We are extending LANs out to transmitter sites for many reasons; backup audio, control and monitoring, security systems, VOIP phones, etc.
I am casually (very casually) toying around with creating my own Linux based remote control system. The ongoing Windows 10 upgrade debacle continues to not end, I can’t help but think that there are many potential clients who could use a reliable transmitter/studio remote control and monitoring system based on a stable operating system. Hmm, sounds like a sales pitch 😉
Anyway, I have run across several Ethernet board manufactures that offer a variety of boards with 8-12 contact closures and a variety of analog and digital inputs. Most new transmitters have some sort of web GUI which are great for transmitter control and monitoring. As we all know, there is more than just a transmitter at any given transmitter site. In addition to the transmitter, I would like to control and monitor things like tower lights, interface and control of coax switches, temperature monitoring, generator status, the old non-web interface backup transmitters, STL signal strength for those old 950 MHz links, etc.
That particular PC board is made in Bulgaria, which is home to this: Mount Buzludzha
What I like about these particular boards is the DRM software (DRM has, apparently, many different meanings) which will run on Linux or Windows. There are also iOS and Andriod applications that can be used as well. It appears that the GUI can be customized for various uses. This seems like it is written in Java, so perhaps I could have some Java expert customize it for radio use. It looks like up to 32 boards can be controlled by a single instance of the DRM software. Alarm reporting would be via SNMP trap and email.
I don’t know, there is one particular cluster of stations that needs new remote control gear at almost every transmitter site. Perhaps a little alpha testing is in order? It could be fun…
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.
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 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.
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.
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:
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.
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)
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.
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:
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:
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:
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.
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:
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:
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.
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.
It was warm enough that the wasps were active, if not a little bit lethargic.
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.
Catwalks to the other towers. At least the swamp grass has been cut this year, it is only four feet tall instead of ten.
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.
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.