[TowerTalk] Remote balun 4:1 or 1:1 ?
Bill Coleman
aa4lr at arrl.net
Fri Sep 10 19:25:40 EDT 2004
On Sep 9, 2004, at 11:38 PM, Steve Forst wrote:
> Tried running ladder line into the shack, but too much RFI. Most
> sellers
> of remote baluns offer 4:1 baluns for connection between coax from
> the
> tuner and 450 ohm ladder line to the antenna.
The key thing here is that it doesn't matter what the balun ratio is so
much for two reasons.
First, you're not running the transmission line in a matched condition
-- it has a complex impedance on it, that changes with frequency and is
no where near the characteristic impedance of the line.
Second, the transformation ratio of a balun only applies for resistive
impedances. Because the impedance is complex, who knows what the
transformation ratio might really be.
> The guys at DX Engineering have a 1:1 balun for this purpose, which
> they of
> course say works better They also say to use 300 ohm line instead
> of the
> 450 line.
A lot of the 4:1 baluns used a voltage balun design on a powdered iron
core. Voltage balun designs are old and do not do too much to enforce
current balance.
Current baluns do a better job of enforcing current balance. A typical
current balun consists of a ferrite core or a series of ferrite beads.
It suppresses common-mode currents while not affecting differential
currents.
My recommendation -- use a 1:1 current balun. Also, use whatever
transmission line has the largest conductors. The object is to minimise
loss. Since the losses at HF are mostly resistive, a larger conductor
will show less loss in the transmission line.
Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr at arrl.net
Quote: "Not within a thousand years will man ever fly!"
-- Wilbur Wright, 1901
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