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DL8OL > HF 09.09.03 12:43l 75 Lines 3161 Bytes #999 (0) @ WW
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Subj: The story of a "chicken ladder"
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From: DL8OL @ DB0ABZ.#NDS.DEU.EU (Klaus)
To: HF @ WW
I am a little bit confused about the discussion concerning the "chicken ladder
feeder"
My both antennas are fed with this sort of feeder and I use a single coil
Z-match, even known as a "variable Balun".
The dipole here, to me it is a 41,12 Mtr Transformer wire made with the
feeding point of 8,86 Mtr from one end works fantastic on all bands.
Here the feeder is apprx 8 Meter long, going to two car plugs for lighting
protection through my window frame to the ATU.
Feeder is made even by transformer wire with a space of 5 cm and pse do
not ask me about the impedance. Should be some where around 450 Ohm, I guess!
A little bit more funny it looks to my beam which is installed on the top
of the roof of my single family home.
Here I had to go up to the rain pipe, where I installed a distance holder of
30 cm and led the feeder over the roof in a diagonal angle to the antenna.
(g4zu, off course homebrew like the ATU. )
I think it is no real worry about the distance of such a feeder to the
rain pipe and/or wall. If you have a distance of 10 cm, nothing negative will
occur.
One word to the balun on the exit of an ATU. I do wonder that such "con-
structions" are still used even in commercial designed ATU's
The disadvantage is immense.
Due to my skills of English I zitate it from an article by AG6K:
(partwise):
"As a rule of thumbs, a balun should have about four times as many reactive
ohms as there are resistive loads.
This means, that, for use with a 600 Ohm balanced load, a secondary-
winding reactance of about 2,4 kOhm.
For 80 Meter operation, this works out to be more than 100 microHenry of
Balun inductance!
To create this much inductance on an appropriate MF/HR-rated (u=40) ferrite
core, an impractically large number of turns should be required.
The use of a balun in high impedance circuits in-evitably creates two
very sticky problems:
Nore turns means more ampere-turns of magnetic flux and high magnetic flux
densities can cause ferrite core to saturate...."
Richard continues, that high number of windings have the follow of unwanted
capasities which brings it to an unwanted imbalance.
It will come to a higher voltage in the balun as higher the frequency is.
The imbalance causes different current flow to the tuner..
Richard explains a number of disadvantages like the possible destroying of
the ferrite and even the disadvantage of possible disturbtions to BC and TV-
receiver...
The article is too long to be copied here
but the right solution is the so-called "Current balun" which has to be in-
stalled in the entrance of the ATU like e.g.
o----////-------II---I------II--------o
/
/
/ (variable coil, II = variable capasistor)
/
o---///--------------I---------------o symmetric outlet, ATU not on earth !
Here you avoid all the problems as you may find with the solution of a
balun on the end!
But as said, the single coil Z-match is the very best way of an ATU - and
very easy to be build up...
Three cheers for the ladies and the gunpowder
Klaus, voice of reason
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