I took great care in the construction of my rotary dipole and feed
system to make it as balanced as possible. I made a clamp-on device from
a split ferrite bead that I can use to measure the relative currents in
the open line conductors, and they are very close.
There is one direction in which there is measurable unbalance due to a
nearby object, but unfortunately this is also the direction of my major
source of noise, another building 100m away, so it's not possible to
tell whether the unbalance contributes to the noise or not.
Someone else said that balanced lines don't reject common mode noise.
It's true that common mode noise currents can flow on the feedline, but
they are rejected by the balanced link-coupled tuner at the transmitter end.
I unfortunately don't have a choked coax-fed dipole to compare it to,
but it is much quieter than the previous antenna, a coax-fed multiband
vertical. And it is also a far better transmitting antenna.
Let's hear it for 1930s technology!
73,
Victor, 4X6GP
Rehovot, Israel
Formerly K2VCO
CWops no. 5
http://www.qsl.net/k2vco/
.
On 17/07/2020 0:41, Fred Jensen wrote:
Indeed! Probably the least unbalanced of "balanced" antenna systems
that ever existed were the HF point-to-point rhombics and V-beams at
the RCA, Mackay, and Marconi shore stations in the first 60 or so
years of the 20th century. Despite very precise engineering to make
them balanced, the RF currents in each side of the open feeders were
never exactly the same.
As a teenage ham, I tended to think in absolutes and exactitudes. If
my Heath MM-1 multimeter said the screen voltage was 176.5 V, I
believed it was, exactly, and if the spec said 177.5 V, I needed to
do something to "fix" it. If the ARRL Handbook said the two halves
of my 40 meter dipole needed to be exactly 32.9114 feet, I believed
that the antenna would not work if I didn't assure my dipole was
exactly 32.9114 on each side. As I grew older, both in age and ham
longevity, I realized my Elmer was right when reminded me on multiple
occasions, "We're amateurs. Most often, 'close' is good enough."
Seems like many today are convinced that 32.9114 ft will work but
32.9000 ft won't.
73, Fred ["Skip"] K6DGW Sparks NV DM09dn Washoe County
On 7/16/2020 1:00 PM, Jim Brown wrote:
On 7/16/2020 1:14 AM, Victor Rosenthal 4X6GP wrote:
If the antenna is well-balanced and fed via a true balanced
antenna tuner (preferably link-coupled) then there shouldn't be a
problem with common mode currents.
Few ham antennas are perfectly balanced -- they are often
unbalanced by their surroundings. For example, ground slope,
unequal heights of the two halves, other conductors around the
antenna, even vegetation. And yes, all elements of the antenna
system, including the feedline, the antenna, and matching at both
ends, contribute to the balance of the SYSTEM.
THAT'S why it's wrong to talk about "balanced line," using
"parallel wire" or 2-wire" line instead.
73, Jim K9YC
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