Antenna Spoke Base Series
Page 9
Tuning Tips When Using the Antenna Spoke Base Series
A pair of HamStick-style antennas is a shortened halfwave dipole. When multiple pairs of dipoles are installed on
the Antenna Spoke Base, it becomes a type of fan dipole. This type of antenna works because the impedance at
resonance for any of the dipoles is around 50 Ohms, but for the other non-resonant dipoles it is very high. There
will still be a small amount of interactions with the other bands, usually causing the antennas to resonate at a
somewhat lower frequency and have a slightly reduced bandwidth than they would, if used separately. One case
that may cause more significant interaction are antennas for bands that are close to the third harmonic of a lower
frequency band being used, such as using 40 meter and 15 meter antennas together. Table (1) shows the band
combinations that should be avoided. Also, do not use two pairs of antennas for the same band.
Table 1. Band Combinations Not to be Used Together.
When tuning the installed antennas, start with the lowest frequency band and work upward. Using the example in
figure (1), tune the 40 meter dipole for resonance, then 20 meters, then 17 meters, and finally 10 meters. You should
adjust both whips the same length for each band pair.
Also, be sure to follow the manufacturer’s tuning instructions
provided with the antenna.
Note: the generic HamStick-style mobile antennas used during testing of the Antenna Spoke Base were very long.
With the stainless-steel whip fully extended, the 40 meter antenna resonated at 6650 KHz and had a 2.0:1 bandwidth
of 60 KHz. The initial SWR at 7200 KHz was 16:1. The stainless-steel whip had to be inserted 12 inches into the
fiberglass base to shorten the antenna enough to resonate in the lower part of 40 meter ham band. When pushed
in around 13 inches, the whip began interfering with the loading coil in the fiberglass base, as indicated by an
increased SWR at resonance. The shorten the antenna further, it would have been necessary to physically trim the
base of stainless-steel whip. The minimum SWR achieved (at resonance) was 1.5:1. The 20 meter pair we tested
exhibited similar characteristics, initially resonating at 13,070 KHz with a 2.0:1 bandwidth of 240 KHz.
Selection of Antennas for the Antenna Spoke Base Series
The mobile antennas do not come with the Antenna Spoke Base Series. Several manufacturers make HamStick-style
mobile whips and almost any monoband mobile antenna w
ith a 3/8” x 24 stud mount can
also be used. Two of the
more readily available choices are shown below and are provided as examples of the style of antenna intended to
be used with the Antenna Spoke Base Series. Chameleon Antenna
TM
does not endorse any particular product or
warrant their performance. They are available from the quality retailers that also sell Chameleon Antenna
TM
products.
1.
MFJ 16__T Ham-tenna
TM
series monoband HF mobile antennas (fill in the band for the model number).
Available from 75 to 6 meters. See
, and
WHEN
USING
DO NOT
USE
80m
30m
60m
20m
40m
15m
30m
10m
17m
6m
