KX3 horizontal 40 meter antenna loop feed connection - HOW?

[Esquer Dave]

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?

I am stumped, confused and bewildered about how to feed my KX3. Opinions are ALL over the map with NO consistency. I have researched the Skywire google and yahoo groups and the ARRL docs.  Just tell me what works best for my KX3 and how to do it.

If I feed with 450 ohm ladder line, how do I transition from the ladder line to the rig? Is it antenna ends to 20 ft of ladder line to the balun and then a short stub of coax? What is short, 1 to 5 ft? Do I cut the ladder line long and what do I do with excess? The ladder line has to stay mostly vertical?

I have the BNC to binding post adapter as well. Can I just feed ladder line to the binding posts directly with maybe some kind of strain relief?

It sorta works now with the antenna loop connected to an MFJ913 4:1 balun to RG8X coax to rig. SWR on 40 meters is about 2.0 to 2.2 across the band (low to high) as measured externally before connecting to the rig. Am I good enough? The ATU handles it quite nicely bringing it down to 1:1.

I’ve tried feeding the coax directly, with no balun, but that made the SWR much worse externally, 5:1 and up.

Remember, be gentle to this newbie and TIA,

Dave Esquer, K6WDE
[hidden email]






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[Ross Primrose]

Ladder line doesn't like to be near "stuff"...  Feed it with coax, even
with a high SWR on the coax your losses will be negligible over that
length.  A 1:1 or 4:1 current balun at the feedpoint would probably be a
good idea, but isn't a deal breaker...

73, Ross N4RP, who needs to get his 160m loop back up before the leaves
come out ;)

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?
>
> I am stumped, confused and bewildered about how to feed my KX3. Opinions are ALL over the map with NO consistency. I have researched the Skywire google and yahoo groups and the ARRL docs.  Just tell me what works best for my KX3 and how to do it.
>
> If I feed with 450 ohm ladder line, how do I transition from the ladder line to the rig? Is it antenna ends to 20 ft of ladder line to the balun and then a short stub of coax? What is short, 1 to 5 ft? Do I cut the ladder line long and what do I do with excess? The ladder line has to stay mostly vertical?
>
> I have the BNC to binding post adapter as well. Can I just feed ladder line to the binding posts directly with maybe some kind of strain relief?
>
> It sorta works now with the antenna loop connected to an MFJ913 4:1 balun to RG8X coax to rig. SWR on 40 meters is about 2.0 to 2.2 across the band (low to high) as measured externally before connecting to the rig. Am I good enough? The ATU handles it quite nicely bringing it down to 1:1.
>
> I’ve tried feeding the coax directly, with no balun, but that made the SWR much worse externally, 5:1 and up.
>
> Remember, be gentle to this newbie and TIA,
>
> Dave Esquer, K6WDE
> [hidden email]
>
>
>
>
>
>
> ______________________________________________________________
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--
FCC Section 97.313(a) “At all times, an amateur station must use the minimum transmitter power necessary to carry out the desired communications.”

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[Don Wilhelm-4]

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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[Edward R Cole]

An alternative to ladder line is running two parallel runs of coax
(RG-213) with only center conductors connected to the loop and the
radio end center conductors to a balun which solves the unbalanced
issue.  The two runs of coax can be taped together.

Two runs of coax will have something like 200-ohms impedance (if I
recall correctly).  The shields of the lines do not connect to
anything but do make the line less susceptible to detuning when run
up thru the building structure.

Since you are running 10w with a KX3 and internal ATU, there is less
things to worry about.  Of course the balun could be installed at the
antenna and single run of coax to the radio, as Don suggests.

I was going to put up a one WL horizontal 80m loop at one time and
was considering this feed.  We moved and the new property is not
conducive to putting  up something this large.

73, Ed - KL7UW
http://www.kl7uw.com
     "Kits made by KL7UW"
Dubus Mag business:
     [hidden email]

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