Fellas,
I think this subject is being over thought. An OCFD is nothing more
than a dipole cut for the lowest operating frequency with an off center
feed. By doing this, it allows reasonable match for multiples of the
fundamental frequency.
If a center fed dipole is ~50-75 Ohms at say 80 meters, it will be
somewhere around 700-1000 Ohms on even multiples. Feeding the antenna
off the center just puts the feed at a point that reflects a more
reasonable impedance. At this point there are two ways to go, the
Carolina Windom way or the straight OCFD. I have used both
configurations and they do really work well if you can get the antenna
up in the clear. Feeding it with something like LMR-400 keeps the losses
reasonable even with an SWR that may be 4 or 5:1. In both cases a 4:1
balun should be used, a current type if you use the simpler OCFD. Every
Elecraft tuner I have used handles this without breaking a sweat. Happy
DXing!
73,
Barry
K3NDM
------ Original Message ------
From: "Don Wilhelm" <donw...@embarqmail.com>
To: elecraft@mailman.qth.net
Sent: 9/18/2017 10:20:22 PM
Subject: Re: [Elecraft] Question for Wayne about your OCF Dipole
Alan,
The loss is determined by the current in the coax, not that it carries
1/2 the power. It is the same as for a parallel line - the current is
equal and opposite, so both conductors contribute to the loss.
The total loss should be the same as two runs of coax, but that should
be matched loss. This situation is likely mismatched loss - which
according to the feedline loss with SWR can be relatively small, but
present. Consult the charts in the ARRL handbook and the Antenna
handbook for the loss due to SWR - for reasonable SWR and low loss
feedlines it is minimal. The loss due to SWR is minimized for lines
that have a small matched loss. The specification for LMR-400 is 0.7dB
at 30 MHz for 100 feet, so it is small at HF.
In contrast, RG-58 at 10 MHz has 3.6 dB attenuation at 10 MHz per 100
feet.
I may be mistaken (it has happened before), but I am certain that the
loss (in dB) is twice the loss in a single run of coax because the
total length of coax is doubled.
bottom line, with LMR-400 at HF that loss is likely minimal.
For installations that would lead to sharp bends in ladderline or
vintage 300 ohm line or close proximity to conducting surfaces, the use
of parallel coax is a good choice for multiband antennas.
Real open wire may be a better choice if it can be spaced with tension
on the wires and run in spaces that are void of nearby conductors, but
that is difficult to achieve in many situations.
Wayne's "run under the house" may negate the advantages of using
parallel conductor lines and make the parallel coax a better choice.
Your situation may vary, but for low loss coax like LMR-400 at HF, that
may be a good choice for multiband antennas. The only "problem" is the
weight of 2 runs of low loss feedline to the antenna feedpoint. If the
feedpoint is supported by a support pole, then that obstacle is not a
problem.
73,
Don W3FPR
73,
Don W3FPR
On 9/18/2017 9:19 PM, Alan wrote:
Yes, I agree the shields should be connected together at both ends.
I believe the loss is the same as a single run of coax. Each coax has
only half the loss since it carries only half the power, but there are
two of them so the total loss is the same.
That assumes the SWR is the same in both cases. If 100 ohms is a
better match than 50 ohms, then the loss will be lower in the parallel
configuration.
Alan N1AL
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