM array poses an unacceptable risk from a health and safety perspective. The five towers are experiencing serious structural degradation and also require repairs to the aviation safety lighting system. In addition, the building which houses the transmitter has shifted off its foundation (as have several of the individual tower sheds).

Given these problems, Bell Media would need to make a significant financial investment to bring CKSL-AM’s transmitter up to compliance with Human Resources Development Canada, Industry Canada and NavCanada operational codes and standards, all of which is estimated to exceed $3 million dollars.

From a market perspective, CKSL-AM has consistently ranked last out of all ten commercial stations in the London market, both in audience share and revenue generation, over the last several years. In fact, since 2013 the London market has seen radio revenues drop 4% and CKSL-AM generates the least amount of revenue of the stations in the market. Even with a significant investment in programming, this trend is unlikely to be reversed.

In light of the significant capital costs coupled with the absence of revenue and audience share, Bell Media is respectfully requesting the revocation of the CKSL licence.

Well, 24/7 comedy will do that to you. Somebody in the business said to me recently “The listeners are abandoning radio!” No, it is the broadcast station owners who are abandoning their listeners and their cities of license. I have a news flash for all current broadcast station owners; as surprising and radical as this might sound, bland, boring, canned, completely irrelevant, dismal, uninformative, unimaginative, unentertaining, dreary, stale, unenjoyable programming will drive away even the most loyal listeners. People really want to listen to the radio, it is an easy habit and readily accessible. Radios are ubiquitous; they are in our kitchens, bedrooms, cars, hotel rooms, offices, restaurants, barber shops, etc. That, however, may not always be the case, as more and more people move to Spotify, Pandora, or Apple radio when they are tired of the disappointment. I was listening to a certain sports radio format the other day and I kept waiting for something interesting to happen. I waited and waited. I would say to myself; okay, this will be the segment when I will learn something or be entertained. This upcoming guest will say something interesting. Sadly, those expectations were never met and I will never tune into that station again. Elevator music would have been better. Worse than sports radio, 24/7 comedy is the absolute death knell. This is like saying; we are out of ideas and we do not care.

Here are a few pictures of the former CKSL-AM transmitter site:

CKSL_transmitter — CKSL transmitter building

CKSL_transmission — CKSL transmission line bridge

Actually does not look too bad, at least the field is mowed. I have seen much, much worse. Those bolt-together towers, though. I would bet that they are the real problem, bolts are deteriorating faster than the tower steel. Very likely all the towers need to be replaced and that is why the license is being surrendered.

If you are a radio geek, get out there and take some pictures of your favorite radio station. If the current trends continue, eventually they will all be gone.

The Horns of a Dilemma

Alternate title: Building and ATU in a truck body toolbox.

Alternate title II: I should get paid extra for this shit.

There is an AM radio station that is near death but the owners do not want it to go away. Nor do they want to spend very much money to keep it around, thus the dilemma. At the transmitter site, there is a multitude of problems; leaking roof, very old rusty ATU, rotting support posts and transmission line bridge, equipment racks rusting out, nothing is grounded properly, the building is full of junk, snakes, and mice have moved in. To further complicate things, the tower and transmitter building serves as an STL relay point for two of the market’s FM stations. There is also two translators with antennas on the tower. The ATU and tower light choke box are rusting through, which is causing arcing and broadband RF noise that is interfering with the FM station’s STL receiver. There was a homemade isocoupler for one of the translators that were allowing AM RF back into the building which was creating havoc with everything. Because of this, the AM station is currently silent. In short, it is a mess.

The red box on the bottom is the ATU, the plywood box on the top with the peeling yellow paint is the homemade isocoupler, and the tower light choke box is behind the isocoupler.

Crumbling old ATU output capacitor in series with tower

This was the capacitor that was feeding the antenna, .0041uf, 10KV 8 amps.

We started remediation on this last February, which is not the optimum time for replacing rotting wooden posts. However, we were able to clean out the building. The leaking roof has been repaired. I was able to find a few old racks from a Schafer Automation system to replace the rusted-out original racks. I began the process of grounding the equipment racks, the incoming transmission lines for the STL, etc.

Cool morning, Garter Snakes warming themselves on top of a Moseley DSP-6000

We will have to find out how they are getting in, the plug up those holes.

Then there were the ATU and tower light choke enclosures. Original to the 1952 sign-on, they were past their serviceable days. Since this is all being done on a budget and nobody wants to spend money on an AM station that has little or no listeners and even less revenue, we had a problem.

Then somebody suggested building an ATU in a truck body tool box. Well… This isn’t the Meadowlands, so if there are no other alternatives then okay, I guess. Off to Amazon to order a toolbox. This particular unit seems fine, my only comment is on the gauge aluminum (or aluminium if you prefer), which is slightly thin for holding up all those parts.

ATU built in a truck body tool box — Fabrication shop, ATU built in a truck body toolbox

Still, the box itself is nice enough and certainly better than the old one. I was able to reuse the inductor and the Delta current meter but the old Sangamo capacitors crumbled in my hands when I removed them. I also saved the feed through bowls, J-plugs, and other parts. I used some copper strap to run a good RF ground from the input to the ground connection. Overall, I am pretty pleased with the finished product. It is a little bit tight in there, but this station only runs 1 KW, so it should be fine.

So, new pressure-treated posts were installed, the box was mounted and the transmission line was connected.

Reused Schaffer Automation racks, much better than the 1950's Gates racks — Reused Schafer Automation racks, much better than the 1950’s Gates racks

The reused racks are old but serviceable and a big improvement over the old, rusting-out racks. I was able to bond each rack to the ground strap that was used to connect to the RCA BTA-1 transmitter. There is one more rack to install to the right of these two. That should give us more than enough rack space for this site.

The station is back on at full power and not interfering with the FM STLs or the translators. You can actually touch the rack and not get an RF burn!

We are also working on an air conditioner.

Other work at this site; cleaning out the building, replacing the tower light photocell, installing a ground buss bar, some STL lightning protectors, dressing the transmission lines, etc. It is a work in progress.

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 facility for WGHQ, Kingston, NY.

This is part II of that process.

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.

The directional array had 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.

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

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 crossbar made a good connection point.

RF feed correctly connected to the tower

After soldering, I cleaned up and sprayed some grey primer on it to prevent rust from 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 part of a directional antenna system, the ATU design is pretty straightforward. 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.

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) = X_L / 2πf(MHz)

And

C (μF) = 1 / 2πf(MHz) X_C

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 canceled 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 canceled 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 a 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:

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 a 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

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 the tower (855 feet, 260.6 meter), which works out to be 0.93 wavelength allowing for the 86% velocity factor of the transmission line.

Time to pack up and go home.