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SWR-584C Instruction Manual
at the far end of the cable. If it is elevated or the Impedance (Z) fluctuates very much as you
tune the analyzer's VFO, suspect defective cable.
7. Lossy Cable: Coax may exhibit excessive loss from contamination or may have too much
normal attenuation for use at higher operating frequencies. To measure loss, unhook the cable
and use the analyzer's Coax Loss mode to check it against the factory specifications.
6.2 The Coax-Fed 1/2-Wave Dipole:
Baluns (again): The 1/2-wave dipole is a balanced radiator and coaxial cable represents an
unbalanced feed system, so it's especially important to install a balun at the feedpoint. Your
balun could be a number of turns of coax wrapped into a coil several inches in diameter and
taped to the feedpoint, or it could be a complicated affair with many interconnecting windings
on a ferromagnetic core. The best choice is a simple 1:1 Guanella "current" balun wound on a
toroid core made from material with the appropriate permeability (43-Mix preferred for most HF
applications). If the feedline isn't isolated, it may load the element and impact a number of
parameters -- including your calculations for the correct element length.
Tuning to Frequency: A dipole's minimum-SWR frequency is mainly determined by element
length. If it's too long, minimum SWR will occur below the target frequency. If too short, it will
occur above. The formula for length is shown below:
Length in Feet =
Length is primary, but other factors may also enter in. "Fat" wire or larger tubing tends to lower
the frequency below formula by introducing capacitive loading. Dipoles made with jacketed
wire may resonate low because the velocity of propagation (Vp) is slowed by the insulation.
Ground conductivity, soil moisture, proximity to other wires, nearby structures, and metal
surfaces also have an impact. And, some specific types of dipoles (inverted "V", NVIS, OCFD,
etc) may tune a little differently because of how they are configured.
Scaling: With so many variables, it's usually better to cut your antenna 5%-10% longer than
formula, put it up, measure the SWR, and calculate a Scaling Factor to nail down the exact
length you need at your specific location. To scale for length, follow this procedure:
[ ] Calculate element length, then multiply the result x1.05 to make it 5% longer.
[ ] Built it, then measure and write down the exact length in feet. Call it L
[ ] Put the antenna up in the location where you intend to install it permanently.
[ ] Set the analyzer for SWR (R&X) and tune the VFO for the minimum SWR reading.
[ ] Write down the minimum SWR frequency as F
[ ] Write down the target operating frequency as F
[ ] Calculate a Scaling Factor (S
If F
is a lower frequency than F
R
If F
is a higher frequency than F
R
1.85 Mhz = 253'
468
1.925 MHz = 243'
Freq. in MHz
3.6 MHz = 130'
3.85 MHz = 121' 7"
) to determine the amount of change needed:
F
, let S
T
, let S
T
7.15 MHz = 65' 5"
10.12 MHz = 46' 3"
14.25 MHz - 32' 10"
18.11 MHz = 25' 10"
(reference frequency).
R
(target frequency)
T
= F
/F
(scaling factor <1.0).
F
T
R.
= F
/F
(scaling factor >1.0).
F
R
T
HF/VHF/220MHz SWR Analyzer
21.25 MHz = 22'
24.93 MHz = 18' 9"
28.5 MHz = 16' 5"
50.15 MHz = 9' 4"
1.
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