Article One – The planThis is the first of three DIY articles on building a 160 meter, 3/8th wave, inverted ‘L’ antenna.
The original goal was to throw up a wire that would do 160 meters and with the use of a tuner be able to make contacts on other bands. My current G5RV antenna runs east/west so the lobes naturally extend south/north. This configuration seems to give me many contacts south as far as Brazil and has far north as Finland, Russia, and Poland going over the North Pole. However, going east/west has always been a challenge unless propagation was very good.
I started my research with the thought of hoisting a full wave antenna hung in a square configuration around the shack. Using 12-gauge stranded coated wire I would need to start with `530 feet and reduce or add wire to get the best resonance on a host of bands from 15 - 160 meters. After a friend ran my design on an antenna program it was discovered this would not be an optimal use of my available assets - Financial nor time.
First, the challenge of putting up the long wave antenna was that the four trees I had picked out were at different heights over uneven ground. Another friend suggested I look at a 160 meter ‘trap’ dipole antenna. The traps would be for 40 and 75/80-meter bands.
This idea had merit however I had a hard time finding commercially manufactured 40 and 75/80-meter traps for sale. The main manufacture for traps, Unadilla
1 would not sell retail and they were on back-order at all the retailers I checked.
Now of course I could wrap my own traps however, at about this time I was over at the President of my local amateur radio club’s house for the annual Pennsylvania QSO party and noticed an odd long wire that ran from the peak of his barns roof to the top of a 65’ tower and then angled down to a balun. The antenna wire was attached to one side of the balun and a 10-gauge wire ran to a set of fanning wires used as a ground plane on the other. Then he had RG-58 coax running from the balun to the shack.
I asked him about the antenna and was told that it was a 160 meter 3/8th wave, inverted ‘L’ antenna. My friend went on to say that he can get other bands from the antenna using a tuner and had made hundreds of contacts all over the globe with it on many of the bands. Once I picked his brains some more I started my education on inverted ‘L’ antennas.
The education started with looking up inverted ‘L’ antennas on line and reading up on the process in the ARRL Antenna book. A great source of info came from L. B. Cebik, W4RNL
2 (Went Silent Key in 2008) who wrote a twenty-six-page synopsis on the inverted ‘L’ antenna. Here is a quick and dirty line drawing of a ¼ wave inverted ‘L’ I copied from his article.
Again, my friends 160 meter inverted ‘L’ was a 3/8th wave so in essence ~200 feet not the 124 feet shown in this diagram. A true ¼ wave would be 132.5 feet in length
I use the true and tested formula of 1005/Freq in Mhz = length in feet (1005/1.900 Mhz = 529 feet) which I multiplied that length by 3/8’s or .375 (529’ x .375= 198.4 feet).
Now that I had the length of the wire figured out an inverted ‘L’ is best when 50% is horizontal and 50% is vertical. So, I needed two trees that were separated by 100 feet plus and I wanted them positioned so the antenna ran north/south vs. my G5RV which runs east/west.
Unbelievably this took quite a few tries. Once I found two trees that met these criteria I then measured the height using a level and a range finder an old trick I learned many moons ago. I positioned the level so the 45- degree bubble was centered while I sighted down the top of the level like the barrel of a shotgun. Then moved back and forth until the top of the level was aligned with the top of a tree. Once this was done I measured the distance from my position to the tree trunk using the range finder and then added 6 feet for my height.
More times than not I had to start the whole process over again as the trees chosen where too short or not in a north/south position. However, through persistence I figured out the two perfect trees. Marked them with surveyor’s tape so I wouldn’t forget which ones I had chosen.
The south pine tree was ~120 feet tall, while the north pine tree was ~100 feet tall. The ground sloped to the south at about a 10% grade from the north tree, so in my mind the horizontal part of the antenna would be relatively level. As a side note, I like using pine trees for semi-permanent antennas because unlike hard woods, pine trees have some give. A tree that has give has less chance of falling in strong winds and the branches tend to act as natural shock absorbers.
I contacted Balun Designs
3 in Denton, Texas concerning the type of balun I should use. Robert (Bob) Rumsy from Balun Designs promptly responded asking several questions. Length, size, and type of wire I will be using, type of ground I would hang the wire over, bands I would like to work plus 160 meters, et cetera. Based on my answers he recommended their 4:1 unun (Model 4134s). I promptly ordered it and once it came in it was worth every penny of the $80- I spent. It was a quality piece of hardware.
Once that was ordered I headed off to Home Depot for wire and some PVC pipe and placed an order with West Marine
4 for rope, thimbles, and sailboat blocks. Here was my shopping list and approximate prices:
> 500 feet of 12 gauge, coated stranded wire in white (Harder to see color) ……. $ 60-
> 600 feet of ¼”, 3-strand, polyester rope…………………………………………………………..... $150-
> 100 feet of RG 213U marine grade coax…………………………………………………………...... $105-
> 2, ¼” galvanized thimbles (only needed one but one is none and two is one)….$ 6-
> 2, 1/16” galvanized thimbles (Didtto)……………………………………………………………........ $ 6-
> 4, ¼”-5/” Harken Bullet blocks (Ditto)………………………………………………………….......... $ 80-
> Scrap, 1 foot, 1 ½” PVC……………………………………………………………………………………........$ 1-
> 2, 8’ copper ground rods……………………………………………………………………………….......….$ 25-
> 6, ground wire clamps…………………………………………………………………………………….........$ 12-
>
Assorted butt connectors………………………………………………………………………………....…$ 4-Total……………………………………………………………………………………………………......... $449-I only used one block, one 1/16” & ¼” thimble, and four ground wire clamps; However, I tend to buy more parts than needed when I do a project. Better to have too much on hand then to little I always say. I also had bits and pieces left over from other projects and they were:
> A ¾” 90-degree PVC elbow,
> RG-58, PL-259 connectors for the coax,
> Solder,
> Amalgamated tape, and
> Other bits and pieces.
As the parts started to pile up I had to figure out how to get the messenger line, rope, and blocks, then eventually the antenna up. My friend who inspired me to put the 3/8 wave inverted ‘L’ up had a great contraption he made which I borrowed to complete the deed. It was a potato gun
5 with a spinning reel attached. Instead of launching a potato you used a mortar round as the projectile and instead of an accelerant like hair spray, you use compressed air.
His potato/antenna gun also had a PSI dial on it, so you knew how much pressure you were using. In the end it took 120 PSI to launch the mortars over the two trees. The temperature was in the mid-teens, so I had to bring the gun into the cabin between launches or the pressure leaked out around the ball valve used to trigger the devise. I am sure if you used a brass ball valve rather than the PVC one, you would not have the issue of leaking air from the plastic seal.
I also planned on using a rope/pulley system to hoist and keep aloft the antenna. I have used this system to good affect before and antennas I have put up in the past remain up. Here are two-line drawings from Radio Works
6 of Virginia Beach’s site.
Below is a quick, down, and dirty diagram of what I wanted to end up with when done launching the appropriate lines, blocks, and antenna wire.
In my next installment on my 160-meter, 3/8th wave inverted ‘L’ I will discuss the process of launching the antenna, hooking it up for the first time to my radio and the contacts I made.
Go
here for Article II
Footnotes 1
Unadilla Traps2
L. B. Cebik, W4RNL, “Straightening Out the Inverted-L3
Balun Designs4
West Marine5
Potato Gun6
Radio WorksArticle 2 of 3 part seriesArticle 3 of 3 part series
