|[AMRadio] Re: Open Wire Line|
k4kyv at charter.net
Wed Nov 14 14:02:48 EST 2007
> In addition, VSWR on open wire will radiate and can cause all sorts of
> problems from RFI/TVI, telephones, and strange pattern distortion not
> encountered with coax using a sleeve choke at the antenna.
I have to disagree an that. If the rf currents in the open wire feeders are
closely balanced for the full length of the transmission line, there will be
negligible radiation regardless of VSWR The out of phase currents in the
adjacent wires almost completely cancels out and this is unrelated to SWR.
There is an infinitesimal amount of radiation because there is a finite
amount of spacing between the wires, but this can be neglected if the wire
spacing is a very small fraction of a wavelength, as is the case with 2-6"
spacing at HF.
Radiation will occur only if there exists common-mode current on the open
wire feeders, but the same goes for coax cable, where the common mode
appears as rf current on the outside of the shield. This current will have
You can think of coax as a three conductor line: the centre conductor, the
interior of the shield and the exterior of the shield. The balanced
transmission line currents exist between the centre conductor and the
interior of the shield. There should be zero current on the exterior of the
shield. That is the purpose of a balun, to suppress this current over the
exterior. Any current over the exterior of the shield is "common mode".
One of the advantages of open wire line is that its loss is negligible even
with VSWR as high as 10:1. That is the definition of a tuned feedline. An
untuned line is one that operates flat, or as near to 1:1 as is practicable.
Tuned feeders allow the dipole to operate at nearly equal efficiency from
one end of the band to the other. By definition, this requires a
substantial SWR on the feeders.
With coax, there is way too much anxiety amongst amateurs over SWR. It is a
waste of time to spend hours and hours trying to trim a 1.2:1 SWR down to
1:1. I use about 140' of RG-213 between my transmitter at the shack and the
antenna tuner shelter at the base of the tower, where I have separate tuners
pre-tuned for each band. I don't worry about SWR's below about 2.5:1 on
that coax transmission line, even at maximum power.
I have found measurable loss on that 140' run, even on 160m with the SWR
adjusted exactly to 1:1. It has measured about the same with RG-213 and
RG-214. I replaced the 214 because it jacket was deteriorating from UV.
Apparently it was not designed for exterior use. I now run 213 directly
buried, using a variety designed for that purpose. Regardless of the coax
used, at 100 watts at the transmitter, I measure no more than about 93 watts
at the opposite end into a 50-ohm dummy load. The old RG-214 had
deteriorated to the point that I measure only about 80 watts at the far end,
due to moisture contamination from the deteriorated jacket, and creatures
that ate tiny holes in the jacket of the buried section.
> Whether it is coax or open wire the tuner losses themselves will be more
> than the feedline in most cases, even with the best designs.
Plus losses in the PA tank circuit. The combined losses of tank circuit,
tuner and feedline are substantial when added together. The 70% or so
efficiency given in the tube manuals represents the plate loss in the tube
itself. Combine all these losses, and a class-C final is apt to be more
like 50% efficent in terms of DC input VS rf input to the radiating element
of the antenna.
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