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Homebrew Linear Loaded
Multiband Semi-Helical Inverted L
Vertical
The "IT" ANTENNA!
by
KG4FBY
Dave Cunningham
"Poor
boy ham
radio"
This
is my attempt to build an antenna for 80 and 40 meters that will
accommodate my installation limitations....lack of space!
I rent a
house so nothing too permanent. My landlord is great , but I don't want to
push it.
My requirements for the
"IT" antenna:
1. One requirement was that it could be
handled by one man.
2. 80 and 40 meters. The 80 and 40 meter
requirement is mainly because we have little or no sunspots and few bands
useful now for DX.
3. Very affordable....... since I don't
have an oil well in the back yard.
4. Oh yea, no useful trees
in the area so must be easy to support.
5. I wanted to be able to
operate on 80 meters with limited space, plus 40 meters.
I
had already tried a homebrew G5RV but couldn't get it more than 18' over
the roof so it was purely NVIS. I also tried a center fed helical vertical
dipole design up the full length of the mast. Made a fine dummy load.
Helicals do work but tend to have negative gain, often -2 to -4db. Linear
loaded antennas don't suffer near the loss but need a bit more space. I
tried to put this whole thing on the current spreaders with the feed point
in the middle and half the ground side linear loaded as well, on the same
mast. Better but still not great.
OK, Ground
planes are not the best, but I will make one anyway.
Hey wait a minute.
Ground planes have a much lower take off
angle at lower heights. I've seen several really nice ones but they all
had at least a 25' aluminum primary radiator and aluminum is quite
expensive these days. Anything you can make with aluminum can be made with
wire and PVC. No it won't survive 100 mph winds but neither will most
aluminum antennas. If mine gets trashed in a storm, I can rebuild it for
less than $50. How much to replace that shiny Cushcraft? BTW, a fine
antenna, out of my price range.
My Design
for the 80/40 meter Semi-Helical Inverted
L Vertical Explained:
This design is basically 2, 1/4 wave
lengths of wire, one for each band, with one band (40
meters), partially helical wound on the mast, and the other band
(80 meters), partially linear "wound", all fed from the same
feed point. Using this method of construction has enabled me to
"squash" the total lengths of each antenna within my limited space
and still let me operate on those
bands.
This
contraption is only 18' bottom to top with only 14' of antenna vertically
(including the balun). Spreaders are just 2' tip to tip. Horizontally at
the top you need about 25' to stretch out the wire to tie it off. Pretty efficient on space for a full quarter wave for
80m.
Drawing above shows completed
installation including guys, mast, etc but does not reproduce well.
See drawing below for more
detail.
Here is a general
representation of my process of assembly. Adjust to your
preferences and available materials. I built mine quick and dirty. I just
wanted to try it out. If you like it then clean it up a
bit.
First I made the mast from (2) 10' PVC
sections. The bottom is 2" and the 1 1/2" telescopes into it nicely. I
used a 1 3/4" u-bolt to keep the smaller tube where I wanted it while
drilling holes for the bolts and realized it would make a fine place to
tie off guy wires. It stays. I used (3) 3/8" by 2 1/2"bolts from 3
different directions to join 2 pieces of PVC 24" telescoped. Drill one,
then slide in the bolt or it will move while you drill the next.
Next I drilled the mast for the dowels. Remember to drill one slightly
higher than the other. The bottom one is 6' from the bottom, middle at 11
3/4, and the top is about 4" from the tip. I also drilled a extra hole
about 2" from the tip. You'll notice the horizontal wires go through these
holes before going to the spreaders. Too much pressure on the spreaders if
you don't use this or some other way to relieve the strain. Now just slide
the spreaders into place. I used wire ties on each side of the dowels
against the PVC to hold them in place. If they fit snug enough you may not
need to.
