If the antenna system can be satisfactorily matched to the transmitter
output impedance, it would seem to matter little whether the feed line
currents are balanced, or, if they are not, by how much.

If there are no obvious common mode currents causing problems in the
shack, then of what consequence would an imbalance be, other than to
modify the radiation pattern, perhaps even beneficially?

Chuck, NT3G

Cecil Moore wrote:
Big Endian wrote:

An unbalanced condition would have a meter indication, like current
flow? Balanced the meter needle would not move?


Yes, for a balanced condition, the meter needle should
not move.

chuck wrote:
If the antenna system can be satisfactorily matched to the transmitter
output impedance, it would seem to matter little whether the feed line
currents are balanced, or, if they are not, by how much.

If there are no obvious common mode currents causing problems in the
shack, then of what consequence would an imbalance be, other than to
modify the radiation pattern, perhaps even beneficially?

You have to remember that the common mode feedline current doesn't stop
at the rig. It continues to the Earth by whatever means are available.
So your house wiring, appliance cords, and other odd conductors often
become part of your antenna system. These aren't likely to be very
efficient radiators. People often go to a lot of trouble to put their
antennas high and in the clear. That doesn't make much sense if you're
going to have your house do a good part of the radiating.

But you can still talk to lots of stations even if some of your
radiating is being done by your feedline and house wiring. And that's
enough for a lot of folks.

Roy Lewallen, W7EL

If the antenna system can be satisfactorily matched to the
transmitter
output impedance, it would seem to matter little whether the feed
line
currents are balanced, or, if they are not, by how much.

If there are no obvious common mode currents causing problems in the
shack, then of what consequence would an imbalance be, other than to
modify the radiation pattern, perhaps even beneficially?

Chuck, NT3G
==========================================
I agree with what you say.

But antenna (or line) unbalance and line-to-antenna impedance mismatch
are not entirely independent of each other.

An unbalanced line or antenna causes a small impedance mismatch.
Because it is small is the reason why it very often happens it doesn't
matter very much whether or not a balanced feedline is used.

For example, a coax line can be used quite successfully to feed a
balanced dipole.

And, in practice, no antenna is perfectly balanced about ground.
----
Reg.

chuck wrote:
If there are no obvious common mode currents causing problems in the
shack, then of what consequence would an imbalance be, other than to
modify the radiation pattern, perhaps even beneficially?

The purpose of the antenna is to radiate. The purpose
of the transmission line is to transfer the energy from
the transmitter to the antenna with as little loss as
feasible. How much an antenna system is allowed to
deviate from its purpose is up to the individual. When
I was in high school, I didn't much care about the purpose
of an antenna system and burned a hole in my lip.
--
73, Cecil http://www.qsl.net/w5dxp

As a follow-up, is there a practical way to determine how much current
unbalance will cause a one dB reduction in power delivered to the
antenna, the "lost power" being that power radiated by the transmission
line?

It seems like a rather complex modeling problem.

Thanks!

Chuck, NT3G







ig Endian wrote:
How does one check the balance between two parallel feed wires into a
doublet antenna. Neon bulbs or some sort of meter gizmo?

tnx

d

chuck wrote:
As a follow-up, is there a practical way to determine how much current
unbalance will cause a one dB reduction in power delivered to the
antenna, the "lost power" being that power radiated by the transmission
line?

It seems like a rather complex modeling problem.

Thanks!

Chuck, NT3G

Nope. You can't generally say that one part of an antenna is radiating a
particular amount of the total power. Each part of the antenna creates a
field, and it interacts with the fields from all other parts of the
antenna. The total power radiated has to equal the total power input
less loss, but that's all you can say for sure. An example will help
illustrate the problem. Consider a parasitic element in a Yagi. It has
considerable current and contributes a great deal to the overall
pattern. Yet the total power input to the Yagi element is zero. With
zero power input, it can't, by itself, be radiating any power. What it
does is intercept some of the power radiated by the driven element and
re-radiates it with a different phase and amplitude. So how would you
apportion the power radiated by the driven element and the parasitic
element?

You might take a look at the current in the driven element and note that
it increases or decreases as you put the parasitic element in place and
remove it. But the current can either increase or decrease, depending on
the length and spacing of the parasitic element. So has the parasitic
element increased or decreased the power radiated by the driven element?
There's no answer.

You can look at the change in pattern in some idealized cases by
modeling. This can tell you what range of effects you might expect in a
real situation.

Roy Lewallen, W7EL

I keep forgetting some of these principles.

Thanks very much for the detailed and helpful explanations, Roy.

73,

Chuck

Roy Lewallen wrote:
chuck wrote:

As a follow-up, is there a practical way to determine how much current
unbalance will cause a one dB reduction in power delivered to the
antenna, the "lost power" being that power radiated by the
transmission line?

It seems like a rather complex modeling problem.

Thanks!

Chuck, NT3G


Nope. You can't generally say that one part of an antenna is radiating a
particular amount of the total power. Each part of the antenna creates a
field, and it interacts with the fields from all other parts of the
antenna. The total power radiated has to equal the total power input
less loss, but that's all you can say for sure. An example will help
illustrate the problem. Consider a parasitic element in a Yagi. It has
considerable current and contributes a great deal to the overall
pattern. Yet the total power input to the Yagi element is zero. With
zero power input, it can't, by itself, be radiating any power. What it
does is intercept some of the power radiated by the driven element and
re-radiates it with a different phase and amplitude. So how would you
apportion the power radiated by the driven element and the parasitic
element?

You might take a look at the current in the driven element and note that
it increases or decreases as you put the parasitic element in place and
remove it. But the current can either increase or decrease, depending on
the length and spacing of the parasitic element. So has the parasitic
element increased or decreased the power radiated by the driven element?
There's no answer.

You can look at the change in pattern in some idealized cases by
modeling. This can tell you what range of effects you might expect in a
real situation.

Roy Lewallen, W7EL