Hi Ian and others:
Is common mode current the same as rf current flowing on the coax shield
because it is connected to one half of the antenna?
If so, no wonder the feed point is unbalanced; there are two return current
paths for any voltage present on the shield side of the antenna. The "hot"
side of the antenna reflection sees the only the coax center conductor and
associated impedance. The other side of the antenna reflections sees the
inside of the coax and associated impedance in parallel with the outside of
the coax and it's associated impedance. The total impedance then has to be
less than the "hot" side.
So, there is a balanced antenna feedpoint with say 50 ohms referenced to
ground on one side and maybe 35 ohms on the other side reference to ground.
The voltage would have to divide 35/50 in that case.
Providing a means to reduce the current on the shield has to improve the
voltage ratio on the two halves of the antenna as that makes the impedance
referenced to ground more nearly equal. Series choke losses also have a
parallel equivalent that would tend to degrade the balance. So, it seems the
choke can help balance to a large degree but never completely restore balance.
Any corrections to this understanding are cheerfully accepted.
73, Gerald K5GW
In a message dated 4/20/2012 11:57:10 A.M. Central Daylight Time,
>Hi All, at the risk of really showing some ignorance, a question:
>If the choke is doing a good job and located at the feedpoint, isn't the
>common mode circuit quite short?
>Seems that the common mode current is the cause of the feed point
> rather than the imbalance causing the common mode current. Chicken first
>or egg first?
Neither; or both.
The only parameters that can truly be labeled "first causes" of antenna
behavior are the physical dimensions of the antenna elements and
feedline, their location relative to ground and other nearby objects,
and the frequency and power level of the applied signal.
These are the parameters that constrain the antenna's behavior according
to the laws of EM physics. It is no coincidence that these are also the
inputs required by an antenna modeling program, which then computes the
one-and-only solution that will fit all the facts supplied.
Antenna currents, feedline currents (both CM and differential) and all
the related voltages and impedances are all related parts of the
antenna's behavior - but they are all *consequences*, not causes.
Because these are all *related* consequences, a change in one will be
accompanied by a change in all the others. But "chicken or egg"
arguments are futile because none of them has been the root cause of the
changes we're talking about.
So here we are, with an antenna system in which some unwanted CM current
is flowing on the feedline. If we now install a CM choke, we are adding
completely new constraint upon the system's behavior, in the form of a
high impedance at that particular location (which modeling software
would call a "load").
The key concept is that we are deliberately forcing the ENTIRE
antenna-feedline system to behave differently from the way it did
without the choke. Everything changes to accommodate this new
constraint, so NONE of the RF currents and voltages will be the same as
Many of the "chicken or egg" arguments that we've seen in the past few
days contain a hidden assumption that CM voltages or currents will be
the same both before and after the choke is inserted. They won't! The
only valid solution is to recompute the behavior of the entire
antenna-feedline system. Unfortunately that will only produce an answer
which is valid for a particular situation; but it also explains why
anecdotal reports can be so variable.
73 from Ian GM3SEK
TowerTalk mailing list
TowerTalk mailing list