>
> Your thoughts? Please restrict comments to the ferrite
choke topic.
> Tnx.
Jim,
A few comments.
To be most effective the ferrite needs to be placed at a
point where common mode impedance is low. It also has to be
placed at a point where the change it makes reduces current
in the feedline.
You should consider the antenna the source and then look
along the line at distance to estimate effects of grounds
and chokes. Consider the case of a 1/4 wl of feedline
hanging from a dipole. Say you have the dipole 30-35 feet
high on 40 meters.
If you simply grounded the feedline at the earth about 35
feet from the antenna, the dipole end of that line would
have a very high impedance. Very little current would flow
over the shield. If you added beads just before the ground
in this example the beads would cause an INCREASE in the
unbalance at the antenna. They would cause feedline common
mode current between the ground and the antenna to increase.
If the feedline was 1/2 wl long to the ground, the opposite
would occur. Connecting the feedline shield to ground would
increase unbalance of the antenna and increase feedline
radiation. Adding beads would reduce unbalance of the
antenna and reduce feedline radiation.
Beads strung over shields are popular, but they are also a
very ineffective way to use material to make a balun. You
double cost every time you double impedance. If you used a
multiple turn winding through a toroid you would very
roughly quadruple impedance every time you doubled the
turns.
The 73 Material beads roughly have about 50-100 ohms
impedance per linear inch of material for a single pass
through the bead on 2MHz. That means 10 inches of bead
length along the cable has 500 to 1000 ohms, with R and X
being about equal at 2 MHz. As frequency increases impedance
stays approximately the same with reactance decreasing.
That's because effective permeability decreases with
increasing frequency, and losses increase with increasing
frequency. I'm not sure 73 material is the best choice for
the entire HF spectrum in your application regardless of how
popular it is, especially if you run high power with an
antenna that produces substantial common mode current.
I was working on an antenna design that had an offset feed
and I could never build a string of beads balun that worked
regardless of how much money I threw at the problem. The 73
material beads would overheat and crack or damage the jacket
of Teflon coax at several hundred watts even when the string
was two or three feet long!
The entire balun issue is actually pretty complex since it
is a complex circuit with many variables. It is so
complicated every book or article I've seen overlooks
important points. Sevik tested a dipole with the feedline
about the optimum length to NOT require a balun, and then he
concluded a balun wasn't necessary in any dipole
application!
Maxwell suggested a string of beads is almost always
adequate, yet they often are not.
Many articles assume the choke impedance needs to be several
times the line impedance and that very often is not true.
When we add the facts that your antenna has significant
common mode excitation of the feedline and you want to use
the antenna on multiple bands at high power, it really could
be a big can of worms. Luck or the Edisonian method might
serve you better than careful planning.
73 Tom
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|