February's Transmitter hunt is on the 13th and starts at the Southern starting point at 2pm, staging & mileages will be taken at 1:30pm. Coordination or assistance is on the H2 repeater 449.925 (-) pl 131.8Hz . Robert N7FLR will be hiding the transmitter.
December 27, 2020
“Visiting a 50,000 watt Broadcast Transmitter Site”
Ever seen little buildings out in the middle of nowhere or along the highway in a open field, with nothing close by except a few tall red & white radio towers?
#1 1140 transmitter site By 15 and Nellis AFB
#2 840 tx site by 15 and Great Basin Highway North of Apex
#3 10 Ghz Program and control link from the studio
#4 Generator Back up power
480 volt, three phase, 100Kw Generator
#5 100Kw UPS Battery Back-up supply
#6 55Amp AGM Batteries
There is (40) 12Volt 55Amp AGM batteries, producing 480Vdc for the inverter. These batteries cost about $400.00 each. $400 X 40 = $16,000. The charger produces 480 Volts at 55Amps float current and the batteries are charged and analyzed continuously.
#7 Andersons connectors
The 400 Amp Anderson connector with the 15/30/45 Amp version hams use for their electrical connections in comparison.
4 inch copper strap connects all the equipment to each other for safety and to keep the RF noise out of the audio cables. This connects to the station’s RF grounding system outside which you will see later.
50,000 watt solid state AM transmitter, each 2-door cabinet is about 5 feet across the transmitter is close to 15 feet long.
#10A Status panel
#10B Circuitry for panel
Control signals from switches, sensors, remote control complete with 3 “AA” batteries as battery backup to the control system.
#11 Amplifier TUNE & LOAD controls
Only adjustments for the output stage…..TUNE & LOAD. The windows have a 0000-9999 digital counter for precise adjustments for logging and maintenance.
#12A Built -in 100Kw wattmeter.
We are currently showing 25,000 watts which was the nighttime RF level.
#13A Amplifier boards
One Mosfet transistor produces 45.365 watts, each amp board has 8 Mosfets. Each module produces 365 watt from the 8 Mosfets.
There 137 modules, these are combined to create the RF output signal.
All the modules are sent a binary word that is modulated at the carrier frequency of 840 Khz which creates the 840 Khz AM signal to the antenna. COMBINED RF GENERATED IS: 365 X 137 = 50,005 WATTS TO ANTENNA SWITCH
The DC power supply that operates these RF amps is 200Vdc @ 550 Amps!
#14 Amplifier boards output
The RF output is on the 1″ wide by 1/2″ thick solid copper bar, this feeds a flexible pair of 1″ wide copper straps to the first inductor below in the tank circuit.
The far end of this coil goes straight up to the wattmeter sensing circuitry and the rest of the tank circuitry.
#16 Wattmeter Sensors board
The wattmeter board has multiple functions. Measures forward RF energy, measures reflected RF energy and actual RF level to the amplifier tank circuitry.
#17 Tank circuit
The black capacitors are .001mfd @ 30,000 PWV and 16 Amps RMS at 1000 Khz (1 Megahertz).
The inside of this cabinet will kill you easily when it’s hot.
#19 Antenna switches/ Phasers
The 4 inch copper pipe at the top of the cabinet is the feedline from the transmitter.
#20 Phasers feed point (upper)
Notice the copper pipe feedlines going into the next section on the right.
#21 Phaser insides (lower)
#22 Audio rack
The program rack has all the program audio that comes to the site via Telco T-1 phone lines as well as the 10 Ghz microwave back up.
#23 1990’s typical interconnect wiring for program audio & control.
Some data on the feedlines and antennas…
ANDREWS HJ-11-50 4 Inch Standard Air-dielectric heliax cable
> WORKING FREQUENCY — 1-1000 MHz
> RF POWER CAPABILITIES — 1.1 Mega Watts
> VELOCITY FACTOR — 92%
> IMPEDENCE — 50 OHM
> ATTENUATION PER 100 FT — .14 db AT 146 MHz
> ATTENUATION PER 100 FT — .268 DB AT 449 MHz
> DC RESISTANCE — .11 OHM PER 1000 FT CENTER
> DC RESISTANCE — .04 OHM PER 1000 FT OUTER
> MINIMUM BEND RADIUS — 40”
A 100 ft roll of 4″ heliax being delivered
This is a Right-angle 4″ heliax connector, It connects the heliax to teh transmitter or to another piece of heliax. This 4″ coupler is machined out of solid copper and brass and has a weight of 44 pounds and the cost was $3,400 each. The “Bullet” (with the red cap) is what goes into the next connection’s center conductor.
Anyone know why the “Wires” of this feedline is not made out of solid copper pipes?
Most AM transmitting towers are a ¼ wave verticals at the operating frequency, in this case the frequency is 840 Khz. Each one of these towers is 278 feet, 6.9 inches tall, and separated from one another by almost 300 ft, end to end distance is about 1,200 ft. These antennas are normally installed above a water table for better conductivity. For obvious safety reasons radio tower antennas are fenced to keep out wandering humans, and some turtles.
Most towers have 8 Ga. ground radials extend out over 290 feet from each tower base. These radials are mounted to the grounding base plates at 2 degree increments. The total radials (180) for each tower totals almost 9.9 miles of wire. When radials from one tower crosses over radials from another tower they are bonded together where they cross, creating an extensive buried mesh or grid type grounding system.
Each tower’s radial grounding plate has a 4” copper strap that runs to a central connecting point for all towers.
The 4” grounding strap from inside the building is connected to this rf ground system at that point, all equipment, antennas and feedlines are all at the same ground potential.
#27 Tower lighting and lightening protection
AUSTIN RINGS are a pair of toroid coils that create a toroid transformer, that transfer the 220vac lighting supply voltage to the other Austin Ring that connects to the tower side marker lights and the top beacon. The tower is HOT so there can’t be a direct connection.
The large balls are the Spark Gap to drain lightening strikes to ground.
#28 Tower Insulator
The full weight of the tower, plus the weight of the guy wires pulling down on the antenna itself is being supported by the single brown ceramic insulator.
( And NO, I don’t know how many pounds per square inch that is)
Now you know, what’s up with those little buildings close to antenna towers out in the dirt fields and along the highways.
Thanks to Tony WB6MIE for assistance in this presentation.
Tonight’s audio stream is provided by Dave W3QQQ:
Next time we are going to visit a 40,000 watt combined four 10Kw transmitters into one antenna site that was installed in 2017-2020.