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N4JTE 6 BAND "RIBBON" ANTENNA
$35
This antenna article is
geared towards new Hams and antenna builders
looking for a very
inexpensive 6 band antenna that can be efficiently fed
with 50 ohm coax
without a tuner.
The inspiration for this design resulted from a visit
to my 82 year old
neighbor's home who had asked me for some help in
dismantling his
amazing and extremly beautiful model train set, and box
up for his
grandson. During this process I was intrigued by his use of
5 and 10
conductor 18 gauge flat insulated ribbon cable for all of the
L.V. switching
and action devices.
So Begins the
Adventure.......
Antenna shown resting on the driveway. Colored
lines show each section.
5 conductor computer ribbon cable
To be
honest I am getting more and more frustrated with some of the
latest
marketing ploys being used by commercial antenna manufacturers
and their
incredible, misleading and unsubstantiated miracle "all band"
antennas that
will sucker in some poor unsuspecting new ham who will
spend his money
on a heavily marketed, over priced, and in some cases,
amazingly reviewed
antenna // Toaster.
My plan was to use this
relatively cheap insulated wire and find out if was
actually possible
to get 6 bands cleanly matched to 50 ohm coax. As this
antenna was
basically built for testing and performance evaluation the
construction
details are limited and somewhat primitive by most standards
so I will
leave it to others to refine, hi.
When I envisioned this concept my
only real concern was how all the close
spaced wires would interact.
The shorter dipoles will all present high
impedance at the feed point
when they are not driven forcing the feed line to
pick the path of
least resistance and best match for the frequency.
The 40 meter
wire will serve well on the 15 meter band as a center fed 1.5 wl
wire.
I am aware that a fan dipole uses the same
single feed concept but I believe
the ribbon antenna eliminates all
those extra tie off points while maintaining
the resultant extra
effective radiated height, especially if used in a flat
top
configuration. Certainly less obtrusive and much less work. Below
is the basic layout with lengths for each half of the
dipole.
Final lengths per side of each dipole:
75 meter
length 58ft
40 meter length 32ft (also used on 15 meters)
20
meter length 16ft
17 meter length 12ft
10 meter length
8ft
Well
you have to start somewhere, so I chose the OMISS net
frequencies
available at http://omiss.net/,
as my starting point for the band frequencies
and wire lengths.
I
have built more than enough monoband Inv. Vees at this location with
insulated wire so I use my own formula of 450/
freq, to achieve what I'm after with minimal pruning.
You
need to approach this antenna one side at a time. The ribbon wire
I
purchased was only available in 50 ft. lengths so I knew the 75 meter
wire
would need about 8 more feet added to each end to reach 75 meters.
It's best to unroll and stretch the wire out to remove the "wire
memory".
With that accomplished, measure out your 5 chosen
1/4 wl lengths and mark or tape off.
As the 75 meter wire is
pretty much done after adding the required wire, separate the next wire,
and carefully peal back to the taped off marker and cut off excess.
Continue this process for the remaining wires up to the 10 meter
point.
Yeah I know it seems like a lot of wasted wire but at 6 cents a
foot
you'll get over it !
Repeat the process
with the other half of your antenna. I just laid them along
side each
other and matched all the lengths. Be careful to use a dull knife
or
fingernail file to separate the wires so as to not break the
insulation.
After cutting all your wires to length you will need to
have some kind of
center support and feedline connector in mind before
stripping and soldering
the 5 conductors together. In my case I pushed
each one thru a ceramic
insulator and then carefully stripped and
soldered together in preparation for
feedline attachment.
The
ends of half wave dipoles are at high rf voltage and if too close to
others
will add unwanted capacitance and tuning problems. For starters
I just
separated the adjacent wires by about a foot and let them dangle
down.
FIRST ATTEMPT.
My goal with
this initial attempt was to see basically where or if, I would get
full
power out. This would give me the best indication of what was
actually
radiating and at what frequency. You cannot expect, nor limit
yourself, to a
1 to 1 swr as being your goal.
The meter will serve
only as a guideline to where your wire length and height works in the real
world of your backyard.
A quick look at any antenna book will show
the relationship between height
above ground and radiation resistance
in ohms. This inv. Vee at 43 ft. is
even a little more tricky to
predict especially when the 75 meter antenna is
only at .175 wl high
and the 10 meter antenna is at 1.2 wl above real ground.