Now it's time to cut some wire. I just used the
standard method to figure a quarter wave for 80 meters. 234/frequency in
MHZ. That comes to about 61 1/2' but since it is harder to add than cut, I
started with about 64' and trimmed a 1/2" at a time till I was where I
wanted to be. I also cut 3 ground radials the same length and trim them
the same with the radiator. Remember the longer the wire, the lower the
resonant frequency. Figure your length for the center frequency you wish
to operate and add a little just in case. You can size this to any HF
band.
When I made the radiator for 40 meters, I figured for
7.225 MHz and got almost 33.5' so I cut 1 vertical and 3 radials
34". Wire choice is up to you. Thicker wire has wider bandwidth.
Thinner is much cheaper. I found 500' of insulated multi-strand 12 gauge
for $35. I wouldn't go smaller than 16 and no larger than
10.
Before you can pull the wire, you need to
support he mast from the top 4" and below the bottom set of spreaders. A
couple kitchen chairs will work. Just need to lay it horizontal but not
allow the spreaders to touch the ground.
To pull the
wire through it, wire tie it to the bottom mast with 2" overlap a foot or
so down from the bottom of the spreader you want to start with. Just pull
the complete pre-cut wire for 80 meters through each spreader, one at a
time. Please refer to the drawing and
instructions below for a suggested wire
pull sequence.
Spreader bars shown only in drawing above for
clarity.
Note that your wiring sequence may be different than mine, but
if you
follow the sequence below, it may help you to visualize
the way I did mine.
From the center conductor of the SO-239, follow this path
using the letters in the drawing above for each separate
antenna in this order.....the letters represent the outer portion of
the spreaders.
80 meter wire:
C, G, K, over
to spreader J,
Then down to F, B, over to A, then to E, to I,
and then to the mast for a strong tie point for the rest of the 80
meter wire length to an insulator on the end.
40 meter wire:
From the center conductor of the
SO-239 wire to D, H, then over to the mast above where the spreaders
are attached to the mast and wind your helical coil on the mast
toward the top. Then at the end of the helical coil to
a secure tie point for the rest of the 40 meter wire to an insulator on
the end.
(The above method should be clear enough,
but you may find by experimenting that other ways may be just as good. The
idea here is to keep the 80 and 40 meter wire sections AWAY from each
other using the ends of the spreaders for separation and not
cross each other on the center mast.)
I followed a path
around the mast so as not to have any wires to close or crossing the
center mast. Pull to the top, pull to the nearest spreader to the left,
pull to the bottom, to the spreader on the left, pull to the top, loop in
the extra hole at the top of the mast, pull out the center of the top of
the mast and attach to a rope long enough to reach something about the
same height if possible. The more downward the "L" turns, the more NVIS.
Tighten it up as you go . .
.
Great, Go ahead and tie your guy ropes to the u-bolt and
were ready to raise it. Raise it into position and strap it to something.
Use U-bolts, ratchet straps, several large wire ties, rope, shoe-strings,
bread ties, whatever ya got handy. Or plan ahead and buy something
specific but it won't have as much character. While your there, you might
want to pick up a couple hundred feet of rope. If you take the clothes
line, you won't be allowed to make any more antennas with stuff around the
house.
Now we need to make a quick balun. It keeps the
RF up there instead of in the shack. Ideally we would use a 3" to 5"
piece of PVC for the core. I don't have any, just wrap it around the mast.
Cut a length of 50 ohm coax between 18' to 21'. I cut mine 20' total.
Strip about 1 1/2" and separate the center from the shield on both ends.
Take one end and twist the center of the coax to the radiator that is
strapped to the mast. Wire tie it to the mast. With some variety of tape
in your pocket, begin wrapping the coax around the mast with each wrap
touching the previous. Tighten it up as you go if needed. Once you have
about a 3-4 inch tail left, wrap it with tape before it moves. Use another
tie at the bottom.
Attach the ground radials to the
braid side of the S0-239 at the top of the balun. Spread
them out in equi-distant fashion and attach to some rope to keep it off
the ground and at least 3 foot from metal if possible. I just tied mine to
a couple bricks, the neighbors roof, my landlords swamp cooler. If you
have trees, they work well too. These are not load bearing radials, just
spread them out, slanting down away from the antenna. If you haven't
already, go ahead a tie off the guy ropes. Future improvements include
finding something I can use as a ring around the mast to allow addition of
more radials.
