Baluns, a VOA +100,000 watt and others
Updated: Nov 24, 2021
This is a 50 to 600 ohm balun removed from a VOA site in CA that I saved. It is a 300 lb pole mount 12:1unit.
I plan on using it, this is a part of Rhombic Antenna history, it needs to be "On the AIR"
Below is a 10KW DXE Balun and the VOA unit, side by side!
Below is an assortment of of baluns used in amateur radio
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
Balun Design, only makes a 9 to 1 Unun, he will not make a 12:1
You could use a exponential taper feed and a1:4 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 in the pictures above. 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 ¼ lambda at your lowest frequency.
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 on now also