wrote:
I use an ugly balun with an attic GRrV and do not suffer significant
back-RFI to my transceiver in the regular frequency ranges. I think
that by chosing 1Kohm that may be a bit conservative. In (old)
engineering school, we tended to use an order of magnitude (X10) as
our highly arbitrary cut-off point for impedances that have a
significant effect. You are doing that too but by using 1000 ohms, you
are using (X10 times 2) as your arbitrary cutoff point. Since the
filter is an exponential curve, if you chose 500 ohms instead of 1000
ohms, you might even get a 4 or 5 to 1 frequency range. In my case I
use two different turns chokes so that is why I think I am covered
pretty well. Your information is very interesting; good to see people
are actually measuring things!

Actually, 1000 ohms is pretty liberal. For instance, on
15m, the G5RV coax sees 36+j230 ohms or about 233 ohms.
The balun needs to be 10x that value or 2330 ohms.
--
73, Cecil http://www.w5dxp.com

On Sep 1, 3:25*pm, Cecil Moore wrote:
wrote:
I use an ugly balun with an attic GRrV and do not suffer significant
back-RFI to my transceiver in the regular frequency ranges. I think
that by chosing 1Kohm that may be a bit conservative. In (old)
engineering school, we tended to use an order of magnitude (X10) as
our highly arbitrary cut-off point for impedances that have a
significant effect. You are doing that too but by using 1000 ohms, you
are using (X10 times 2) as your arbitrary cutoff point. Since the
filter is an exponential curve, if you chose 500 ohms instead of 1000
ohms, you might even get a 4 or 5 to 1 frequency range. In my case I
use two different turns chokes so that is why I think I am covered
pretty well. Your information is very interesting; good to see people
are actually measuring things!

Actually, 1000 ohms is pretty liberal. For instance, on
15m, the G5RV coax sees 36+j230 ohms or about 233 ohms.
The balun needs to be 10x that value or 2330 ohms.
--
73, Cecil *http://www.w5dxp.com

Why not 500 ohms, assuming a 50 ohm source and transmission line?

wrote in news:6f4f9e36-af26-4f1b-9244-383494f77b26
@c58g2000hsc.googlegroups.com:

On Sep 1, 3:25*pm, Cecil Moore wrote:
wrote:
....
Actually, 1000 ohms is pretty liberal. For instance, on
15m, the G5RV coax sees 36+j230 ohms or about 233 ohms.
The balun needs to be 10x that value or 2330 ohms.
--
73, Cecil *http://www.w5dxp.com

Why not 500 ohms, assuming a 50 ohm source and transmission line?


The common mode impedance of the balun acts in the common mode
transmission line (which is mutually coupled to the nominal radiator).

How is the differential mode transmission line characteristic impedance
relevant to the determination of common mode current in the antenna
system scenario described?

Owen

On Sep 1, 9:52*pm, Owen Duffy wrote:
wrote in news:6f4f9e36-af26-4f1b-9244-383494f77b26
@c58g2000hsc.googlegroups.com:



On Sep 1, 3:25*pm, Cecil Moore wrote:
wrote:
...
Actually, 1000 ohms is pretty liberal. For instance, on
15m, the G5RV coax sees 36+j230 ohms or about 233 ohms.
The balun needs to be 10x that value or 2330 ohms.
--
73, Cecil *http://www.w5dxp.com

Why not 500 ohms, assuming a 50 ohm source and transmission line?

The common mode impedance of the balun acts in the common mode
transmission line (which is mutually coupled to the nominal radiator).

How is the differential mode transmission line characteristic impedance
relevant to the determination of common mode current in the antenna
system scenario described?

Owen

For a balanced transmission line, the characteristic impedance is not
expressed in differential mode terms, it IS common mode so I do
not know why you ask about differential mode characteristic impedance.
I never mentioned it. The characteristic impedance of a ladder line
for example
might be expressed as 600 ohms. That 600 ohms assumes common mode
conduction, as charateristically transmitted in a balanced line,
Differential mode impedance is assumed for un balanced transmission
line conditions.

wrote:
For a balanced transmission line, the characteristic impedance is not
expressed in differential mode terms, it IS common mode so I do
not know why you ask about differential mode characteristic impedance.

Well, there's your problem. In a properly functioning
transmission line, whether balanced or unbalanced, the
currents in the two conductors are differential, not
common-mode. The two currents are equal in magnitude
and 180 degrees out of phase and their fields tend
to cancel which minimizes radiation.

