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