Vectronics SWR-584C - Manual (Page 21)

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 1/4λ and odd multiples (1/4λ, 3/4λ, 5/4λ, etc), terminate the cable with an open.
[ ] For 1/2λ and even multiples (1λ, 1-1/2λ, 2λ), 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 1λ in feet = (983.6 / Freq. MHz), or 1λ in inches = (11803 /Freq. MHz)
[ ] Multiply 1λ by the fractional length you need (eg λ x .25 for 1/4λ, 1λ x .5 for 1/2λ, 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.