Topband: Balun Discussions

[Sinisa Hristov]

John Kaufmann wrote:

> You can actually see the impedance go through resonance at some frequency by connecting to an impedance analyzer and sweeping the frequency.  Below resonance, the choking impedance increases with increasing frequency, and as you get near resonance it goes up very sharply.  Just on the high-frequency side of resonance, the impedance starts to drop very sharply and continues to decrease (although less sharply) with increasing frequency.


A choke should be regarded a short-circuited transmission line,
with input [near] end being at choke treminals, and shorted output
[far] end being in the middle of the choke. So the [differential]
wave travels from ends toward center, reflects and travels
back toward input terminals, etc.

The lowest resonance is parallel (transmisiion line is lambda/4),
next resonance is series (transmision line is lambda/2), etc.
Resonances alternate the same way as in any other shorted
transmission line. Quality of resonances depends on quality
of the line. Outer plastics of RG213 are not particularly good,
so balun resonances get somewhat distorted, but they are there.
A good quality choke presents a nice "comb" impedance sweep.

A couple of years ago I helped YU1AW in designing anode choke
for his non-switching 50 MHz / 144 MHz amplifier. The choke operates
on first parallel resonance for 50 MHz, and on second parallel resonance
for 144 MHz. This provides sufficiently high choking impedances
on both bands, without switching.


73,

Sinisa YT1NT, VA3TTN