Alright, just need to connect that 50 ohm coax
to the bottom of the balun. If you are committed enough, solder the SO-239
to the bottom. I just twisted the centers together as well as the braided
wires. Solder the connections well and seal of course for long term use.
I'm still tinkering with it so I haven't soldered it yet. Note that you can always eliminate the SO-239 and
just attach everything on the end tail of the Balun coax if
needed by using just one unbroken length of coax including the balun. Your
choice.
If you want to add extra bands, just use the
formula (234/freq in MHz=length in feet) to cut your radiator. (Add more
spreaders with different lengths as needed.)
Make sure to cut at
least 2 ground radials the same length for each band. Wire them directly
into the same feed points you used for the 80m. I did that with 33' of
wire for 40m. Simply fed it through the first 2 spreaders, then spiraled
up the mast from the center to top, fed through the other hole at the top
of the mast and the rest mostly horizontal to a small tree at the corner
of the house, tied off with some more of that cheap
rope.
Balun (Air Choke) shown mounted on mast. Feed point
just above it.
Check SWR on the top of the
band first, then at the middle, then the lowest frequency. SWR should be
best at the lower frequency. If so, cut 1/2" or so at a time till SWR is
lowest at your preferred center frequency. I don't have exact lengths for
wire because that varies with your preferred center frequency, distance of
the antenna from other metal structures, method of loading, etc. That's
why I just start a little long, and prune it down to my preferences and
abilities given the installation environment.
So far this has been very
durable in the wind although one of the top spreaders broke with a
previous version. That's part of the reason the 40 meter part is helically
wound. That and I wanted as much as possible in a vertical orientation.
The horizontal section makes it a inverted "L" with some NVIS and some
lower angle radiation. No I haven't modeled it. Haven't really had the
time to figure out modeling yet. Just stands to reason based on
known attributes of other antennas.
Mine is feed with 35'
of RG58x. Yea. I have to walk around all the wires on the roof, but this
thing actually seems to work! I can use both 80 and 40 meters without the
tuner (about 1.5 to 1.8 SWR) and can tune 10, 12, and 15 with my $30 MFJ
tuner. 20 meters is tough to get below 2.5 but I will probably just
add another radiator for that later.
I don't claim to
be an engineer and I don't have an antenna analyzer or a lot of money. I'm
more of a "run what ya brung" builder that benefited from the myriad of
great homebrew articles. You can combine the ideas of others into
something that works for your situation.
That's why I made this.
It amused me for several weekends, trying various things and I learned a
bit about antennas. I read many great articles about linear loading and
helically wound antennas and such. I never found anything quite like this,
so I had to try.
So far I have made a few contacts with it. I am
in El Paso TX and have made contacts in Illinois, California, Michigan
southern Mexico and a few other places. I only have 1 to 3 hours on the
weekends and I listen more than talk. Receive is much stronger on this
version as is the noise but I can live with it.
By the way I
use a Yaesu FT-757 ($240 on EBay) and the power supply is just 12 amps
(bought for my vhf/uhf radio) so I only transmit at about 35 to 40 watts.
Will be adding a deep cycle battery soon so I can run full power and have
backup power as a bonus.
I should also mention, the whole
thing weighs about 20 lbs and can easily be lowered by removing the
ratchet strap at the top of the swamp cooler and the rope at the base. PVC
probably wouldn't survive many winters up north but seems to do fine
here.
I have a hard time describing my
antenna to people so I figured a picture or two and
this project is worth a thousand words. I also hope this
may encourage others with little money and a bit of determination will go
ahead and give "it" a try, whatever "it" is.
Hey, if this
ain't poor boy ham radio, I don't know what is.
I
love this hobby!
KG4FBY
Dave
Cunningham
excalibrate@gmail.com
If you feel that
this article has been of value to you, please make ANY donation to
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kindness!
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