Inexpensive 17-Meter Vertical – from K5DVW
Used with non-commercial rights from Eham.net  All Rights Reserved

A low cost, low profile DX antenna

We hams love our antennas. So much so that it’s not unusual to see an amateur erect a 100 ft tower and sprinkle it with beams, and wires. Unfortunately for most of us, we either don’t have the space, funds, or accommodating neighbors for such a system. But, all isn’t lost when it comes to chasing DX. This article focuses on one way to accomplish a decent inexpensive antenna that is capable of 17 meter DX.

 

It’s well known that a properly installed vertical antenna has low angle radiation to the horizon, the favored direction of DX propagation. It’s also well known that a horizontal dipole will have excellent low angle radiation too, but only if it’s high enough off the ground. One half wavelength or higher is the target for a DX oriented dipole. On the higher HF frequencies, that’s not usually a big issue as � wavelength on 10m is only 15 feet. But, a horizontal dipole will need three tall support structures spaced at proper horizontal distance and may not be so neighbor friendly. Enter the vertical.

 

I’ve used a ground mounted HF vertical for years with good success. The secret to a good ground mounted vertical is to put many, many radial wires from the feed point along the ground like spokes of a bicycle wheel. This has the effect of reducing the ground loss in the near field of the antenna by shielding the antenna’s image from lossy soil. These ground losses can be significant and reduce a vertical to little more than a dummy load if they are not reduced. On the order of fifteen 0.1 wavelength radials are the absolute bare minimum if you don’t want to lose a lot of signal to the earth below. Hardcore vertical users put upwards of 100 half-wavelength radials to squeeze the last dB out of their installation. Still, I like verticals. My main HF antenna is a popular commercial trapped model and it doesn’t cover 160m, 60m or the higher WARC bands (17m, 12m). So, I set out to develop a simple antenna to fill in the gaps in my HF coverage. I wanted to focus on the higher HF band coverage for the time being, leaving the low band challenge for later. Specifically I wanted to target 17m.

 

Fortunately there is an alternative to good vertical performance without all those radials. An often used technique to making a vertical work without a massive radial system is to elevate the entire system, radials and all, off the lossy ground.. As little as 0.1 wavelength separation from earth does wonders for efficiency. In fact, at such a height or higher, the number of radials required can be reduced significantly without much performance degradation compared to a ground mounted system with 100 radials. Believe it or not, at sufficient height, reasonable performance can be had with only two tuned radials! The height target for good ground decoupling is greater than 0.1 wavelength high and at 17m that’s only a little higher than 5 ft off the ground. That’s to say that the ground radials must be 5 feet or higher off the ground at their lowest point.

 

With the idea to develop an elevated vertical for 17m that is not too complex or expensive as my goal, I started reasoning through my options. If you are anything like me, you’ve probably tried HF mobile before and you may already have the main piece to build this project; a hamstick antenna. The hamstick is a mobile style HF antenna for single band HF coverage. The price is right and every hamfest I’ve been to has a vendor selling them inexpensively. The hamstick construction is that of a bottom loading coil section wound on a � inch fiberglass tube with a resonating whip above. The total length is about 7 feet and the base is a 3/8 inch 24 TPI (turns per inch) bolt with male threads. These antennas are built to take abuse, so they’re not fragile and a good candidate for outdoor use.

Now I had the idea that I could build an elevated 17m vertical using a hamstick as the main element, it was time to design and test it.

 

Construction

PVC drain pipe is a wonderful material to homebrew with. It’s lightweight, strong, and inexpensive, plus, your local hardware store has plenty of it. It just so happens that the cap for the 1 1/2” drain pipe is flat on the top which makes for a great mounting surface, the smaller pipe caps are curved and would be difficult to use without sanding them flat. I started the project by obtaining 10′ and 5′ sections of SCH 40 PVC, 1 1/2” drain pipe, a pipe coupling, and a cap. I also picked up a 1” long 3/8” 24 TPI bolt, a couple of washers, a �” 24 TPI threaded coupler and an electrical eyelet large enough to fit over the bolt. For feed line I decided on a 15′ length of RG58 with a PL256 connector on one end. Any length is fine as long as it reaches your radio. The radial system is made using #14 solid copper wire which can also be found at the hardware store. Any gauge of wire is fine, but the thinner stuff can be fragile. Be sure to have at least 30′ of it on hand for this project. It doesn’t matter if it’s insulated or not. Figure one is a diagram of the cap and feed point.

Figure 1. PVC pipe cap and feed point detail

The PVC cap I found had raised lettering on the top of it which disturbed the flatness of the surface, so I first sanded it down flat using sandpaper. I then drilled a 7/16” hole in the center of the cap. I also drilled two 1/8” holes for the radial wires on opposite sides of the top of the PVC cap about �” down and a 3/8” hole about 1ft down from the top on the 5′ PVC pipe to route out the coax feed line along the outside of the pipe.

The coax feed line is prepared by placing the free end through the hole in the side of the 5′ PVC pipe section and bringing it out the top open end. Strip the jacket insulation off the free end about 1.5” and separate the shield from the center conductor. Strip the center conductor insulation back about �”. The electrical eyelet should then be soldered on to the center conductor.

Next, prepare the feed point by inserting a 1/2 foot section of the stripped #14 wire through the two drilled radial holes in the pipe cap. The idea is to connect the radials later, but to have a continuous length of wire running through the pipe cap for mechanical rigidity. Solder the radial wire inside the pipe cap to the shield of the coax cable. Be sure to not let the wire get too hot as it will melt the PVC material.

