[Elecraft] Toroids frequency range and baluns

[Wes Stewart]

Bob W1SRB writes:

>    There have been several e-mail threads recently about ferrite
cores and about baluns and transmission line loss with high SWR. Related
to this is a question that I haven't seen addressed anywhere - toroids
all have a specified frequency range but what happens when they're used
outside of that range?

>   To be more specific, I have a multiband fan dipole fed with
ladder line into a 4:1 current balun that then connects to my rig
through about 5 feet of RG-8. I made the balun using a pair of T200-2
powder iron cores, which have a specified frequency range of 0.25 to 10
MHz. Since I'm using this single antenna from 160m to 10m, I'm way
beyond the specified frequency range of the cores - is the balun likely
to be very inefficient above it 10MHz? The antenna seems to work very
well on 40m and 20m and OK on 15m. Since the band conditions haven't
been particularly good in the last few years that I've been using this
antenna, I can't tell whether my lack of many QSOs on 15m and above is
due to inefficiencies in the balun or to band conditions? What do you
think?


Bob, you are mixing apples and oranges.  Current (choke) baluns (that use cores) are almost always built using ferrite cores, not powered iron.  If you wanted to tune one to parallel resonance at some particular frequency, you might use iron, otherwise, you want ferrite that provides a high impedance to common-mode currents over a broad frequency range.   A resistive component in this impedance is not a bad thing.


Roy, W7EL, has a good description of balun operation here:

http://www.eznec.com/Amateur/Articles/Baluns.pdf

Wes N7WS