- 12 -
If you purchased the
DXE-UWA-KIT
and would like to add the coax strain relief shown in
Figure
17
, order
DXE-CSR8X-1
for RG-8X
,
or
DXE-CSR213-1
for RG-213 from DX Engineering.
Multi-Band Center Fed Shortened Dipole using Ladder Line Feedline
A simple multi-band dipole may be constructed by first choosing the lowest band on which
operation is desired. The overall length of the dipole antenna should be a shortened half wavelength
as shown in
Table 1
. This antenna will be fed with ladder line and an antenna tuner with balanced
connections. You can also use a DX Engineering external balun connected with coaxial cable to an
unbalanced tuner for tuning the different bands.
Although it may not seem logical, shortening a multi-band dipole intended for 160 through 10 meter
operation to less than 220 Ft. will actually help your wide range antenna tuner cover the lower
frequencies easier. That is because you are using a non-resonant antenna system when you use
ladder line feed systems for multi-band operations. Changing the length of the ladder line will alter
resulting impedances enough so that the tuner may be able to reach a certain frequency that was
giving it trouble. The coax from a DX Engineering 1:1 Balun to the tuner should be kept short;
typically 5 to 15 feet is best.
The
DXE-LL300
- 300 Ω ladder feedline for a multi-band dipole
must
be in
odd
multiple lengths of
1/8 wavelength on the lowest operating frequency, used to optimize the impedance presented to the
balun and tuner over the frequency range of the antenna. This length can be calculated using the
following formula or use
Table 1
. The DX Engineering 300 Ω ladder feedline has a VF (Velocity
Factor) of 0.88.
Formula:
Where:
123 = 1/8-Wavelength Factor, Freq = Frequency in MHz,
0.88 = Velocity Factor of
DXE-LL300
300 Ω Ladder Feedline
Multiply the result times the odd multiple (1, 3, 5, 7, etc) to get the correct length closest to your
required feedline length.
Table 1
Recommended Antenna and Feedline Length for
Shortened Multi-Band Dipoles for easier tuning
Frequency
(MHz)
Shortened
Dipole (Ft.)
Make feedline an Odd Multiple of
this length in Feet (x 1, 3, 5, etc.)
1.8
220
60.1
3.5
110
30.9
5.3
76
20.4
7
55
15.4
10.1
41
10.7
14
29
7.7
18
22
6
21
19
5.2
24
19
4.5
28
19
3.9
Note:
If using an external balun, the feedline length should be calculated to the balun.
