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SWR-584C Instruction Manual
HF/VHF/220MHz SWR Analyzer
[ ] Calculate the new (target) antenna length (L
) as follows: L
= L
x S
.
2
2
1
F
Example: Your 132-foot dipole has minimum SWR at 3.750 Mhz. You want it at 3.900 MHz.
The element is presently too long, so: S
= F
/F
= 3.750/3.900 = 0.96.
F
T
R
The new length will be: L
= L
x S
= 132' x .96 = 126.7'.
2
1
F
Scaling eliminates time-consuming "cut-and-try", but it only works on full-size dipoles and
verticals with no loading coils, matching networks, traps, stubs, capacitance hats, etc.
Dipole Height: Dipole resistance is mainly influenced by proximity to ground. Most dipoles
match quite well through 50-Ohm coax at normal residential mounting heights (hence their
popularity). By altering height, you may be able to improve your match. However, don't let
SWR be your only consideration! If the change places too much stress on high tree branches
or changes the angle of radiation unfavorably (local vs DX), why change? A small
improvement in SWR will have little impact on your station's overall performance.
Radios, Impedance, and SWR: When antenna and feedline aren't precisely matched, we
know the coax will act like a transformer and modify the value of the load appearing at your
radio (same dynamic that applies to your analyzer's calibration plane). However, if you use
good-quality 50-ohm cable and set your station up properly, most solid-state radios have no
problem handling these impedance excursions as long as the SWR remains below 2:1. As
Section 6.1 pointed out, your SWR shouldn't change even though the impedance may shift
around because of phase rotation in the cable.
6.3 Verticals:
1/4-Wave Vertical: All 1/4-wave monopoles require a good ground system. If your radial
system is poor, the "good earth" will swallow up a large portion of your signal as ground loss.
In fact, you can often gauge the integrity of your radial field by measuring the antenna's driving
resistance (the value of R when X=0). It should be low -- around 25-35 Ohms. If your 1/4-wave
vertical shows a tidy 1:1 match through 50-Ohm cable, that's your cue to lay out more radial
wire. Radials placed on the ground need not measure exactly 1/4-wave to be effective, but a
minimum of 16 is recommended and more is always better. Antenna books cover radial
systems extensively, although not all authors may agree on the best way to construct them.
A 1/4-wave vertical tunes the same way as a dipole -- lengthen to lower operating frequency
and shorten to raise it. Monopole length can be scaled unless the element is loaded with a hat
or a loading coil. Because the impedance is (or should be) quite low, most require matching at
the feedpoint to make the transition up to 50 Ohms (shunt matching coil, L-network, auto-
transformer, etc.). HF monopoles with a good radial system underneath are efficient, have a
very low angle of radiation, and make excellent DX transmitting antennas.
Ground-Independent Verticals: These antennas don't require radial systems because they
have a counterpoise of some sort built in. Most are configured as multiband OCFDs (off-center
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