[Elecraft] The K2 and Random Antennas
Ron D'Eau Claire
rondec at easystreet.com
Sun Jun 12 15:20:57 EDT 2005
Tom W3QS wrote:
I'm about to order my K2 and I have a load of questions, but just one for
now.
I am presently restricted to a random wire (perhaps 60 feet or so - OK,
about 20 meters) sloping up to the top of a tree. I am feeding this with 50
feet of RG-8 and have a ground system consisting of two 1/4 wave wires for
40 through 10.
Will the K2 and the KAT2 tuner handle this antenna? Anyone have similar
setups?
--------------------
How are you feeding the wire, Tom? At the end or the center?
Since you are using coax, I'm guessing you are planning on center feed. That
is, you are erecting a "doublet" fed with coax.
If so, then you will experience very high relative losses in your coax. The
antenna will show a lot of reactance on all bands except maybe 40 and 15
meters if you can get 66 feet of wire up. A 66 foot wire is self-resonant at
40 meters. That is, it will show very little reactance. The impedance at the
center will be resistive and somewhere in the vicinity of 50 ohms - a good
match for 50 ohm coax. It will show another resonance at about 21 MHz (15
meters) where it will again show a decent match for coax.
On the other bands the SWR will be very high and so your coaxial line losses
will be very high. Most operators using a non-resonant doublet like that
will choose open wire line or 450 ohm ladder line to minimize these losses.
Those lines have much, much lower losses than coax primarily because of the
lower SWR on the line. For example, if you have a 60-something foot radiator
fed at the center, on 20 meters it will be close to two half waves,
presenting a very high impedance at the center feed point. In the practical
world, the impedance will probably be something on the order of 2000 or 2500
ohms, maximum. If you connect 50 ohm coaxial line to the center, the SWR
will be >40:1. If you use 450 ohm ladder line the SWR will be about 5:1.
That's a huge difference that will save you a lot of RF being wasted as heat
in the coax.
If you simply cannot run the ladder line or open wire line all the way to
the rig, use a short piece of coax for the final few feet, but keep it as
short as possible. Connect one side of the ladder line to the center
insulator and the other to the braid. The coax simply makes a shielded wire
to run the last few feet.
If your feed line is "in the clear" running up to the antenna, I'd not
bother one bit about trying to "balance" it with a balun. Baluns are
unpredictable and often lossy in lines with high SWR. Just hook one side of
the feeder to the center pin of the antenna connector and the other side to
the ground. If you want to try a balun, put it at the end of the open wire
section of your feed line. I'd recommend using a 1:1 balun although
sometimes if you have a very high impedance at the end of the open wire
feeder, a 4:1 balun will bring the value down into the range for the tuner.
If you are feeding your 60-something foot wire at one end, don't use coax at
all. Just bring the end to the center pin at the ATU output. Avoid running
the line any distance inside the house. It may cause RFI and pick up noise
from wiring and appliances. To 'ground' your rig effectively, connect a 1/4
wave long wire to the case of your rig for each band you are operating on.
The ground wires must be insulated at the far end. They will be hot with RF
at the ends. You can run them indoors (along baseboards, etc) or outside.
Just be careful to keep the ends insulated and away from people and pets.
They can inflict an RF burn if touched while transmitting.
You mentioned 40-10. If you can put up 60+ feet of wire, you'll find that it
will work FB on 80 meters as well, either center fed or end fed.
The Elecraft tuners have excellent matching ranges, with the exception of
perhaps the built-in KX1 ATU. The KX1 ATU
was designed to fit in a tiny space, so it doesn't offer the wide range of
the other Elecraft tuners. It is possible to arrange an antenna they can't
match on one or more bands because of the impedance extremes. The usual way
to deal with that is to change the length of your feedline.
Ron AC7AC
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