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

John Smith wrote:
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.

Or it could be wound as an autotransformer.
--
73, Cecil http://www.w5dxp.com

On Feb 7, 8:38*am, Cecil Moore wrote:
John Smith wrote:
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.

Or it could be wound as an autotransformer.
--
73, Cecil *http://www.w5dxp.com

I still prefer just a simple coax series transformer.
As an example, for a 40m loop, about 22 ft of
75 ohm coax will do the trick.

wrote:
I still prefer just a simple coax series transformer.
As an example, for a 40m loop, about 22 ft of
75 ohm coax will do the trick.

But that wouldn't meet the 2:1 transformer requirement.
--
73, Cecil http://www.w5dxp.com

On Feb 7, 4:46*pm, Cecil Moore wrote:
wrote:
I still prefer just a simple coax series transformer.
As an example, for a 40m loop, about 22 ft of
75 ohm coax will do the trick.

But that wouldn't meet the 2:1 transformer requirement.
--
73, Cecil *http://www.w5dxp.com

Should be pretty close. But calculating it, I show
about 22.7 feet would be the best length for 7.150mhz.
Assuming a 52 ohm feedline, and 75 ohm 1/4 wave
section and a Zr of 120 ohms.
I've used them before. Not really that critical on the
precise length as long as it's pretty close.

wrote:
Should be pretty close.

Sorry, I thought you were talking about an ugly balun.
--
73, Cecil http://www.w5dxp.com