Insert the 1” long 3/8” 24 TPI bolt through the eyelet on the center conductor, through a washer, then into the hole in the pipe cap. The head of the bolt should be inside the pipe cap with the threaded end sticking out the top. Place a washer on the bolt on the outside of the cap and using a wrench, tighten the spacer onto the bolt. It’s a good idea to wrap the radial wire with electrical tape where it is soldered to the coax shield so that it doesn’t contact the bolt head. Place the pipe cap onto the 5′ section of PVC pipe, and your antenna feed point is now finished! Figure 2.

Figure 2. Pipe cap assembly with radial pig tails

I added some RTV to the holes where the radial wire and feedline comes though the pipe cap but I did not glue the pipe cap onto the pipe. I don’t think it’s necessary to use glue since the downward weight and pressure of the antenna and radial wires should hold it in place and it’s a snug fit. Tie wraps are a good idea for keeping the feedline from blowing around in the wind.

With the remaining #14 wire, it’s time to make radials. Since we’re just using two, cut them to the appropriate length for the band your hamstick is designed for [L=243/(MHz)]. In my case the radial lengths for 17m are 13.5 feet each. Strip back and solder the radials onto the wires which come out of the pipe cap again being careful not to melt the PVC. The 5′ section of pipe can now be connected to the 10′ section by use of the pipe butt coupler. Now you have your mast. Again, I don’t see the requirement to cement them together, but you certainly can. You can even paint the pipe. Next, screw the hamstick onto the 24 TPI coupler. I decided to use the coupler for two reasons. One, the thread on the antenna didn’t seem long enough to go thru the pipe cap and washer and, I could change antennas without disturbing the feed point connection inside the pipe cape. Your antenna is now complete! Figure 3.

Figure 3. Complete antenna hidden by a tree.

Installation

The hardest part of this project is deciding where to put the antenna. A site clear of metal obstructions and far from the house is best. A bonus would be if it could be hidden from view. Wooden fence posts make an excellent mount and the radials can be anchored to the fence itself. I decided to strap my antenna to a short tree and let the radiating element protrude above the tree top. My radials are anchored to a nearby wooden privacy fence. In this configuration, I didn’t need to guy the mast since the tree and radials did that for me. For best results and a better SWR match, the radials should be angled down from the feed point and kept symmetrical. Angles from 30 to 60 degrees provide a good match, with 45 degrees usually being optimum. Of course, you will need to adjust the whip on the hamstick to tune to lowest SWR. My SWR plotted over 17m is shown. Due to the low feedpoint impedance of a shortened antenna, you can expect SWR to run a bit higher than with a full sized version. In other words, don’t expect to find a 1:1 SWR match

Simulation

Always curious how my antenna designs perform, I like to simulate them. Using NEC-2, I simulated a wire structure representing my antenna with the feedpoint 15 feet over average ground. I used distributed loading to approximate the bottom loading section of the antenna. Since I couldn’t measure the value of the loading coil directly, I played around with the simulation values and found where it would resonate at 18.1 MHz with a 3.5 foot whip by adjusting the loading inductor value. I then noted the simulated feed point resistance at resonance in the simulation was lower than the 35 Ohms I measured with my impedance bridge, so I added resistive losses to my loading coil until the simulation predicted a feed point impedance of 35 ohms. The final values for the distributed loading coil are 1 ohm in series with 0.42 uH. My simulated SWR curve matched very closely the measured data.. NEC predicted that the maximum gain direction is at a low elevation angle with approximately 0 dBi. Recall that a dipole should have about 2.4 dBi, so it’s reasonable to expect we’re losing a bit of gain over theory with this setup, and this simulation doesn’t include any effects of the transmission line, or unbalanced radial current, but it does show a trend. It shows that most of the RF energy is at low angles leaving this antenna! Just what we want for DX. Figure 4

Figure 4. NEC simulated pattern

Figure 5. Measured SWR

Operation

Now I’ll be the first to admit that this is a compromise antenna and it’s not meant to perform like a tower and beam or a high dipole. There are plenty of discussions around about how inefficient verticals can be, and especially bottom loaded verticals. On the other hand, you’ll see some of the best low band DXers using verticals, so they can and do work very well with low angle skip. By elevating the radials from ground as in this design, one major source of loss has been reduced, the dirt below. The loss of the loading section is still there, but I think it can be tolerated; after all, thousands of mobile stations make excellent contacts with these same antennas. The elevated vertical explained here should operate certainly as well as if not better than a mobile installation. So, how has mine worked?

As for on the air results, within the first 30 minutes of operation I made contacts to Portugal, Ireland, Argentina and Azores from my central US location. I’d say the low angle skip is definitely being worked!

Conclusion

The benefits of the antenna that I have presented here are a simple construction technique, easy match to 50 ohm cable, good low angle omni-directional performance, portability and ability to install most anywhere. Drawbacks are certainly the lossy nature of a short, loaded vertical when compared to a full sized vertical antenna, or a dipole up � wave length.

Please treat this article as only one suggestion and expand on the idea. Try different mounting locations, modify the construction techniques and play with the radial orientation. Try it camping or at your next field event. Experiment and have fun! Hamsticks are available for any HF ham band so it’s possible to go lower than 20m with this idea. One note of concern, however, is that going below 40m in frequency may reach a point where the loading coil losses become very large, but the low angle radiation pattern will be the same. If anyone tries it, please write me with the details! Sixty meters anyone?