----- Original Message -----
From: "D. Chester" <[EMAIL PROTECTED]>
To: <[email protected]>
Sent: Wednesday, November 14, 2007 2:02 PM
Subject: [AMRadio] Re: Open Wire Line
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.
In a perfect world only.
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.
Commercial and shortwave broadcasters went to extremes to minimize feedline
radiation. I doubt that many ham installations can claim that. Twist, wind,
rain, less than perfectly vertical drops, feedthrus/standoffs, ground
reflections can all affect radiation. The classic photo of a feedline
swooping away down and horizontal is a classic example of a radiation
generator.
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 standing waves.
Radiation will ALSO occur from common mode currents developed by the
mismatch of the line to antenna impedences.
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".
No offense but I dont need a tutorial on common mode issues, Ive authored
several papers on the subject over the past 25+ years.
A balun in the true sense of the definition which is a voltage device does
not suppress common mode; installing one at the antenna feed point performs
the function of a 2 port device. What is commonly called a sleeve, common
mode, or choke balun is really just a RF choke performing a similar function
to slipping a ferrite bead over a transistor lead to stop parasitics. It
forces (not transforms) almost equal currents into a dipole. Many yagis use
a coiled up length of coax to perform the same function.
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.
A dipole will operate efficiently no matter what is used for a feed line;
you are mixing antenna and feedline theory together. What goes in to a
dipole or any other antenna gets radiated minus I squared R loss. I refer
you to the Antenna Engineering Handbook by H. Jasik.
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 completely agree. As I said earlier, if my amp will load into the cable I
could care less at HF.
Maxwell has been trying to get this thru many thick heads for decades. It
would also result in a lot less junk tuners being sold.
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 regularly operate with up to 4:1 when covering the 80/75M band with the
high dipole. Its never stopped me from working anything I can hear or
maintaing QSO runs in a contest. With ~ 450' of CATV 3/4" hardline my
maximum VSWR loss is under 0.4dB. Total loss is then around .8db at the high
end of 75; I can live with that. A 160M dipole is in parallel and that loss
never exceeds 0.4dB. I could never do that good with open wire on either
band by running 250' thru the woods and then up the tower. That same antenna
is used on all 3 WARC bands sans tuner and Ive WAZ plus around 300 countries
on all 3 bands. I shudder at the 12M loss!
I forgot about a more recent experience with open wire which was about 15
years ago. I tried to make a 8JK work on 80/75 but it performed so poorly
compared to either dipole or the 4 Square that it came down in a month. It
was 80' high strung between 2 other towers and the feed came straight down
to the tuner.
I have found measurable loss on that 140' run, even on 160m with the SWR
adjusted exactly to 1:1.
If you measured over 0.375 dB loss then either the cable or measurement was
defective. I dont consider that a loss to even talk about.
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.
It is rated used for exterior applications but the silver plated shield
reacts with the jacket and dielectric over time. That is why most silver
plated cables are Teflon.
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.
Thats close enough for government work to published loss. Again it is
inconsequential and as useless as worrying about VSWR on HF.
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.
Disagree there. Using a pi network and 50 Ohm load at the TX you will
determine the transmitter efficiency and it should be at least in the high
60's for Class C. My 3-500Z linears are around 62% right up to 10M. Hook the
Bird and load at the far end of the coax and you then have the combined
efficiency. Antenna efficiency is something alltogether different.
I have no idea what PP amp link coupled efficiency runs at. End link can be
measured at 50 Ohms but how the heck do you determine center link accurately
at 600 Ohms? A 12:1 balun with 98-99% efficiency into a load right there
will be the most accurate IMO. RF ampmeters are notoriously inaccurate and
good for only a relative tuning indication. I suspect that the link
efficiency is not as good as many believe. One way to statically test is
with a VNA and the tank coil terminated in the tube load impedence. Ive a
VNA here and use that method to look into VHF/UHF amps I design and build as
well as the occasional customer HF industrial amp.
The N2PK developed, PC based VNA project that has been getting very popular
lately will be something that all builders should be considering. It can
take most of the guesswork out of all sorts of projects.
Carl
KM1H
Don k4kyv
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