"Cecil Moore" wrote in message
...
Ed Cregger wrote:
Such things as variances in construction materials from one batch to
another and the variations that one human will introduce to construction
versus another human also induce characteristics that do not always jibe
with theory.

IMO, the major problem with baluns is that they are
designed for specific impedances and most often used
with unknown impedances.
--
73, Cecil http://www.w5dxp.com

----------

Very true, but often times they work well enough with varying impedances to
get us on the air. If it is that or nothing, then I'm all for doing it.

Ed, N2ECW

Ed Cregger wrote:

I am beginning to suspect that traditionally made baluns are not as exact in
practice as they are theoretically. This is not a surprise, really. Few
things in electronics are exact as we humans like to assume, as you well
know.

Good seeing your post, OM.


Ed Cregger, N2ECW former NM2K

The whole problem is "liking to assume" that things are simpler than
they are. When the theory you apply is too simple, guess what -- you'll
find that the real thing doesn't behave as your oversimplified "theory"
predicts. Theory works just fine, and accurately predicts how a real
object will work. Oversimplified "theory" often doesn't work so well.

A well made balun or RF transformer behaves reasonably well like an
ideal transformer, that is, infinite winding inductance, no coupling
capacitance, zero leakage inductance, no loss, and so forth, but only
under quite a narrow range of circumstances. Those circumstances include
being terminated with a fairly narrow range of impedances and over a
limited frequency range. Usually, one side is designed to be terminated
with 50 ohms, purely resistive. That means the other side of a 4:1 balun
has to be terminated with something fairly close to 200 or 12.5 ohms
(depending on how it's designed), also resistive, in order for it to
work as intended. If the impedance differs very much at all from that
value, you'll find that the transformation ratio is no longer 4:1, and
that the balun will add a series and/or shunt impedance to the circuit.
This can be accounted for by theory, but only with great difficulty
since it requires careful characterization of the core and windings.
People tend to design, and often test, a 4:1 balun in a 50 ohm
environment, then attach it to a multiband antenna that has rather
extreme (but entirely predictable) impedance variations. Then they're
surprised because the impedance seen looking into the balun isn't 4
times or 1/4 times the antenna impedance, but is something wildly
different. They shouldn't be. A 4:1 balun or transformer that effects a
nice 4:1 impedance transformation when presented with a very wide range
of termination impedances simply doesn't exist. Any "theory" that
predicts it is oversimplified and invalid.

Roy Lewallen, W7EL

"Roy Lewallen" wrote in message
treetonline...
Ed Cregger wrote:

I am beginning to suspect that traditionally made baluns are not as exact
in practice as they are theoretically. This is not a surprise, really.
Few things in electronics are exact as we humans like to assume, as you
well know.

Good seeing your post, OM.


Ed Cregger, N2ECW former NM2K

The whole problem is "liking to assume" that things are simpler than they
are. When the theory you apply is too simple, guess what -- you'll find
that the real thing doesn't behave as your oversimplified "theory"
predicts. Theory works just fine, and accurately predicts how a real
object will work. Oversimplified "theory" often doesn't work so well.

A well made balun or RF transformer behaves reasonably well like an ideal
transformer, that is, infinite winding inductance, no coupling
capacitance, zero leakage inductance, no loss, and so forth, but only
under quite a narrow range of circumstances. Those circumstances include
being terminated with a fairly narrow range of impedances and over a
limited frequency range. Usually, one side is designed to be terminated
with 50 ohms, purely resistive. That means the other side of a 4:1 balun
has to be terminated with something fairly close to 200 or 12.5 ohms
(depending on how it's designed), also resistive, in order for it to work
as intended. If the impedance differs very much at all from that value,
you'll find that the transformation ratio is no longer 4:1, and that the
balun will add a series and/or shunt impedance to the circuit. This can be
accounted for by theory, but only with great difficulty since it requires
careful characterization of the core and windings. People tend to design,
and often test, a 4:1 balun in a 50 ohm environment, then attach it to a
multiband antenna that has rather extreme (but entirely predictable)
impedance variations. Then they're surprised because the impedance seen
looking into the balun isn't 4 times or 1/4 times the antenna impedance,
but is something wildly different. They shouldn't be. A 4:1 balun or
transformer that effects a nice 4:1 impedance transformation when
presented with a very wide range of termination impedances simply doesn't
exist. Any "theory" that predicts it is oversimplified and invalid.

