> >Has anybody made a successful RG213 choke balun @ 1.83
I looked at this one time for a manufacturer.
The most efficient way, cost wise, to make a balun for low
frequencies is to use an air core coil of coax.
If you use an air-core solenoid of coax, impedance approximately
squares with the number of turns. Twice the material is four times
the impedance.
If you use beads, the increase is equal to length change. Twice the
material length is twice the impedance.
45 feet of RG8X coax on a 4 inch diameter non-metallic form has
500 ohms choking impedance on 160 meters, about the same as
20 linear inches of 77 material beads along the coax.
That means you need at least 20 linear inches of beads to make
what would be considered a marginal balun at 1.8 MHz.
40 linear inches of 77 beads would be better, as would 70 foot of
coax wound on a 4 inch form.
Another difference is at 2 MHz the 77 material behaves like an 85
ohm resistor is in parallel with every 30 ohms of inductor, so power
dissipation can be quite high in many cases!! If common mode
voltage is high, you might never be able to use high ui beads
without overheating problems!
The air-wound coax acts like a nearly pure inductance, and doesn't
dissipate any noticeable power while choking off RF. You have the
small additional loss of 45 feet of coax, that's all!
Air-core choke baluns are cheap, handle large amounts of common
mode voltage, easy to build, and relatively immune to damage.
Ferrite sleeve baluns are smaller diameter and easy to build and
look very "official".
73, Tom W8JI
w8ji@contesting.com
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