[Elecraft] KX3 horizontal 40 meter antenna loop feed connection - HOW?

[Don Wilhelm]

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?
>
>