On Fri, 8 Oct 1999 13:49:42 -0500 brunet@us.ibm.com writes
>
> Are their designs for 80m dipoles covering 3.5 through 4.0
> preferably without a trip to the antenna?
> 1) a pair of dipoles with one feedpoint, one cut for each end of the
> band
>
>
> Thanks, Pete ws4g
>
This will work fine if the antennas do not couple to each other
which can be achieved by placing them at 90 degrees to each other.
Another approach is described below by W4RNL based on
a piece by W6NL. I believe K4VX may have written on this subject also.
de Tom N4KG
......................
Dave Leeson brought to our attention an interesting technique for
achieving
wide-band operation on the lower HF bands, derived from mentions in texts
and references in ARRL publications by Frank Witt, AI1H. The technique
involves choosing a geometric average frequency between two frequencies
of
interest--then, for that frequency, cutting a length of 50-ohm coax a
multiple of 0.5 wl (allowing for velocity factor), with a 0.25 wl length
of
75-ohm coax (again, allowing for velocity factor) at the station end of
the
line.
This is a bit of follow-up that seemed interesting as the numbers emerged
from some modeling exercises. I thought I would pass them on.
The SWR at the antenna relative to 50 ohms does not change, but line
losses
at the lower HF bands are not--for many purposes--sufficiently large to
make a case against this or other wide-banding techniques with coaxial
feed
lines. The factors that produce wide-band operation (using the
conventional <2:1 SWR measure for convenience) include the impedance
transformation along the transmission line at frequencies above and below
the dipole resonant length and the physical lengths of coax cut for that
resonant frequency.
Since the situation described by Dave can be modeled directly in NEC-2 or
NEC-4, using the transmission line feature available on NEC, I decided to
look at some SWR curves across 80 meters. My dipole was resonated at
3.75
MHz to ensure that the 2:1 SWR points fell within the band. I used the
NEC
mathematical models of 50-ohm, 0.765 VF transmission line for 0.5, 1.0,
1.5, and 2.0 wl, followed by a 0.25 wl section of 75-ohm, 0.66 VF cable
to
the feedpoint/station end.
My dipole at 120' over level medium ground had an independent feedpoint Z
of 76 ohms. I am reading from graphs at this point, but hope to make the
data more precise later.
50-ohm Length Lower limit Upper limit Bandwidth Lowest SWR
0.5wl 3.57 3.96 0.39 1.45
1.0 3.55 3.96 0.41 1.30
1.5 3.57 3.93 0.36 1.10
2.0 3.58 3.91 0.33 1.05
The table has several interesting features. First, for a 0.5 wl 50-ohm
run, there is only one SWR minimum, roughly at the self-resonant
frequency
of the dipole. With an independent feed Z of 76 ohms, the SWR shows a
shallow curve.
Second, for lengths of 50-ohm coax of 1 wl and up, the double minima
curve
emerges. With the given independent dipole feed Z, bandwidth is greatest
with a 1 wl run and diminishes above that. In fact, as the length of 50-
ohm coax is increased, the rise in SWR is steeper at both the low and
high
ends of the band. However, the minimum SWR become lower with increases
in
50-ohm line length. The SWR at the dipole's self-resonant frequency
remains almost unchanged (1.4 to 1.5) throughout.
I reran the exercise, each time lowering the dipole height by 10' in
order
to see what effect an increasing independent feed Z might have on the
curves. I adjusted the independent dipole length as necessary for
resonance and imported that length to the model with transmission lines.
First the numbers:
110' up" Z=83 ohms
50-ohm Length Lower limit Upper limit Bandwidth Lowest SWR
0.5wl 3.55 3.97 0.42 1.35
1.0 3.54 3.96 0.42 1.25
1.5 3.56 3.93 0.37 1.10
2.0 3.58 3.91 0.33 1.05
100' up" Z=89 ohms
50-ohm Length Lower limit Upper limit Bandwidth Lowest SWR
0.5wl 3.53 3.97 0.44 1.30
1.0 3.53 3.96 0.43 1.20
1.5 3.56 3.92 0.36 1.05
2.0 3.58 3.90 0.32 1.05
90' up" Z=92 ohms
50-ohm Length Lower limit Upper limit Bandwidth Lowest SWR
0.5wl 3.53 3.99 0.46 1.25
1.0 3.54 3.96 0.42 1.10
1.5 3.56 3.92 0.36 1.01
2.0 3.58 3.90 0.32 1.05
As the independent feedpoint impedance of the dipole increases (within
the
boundaries of the test runs), the performance of the 0.5 wl 50-ohm coax
run
improves. The curves over all the tests for this length of line are
largely congruent, and the improved performance with increasing feed Z
occurs because the impedance presented to the 0.25 wl 75-ohm matching
section grows closer to the value needed for a 50-ohm impedance at the
transmitter end.
Although not especially extreme, the slope of the SWR curves for the two
longest runs of 50-ohm line grow steeper with increasing independent
dipole
feed Z. Band edge values are about 5:1 for 2 wl runs and 4:1 for 1.5 wl
runs. By contrast, with a 1.0 wl run, the band-edge SWRs are close to
3:1,
while with a 0.5 wl run, the band-edge values are about 2.7:1 for the
worst
case and 2.5:1 for the best case (at the lower end, with lower values at
the upper end of the band). These values do not account for dissipative
line losses that ordinarily show up at the shack end of the line as
slightly lower SWR readings.
So, what is the use of all this modeling? If all one needs are two
low-SWR
points within the band, then any of the 50-ohm lengths might be in order.
However, if one is seeking the maximum possible coverage of 80-75, then
one
might consider restricting the length of initial 50-ohm coax run to 0.5
wl
or at most 1.0 wl. One can insert the 75-ohm matching section at this
point and use 50-ohm coax the rest of the way to the shack. Since the
impedance values fluctuate across the band by as much above 50 ohms as
below it, some further impedance transformation will occur, but it will
be
in virtually all cases less radical at the band edges than would be the
case of using the longer initial 50-ohm runs indicated in the charts.
The numbers and suggestions are limited, of course, by the limitations of
the models and modeling program. They may require field adjustment in
accord with the circumstances of any given installation. However, I hope
they are useful to those thinking about using the feed system Dave Leeson
has brought to our attention.
-73-
LB, W4RNL
L. B. Cebik, W4RNL /\ /\ * / / / (Off)(423)
974-7215
1434 High Mesa Drive / \/ \/\ ----/\--- (Hm) (423)
938-6335
Knoxville, Tennessee /\ \ \ \ / / || / (FAX)(423)
974-3509
37938-4443 USA / \ \ \ \ ||
cebik@utk.edu
URL: http://funnelweb.utcc.utk.edu/~cebik/radio.html
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