All 6
antennas (the dipoles) should range between 20 and 90 ohms with the
ends at 10 ft. above ground. The results of the first attempt were very
interesting in that at least I was getting full power out somewhere close
to the 6 bands of interest.
I had my doubts because of introducing the
17 meter antenna into the mix
which is not an even multiple of the
lowest band, usually considered a no no
on multiband
antennas.
--------------------------------------------------------------------------------------------
SECOND ATTEMPT:
I use an eye hook stuck into the
top of the fiberglass pole with a masonry
string to allow easy up and
down access for the cutting and tuning process.
I started by adding
about 4 ft to each end of the 75 wire and after some
diddling ended up
with around 3.95Mhz. The only band affected by this change when scanning
thru the bands was the 40 meter wire which changed it's apparent resonant
frequency to 6.9Mhz and the 15 meter wire also dropped in frequency.
A
quick trip to the backyard to shorten each end of 40 meter wire by
one foot made no change on the resonance. I then separated the 40 from the
75 by dangling about 3 ft. at each end. Obviously the end effect was
kicking in because I ended up at 7.1Mhz, close enough for my needs and the
15 meter wire was happy in the middle of the SSB
portion.
FIRST TRY
RESULTS
75----------4.179Mhz short
40----------7.290Mhz
short
20----------14.190Mhz okay
17----------17.800Mhz
long
15----------21.553Mhz long
10----------27.713Mhz
long
SECOND
TRY.
75--------------3.95Mhz
40--------------7.1Mhz
20--------------14.190Mhz
17--------------17.800Mhz
15.--------------21.300Mhz
10---------------27.717Mhz
THIRD ATTEMPT:
The only bands left to fine tune
were the 17 and 10 meter wires which were
still too long, so I cut off
6 inches from both ends of each antenna. The 17
meter ended up at
18.135Mhz and the 10 meter at 29.5Mhz in the FM portion of the band which
I actually prefer these days.
THIRD, FINAL
TRY, I'm
lazy:
75----------------3.95Mhz
40.---------------7.1Mhz
20----------------14.190Mhz
17----------------18.135Mhz
15----------------21.360Mhz
10----------------29.500Mhz
RECAP:
The whole tune/recut
exercise took about 6 hours and resulted in a 6 band
antenna that will
radiate full power out without a tuner.
Due to the fact that
there are no traps, no loading coils, no tuners and no ladderline needing
a balun to match, the only losses will be in the feedline due to it's
length and not the result of any mismatch at the antenna feedpoint. I do
not have the necessary brain power to model this design and would
appreciate a peer review on the modeled radiation resistance and resultant
antenna patterns.
Point of
interest; The 60, 30 and 12 meter bands had around a 3 to 1 swr
and
were showing 60 to 70 watts out without a tuner. Probably will work
well with an auto tuner.
IF YOU BUILD
IT:
1. A simple Plexiglas T, or equivalent with double slots for
the ribbon wire
and two small holes to tie wrap the coax should be more
than enough after
waterproofing to ensure stability and strength for a
center supported light
weight antenna such as this.
2. The 75
meter wire ends up holding the whole antenna up, so I would
attach
some masonry string to the center insulator and tie wrap at the
dipole leg end
dangles and a couple more at the 40 and 80 wires with
the string going to
tie off points. By tie wrapping you will also
prevent the wires from
separating at band junctions.
3. Attach small
non conductive weights at the drop wires after final
tuning.
4. Use a tuner at higher powers to attenuate
harmonics and any possible
spurious transmitter outputs.
5. If 10
conductor wire is available you can double up the wires for each
band
providing an increase in bandwidth and power handling. FYI, I had
no
problems with the 5 wire at 500 watts ssb.
FINAL
COMMENTS:
I am not going to waste everyone's time by recounting
all my log entries
while testing this antenna. I will tell you it is a
joy to run from 75 to 10
meter fm and be able to hear what is going on
and respond to a cq without
fumbling around with antenna switches and
tuners. This antenna is nothing
more than 6 inv vees at 43 ft. that
perform to the laws of physics and will
serve you well if you are
committed to a little sweat equity to get it
working
efficiently.
You will not be disappointed with this $35
antenna.
BUILD.... DON'T BUY !
Tnx for
reading,
Bob N4JTE
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