14
Antenna Considerations
An antenna tuner will not
improve
your antenna–it can only tune out the antenna's reactance. In general,
use the longest, highest antenna you can put up, and keep the antenna away from power lines and buildings.
The KAT1 will work with a wide variety of antennas, some of which are covered below.
Ground System
A good ground system will provide efficient radiation, better antenna loading characteristics, less RF on the
chassis, and safer operation. Use a minimum of two radials, cut to 1/4 wavelength on the lowest band used.
Do I Need a 1:1 SWR
?
No—this is a common misconception. For example, if the SWR is 2:1, the loss in transmitted signal
strength will be only about 0.5 dB. Of course the KAT1 always tries to hit 1:1, and with most antennas it
will find an SWR below 1.5:1 on most bands. If the SWR is higher than 3:1, try using a balun (see below).
Resonant and Multi-Band Antennas
With resonant antennas such as dipoles, beams, or verticals, the KAT1 will typically improve the match on
each band used. It may also allow operation on additional bands, although with beams, the radiation pattern
may change completely. With short, loaded whips the tuner may provide multi-band coverage, but such
antennas are generally ineffective due to the low radiation resistance. We recommend random-wire
antennas for portable use (see below).
One useful trick with coax-fed antennas is to short the center conductor and braid together and treat the
system as if it were a random wire. This may allow you to operate well below the antenna’s intended bands.
Random-Length Antennas
The KAT1 is optimized for use with long, random-length wire antennas, since these are the easiest
antennas to set up in the field (you just toss the wire in a nearby tree). In most cases you can connect such
an antenna (and a few ground radials) directly to the K1, with no feedline. However, watch for RF
problems, especially if the wire is exactly a half-wavelength long or any multiple thereof on a given band.
The KAT1 also works with loops. An untuned loop of wire 30 feet long (or longer), formed into any shape,
can be matched on most bands. A loop may work well even if you can’t lay out a lot of ground radials.
Using Baluns and RF Isolation Chokes
A balun converts a balanced antenna or feedline to unbalanced (one side grounded) to work with your
tuner. A balun can also perform an impedance transformation and/or help isolate the antenna from the rig 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 or 4:1 balun is often used
with open-wire line. A 1:1 balun or coaxial isolate choke (current balun) will also work in many cases.
Balun design is covered in many publications, including the
ARRL Handbook
. Also check the Elecraft web
site (
www.elecraft.com
) for antenna-related application notes.
