Just got an e-mail asking "so where is it best to use a 4:1 balun?"
Remember when we learned that a folded dipole antenna exhibited a typical
feedpoint impedance of some 300 ohms? Well the 4:1 balun works just great
transforming the 300 ohms to 75 ohms. Years ago the Pi network in our
transmitters would match 75 ohms without a hitch plus RG-59 was very
available and inexpensive. Now, bring the folded dipole closer to ground
and that 300 ohm impedance will drop to something like 200 ohms. Add a 4:1
balun and one has a resonant antenna, the balun is operating between matched
impedances, and your transmitter sees something like 50 ohms.
Of course the folded dipole is a single band antenna so don't expect to use
a tuner and make it work other bands. But it is one of the quietest
antennas of any type, form or function plus likely covers the 80M band with
less than a 1.5:1 SWR and NO TUNER.
I have one cut for 3885 and it out performs the center fed dipole, the
balanced fed dipole and the full wave loop. I just wish I had resources and
supports to put up one for each band. I construct mine from heavy duty 300
ohm window type line supporting the ends and center with home made strain
relief supports that does not connect to the conductors but clamps on the
outer jacket. They stay up through wind and winter snow and ice.
Just remember that using a 4:1 balun that one is not matching the feedline
impedance. Many think that if one uses 450 ohm line that a 4:1 balun is
required. If that antenna is close to the ground the center feed impedance
is most likely about 25 ohms. A 1/4 wave of open wire line transforms it to
about 50 ohms. A 4:1 balun then drops it to about 12.5 ohms. Tests have
shown that tuners operating at lower impedance values typically have higher
loss. It would then make sense to use a 1:1 balun thus the tuner would
operate at about 50 ohms. Or perhaps a tuner would not be needed at all.
73
Bob, K4TAX
----- Original Message -----
From: "Robert Mcgraw" <rmcgraw@blomand.net>
To: <n4py3@earthlink.net>; "Discussion of Ten-Tec Equipment"
<tentec@contesting.com>
Sent: Wednesday, July 10, 2013 3:45 PM
Subject: Re: [TenTec] OCF antennas evolution
I agree 100% with Carl and Stuart on this. One caveat, please use a
balanced tuner an not a poor performing balun to get from unbalanced to
balanced configuration. Also most likely a 1:1 current balun will
exhibit lower loss, handle higher power than a 4:1 internal balun.
Remember the power ratings on a balun are for MATCHED conditions, which
by the way is highly unlikely in a configuration used for multiple bands.
If you plan to run anything near legal limit power then a balun power
rating of 5KW to 10KW is reasonable.
73
Bob, K4TAX
I totally agree with you Stuart, the 135 foot dipole fed with open wire
line and a balanced tuner is the best all band antenna I can think of.
Carl Moreschi N4PY
121 Little Bell Dr.
Hays, NC 28635
www.n4py.com
On 7/10/2013 4:11 PM, Stuart Rohre wrote:
Many hams have used some form of OCF antenna. Not all are horizontal.
For example, My Gap Titan vertical is technically an OCF antenna, since
electrically it is longer on one side of the feedpoint than the other.
The original OCF was probably the "Windom", which was fed with one wire
to the rig. Balance was not a concern as most rigs had single wire
feeds
against Earth. Enough power was used to radiate some and work stations.
Later in the application of the antenna, coax was adapted to feed the
OCF.
Well, the first problem was Windom was an out of balance antenna in
that, unequal currents would be found in the differing length
(resistance) radiators.
To feed with coax, you had to step up to the impedance of the tap point
which was considered to be about 300 ohms, or that was the line used to
feed an OCF converted from Windom feed of single wire to parallel feed
in the 50's.
Now, using balanced 300 ohm line, you had still, unequal currents in
each radiator leg. (The legs were differing impedances with more copper
on one side).
Later, coax became popular. Attempts to feed the OCF dipole with coax
and step up transformers, (balun), still faced the unequal length
radiators and hence unequal currents. Coax feeding a balanced antenna
will have some added radiation on the shield which encloses the center
conductor. The shield can be shown to consist of two conductors, the
outside of the shield and the inside of the shield. Mismatching at some
frequencies resulted in radiation from the outer shield, but also pick
up of vertically polarized local noise.
To further "fix" the OCF, cable chokes were added (also called coax
isolators), usually cores applied to the outside of the coax. Finally,
the OCF might become quiet in an urban noise environments. But, it
still
might radiate a little vertical component, and still was feeding an
inherently unbalanced point having unequal currents in the dipole wires
of unequal length.
I just like the inherent simplicity of the equal legs dipoles of 135
feet total, fed with parallel line, and a tuner; hopefully a balanced
tuner like a double PI Net, which would finally afford the chance to
have equal currents in all parts of the antenna. These have given good
accounts on all bands, and are simple for home construction, with less
weight, typically, than an OCF with its added matching and choking
components.
I would expect an OCF to have some directionality toward one end vs.
the
other, but have never seen this written up. Refined versions like the
"Carolina Windom" (which is not single wire feed, and hence not a
"Windom"), do work well for many folks, but you seem to have to spend
more money and have more weight issues to support the OCF version of
dipoles.
Stuart Rohre
K5KVH
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Disclosure:
I am a Tentec Ambassador and compensated according to the Tentec
Ambassador plan. I serve as a volunteer beta test person for the Omni
VII, Eagle and Argonaut VI products. Otherwise, I hold no business or
employment interest with Tentec.
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