[Elecraft] Toroids frequency range and baluns

[Ron D'Eau Claire]

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?

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The balun doesn't have any effect on the efficiency of the antenna unless it
involves lots of coax at high SWR such as the big coil of coax some Hams use
for a "choke balun". (They're FB as long as the SWR is low but, like any
coax, the losses go up with the SWR.) 

The only function of the balun in your setup is to manage the flow of RF
current to keep it off of the *outside* of the coax and your rig. That's
only a concern if the voltages are sufficient to cause your rig to be "hot"
with RF so that touching the rig changes the antenna tuning or you get RF
feedback in to the rig through the mic or you have other operational issues.


As long as you don't have those issues, you don't need a balun at all.

To more directly answer your question, the balun you have probably works as
well at 30 MHz as it does at 1.8 MHz. The T200-2 cores will show a high
impedance to the RF currents across the HF range. That's all you need. 
 

Ron AC7AC