Re: KX3 Field Ant. for 80/40/30

Posted by Bob McGraw - K4TAX on Nov 22, 2017; 2:40am
URL: http://elecraft.85.s1.nabble.com/Re-KX3-Field-Ant-for-80-40-30-tp7636045p7636165.html


A relatively new ham asked for a copy or diagram of my portable antenna
design.   Here's my response.

********************

Welcome on being a new ham.  As to the antenna design..........well
there is no such thing in existence.

We usually camp in National parks and State parks.  There's usually a
few to a lot of trees around.  I carry a 100 ft spool of #22 insulated
hookup wire wound on a plastic spool.  That was the way I purchased
it.   Once the travel trailer is in position, I will scout out a nice
tree limb some 15 to 30 ft off of the ground.  I then unroll about 50 to
75 ft or so of the wire from the spool, drop a half hitch around the
spool to prevent the remainder of the wire from rolling off of the
spool.  Using a method similar to that of "David when he slew the
Giant", I hurl the spool over a limb.  Of course sometimes my estimated
distance or height is in error and I find it necessary to make another
attempt or two.  The spool and wire simply keep the wire taught over the
limb and to the trailer.  On the trailer, I have a feed through which
was originally for a TV antenna, no longer used for that purpose.  To
this I attach the end of the wire to the F connector.  Inside I have a
length of RG 58 coax that runs to the F connector and terminates into a
BNC connectro.  This attaches to my home made L network for the tuner. 
The L network is comprised of a single variable C of about 500 pF and an
air wound inductor which is  1 1/2" in diameter and about 6" long.  I
have a 20 position tap switch that is configured so I can use individual
tap or select turns of the inductor.  This is mounted in a4" x 4" x 8"
aluminum  box with a BNC on each end.    I recall having about every 2
turns tapped to the switch.  Using this L network I can find a point of
C and L values which produce suitable SWR, less than 2:1 for my
operation.   After all my RG-58 is only 6 ft long so there is little
loss on HF in the "feed line".   If this doesn't find a match, I just
reverse the IN and OUT to provide a different impedance range.    
Should I then have difficulty in obtaining a match, I will loosen the
wire, allow more to spool off and then try again.   Usually a length of
some 50 to 75 feet will work about any band except 160 M.   I need more
wire for that. When we are finished camping, I turn the end of the wire
loose at the trailer allowing the spool and wire to fall to the ground,
wind it up and stow it for the next adventure.    All of this is best
described as an end fed long wire with a L matching network.    There is
nothing exotic about it, no baluns, no specific lengths, just use the
wire length and space available.

In reading the various posts and opinions on the topic, I'm amazed at
the lengths, expense and trouble many hams go to in order to put up an
antenna.

Hope this helps

***************************

73

Bob, K4TAX


On 11/21/2017 6:06 PM, Ignacy wrote:

> Antenna performance does not depend on how it is fed (if feeder losses are
> low) but on height and ground type. For KX3 with AT, any special length
> makes little sense, especially in portable conditions. KX3 matches most
> random wires, and KX3 with 4:1 balun matches any wires. Lack of balance is
> not too important with battery operation. I try to make random wire +
> counterpoise at least 1/4 wave on the lowest frequency for reasonable
> efficiency.
>
> For 1.5 KW, an endfed with a 49:1 or so transformer makes lots of sense as
> wideband tuners for QRO are rare, expensive and heavy. "Myantennas.com"
> perfected a  transformer that has very small losses and does not
> self-destroy at high power. But in inv V configuration, it is way down from
> a flattop.
>
> Ignacy, NO9E
>
>
>
> --
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