Roy Lewallen, W7EL

------------

Roy, that is precisely what I said, but rather imprecisely.

Ed, N2ECW

Roy Lewallen wrote:
Ed Cregger wrote:

I am beginning to suspect that traditionally made baluns are not as
exact in practice as they are theoretically. This is not a surprise,
really. Few things in electronics are exact as we humans like to
assume, as you well know.

Good seeing your post, OM.


Ed Cregger, N2ECW former NM2K

The whole problem is "liking to assume" that things are simpler than
they are. When the theory you apply is too simple, guess what -- you'll
find that the real thing doesn't behave as your oversimplified "theory"
predicts. Theory works just fine, and accurately predicts how a real
object will work. Oversimplified "theory" often doesn't work so well.

A well made balun or RF transformer behaves reasonably well like an
ideal transformer, that is, infinite winding inductance, no coupling
capacitance, zero leakage inductance, no loss, and so forth, but only
under quite a narrow range of circumstances. Those circumstances include
being terminated with a fairly narrow range of impedances and over a
limited frequency range. Usually, one side is designed to be terminated
with 50 ohms, purely resistive. That means the other side of a 4:1 balun
has to be terminated with something fairly close to 200 or 12.5 ohms
(depending on how it's designed), also resistive, in order for it to
work as intended. If the impedance differs very much at all from that
value, you'll find that the transformation ratio is no longer 4:1, and
that the balun will add a series and/or shunt impedance to the circuit.
This can be accounted for by theory, but only with great difficulty
since it requires careful characterization of the core and windings.
People tend to design, and often test, a 4:1 balun in a 50 ohm
environment, then attach it to a multiband antenna that has rather
extreme (but entirely predictable) impedance variations. Then they're
surprised because the impedance seen looking into the balun isn't 4
times or 1/4 times the antenna impedance, but is something wildly
different. They shouldn't be. A 4:1 balun or transformer that effects a
nice 4:1 impedance transformation when presented with a very wide range
of termination impedances simply doesn't exist. Any "theory" that
predicts it is oversimplified and invalid.


Thanks for the information!

Ed, good to run into you again! What brought
you to Georgia?

Ray - I found a Dover edition of Transmission
Lines, Antennas and Waveguides. Thank you for
the suggestion!

One final question about the 4:1 balun:
Assuming a single band Delta Loop with a feed
point impedance of approx 100 ohms, with or
without a 4:1 balun you have approximately a 2:1
SWR - so why use the balun?

Thanks and 73,

Tad Danley, K3TD

"Tad Danley" wrote

Ed, good to run into you again! What brought you to Georgia?

Thanks and 73,

Tad Danley, K3TD

----------

Dupont decided that they needed the wife in Chattanooga, TN. That was
slightly over ten years ago. We are living in the Northwest corner of
Georgia, just below Chattanooga, TN. It is a very, very nice place to live.

What the heck are you doing in Texas?

Ed, N2ECW

Tad Danley wrote:
One final question about the 4:1 balun:
Assuming a single band Delta Loop with a feed point impedance of approx
100 ohms, with or without a 4:1 balun you have approximately a 2:1 SWR -
so why use the balun?

The resonant feedpoint impedance of a loop is often a
little higher than 100 ohms, e.g. 115 ohms according to
EZNEC, in which case a 4:1 balun will lower the SWR -
sometimes alleviating foldback problems.
--
73, Cecil http://www.w5dxp.com

Tad Danley wrote:
I have started to experiment with EZNEC and am modeling a couple of loop
antennas including some delta loops. I see references to hams using 4:1
baluns with these antennas, but the models I see show a feed point
impedance of roughly 100 ohms. I'm not sure how a 4:1 balun would help
- what am I missing?

The resonant feedpoint of the 80m loop that I modeled
with EZNEC is 115 ohms. Without a 4:1 at the feedpoint,
the 50 ohm SWR is 2.3:1 inviting foldback. With a 4:1
balun, the 50 ohm SWR is 1.7:1 with no foldback. It is
rare for the feedpoint resistance of a loop to be exactly
100 ohms.

Of course, if the loop is fed with high-Z0 ladder-line,
the 100 ohm feedpoint resistance is transformed to a higher
impedance value where a 4:1 balun might be more effective.

