Some call it a Skywire Loop Antenna, some call it a W0MHS Loop
Skywire Antenna, nowadays it's called a Full Wave Loop Antenna
but it still is a force to be reckoned with. In the first couple hours of
having it up I was able to hear stations I could not hear before.
The skywire loop is an excellent antenna - they can be a bit of work to get
up in the air if you have a lot of obstacles (trees, xyl, etc.) but it is
worth it.
What's the right length?
574' feet for 160-15 meters (10 meters also, but with a short
feedline). Anything shorter becomes a challenge to match on the upper bands
and increases the chances of the feedline becoming the radiator and a source
of RFI.
The length of a loop best suited for operation on 160 meters
is too short for use on the higher bands.
The optimal choice is to cut the loop for resonance on the highest band you
plan to use, however it becomes difficult to use a giant 160 meter loop on
10 meters - your feedline length must not be a fractional wavelength of the
band you want to use or else you'll wind up using your feedline as the
radiator!
If you plan to use the loop for 160 meters 99% of the time then cut it for
160 meters @ 544' - but be prepared for difficulty and reduced performance
above 7MHz.
I recommend a different loop for use on 20-10 meters if those are
your favorite bands, mostly because it's easier to put up 71 feet of wire
(14MHz) with only 18 feet per side. It's also easier to make a 23' diameter
loop for 20 meters more circular instead of square shaped. Remember, the
ideal loop is a perfect circle - it's not too hard to make an 8-sided loop
for 20 meters using PVC pipe and some dacron rope to hold it rigid.
Formula for length of full wave loop
The formula for a calculating the length of a full wave loop antenna is: Length (feet) = 1005/f
MHz.
544' is comfortably resonant at 1.847kHz, 3.694kHz, 7.389kHz, 14.779kHz and 29.558kHz. Not an optimal length.
560' is comfortably resonant at 1.794kHz, 3.589kHz, 7.178kHz, 14.357kHz and 28.714kHz. Not an optimal length.
565' is comfortably resonant at 1.778kHz, 3.557kHz, 7.115kHz, 14.230kHz and 28.460kHz. Still a little short.
574' is comfortably resonant at 1.750kHz, 3.5kHz, 7.00kHz, 14.000kHz and 28.000kHz. My personal preference.
Performance Observations
Noise seems to be greatly reduced with the loop compared to dipoles. My
dipoles are mounted on a pole at the end of the house and some of the
noise may be due to their height (35 feet for the dipoles vs. 30 feet for
the loop) and close proximity to the house and stuff inside. The loop is
40 feet or greater from the house in most places. The noise on 20 meters
is sometimes reduced as much as 1 S unit when using the loop compared to
the dipole (even at 3:00am when nothing in the house is on). Once in a
while a station may be slightly stronger with the dipole but the noise is
stronger too - weaker stations are easier to copy with the loop due to
less noise.
Polarity of the loop shouldn't matter but sometimes it does. The
difference in polarity can be heard as some stations being much louder than
others when the polarity is changed - i'm guessing this is due to
cancellation of signals as received by the different length of the sides of
my loop. A circular loop would reduce these effects.
I have an old DPDT copper knife switch that I use to switch the polarity of
the 160 meter loop. My experiments with smaller loops, 20 meter loop for
example, shows that the polarity plays very little role.
Performance is exceptional and rarely is beaten by a dipole at the
same height.
Feedpoint location is important - it's preferable to have the
feedpoint on the north side of the loop (or a side that faces a part of the
world where there is little DX), especially if your loop is square. My
loop's performance was increased by moving the feedpoint to the north side;
stations to the south were easier to hear.
Corner feedpoint locations of square loops increase gain in the
direction of the opposite corner. A loop fed in the southwest corner will
have a few dB (3dB or more) gain favoring the northeast corner.
The loop really, really cuts the mustard on 160, 80 and 40
meters compared to a dipole at the same height. It works just fine and
dandy on 20 meters as well but I'd rather have a dedicated 20 meter loop for
20 through 10 meters. I had a separate loop for 20-10 and it was far easier
to put up and maintain (a lot less wire to be targeted by nearby falling
tree limbs).
Other observations
Signal reports are usually equivalent or better with the loop than my
20 or 40 meter dipoles at 38-40 feet.
QSB seems to be less of a problem with the loop than the dipole - the
fades aren't as deep and sometimes not there at all while the dipole could
be horrible.
The loop is a little quieter than the dipole, especially on the lower
bands.
My experience so far has been that the computerized models of loops are
over-optimistic compared to real operation in the field. You can get a
rough idea with modelling software but that software doesn't take into
account how much the wire droops from the corners to the middle of each
side, etc. The exception to this is corner-fed loops, they do have a strong
lobe as the models predict.
The bottom line is if you can put up a loop with the corners at least 25
feet high it'll work as well as anything else at that height and give you
the convenience of having the other bands available from a single antenna.
Progress
10-Nov-06: Spent almost the entire day cutting trees, limbs, brush and
climbing ladders to tie pulleys to trees and run the 544 feet of wire.
11-Nov-06: Another 3 hours spent cutting more limbs and branches I didn't
see near the end of the previous day.
21-Nov-06: I worked HC8N quite easily on 30 meters with 100 watts - big
pileup but easy to work.
Winter 2007 - 544' to short - added another 20' - 15 meters is much easier
to tune now
Spring 2008 - wire broke - my pulleys are too small, I should have used a 3"
pulley at each corner. Added more wire - 574' is a great length.
Common Misconceptions and Myths
When I read about full wave loops, five things came to mind:
Tuners are a bother
Ladder line is a bother
It's too big
It's hard to put up
It isn't worth it
I was wrong. A manual tuner is as easy as tuning a linear, ladder line is a
misunderstood jewel, there's no such thing as an antenna that is
too big, it was fairly simple to put up (even in my woodsy yard) and the
performance makes up for whatever negatives are left over.
Ladder line has become my feedline of choice - RFI is greatly reduced and
power levels are not a concern. My tuner will burn up before ladder line
will.
Skywire Loop Websites
Ladder Line Safety -
important reading for anyone interested in using open line "ladder line"
Take a look at KA1FSB's loop antenna
page for more details on what i'm doing
I get my antenna hardware from Craig K1QX of Radioware.
Craig has excellent service, advice and selection of the stuff you need.
My fullwave skywire loop is made up of WA1FFL Ladder Lock, 544' insulated Flex-Weave Antenna
Wire held by 3/16" Dacron Rope.
Craig has excellent service, advice and selection of the stuff you need.
Pulleys - BIG ones are better, preferably 3", and get good ones. DON'T
use anything smaller than 2" or the wire in the corners will break after a
couple years.
I use marine-grade
Harken free running blocks (see
West
Marine
for details). Check for strength (200lbs is good), preferably with a sheave
nade if delrin - conductivity should not be an issue even if the insulation wears
through.
