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Antenna Considerations
An antenna tuner will not
improve
your antenna–it can only tune out
the antenna's reactance. Use the longest, highest antenna you can put
up, keep the antenna away from other objects, and use a good ground
system.
Stray RF
At high power, you may experience RF feedback problems when using
electrically short antennas, end-fed antennas, very short feedlines, or
any combination of these. Improved RF isolation or grounding may be
required, as discussed below.
Ground System
A ground system will provide efficient radiation, better antenna
loading characteristics, less RF on the chassis, and safer operation. A
buried ground rod and a large array of radials is best, but connections to
plumbing and house electrical ground wiring will often suffice.
SWR Requirements
A low SWR (less than 1.5:1) won't necessarily contribute to a better
radiated signal. For example, if the SWR is 2:1, the loss in transmitted
signal strength will only be around 0.5 dB relative to 1:1. However, a
low SWR
can
be important at higher power levels. The K2/100, like
other full-power transceivers, will automatically limit maximum power
output to protect the final transistors if reflected power is too high.
Of course the KAT100 always
tries
to hit 1.0:1, and with most
antennas it will find an SWR below 1.5:1 on most bands. On antennas
or bands where this is not possible, you may want to reduce the
POWER setting manually.
Resonant and Multi-Band Antennas
The KAT100 will tune up nearly any antenna cut for 40 or 80 meters
on all bands. With multi-band antennas such as beams or verticals, the
KAT100 will typically improve the match on each band used. It will
also allow operation on additional bands.
Random-Length Antennas
A wire antenna, end-fed or fed with balanced line, is an excellent
choice for use with an ATU. Nearly any piece of wire over about 30
feet long and end-fed can be tuned up on 80-10 meters (preferably
working against a good ground or at least two radials). A longer wire or
additional inductance in series will be needed to cover 160 meters.
For portable operation (Field Day, etc.) a good strategy is to put up
two
wire antennas. You can then do quick comparison tests as often as
necessary using
A N T 1 / 2
. With two long-wires oriented in different
directions, differences of from one to three S-units may be observed on
some signals.
Using Baluns and RF Isolation Chokes
A balun is a device that converts a balanced antenna or feedline to
unbalanced (one side grounded) to work with your ATU. A balun can
also perform an impedance transformation, and may help isolate the
antenna from the tuner to reduce RF pickup. You may need a balun if:
(1) you're using balanced feedline; (2) you're having trouble matching
an antenna on one or more bands; (3) you notice RF feedback or get
an RF burn. A low-loss, broad-band, 4:1 balun is a good choice, and
may solve all three problems. A 9:1 balun is often used with 450 or
600-Ω open-wire line, but since the tuner can handle a wide range of
impedances, 4:1 may work just as well. A 1:1 balun will also work in
most cases. All baluns exhibit some loss, but the loss may be negligible
if the balun is used properly.
RF isolation or "choke" baluns can be very effective at reducing RF
feedback. These baluns use a large number of ferrite beads or cores
slipped onto coax, and can be used between the transceiver and the
ATU, or between the ATU and antenna (if the antenna is coax-fed).
Commercially-built units (usually in the form of large tubes) can be
obtained from many different suppliers.
