On Mon, 15 Feb 2010 06:34:40 -0800 (PST), ve2pid
wrote:

I have a question about OCF dipoles; it refers to the choice of the
type of balun transformer (voltage or current) to be used with that
type of antenna:

What is most important is the choking action which then demands a
current BalUn. As the transmission line falls away from an unbalanced
dipole, it also upsets they symettry further by its presence. This
further demands you choke it at quarter wave intervals away from the
feed point. As the OCF Dipole is meant to be multiband, this then
demands a number of such quarterwave-away chokes. My solution has
been to place a bead every foot along the entire drop.

The next trick is with the BalUn ratio. Few describe that accurately,
defend their choice, or even pretend to know. Further, the degree of
off-centeredness is another wild guess that none suggest makes a
difference.

73's
Richard Clark, KB7QHC

Richard made a good summary about the problem with common mode currents.
I'm a little leery, however, of the suggestion of putting a bead every
foot. The reason is that a common mode choke (aka current balun, or
bead) has low loss if its impedance is either much higher or much lower
than the common mode impedance. Loss is maximized when it's roughly the
same. So I can see situations where the distributed beads might end up
dissipating a significant fraction of the applied power. But I've never
set up and measured such a system, or modeled one, so don't have any
evidence this would happen.

As the developer and seller of antenna modeling software, I also caution
people that models of the types of baluns and transformers commonly used
for OCF dipoles are greatly in error when used as an OCF feed. The
problem is that on many bands the transformer or balun is operating with
impedances well outside its design range, and consequently adds series
and shunt impedance and exhibits a changed transformation ratio.

Here are a couple of postings I made on this forum some time ago:

3/10/07

Here are some characteristics of off-center fed dipoles which I've
observed in doing careful measurements:

1. The feepoint balun is commonly a voltage balun, which may have the
nominal claimed impedance transformation ratio over most of the HF range
when terminated with 50 + j0 ohms times the tranformation ratio.
However, the antenna doesn't have this impedance at hardly any
frequency, and can be very different at some frequencies. When presented
with load impedances typical of the antenna, the transformation ratio is
way off and becomes complex, and the balun adds considerable shunt and
series reactance.

2. Whenever a voltage balun is used to feed an asymmetrical antenna, it
creates an imbalance current in its attempt to equalize the voltages at
the two halves relative to the "cold" side of the input. This imbalance
current flows down the feedline as a common mode current.

3. Additional common mode current results from the unequal mutual
coupling between the feedline and unequal antenna halves.

4. It takes very concentrated efforts to reduce the common mode current
to a low level on all bands. Multiple current baluns (probably what the
CW calls and "isolator") are required, and even then it might also
require feedline length adjustments to get low common mode current on
all bands.

5. Without being able to quantify what the feedpoint balun will do in
terms of transformation, reactance, and common mode current generation,
it's impossible to build a model of one of these antennas with any
confidence, even if the feedline is included in the model. The best
efforts I made to measure a real antenna and its balun and build a model
based on the measurements led to generally poor agreement between the
measured and model impedance. Consequently I'm extremely skeptical of
any model that purports to predict anything about OCF dipole performance.

9/22/08

A while back I did some pretty careful measurements of an OCF dipole. I
found that ferrites were required at both the feedpoint and at one or
more places along the feedline. The ferrites at the feedpoint suppress
the conducted common mode current (which is actually forced to exist by
the voltage balun). But the asymmetry of the antenna results in common
mode current being induced onto the feedline by mutual coupling to the
antenna. This isn't a problem in a symmetrical dipole if the feedline is
positioned symmetrically relative to the antenna, since the currents
induced by the two equal halves cancel. But the OCF dipole can result in
quite a lot of induced common mode current.

Ideally, you'd put at least a second bunch of snap on cores about a
quarter wavelength from the feedpoint. But one of the main reasons
people use OCFs is for multi-band operation. So the thing to do is to
place the cores for maximum effectiveness on the band(s) where you have
the most trouble -- the common mode current also depends on the feedline
length and position, and will vary considerably from band to band even
if you do nothing.

My opinion is that users of OCF dipoles are just about always going to
have to deal with some amount of common mode current, and the best you
can do is reduce it to a level you can tolerate.

Roy Lewallen, W7EL

Roy Lewallen wrote:

My opinion is that users of OCF dipoles are just about always going to
have to deal with some amount of common mode current, and the best you
can do is reduce it to a level you can tolerate.


