SWR-584C Instruction Manual
HF/VHF/220MHz SWR Analyzer
21
fed dipoles) with the longer leg being the dominant vertical radiator. The counterpoise is often
fanned out at the base as sloping radials or as a capacitive-hat structure. Ground-independent
verticals are really "dipoles" oriented in the vertical plane, and they tend to work more
efficiently when elevated well above ground. Mounted close to the soil, they tends to detune
and radiate less efficiency because of ground losses. Most are multi-band arrays with the
larger vertical radiator using resonant elements connected in parallel, traps installed in series,
or a combination of both. Also, most utilize a built-in matching network and balun at the
feedpoint to match into 50-Ohm coax. Tuning procedures may be somewhat complex and
interactive for arrays with multiple bands, so its important to follow the manufacturer's
procedure for tune up.
7.0 Special Measurement Procedures
7.1 Overview:
There are a number of
specialized measurement procedures
you can follow to
extend the versatility of your SWR-584C. These include the following tasks:
•
Measuring and cutting a transmission line stub to
Electrical Length
.
•
Measuring the
Velocity Factor
(
Vf
) of an Unknown Cable.
•
Finding the
Impedance
(
Z
) of an unknown feedline or Beverage antenna.
•
Pre-adjusting an antenna tuner.
•
Testing RF transformers and baluns for isolation.
•
Analyzing RF chokes for
Self-resonance
.
7.2 Measuring and Cutting a Line or Stub to Length:
To cut a matching stub or a resonant
length of transmission line, use the analyzer's default
SWR/Impedance
function in the
Basic
Menu
(
R&X
).
[ ] For
1/4
λ
and odd multiples (1/4
λ
, 3/4
λ
, 5/4
λ
, etc),
terminate the cable with an open
.
[ ] For
1/2
λ
and even multiples (1
λ
, 1-1/2
λ
, 2
λ
),
terminate with a short.
[ ]
Coaxial lines
may be piled or coiled on the floor -- no isolation from ground needed.
[ ]
Balanced lines
require isolation from ground. See the setup outlined in Chapter 5.5.
Next, determine your target frequency (
F
T
)
and
estimate cable length as follows:
[ ] Calculate
1
λ
in
feet
= (
983.6 / Freq. MHz
), or
1
λ
in
inches
= (
11803 /Freq. MHz
)
[ ] Multiply
1
λ
by the
fractional length
you need (eg
λ
x .25 for 1/4
λ
, 1
λ
x .5 for 1/2
λ
, etc.)
[ ] Look up or measure your cable's
Velocity Factor
(
V
f
)
[ ] Convert the cable's
Electrical Length
to a
Physical Length
:
L
PH
=
L
EL x
V
f
[ ] Cut the cable
20% longer
than your calculated
Physical Length
:
L
CUT
= 1.2
L
PH
[ ] Connect one end to the analyzer and terminate the other end as specified (
short
or
open
).
[ ] Tune the analyzer
VFO
to find the
frequency of the lowest Impedance null
.
[ ] Fine-tune, watching the
Reactance
(
X
) display. Adjust as close to X=0 as possible.
