[Elecraft] K3 Multi-Band antenna thoughts

[Robert Tellefsen]

Hi Mike
I can address part of your concerns.
Here in my QRP contest station, I have a bunch of wire
antennas, because one alone just won't cut it.

My main antenna is a 40m dipole fed with 450 ohm
window line so I can tune it on other bands.  It works
80-40-30 and 20m.  Above 20m, it starts developing
lobes in directions I don't want, and nulls in directions
I do want.  For 15 and 10m I have nested bisquare antennas
supported by the same mast that supports the center of
the 40m dipole.

To make antenna matching easier for the TX, I have a
homebrew relay box that adds in various lengths of more
450 ohm window line to bring the feedpoint reactance
down to close to zero.  This gives the ATU a chance to
match easily and gives the best 2:1 swr bandwidth.  The
single band antennas are coax fed, but still switched
through the relay box.

In summary, for any serious operating, one antenna
won't really do the job.  I suggest you do the best you
can with the antenna you presently propose, and after
a period of using it, find out what directions you are
having trouble with.  Then consider what you can do
for a supplementary antenna at that time.

I truly recommend EZNEC as an antenna modeling program.
I won't pick up a tool or a piece of wire to do antenna
work untill I've checked out what I plan to do with EZNEC.
It has saved me an enormous amount of wasted time and
effort.

Good luck and 73
Bob N6WG

Wire antenna farm here:
40m dipole for 80-40-30-20m for E-W path
10m bisquare E-W
15m bisquare E-W
55 ft dipole for 40-30-20-15m for N-S path
20m half square for N-S path (also seems to work well on 17 and  15m)
55 ft top loaded vertical for 160m.  Also works well as a
    top loaded vertical half wave for 80m.

----- Original Message -----
From: "Mike Scott" <mike at paxsen.com>
To: <elecraft at mailman.qth.net>
Sent: Tuesday, March 11, 2008 6:54 PM
Subject: [Elecraft] K3 Multi-Band antenna thoughts


I am considering building a multi-band doublet fed with ladder line to
the
window and then 10-12 feet of coax to the K3 from the window
interface. I
would let the K3 ATU tune the beast. I have been playing with a coax
calculator and with low loss coax and 10:1 SWR the losses in the short
run
of coax up to 30MHz are manageable to ½ dB. The ladder line can handle
the
mismatch with small losses. I was thinking of putting a sleeve balun
at the
window interface from the coax to ladder line. I could of course put
two
different length doublets in parallel at the feed point if that would
tame
problem bands. I don't need this antenna to go to 50 MHz as I will
have a
separate antenna for 6M. I don't have antenna modeling software. I
know I
will have pattern issues if I try to push a long antenna too high in
frequency. I am not thinking right now of pushing this antenna to
160M.

So now the questions:

1. Is there a magic doublet antenna length that will end up with less
than
10:1 SWR on the most bands? I know Cebik likes 88 feet or 44 feet
depending
on lowest operating band but I don't know that he has solved my exact
problem for 10:1 SWR or less on all bands.
2. Is there a magic feed line length to make this all work?
3. Have any of you tried something similar to this with the K3 ATU? I
have
read the reports of just tuning up any old antenna that you have and
you get
it to work at some unknown efficiency. I have done this too.

I know it is time for me to bite the bullet and purchase some real
antennal
modeling tools.


Mike Scott
AE6WA Tarzana, CA
K3/100 SN508


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