I've always wondered, when people take these antennas that are likely to
have feedline radiation, and try as they may to stop it, isn't it likely
that they are making the antenna not work as well as it might have if we
just left the feedline radiate as it would? You might be taking away a
major part of the antenna performance?

I have to expect that a radiating feedline would have to be a pretty
inconsistent sort of antenna from one installation to another.

- 73 de Mike N3LI -

Michael Coslo wrote:

I've always wondered, when people take these antennas that are likely to
have feedline radiation, and try as they may to stop it, isn't it likely
that they are making the antenna not work as well as it might have if we
just left the feedline radiate as it would? You might be taking away a
major part of the antenna performance?

I have to expect that a radiating feedline would have to be a pretty
inconsistent sort of antenna from one installation to another.

- 73 de Mike N3LI -

Absolutely. A radiating feedline is part of the antenna, as is the
entire path to the Earth along the outside of the rig, through the mains
power system or whatever other path it can take. So two people thinking
they have the same "antenna" can really have very different radiating
systems.

Sometimes the radiation from the feedline, mains wiring, and stuff in
the shack can be beneficial -- it might, for instance, fill in deep
nulls in the main antenna's pattern and result in a spectacular signal
strength improvement in particular directions. On the other hand, it can
cause lots of problems. For example, when I was testing an OCF dipole a
while back, on one band my electronic keyer would lock up after the
first "dit" due to RF in the shack and on all the station connecting
wires. And having the power wiring be part of your antenna system can
lead to trouble with telephone, TV, and other kinds of interference. But
then you might get lucky and get away with it.

Probably because of the same personality quirk that led me to become an
engineer, I prefer to be able to predict and understand how my antenna
system will work, and design it to work as I want, rather than making it
a crap shoot. But that's surely not the only, or necessarily the best,
way to get on the air and talk to people.

Roy Lewallen, W7EL

On 15 fév, 19:22, Roy Lewallen wrote:

Sometimes the radiation from the feedline, mains wiring, and stuff in
the shack can be beneficial -- it might, for instance, fill in deep
nulls in the main antenna's pattern and result in a spectacular signal
strength improvement in particular directions.

I am slightly OT here, but the same argumentation could be used about
non-resonant antennas like the G5RV. Yes, ATU needed, but L-type
autotuners for example have very small insertion loss.

I am wondering about the real advantage of a razor cut dipole
(resonant anyway on a small region around a freq) versus a multi-band
dipole using a length of feed line matching section like the G5RV or
ZS6BKW/G0GSF Antenna System...

73 de Pierre VE2PID

ve2pid wrote:
On 15 fév, 19:22, Roy Lewallen wrote:
Sometimes the radiation from the feedline, mains wiring, and stuff in
the shack can be beneficial -- it might, for instance, fill in deep
nulls in the main antenna's pattern and result in a spectacular signal
strength improvement in particular directions.

I am slightly OT here, but the same argumentation could be used about
non-resonant antennas like the G5RV. Yes, ATU needed, but L-type
autotuners for example have very small insertion loss.

I am wondering about the real advantage of a razor cut dipole
(resonant anyway on a small region around a freq) versus a multi-band
dipole using a length of feed line matching section like the G5RV or
ZS6BKW/G0GSF Antenna System...

73 de Pierre VE2PID

No, the phenomenon I'm talking about is feedline radiation, which is
very difficult to prevent with a non-symmetrical antenna like an OCF
dipole because of their asymmetry. It has nothing to do with resonance
or non-resonance, or wideband or narrowband characteristics. You can, of
course, cause feedlines of symmetrical antennas to radiate by imbalanced
feeding. But this is as true of a resonant dipole as a random length
symmetrical antenna.

Roy Lewallen, W7EL

On 15 f v, 19:22, Roy Lewallen wrote:
Sometimes the radiation from the feedline, mains wiring, and stuff in
the shack can be beneficial -- it might, for instance, fill in deep
nulls in the main antenna's pattern and result in a spectacular signal
strength improvement in particular directions.

I am slightly OT here, but the same argumentation could be used about
non-resonant antennas like the G5RV. Yes, ATU needed, but L-type
autotuners for example have very small insertion loss.

I am wondering about the real advantage of a razor cut dipole
(resonant anyway on a small region around a freq) versus a multi-band
dipole using a length of feed line matching section like the G5RV or
ZS6BKW/G0GSF Antenna System...

73 de Pierre VE2PID