Baluns, a VOA +100,000-200,000 watt and others
- skylarkcolo
- Nov 17, 2021
- 4 min read
Updated: Mar 11
This is a 50 to 600 ohm Model 555/543 balun (200,000 watt) removed from a VOA site in CA It is a 300 lb pole mount 12:1unit, which is now at my Kansas antenna farm.
I plan on using it, this is a part of Rhombic Antenna history, it needs to be "On the AIR"
ABOVE: IT USES A LARGE FLANGE TYPE HARDLINE CONNECTION TO HANDLE THE POWER
Below is a 10KW DXE Balun, and the Big 200,000 watt VOA unit, side by side!
Below is an assortment of of baluns used in amateur radio
Below: This data shows general info for the 555 and the 543 models, my Balun must be a special made for the VOA unit a 12:1 like the 543, but 600 ohm
ABOVE: Inside the big 1000,000 watt balun, very clean
Here are some links to the better 12:1s that I have used for the Rhombic and Vee beams,
DXE 12:1 is not available any longer
Also: Collins numbers are: (1) 754-9057-001 and described as "1KW Termination Kit" 2) 774-6261-001 " 3) DAA805-68-C-0020 " "Transformer, Radio Frequency /TF506/TRC-136" 4) 758-5322-001 " "50-600 ohm Balun", 7649858-001 also labeled - 764-9604-001-D with "REV B" one - 764 9058 001 with "REV C"
Balun Design, only makes a 9 to 1 Unun, he will not make a 12:1
Below: Is one of my antenna control and balun boxes, that is over 1000 feet from the shack, feeding two antennas. With provisions to add one more. Using DXE 12:1 baluns, with 10 amp switching relays, and spark gap for static and lightning protection.
I use 12 volt relays, powered by solar and a deep cycle battery
You could use a exponential taper feed as a 1:4 balun, to transform the 50 Ohms unbalanced to 200 Ohms balanced. I have done some research on "tapered lines", the result of which is shown below . After several prototypes, at my K0UO test range I settled on the tapered 2-wire transmission line, which is a modified exponential taper. The exponential feeder is used to match (not tune) the feeding end of the antenna. It creates a broadband linear transformer from 900 ohms to 200 ohms. It's basically a poor man's broad-banded transformer. As implies, there is an exponential feedline at the feed end and also at the termination end. The exponential feedline allows the appropriate impedance across the entire useful frequency range of the rhombic. For the higher band use a 20m long tapered open wire feed line, starting with a spacing of 10 mm and tapering out to 300 mm at the end. The taper is made logarithmic, but a a linear taper would work too. The main thing is that it should at least be 1 wave at your lowest frequency.
ABOVE: OPEN WIRE FEEDERS FROM THE ANTENNA an exponential taper feed 800 ohm unbalanced to 50 Ohms balanced termination.
You can also stack rhombics with an exponential matching section, they are connected between the mid point of the stacked rhombics and the feeder at an angle to the horizontal, with a flying spreader at the mid point in the line. The impedance measured at the 600 ohm feeder input of a four element rhombic array.The output impedance of a stacked pair of rhombics is approximately 300 ohms and this must be matched over the operating frequency range to the 600 ohm feeder. The interlaced rhombic array requires three 300 to 600 ohm matching sections. The wide-band impedance matching properties of exponential lines are well known., but not used by many hams,
You could also do this to terminate the antenna, to a 50 ohm high power dummy load can, since large high-power non-inductive power resistors are so hard to find and cost big $$$$s. Use a 16:1 or 12:1 balun to a 50 ohm power resistor (rated a 33 to 50% of the input power), or you could use an exponential taper feed with a1:4 balun, to transform the 600 to 800 ohm unbalanced to 50 Ohms balanced termination.
Also your antenna will need a Ladder line static bleeder of some type.
DXE has one also, see
below
Above: Using a spark gap for static and lightning protection which also protects your terminating resistors. it is a good safety precaution.
Below: I have added static dispenser discharge (Porcupine) wicks at the end of most of the big wire antennas, by combing the stranded wire cable which I use as my antennas wire.
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