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SWR-584C Instruction Manual
reactance into the system. Phase errors compound with multiple half-waves, so it's best to limit
length to one or two phase rotations.
2. SWR, Resonance, and Impedance: Always use SWR rather than Resonance (X=0) or
Impedance magnitude (Z) to optimize simple antennas. SWR is much easier to measure
accurately because feedline length and calibration plane placement aren't critical. Also, if you
cut the element based on Resonance alone (X=0), a significant resistive mismatch may still be
present that shifts minimum SWR elsewhere. And, if you cut for Z=50 Ohms alone, you may
get a large reactive component yielding the same outcome. SWR is always your best bet.
3. Tuning vs Matching: Unlike simple wire dipoles or verticals, many antennas have a built-in
impedance matching networks (Delta, Gamma, Shunt, etc). These antennas may be adjusted
for both resonance and impedance. Begin by setting up the prescribed element length for your
target frequency using the manufacturer's instruction sheet, a model, formula, or X=0
measurement. Then, adjust the antenna's matching network for minimum SWR. In most cases,
the two adjustments will interact, so be prepared to alternately touch them up. Again, minimum
SWR at your target frequency should be the last word.
4. Feedline and SWR: If all is well, feedline length won't have much impact on SWR readings.
You should be able to add or remove cable or measure SWR at any point along the way with
little change. It's normal to see a small SWR drop when adding cable or an increase when
removing it because of resistive loss. But, if you see big SWR changes when altering feedline
length, repositioning the cable, or grounding the shield at any point, suspect a problem.
5. Radiating Feedlines: Common-Mode RF Current flowing along the outer surface of the
coax shield is always be a prime suspect when you see erratic SWR changes. It happens
when the feedline becomes part of the antenna element and radiates RF (hence the sensitivity
to cable length, positioning, and grounding). With a radiating feed, it's virtually impossible to
measure the antenna's actual resonant frequency or SWR because the load presented by the
feedline alters these parameters unpredictably.
With no balun,
"dipole" actually
has three legs.
A Guanella current balun installed at the feedpoint offers the best protection against RF
conducting down the outside of the cable. The balun functions as a choke to isolate the coax
shield from the part of the antenna that's supposed to be radiating. It's also important to run
coax away from the antenna at a 90-degree angle to prevent RF from coupling onto the line by
induction. These two steps not only stabilize SWR, they reduce receiver noise, get rid of RF in
the shack, and allow the antenna to radiate predictably.
6. Defective Cable: Erratic SWR readings will also occur if your coax isn't 50 ohms. Kinks,
water ingress, oxidation, corrosion, bad connectors, improper construction, and even
mislabeling by the manufacturer may be the cause. Check SWR with a dummy load installed
HF/VHF/220MHz SWR Analyzer
Balun "chokes off"
the third RF path
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