MFJ-1795 Vertical Antenna
Instructions
11
IMPORTANT:
A suitable Ground radial system must be connected to the antenna
mounting bracket if the Ground
Coupled Portable™
Antenna Base is not used.
The SWR can be measured by using a transmitter and SWR bridge or an SWR Analyzer. The
measuring device should be connected to the antenna with a reasonably short length of high
quality 50-Ohm coaxial cable. If using a transceiver and SWR meter, set the transceiver to the
lowest possible power to take measurements.
Please read the following hints:
•
The normal resonant frequency of this antenna is at or below the bottom of each
amateur band.
This allows the user to "trim" a small amount off the
inside end
of the
capacitance spokes to raise the resonant frequency. Conversely, adding a longer capacitance
spoke will lower the resonant frequency of a loading assembly. Spare spokes are included in
case you need to lower the resonant frequency of the antenna.
•
If the suspected resonant frequency is lower than your equipment can detect,
for
example below 6.0 MHz, take one 40 meter spoke entirely off the antenna. Measure the
resonant frequency again. If the resonant frequency still cannot be found, check another
band. If none of these tests results in a good SWR on any frequency, substitute a 50-Ohm
load for the antenna to test the feed line. If the SWR checks good, the antenna assembly will
have to be checked for proper assembly, shorts, or openings at the feed point of the antenna.
•
If the antenna operates normally higher than the band after one spoke is removed,
install all the spokes and trim each spoke in 1/4 inch increments. The spokes for each band
should be cut to the same length.
•
Once the resonant frequency is found,
use the chart located under the section Tuning The
Antenna section of this manual to estimate the amount of the spoke length to cut so that the
antenna will resonate at your favorite section of the band.
CAUTION
:
Always start tuning on 40 meters and adjust each band progressively higher in
frequency
. Adjustment of a lower frequency band will always have the most effect on the next
higher frequency band. The tendency of the interaction is that if you move one band higher ALL
the other bands move higher, but only very slightly. It is always best to "shoot for" the lowest
end of the range you intend to use, and "trim in" by adjusting the bottom loading assembly after
the antenna is in it's final location. ALWAYS work from the bottom band up.
Tuning the Antenna
1.
Measure and record the frequency where the lowest SWR occurs on for each band. The lowest
SWR should be at or below the bottom end of each HF band. The SWR should be below 2:1 at
resonance on each band.
2.
The following is a typical chart for initial measurements of a new antenna before tuning:
40 M 1.2:1 at 6.97 MHz
