wrote:
Why worry about VHF though?

Because MFJ advertises 2m operation with the
recommended coil of coax?
--
73, Cecil http://www.w5dxp.com

Chris W wrote:
I have been looking at the MFJ-1622 antenna. Part of the instructions
for antenna set up, which you can find here....
http://www.mfjenterprises.com/man/pdf/MFJ-1622.pdf
say to roll up about 30 feet of coax into a 12" diameter coil. This
they label as a choke. This just seems like a "duct tape" solution to
me. Is there a better way to accomplish the same thing? Would a few
ferrite beads on the coax do the trick or do I just have no idea what I
am talking about?

I have an EXCEL calculator which indicates that particular
coiled coax choke should have maximum effectiveness around
~8 MHz. So it probably does a reasonable job on 40m-20m.
Unfortunately, it will reach 1/2WL self resonance somewhere
around ~16 MHz and thus be essentially useless 17m-2m because
of self-capacitance shorting out the choking impedance. Anyone
who says a particular coiled coax choke will cover 40m-2m is
mistaken. However, the stress on the choke is reduced because
the MFJ antenna appears to be resonant, i.e. a low feedpoint
impedance.

Here are some actual measurements.

http://www.k1ttt.net/technote/airbalun.html

In a nutshell, the coiled coax choke works well over a
frequency range of 2:1 (conservative) or 3:1 (liberal).
The beads in the above measurements work well over a
frequency range of 4:1. It's hard to beat stacked ferrite
toroids of differing materials as a foundation for an
excellent all-HF-band 1:1 choke-balun. I know of no
single balun that can cover 7 MHz to 148 MHz well.
1/2WL self-resonance defeats the choking action when
trying to cover 4+ octaves.

I recently used an MFJ-259B to measure the choking
impedance of 8 turns of RG-400 on a 5.35" form. The
choking impedance dropped below 650 ohms at 4.63 MHz
on the low end and 28.1 MHz on the high end. 650 ohms
is not enough choking impedance for all-HF-band ladder-
line fed dipoles. Assuming SWR ranging up to 18:1, which
is the case for my all-HF-band 130 ft. dipole, the choke-
balun may encounter as much as 8000 ohms looking into
the Z0=450 ohm ladder-line.
--
73, Cecil http://www.w5dxp.com

It's hard to beat stacked ferrite
toroids of differing materials as a foundation for an
excellent all-HF-band 1:1 choke-balun. I know of no
single balun that can cover 7 MHz to 148 MHz well.
1/2WL self-resonance defeats the choking action when
trying to cover 4+ octaves.


Can you direct me to more good information (as in NOT sales literature)
about ferrites and toroids as choke baluns?

The ARRL Handbook mentions the use of 1:1 baluns on p. 21.16
(Transmission Lines):
For 3.5 to 30 MHz, the recommended choke is 10 ft and 7 turns using
RG-8, 58, 59,8X and 213 (I don't think I've even heard of the last two)

W2DU has a 50 bead balun over RG-303.

As you may recall, I am using a homemade G5RV and I simply connected
RG-8 directly to the ladderline portion. I can see this connection
right out my window (2nd floor shack). I am unaware of any significant
problems as it is. Should I consider using one of these methods and
re-do my antenna?

Thanks for being one of the experts that doesn't talk down to the "rest
of us".

John
AB8O

jawod wrote:
As you may recall, I am using a homemade G5RV and I simply connected
RG-8 directly to the ladderline portion. I can see this connection
right out my window (2nd floor shack). I am unaware of any significant
problems as it is. Should I consider using one of these methods and
re-do my antenna?

Are you having problems with RF-in-the-shack? If not,
your feedline radiation may be doing you more good than
harm. I'm a purist. I install a very good balun/choke at
every BALanced to UNbalanced junction. Some people are
good and some people are lucky. If you have a current
maximum common-mode loop in the shack, you may not even
notice it. It's those voltage maximums that tend to bite.

Thanks for being one of the experts that doesn't talk down to the "rest
of us".

Heh-heh, I only talk down to the gurus for the purpose of
creating a crossed transactional analysis. :-)
--
73, Cecil http://www.w5dxp.com

jawod wrote:
Can you direct me to more good information (as in NOT sales literature)
about ferrites and toroids as choke baluns?



Here are some actual measurements of coax and ferrite chokes. Tests
done with a Hewlett-Packard 4193A vector impedance meter.

http://www.k1ttt.net/technote/airbalun.html