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Page 15
A balun is an electrical circuit used to help resolve the inherent problem of feeding an antenna
with an electrically unbalanced (coax) feed line. It is intended to present an infinite impedance
to any RF current that might otherwise flow on the outer conductor (shield) of the coax pro-
ducing radiation from the line. This current, if high enough, can cause heat buildup and po-
tential damage to the radio as well as a distorted radiation pattern.
Why is it Optional ?:
In the normal configuration, ground mounted with 12 or more radials, the ground will bleed/
drain the unwanted RF signal from the coax shield.
When Should You Use A Balun ?:
•
When elevating the base of a vertical
antenna above the ground
•
When only a few radials are used
•
When the coax run is shorter than the radials
•
When the ground condition is poor
•
Unusual SWR readings on one band
Balun Installation
The balun is shown in figure 25. There is a mount-
ing flange with 4 holes molded into the housing.
There are numerous options for mounting the bal-
un—by far the most popular is to mount the balun
on an adjacent post or similar structure, or even on
a piece of wood that rests on the ground. Figure 26
shows how to connect the balun when using the
BigIR without the 80m coil. Figure 27 shows how to connect the balun with a BigIR and op-
tional 80m coil.
Figure 25
Antenna
Radio
Ferrite
Toroidal
Core
Coax
Figure 23
18
Figure 26
Figure 27
Optional (1:1) Balun
Summary of Contents for SmallIR
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