An important correction: I DO NOT get a lower-than-50ohm *impedance* on
any band.

I may be taxing the limits of my '259B but this is what I measure as
far as ballpark magnitude of impedance.

80M 225 ohms
60m 600 ohms
40m 630 ohms
30m 330 ohms
20m 420 ohms
17m 206 ohms
15m 216 ohms
12m 216 ohms
10m 180 ohms

Nowhere is this resistive.

So, maybe the 4:1 current balun is the more appropriate one. How does
the impedance transformation work with reactive loads?

Here are the impedances if anyone needs:
80m 41-j220
60m 500-j320
40m 500+j370 when paralleled with a 1k resistor (300+j1000ish?)
30m 39-j325
20m 300+j255
17m 50-j200
15m 190+j104
12m 70-j205
10m 170+j60

Will a 4:1 current balun transform a random impedance Z1 to Z2 where
|Z2|=|Z1|/4? I know Roy pointed out that this won't work for a voltage
balun...

73,
Dan

wrote:
. . .
Will a 4:1 current balun transform a random impedance Z1 to Z2 where
|Z2|=|Z1|/4? I know Roy pointed out that this won't work for a voltage
balun...

No, it won't either. The amount of error with any balun type depends on
the balun construction. You'll just have to build one and see what it does.

It would be interesting to know what your antenna Z looks like on each
band at the input to the existing balun. That is, what do you get when
you connect your antenna to the balun output and the analyzer to the
balun input?

Roy Lewallen, W7EL

Roy Lewallen wrote:
wrote:
. . .
Will a 4:1 current balun transform a random impedance Z1 to Z2 where
|Z2|=|Z1|/4? I know Roy pointed out that this won't work for a voltage
balun...

No, it won't either. The amount of error with any balun type depends on
the balun construction. You'll just have to build one and see what it
does.

It would be interesting to know what your antenna Z looks like on each
band at the input to the existing balun. That is, what do you get when
you connect your antenna to the balun output and the analyzer to the
balun input?

Since the balun is connected to a tuner, inaccurate impedance
transformation won't matter much. The only concern would be if it's a
sign of some other problem in the balun.

For a monster 4:1 current balun of the type that has two independent
'cores', see page 30 of:
http://www.yccc.org/Articles/W1HIS/C...S2006Apr06.pdf
(or for short: http://tinyurl.com/qnzs3 )

The whole article is about common-mode chokes, and is well worth
reading. It shows what can be done to tame a "noisy" QTH, if you're
prepared to go to extreme lengths... and this guy certainly was!


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

Ian White GM3SEK wrote:
Roy Lewallen wrote:
wrote:
. . .
Will a 4:1 current balun transform a random impedance Z1 to Z2 where
|Z2|=|Z1|/4? I know Roy pointed out that this won't work for a voltage
balun...

No, it won't either. The amount of error with any balun type depends
on the balun construction. You'll just have to build one and see what
it does.

It would be interesting to know what your antenna Z looks like on each
band at the input to the existing balun. That is, what do you get when
you connect your antenna to the balun output and the analyzer to the
balun input?

Since the balun is connected to a tuner, inaccurate impedance
transformation won't matter much. The only concern would be if it's a
sign of some other problem in the balun.


Well, yes and no. Good tuner designers go to a lot of trouble to
maximize the Q of the inductors to minimize loss. It's doubtful that the
Q of the balun inductance is nearly as good, so loss is liable to be
higher if the balun is contributing a significant amount of reactance.
Otherwise, I agree, it doesn't make much difference. The balun might
move the impedance to a point where the tuner can't match it, but it's
just as likely that it'll move an otherwise unmatchable impedance to
within the tuner's range.

Roy Lewallen, W7EL

A choke balun has NO impedance or turns ratio. It is silly to refer to
one.

To make one just wind 15 or more turns of twin speaker wire on a 2"
diameter, one-hole, ferrite core.

A one-hole core is a ring with a hole in the middle.

Ferrite permeability need not be high. 200 or 300 is good enough and
will provide enough inductance to cover the 160m band. Low
permeability materials also have lower loss at the higher frequencies.
Not that a choke balun is a lossy component. Efficiency is extremely
high.

Because the length of wire is only about 1/8th of a wavelength at 30
MHz it will be ok at that frequency too.

All the talk about saturation is so much hot air. You couldn't
saturate it even if you tried. The currents in the two wires run in
opposite directions and cancel each other out.

