Multiband Semi-Helical Inverted L
The "IT" ANTENNA!
"Poor boy ham
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
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.
affordable....... since I don't have an oil well in the back
4. Oh yea, no useful trees in the area so must be easy
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
OK, Ground planes are not the
best, but I will make one anyway.
Hey wait a
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
My Design for the 80/40 meter
Semi-Helical Inverted L Vertical
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
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
Drawing above shows completed
installation including guys, mast, etc but does not reproduce well.
See drawing below for more
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
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
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
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
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
Spreader bars shown only in drawing above for
Note that your wiring sequence may be different than mine, but
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
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
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
(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
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
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
Balun (Air Choke) shown mounted on
mast. Feed point just above
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
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
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.
love this hobby!
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