Common-mode currents only occur when the two currents
are not perfectly differential, i.e. not equal amplitudes
and/or not 180 degrees out of phase.
--
73, Cecil http://www.w5dxp.com

On Sep 3, 7:18*am, Cecil Moore wrote:
wrote:
For a balanced transmission line, the characteristic impedance is not
expressed in differential mode terms, it IS common mode so I do
not know why you ask about differential mode characteristic impedance.

Well, there's your problem. In a properly functioning
transmission line, whether balanced or unbalanced, the
currents in the two conductors are differential, not
common-mode. The two currents are equal in magnitude
and 180 degrees out of phase and their fields tend
to cancel which minimizes radiation.

Common-mode currents only occur when the two currents
are not perfectly differential, i.e. not equal amplitudes
and/or not 180 degrees out of phase.
--
73, Cecil *http://www.w5dxp.com

Nope. The CM choke works precisely because the common mode currents
are mirror images, 180 degrees out of phase. If they weren't, the CM
choke could not act to cancel them and you would need differential
mode chokes to get rid of them. On your balanced feedline, the voltage
at any point is delta between the amplitude of line 1 and 2.

wrote:
Nope. The CM choke works precisely because the common mode currents
are mirror images, 180 degrees out of phase.

You are 100% wrong. From "The IEEE Dictionary":
"common-mode (1)(general) The instantaneous algebraic
average of two signals applied to a balanced circuit,
both signals referred to a common reference."

The "instantaneous algebraic average of two signals"
180 degrees out of phase is ZERO.
--
73, Cecil http://www.w5dxp.com

wrote:
On Sep 1, 3:25 pm, Cecil Moore wrote:
Actually, 1000 ohms is pretty liberal. For instance, on
15m, the G5RV coax sees 36+j230 ohms or about 233 ohms.
The balun needs to be 10x that value or 2330 ohms.

Why not 500 ohms, assuming a 50 ohm source and transmission line?

Be the current making a choice of paths at a junction.
How much of you would flow through 500 ohms and how much
would flow through 233 ohms? (If 500 ohms is the total
impedance seen by the shield current looking back toward
the source, about 1/3 of the current would flow back
through the 500 ohms down the coax.)

http://eznec.com/Amateur/Articles/Baluns.pdf
--
73, Cecil http://www.w5dxp.com

On Sep 2, 7:51*am, Cecil Moore wrote:
wrote:
On Sep 1, 3:25 pm, Cecil Moore wrote:
Actually, 1000 ohms is pretty liberal. For instance, on
15m, the G5RV coax sees 36+j230 ohms or about 233 ohms.
The balun needs to be 10x that value or 2330 ohms.

Why not 500 ohms, assuming a 50 ohm source and transmission line?

Be the current making a choice of paths at a junction.
How much of you would flow through 500 ohms and how much
would flow through 233 ohms? (If 500 ohms is the total
impedance seen by the shield current looking back toward
the source, about 1/3 of the current would flow back
through the 500 ohms down the coax.)

http://eznec.com/Amateur/Articles/Baluns.pdf
--
73, Cecil *http://www.w5dxp.com

I got confused as to whether we were talking "choke" or "balun". For
the balun, you want to be as close to 50 ohms as possible. Actually
233 ohms is not that bad. 1K would be really bad and illustrate that
the balun is not working well. It 233 ohms, it is sort of OK,
especially with a tuner, which you use with a G5RV anyway, along with
the ugly balun. In priciople, the frequency works OK for my G5RV/ugly
balun system.

By the way, I have seen so many articles about baluns written by other
hams and they tend to repeat the same mistakes and assumptions. Most
hams do not understand how a balun works. Some even think you do not
need a balun if the antenna is at resonance which is totally untrue.
Anytime you feed a dipole directly with an unbalanced coax, the
balanced dipole "load" forces current down your ground shield and into
your radio and makes your radio part of your transmitting antenna.
Inserting a balun does not "choke" the current in the shield, it
merely shifts the output phases so that the current (voltage) is
directed towards the dipole at all times (see my other post in this
thread).

wrote:
By the way, I have seen so many articles about baluns written by other
hams and they tend to repeat the same mistakes and assumptions. Most
hams do not understand how a balun works.

Your following statement puts you in that category.

Inserting a balun does not "choke" the current in the shield, it
merely shifts the output phases so that the current (voltage) is
directed towards the dipole at all times (see my other post in this
thread).

This applies to a W2DU balun. Why would you want
differential current fields to be introduced into
the #77 ferrite beads? Their entire purpose is to
provide a large choking impedance and dissipate
common-mode power.
--
73, Cecil http://www.w5dxp.com