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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

 

 

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