Dave,
Opinions on antennas are connected with a lot of myths and "it works for
me" opinions. You mmight want to read the Antenna and Transmission Line
article on my website www.w3fpr.com. It is not filled with formulas and
math, but can give you some idea of antenna system behavior.
You say you have a 2.2 SWR across the 40 meter band, so that is quite
OK, the KXAT3 will handle that with no problems, but you may need a
tuner to keep the transmitter 'happy'.
Your 20 feet of coax will not have significant loss on any of the HF
bands, but do not coil it up in the field of the antenna. If it is too
long, cut it off and terminate it in a BNC connector to match the KX3
(and the QRP 'standard').
Check the feedpoint impedance (or SWR) on all bands of interest. If it
is above 10:1, then you may have some trouble matching it on those bands
even with the KXAT3 - it all depends ---.
You might try substituting a 1:1 balun for the 4:1 to see if you can
achieve a better compromise between bands.
In your situation, use ladder-line only as a last resort.
Keep the feedline running at right angles from the radiator for as great
a distance as possible so the feedline does not pick up RF from the
antenna. You might find benefit from a balun at the antenna and an
additional 1:1 current choke (often called a line isolator) at the
transmitting end. If you do decide to feed with balanced line (300 ohm
or 450 ohm will make little difference), use something like the Elecraft
BL2 in the shack to transition to coax.
With the close proximity for your radiator to the shack, you may
experience problems with RF-in-the-shack just due to that proximity. If
you confine your operation to QRP, it may not give you any problems, but
if you increase power, the problems may become apparent.
73,
Don W3FPR
On 4/1/2014 5:26 PM, Esquer Dave wrote:
Newbie Dave, KX3 owner with built in ATU. I have a loop antenna feed point line
question. I know this is not an antenna group but I want something optimized
for the KX3 and QRP.
I have installed a 40 meter skywire horizontal loop antenna in my attic, 135
feet in length. I hope to use the antenna on 40-10 meters. I live in a condo
complex, fortunately for me, on the 2nd floor. The distance from the feed point
to the rig vertically is about 20 ft and about 5 ft horizontally through a
ceiling.
Coax or ladder line feed, most say ladder line for low loss?
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