Ian White GM3SEK wrote:

For a more broadband solution based on coiled coax, I'd agree with
Cecil's suggestion of cascading a large coil for the lower bands with a
smaller coil optimized for the higher bands. It would also be possible
to cascade a large coil with a small ferrite choke.


Ian and Cecil,

I think you both should disclose any ties you might have to the cable
industry. (hi)

I haven't been able to find out the proper placement of ferrite beads.
Are they placed ANYWHERE along the cable. Do you calculate the
transmission length for voltage peaks?

Thanks,

John
AB8O

jawod wrote:
I haven't been able to find out the proper placement of ferrite beads.
Are they placed ANYWHERE along the cable.

Ideally, they should be placed at a common-mode standing-
wave current antinode (maximum). Aren't you glad you asked?

Most hams install the choke at the most convenient
place. However, at the feedpoint of a one wavelength
dipole is not a good place.

A common place is at a BALanced to UNbalanced junction
where the choke can perform the balun function.
--
73, Cecil http://www.w5dxp.com

jawod wrote:
Ian White GM3SEK wrote:
For a more broadband solution based on coiled coax, I'd agree with
Cecil's suggestion of cascading a large coil for the lower bands with
a smaller coil optimized for the higher bands. It would also be
possible to cascade a large coil with a small ferrite choke.


Ian and Cecil,

I think you both should disclose any ties you might have to the cable
industry. (hi)


Somewhere in there is a very bad joke about "cable ties"...

Cable is much less expensive over here than large ferrite beads imported
from the USA; hence my interest in the coiled cable chokes.

I haven't been able to find out the proper placement of ferrite beads.
Are they placed ANYWHERE along the cable. Do you calculate the
transmission length for voltage peaks?


The place to start is where the common-mode current would be launched
onto the coax. In an antenna fed with coax all the way, that would be
the feedpoint. In an antenna fed partly with coax and then with parallel
line, it would be at the transition point.

But if the system is physically asymmetrical (eg if the feedline runs
back horizontally below the antenna) a single choke may not be enough to
solve the problem. In that case, the next place to think about would be
a quarter-wave closer to the transmitter.

....Doorbell...

Someone stopped by for a long Sunday afternoon chat, and in the meantime
Cecil has provided the rest of the answer.


--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

On Sun, 22 Oct 2006 18:01:24 +0100, Ian White GM3SEK
wrote:


Cable is much less expensive over here than large ferrite beads imported
from the USA; hence my interest in the coiled cable chokes.

The cost of cores from imported from the US in Australia is also very
high.

For those in Australia interested in W2DU style baluns, I roughly
measured the impedance of an inexpensive core from Jaycar using a
Mighty Fine Junk 259B and plotted the results at
http://www.vk1od.net/balun/index.htm . Less than A$20 is probably
sufficient for an adequate balun for 80m to 10m.... depending...

My own view is that the number of cores required for a W2DU style
balun is unrelated to the characteristic impedance (inner to outer
conductor) of the coax as is commonly held, but that the number of
cores / effectiveness varies with location on the feedline and is
highly dependent on the scenario (frequency, topology etc) and the
optimum solution may required more than a single choke.

Ian, perhaps low cost suppression cores are also available directly in
your country, candidates for measurement and reporting!

Owen

PS Interesting plots, better presentation isn't it, the Y axis label
on your phase graph needs a fix.
--