MFJ-259C Instruction Manual
HF/VHF SWR Analyzer
8
Note that the MFJ-259C also has Advanced antenna-measurement modes that are described in Section-5.0.
However, unless you have a strong working knowledge of RF systems, you may find these added modes of limited
value. Most represent more technical ways of expressing the same data offered by the basic modes.
Antenna hints:
1.) Measuring Antenna Impedance: For complex impedance measurements, you must install the analyzer as
close as possible to the element's feed point (within or 1-2 degrees of phase shift) or use a precisely cut 1/2-
wavelength of cable displace the calibration plane by a controlled amount (360-degree phase rotation).
2.) Electrical Half-Wavelengths of Cable: Installing a half-wavelength of cable between the load and the
analyzer will rotate phase a full 360 degrees so that no apparent transformation takes place in the line.
However, the response will only be transparent on one discrete frequency. Even a small frequency change will
begin to skew your impedance readings and may even shift the antenna's resonant frequency because the cable
will begin to introduce its own reactance into the system. Errors will compound with multiple half-
wavelengths, so limit cable to one or two phase rotations at most!
3.) SWR, Resonance, and Impedance: It's always preferable to measure SWR rather than resonance or
impedance magnitude (Z) as the standard for adjusting your antenna. By definition, minimum SWR (1:1)
and maximum power transfer occur when the source, transmission line, and load impedance are all of equal
value. Resonance occurs when reactance fully cancels at the antenna's feed point and the load becomes purely
resistive (Xc+XL=0). Although Minimum SWR and Resonance often coincide closely, they are not directly
correlated and rarely occur on exactly the same frequency. If your antenna doesn't happen to present a 50-ohm
load at resonance, there will still be resistive mismatch in the system and lower SWR may actually occur on
some other frequency. By the same token, if you adjust your antenna for an Impedance reading of 50 ohms, the
load could have a substantial reactive component (e.g. R=46, X=17) that elevates SWR and shifts the
minimum-SWR point to a different frequency. SWR is always your best predictor of antenna performance!
4.) Tuning and Matching: Unlike simple wire dipoles, many antennas such as Yagis and verticals are adjustable
for both resonant frequency and impedance match. Begin by setting these antennas for the element length
prescribed in the instruction sheet -- or the calculated length. Then, adjust the matching network for minimum
SWR. The two adjustments (tuning and matching) are separate but often interact, so be prepared to alternately
readjust both element length and network setting to achieve minimum SWR at your exact frequency of
interest.
5.) Adding and Removing Feedline: You should be able to add or remove lengths of feedline (or measure SWR at
any point along the feedline) without observing a significant change in SWR. It is normal to see SWR drop
slightly as cable is added, or see it increase slightly as cable is removed because of changes in resistive loss.
However
(a.) if your SWR measurements change a lot with relatively small changes in cable length, or
(b.) SWR changes as the cable is moved around, or
(c.) SWR changes when the coax shield is grounded at some point part way between the antenna and the
radio, look for a feed problem!
Here are some possibilities to check:
6.) Common Mode Current: Your coax may be carrying Common-Mode Current on its outer shield and radiating
RF. To eliminate this problem, install a Guanella current balun at the feed point. It will isolate the shield from
the radiating portion of the antenna, stabilize SWR, reduce receiver noise, and suppress "RF in the shack".
Installing a balun is good engineering practice -- and always worth doing!
7.) Defective Cable: Your coax may not really be 50 ohms. Kinks, water ingress, oxidation, corrosion, bad
connectors, improper manufacturing, poor quality, or even mislabeling may be the cause. Check SWR with a
dummy load installed at the far end of the cable. If the SWR is elevated or the Impedance (Z) fluctuates very
much as you tune the analyzer's VFO, suspect defective cable.