It has a a lower loss and higher power-handling ability than a core
wound with the usual very small diameter coax.
----
Reg.

Reg Edwards wrote:
A choke balun has NO impedance or turns ratio. It is silly to refer to
one.

Consider me silly as well as an "old wife". A choke balun has common
mode impedance, and that impedance is its single most important quality.
If the common mode impedance isn't adequate, it won't perform its
function. It can be measured by short circuiting the input conductors
together and output conductors together to temporarily make one
conductor, and measuring the impedance between the ends.

. . .

All the talk about saturation is so much hot air. You couldn't
saturate it even if you tried. The currents in the two wires run in
opposite directions and cancel each other out.

I agree that saturation isn't a problem, but disagree about the reason.
Core flux density is a function of the common mode current, which is in
the same direction in the wires and doesn't cancel out. The objective of
the balun is to minimize this current, but in a high power system even
with an effective balun, the I^2 * R loss, where I is the common mode
current, can still get large enough to make the core hot. However, if
you use a high-permeability, low frequency ferrite, the flux density
will still be way below saturation even when the core is hot enough to
break.

. . .

Roy Lewallen, W7EL

Insert "ratio" and repeat after me -

"A choke balun has no impedance (ratio) or turns ratio."
----
Reg.

Ian... extreme lengths is right! I like the ferrite cores strung onto
the 240V lines.

I'm sure this approach would help if I wasn't on a graduate student
budget and living in an apartment (though maybe I could sneak around
and install snap-on ferrites on everything?)

After reading that article and thinking back a bit, I think a real
current balun would be worthwhile.

A previous incarnation of my "invisible" antenna used *one* length of
magnet wire to a SO-239 tuner center pin and used the balcony rail
connected to the ground on the tuner.

Switching to the "balun" and two legs reduced my electric(al/onic)
noise quite a bit. I think at least trying to enforce balance is
worthwhile. I guess I need to be prepared to spend some money if I
want to be able to choke off common mode currents on all bands .
I certainly am trying to tame a noisy QTH, but I've made some progress.
As is often the case, it was mostly *my* stuff causing the noise.
Still, I've got a few persistent sources. I know *some* of it is
radiated and I'm sunk there. My 6m antenna is a moxon rectangle with a
string of 60 or so #43 beads as a balun mounted on a fiberglass mast.
Everything it's picking up is radiated :-)

Roy, I'll do the impedance measurement on the balun input, just as a
matter of curiosity, and post the results.

First pass, I think, will be the 1:1 current balun, especially if the
MFJ balun's core is of a worthwhile material. I'll let the tuner do
its job.

I'm curious about what would happen with the 4:1 current balun, butI'll
have to order some cores.

-Dan

wrote:
An important correction: I DO NOT get a lower-than-50ohm *impedance* on
any band.

I may be taxing the limits of my '259B but this is what I measure as
far as ballpark magnitude of impedance.

80M 225 ohms
60m 600 ohms
40m 630 ohms
30m 330 ohms
20m 420 ohms
17m 206 ohms
15m 216 ohms
12m 216 ohms
10m 180 ohms

Nowhere is this resistive.

So, maybe the 4:1 current balun is the more appropriate one. How does
the impedance transformation work with reactive loads?

Here are the impedances if anyone needs:
80m 41-j220
60m 500-j320
40m 500+j370 when paralleled with a 1k resistor (300+j1000ish?)
30m 39-j325
20m 300+j255
17m 50-j200
15m 190+j104
12m 70-j205
10m 170+j60

Will a 4:1 current balun transform a random impedance Z1 to Z2 where
|Z2|=|Z1|/4? I know Roy pointed out that this won't work for a voltage
balun...

To the extent that the balun is working, it will work with complex
impedances as well as with a purely resistive ones.

The Trask paper has several designs for 2 core 4:1 current baluns that
should work. They need two cores. If you want really wide band
performance, you can stack a low frequency and higher frequency
ferrite toroid for each of those cores. With this approach you can
achieve a 1000 to 1 frequency range. The long tube bead ferrites make
good baluns in a small space.

The imaginary part of these numbers seem a bit odd (oh no, only three
of them are odd--the rest are even). Is your antenna resonant on any
band?
I'm probably not well-versed in all the impedances an antenna/feedline
can take at the tuner, but would like to know what antenna and feedline
this is.
73,
Glenn AC7ZN