RadioBanter - View Single Post - End-feeding dipoles

Richard Clark · June 21st 07, 07:45 AM posted to rec.radio.amateur.antenna

On Thu, 21 Jun 2007 03:30:53 GMT,
wrote:

On Tue, 19 Jun 2007 16:45:15 -0400, Chuck
wrote:

Hello Richard,

Richard Clark wrote:
It is merely an off-center dipole that hasn't come out of the closet.
The wiring in your shack supplies that other half, and supports the
common mode current/voltage.

Interesting concept. Not technically a
dipole, though. But similarly true of
less-than-perfect, center-fed dipoles, no?

Seperate generic dipoles from resonant dipole halfwave antennas
or other resonat half wave antennas with unusual feeds.
It's possible to have a nonresonant dipole that is an effective
antenna but that a whole different animal.

This doesn't make much sense. A dipole is a generic term for two
poles. There are very few monopoles in nature in spite of the term
being used generally for a vertical. Even then, there is the plane of
ground (or a ground plane when elevated) that serves as the other half
fulfilling the sense of "dipole" (two poles, i.e. points that are
separated by 180 degrees of voltage sense).

Irrespective of being a true or other dipole, resonance comes with the
nature of the system, which may include the feedline, and any wiring
that is substantially (in terms of wavelength) remote from ground. It
takes extreme effort to remove these effects of elevated wiring. These
same wires radiated, even if ineffectively (or even effectively) or,
rather, unmanageably (the operator never intended, or considered them,
part of the radiator).

Be careful as the average OCF is an antenna that longer than a
halfwave. Though for instructional value there is one aspect there
in that at the center the feed point in free space is 72ohms (or there
abouts). As you move the feed point to the ends the feed resistance
goes up untill you reach the and where it's the highest at that point.

That, in fact, is not true. You may find certain "sweet" or "sour"
spots along the length. Some can effectively render a 50 Ohm match
(if you take great care to both choke the feed and isolate it from the
imbalanced fields). You can also elevate the Z too. Transformation
along the length of a wire renders many Rs and Zs. The OCF will
generally resonate at the same frequencies regardless of feed point.
So a half wave of wire can be induced to match different Zs and Rs by
feed placement (although maintaining isolation, as mentioned, can be
exceedingly difficult).

Shack wiring is not RF.

This is the desire of the operator, but rarely given enough
consideration to be generally true. The further your operating
position is from ground, in terms of wavelength, the higher potentials
your position supports in the presence of Common Mode excitation.
Second story shacks are notorious for these problems.

The difference if the home power was DC
it will still require a return path and there is no way to virtualize
it. At RF ground is/can be a virtual thing.

You can force the solution through tuning the ground wire, or
providing for a virtual ground, certainly. This, however, is rarely
practiced. Faith does not replace ground.

Having built end fed half waves (and full waves) for 40-6m
I can say I've had RF problems. There is a reason for that.
If the radiator is not a resonant half wave there will be problems
and that length is affected by position and environment
just like any dipole.

Common mode cares not a whit about resonance. It is a condition
driven by imbalance. That imbalance arrives either electrically or
physically (both of which are so intimately intertwined as to be
inseparable in the common antenna installation).

That is the first step, if the radiator is
not resonant it will induce reflections in the system just like a half
wave dipole.

All antennas that are short are by definition (the halfwave and even
full waves discussed here) are Standing Wave antennas. They ALL
reflect regardless of resonance. You only encounter non-reflective
antennas by one of two means:
1. Large dimensions (Rhombics are classic) that are terminated;
2. Short antennas provided with buffering resistors.

The second reason I've had problems is
going from 50 ohms to around 3500 to 5500ohms (assuming a
resonant radiator) and the network for that if wrong will induce
all manner of problems.

Of course. A Hi-Z feed point is nearly impossible to choke. That
feedpoint looking back at the entire system will endeavor to feed
everything, including the transmission line and thus induce Common
Mode currents. These currents are a random and opportunistic fact of
life, and across the variety of situations will either present
problems or appear to be benign.

When I have it right there is no common
mode RF on the outside of the coax (other than induced by the near
field of the antenna) and this is similar to what would be expected
from a properly fed half wave dipole.

On the face of it, an OCF (or end fed) antenna is distinctly different
from a "properly fed half wave dipole" by simple, geometrical
analysis. The Common Modality follows this with strong correlation. A
standard dipole, "properly fed" assumes a strong symmetry which
precludes Common Mode induction when also properly choked. An OCF
does not exhibit symmetry, and typically suffers considerable Common
Mode induction which often precludes attempts to suppress conduction
through choking.

Voltage fed antennas
(halfwave) have too low a current at the feed point to have common
mode problems

This is simply wrong. There is nothing inherent about Common Mode
that is extinguished by any distinction of being "voltage fed." You
may experience a condition that supports this statement as an example,
but it does not create a rule. Luck does not define the condition.

Allison, you mix so many examples of problematic episodes and
conflicting advice as to negate most of what you offer. Common Mode
is dominated by balance or imbalance. There are certainly
expectations of resonance that follow from need, but they have
absolutely nothing to do with the CM problems that are encountered,
nor dodging the CM bullet through happenstance.

Common Mode issues are induced, controlled, or cured through very
simple considerations and they never appeal to resonance. The problem
for the typical Ham is these simple considerations are not simply
solved. Balance is not achieved by the wave of a hand. You cannot
force CM away through careful tuning. If you don't notice CM, all
fine and well, but that does nothing to solve an imbalance either -
you simply don't care about it is all.

73's
Richard Clark, KB7QHC