For single-band operation, most hams simply feed the loop
with 1/4WL of Z0=75 ohm coax (quarter-wave transformer).
--
73, Cecil http://www.w5dxp.com

Tad Danley wrote:
I have started to experiment with EZNEC and am modeling a couple of loop
antennas including some delta loops. I see references to hams using 4:1
baluns with these antennas, but the models I see show a feed point
impedance of roughly 100 ohms. I'm not sure how a 4:1 balun would help
- what am I missing?

Thanks and 73,

Tad Danley, K3TD

Tad:

A 2:1 construction of a "true" 2:1 balun is possible, however, driving a
100 ohm loop from 50 ohm coax does NOT require one--meaning, a 2:1 "RF
TRANSFORMER" will suit your purposes, more than adequately.

The winding to the 50 ohm source will be half the turns of the 100 ohm
winding--and there is no electrical connection between windings--i.e.,
the 50 and 100 windings are separate on the core.

The turns will depend on the core material/power/freqs of your intended
use ...

However, the focus here is that you DO NOT need a true balun here, since
the loop is inherently free from any adverse influences of using a
voltage balun, a rf transformer is more than adequate for your use--and
will simplify your requirements. You should find adequate construction
data for a "2:1 rf transformer" (separate 50/100 ohm windings) with a
google search ... etc. A ferrite bar or toroid, either, should fit your
purposes, as you choose ...

Regards,
JS

"John Smith" wrote in message
...
Tad Danley wrote:
I have started to experiment with EZNEC and am modeling a couple of loop
antennas including some delta loops. I see references to hams using 4:1
baluns with these antennas, but the models I see show a feed point
impedance of roughly 100 ohms. I'm not sure how a 4:1 balun would help -
what am I missing?

Thanks and 73,

Tad Danley, K3TD

Tad:

A 2:1 construction of a "true" 2:1 balun is possible, however, driving a
100 ohm loop from 50 ohm coax does NOT require one--meaning, a 2:1 "RF
TRANSFORMER" will suit your purposes, more than adequately.

The winding to the 50 ohm source will be half the turns of the 100 ohm
winding--and there is no electrical connection between windings--i.e., the
50 and 100 windings are separate on the core.


Usually the turns ratio of an impedance-matching transformer is the square
of the impedance ratio. If the turns ratio, primary to secondary, is N the
secondary voltage Vo is N times the primary voltage Vi but the secondary
current Io is the primary current Ii divided by N. If the primary is fed
from a source of impedance Zi, and Zi = Vi/Ii, then on the secondary side we
have Zo = Vo/Io = NVi/(Ii/N) = (NxN)Vi/Ii. So Zo = (N^2)Zi or N = square
root of (Zo/Zi).

An impedance ratio of 2 would require a turns ratio 1.4. I wonder if
there's a reason why this case would be different.

Chris

"christofire" wrote in message
...

"John Smith" wrote in message
...
Tad Danley wrote:
I have started to experiment with EZNEC and am modeling a couple of loop
antennas including some delta loops. I see references to hams using 4:1
baluns with these antennas, but the models I see show a feed point
impedance of roughly 100 ohms. I'm not sure how a 4:1 balun would
help - what am I missing?

Thanks and 73,

Tad Danley, K3TD

Tad:

A 2:1 construction of a "true" 2:1 balun is possible, however, driving a
100 ohm loop from 50 ohm coax does NOT require one--meaning, a 2:1 "RF
TRANSFORMER" will suit your purposes, more than adequately.

The winding to the 50 ohm source will be half the turns of the 100 ohm
winding--and there is no electrical connection between windings--i.e.,
the 50 and 100 windings are separate on the core.

- - - - - -

Usually the turns ratio of an impedance-matching transformer is the square
of the impedance ratio. If the turns ratio, primary to secondary, is N
the secondary voltage Vo is N times the primary voltage Vi but the
secondary current Io is the primary current Ii divided by N. If the
primary is fed from a source of impedance Zi, and Zi = Vi/Ii, then on the
secondary side we have Zo = Vo/Io = NVi/(Ii/N) = (NxN)Vi/Ii. So Zo =
(N^2)Zi or N = square root of (Zo/Zi).

An impedance ratio of 2 would require a turns ratio 1.4. I wonder if
there's a reason why this case would be different.

Chris


Ooops, I missed out the important word 'root' in my first line above!

The impedance ratio is the square of the turns ratio.
The turns ratio is the square-root of the impedance ratio.

Chris