Re: Non-resonant attic loop

[Vic Dively]

Gene: Hello.  I'll relay some of my experiences, since I've been limited
to attic antennas, due to living in a townhouse.  I have about 20 X 25
ft. of useable attic space.  I've probably built about 30 different
antennas in the last 4 years, based on about a dozen different basic
styles.  I use EZNEC to model various loops and end fire arrays, center
fed and off center fed versions, loop arrays, etc. and then build them
and test them out to see if the performance matches the predictions from
the computer models.  So far my best multi-bander results have been with
a center fed bent dipole/end fire array.   My shack is in a spare bedroom
directly under the attic, so my feedline is only about 15 feet long.  I
use ladder line.  My current antenna takes up 16 X 10 feet of space.   It
looks like this:


           __________________o____________________
          l                                                              
                        l
          l                                                              
                        l
          l                                                              
                        l
          l                                                              
                        l
          l                                                              
                        l
          l                                                              
                        l
          l                                                              
                        l
          l_________________      ___________________l

The "o" is the feedpoint.

Not exactly to scale, but it is center fed along the top, 8 feet on each
side of the feedpoint, the sides are 10 feet long, and the bottom
elements are 9 ft. 4 inches long, leaving a 8 inch gap.  This is
installed horizontally in the attic, hanging about 1-2 inches below the
roof rafters.  As I'm in a 3 story townhouse, the feedpoint and center
element is about 37 ft. above ground, and the whole thing follows the
roof slope down to around 33 ft. above ground (22 degree sloping roof).
In other words, the antenna is as high up off the floor as I can go.  I
used 1.25 inch copper pipe, and instead of soldering the corner joints, I
cut the copper elbows with a hacksaw, then use muffler clamps to hold the
joints together.  I think you get a much better joint that way, since
solder really isn't good for carrying high currents.  I polish the inside
of the elbows and the outside of the pipe before clamping in order to get
a real good electrical connection with wide contact surfaces.  I use the
new MFJ balanced line tuner and can tune up on everything from 10-80M.
If you don't use fat copper pipe, try fat reefer tubing, and avoid joints
altogether.  It makes a huge difference when you try to use the antenna
on the lower bands.  Ladder line is critical, as you are running very
high SWR on the feedline, ladder line keeps your feedline losses quite
low, especially for these short lengths.

It has useful patterns on all bands from 10-80M... 20M is
omnidirectional, just about 1.5 dB less gain than a regular 20M dipole at
its max point of gain.  For 15M/17M this acts like an end fire array,
with gain perpendicular to the sides.  On 40M and 80M this acts like a
bent dipole, with gain perpendicular to the center element.  The other
nice thing is you can clamp a jumper across the 8 inch gap and now you
have a 16 X 10 loop, circumference of 52 ft.  This has a circular pattern
on 40M. and an oval one for 80M.  It also has good patterns for 15M, at
right angles to the pattern when you don't have the jumper in place.  

The big advantage of this antenna over a ladder line fed dipole, is that
the radiation patterns are different on different bands, allowing you to
work in any direction, depending on what band you are on.  A center fed
dipole is a great multi band antenna, but all your bands are basically
pointed towards the same places.  Where I am, the Caribbean/South America
is at right angles to Europe, so a center fed dipole would get me one or
the other, but not both.  With this antenna, I can work both regions, as
well as all of the U.S.  

I used this antenna, in the loop mode, for the ARCI Spring QSO party.  I
didn't have much time to operate, mostly a little in the evenings, about
4.5 hours total, but I did make 40 Q's, mostly on 40 (18) but also 9 on
80M and 11 on 20M.  I had literally 3 minutes Sunday afternoon to get on
the air and got a couple 15M Q's before running out the door, before
getting back for the last hour of the contest.  By then, 15M was closed.
 

I used a similar version of this antenna for the 2005 ARRL DX CW contest
and made 100 DX q's, working about 10 hours, mostly in the evenings
again, and missing most all of Sunday.  I even managed to reach Tobago,
about 2000 miles, on 80M.  I was really excited about that.  I used yet a
different variation on the same theme (an omnidirectional 20M pattern)
for the 2004 WPX contest, and made a couple hundred QSO's.  I was very
happy that 20M did prove to be omnidirectional, I worked all across
Canada and the US, as well as Europe, Caribbean, South America on that
band.

So I'm currently on version 3, and hoping to be able to give it a good
try in the 2005 WPX CW contest next month.  The big difference from my
last version is that 15M is much more oval, closer to omnidirectional,
giving me hope that I can work Europe and all of the Caribbean and South
America on that band.  I'm also hoping this antenna will give me a
stronger performance on 40M and 80M than last year's efforts, based on
the ability to jumper the gap and give me an improved 40M/80M pattern.
We'll see.

I've also used a 94 foot loop, pretty close to what you are asking about
-  20 X 27 foot, fed in the center of one of the 20 foot sides with 450
ohm ladder line.  That worked great on 40M (didn't have an 80M rig at the
time, so I can't say how it worked from personal experience, but it
models good with EZNEC for 80M).    As a matter of fact, I've toyed with
the idea of making that antenna again, this time with  1/4 inch copper
reefer tubing (which you can get in 50 ft. coils).   It models with
really nice patterns on everything except 20M, where it seems to have
relatively poor gain in all directions... everything else looks good.

I've just recently finished my first Elecraft rig, a K2, which I used for
the ARCI QSO party last weekend.  I'm eager to find out how well this new
rig, along with my new antenna, will perform for the upcoming WPX
contest.

73,
Vic
KG4HTT
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