The antenna, including the 300
Ω
feedline, should also be mounted as far from other structures
as possible. This includes the ends of the wire elements, which are actually the most sensitive
part of the antenna. Any objects, metal in particular, within the near-field radiation pattern can
affect the impedance and radiation pattern of the antenna. The feedline should also come away
from the antenna at right angles for best performance.
Most installations involve compromises due to local terrain, available supporting structures, or
other restrictions. Do the best you can with what you have.
Installation
You should have two rolls of element wire, one 65 foot roll of ladder feedline, and a hardware
packet. Unroll the 2 wire elements and feedline using a hand-over-hand technique. This will
prevent kinks and allow the wires to lay flat.
Feedline
The SWA-148 dipole includes 65 feet of high quality 300
Ω
ladder line used for the feedline.
This length is the minimum for this antenna and should not be shortened. If your installation
results in excess feedline, it can be zigzagged while suspended in air, but it can't be closer than a
few conductor widths to metallic objects and
should not be coiled.
If it is necessary to pass close
to a metallic object, twist the line to partially balance the effect on both sides of the feedline.
If you need additional feedline length, the feedline must be in
odd
multiple lengths of 1/8-
wavelength on the lowest operating frequency, in this case 1.8 MHz, to optimize the impedance
presented to the balun 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 line has a VF of
0.88.
Note:
It might be more convenient to use an external 1:1 balun and convert to coax rather
than add additional 300
Ω
ladder line.
Calculate the Proper Feedline Length
Formula:
123 / Freq (MHz) x VF: Multiply the result times the ODD multiple (1, 3, 5, 7, etc) to get
the correct length closest to your required feedline length.
Where:
123 = 1/8-Wavelength Factor
Freq = Frequency in MHz
- 4 -
VF = Velocity Factor of Feedline
