Tuesday, October 20, 2015

160 Meter Inverted U Antenna

This particular 160 meter antenna looks like a half square, but its radiation pattern and performance is more like a vertical and it is about 40% as big as a half square.  If you have about 100 feet or so space it is a viable 160 antenna.  I have used this antenna on the air with equally good to slightly better result as the vertical discussed in the previous posting (12/4/13).  I have a friend who also use this antenna with the same results. His is inside a 90 foot wide lot.

Advantage of this antenna over the vertical is that it is much less complicated to build and uses a wire mat for the ground. The disadvantage is the space required for the antenna wire.

I started with 138 feet of #18 AWG wire.  Finished length was somewhat shorter after it was trimmed to resonance.  More on trimming and matching later.

Figure 1*

The section labeled 1 is 1 foot above the ground at the end nearest the ground (This section of the antenna should be put inside of PVC for safety due to high voltages).  The section labeled 2 is 28 to 30 feet above ground. Section 1's length will vary with the size of the ground mat and antenna tuning.  Section 2's length may also be varied, but the longer the better. Section 3 is connected to the top end of the matching coil (more on this later). Section 3 is 28 to 30 feet long. Note that sections 1, 2, and 3 are one continuous piece of wire. You trim section 1 to adjust the antenna to resonance. Be sure to put the PVC over section 1 after each change. It will effect the tuning of the antenna. The end of the antenna (section 1) may be as high above the ground as convenient. The shorter section 1 the better the antenna will perform, but not by much.

Note:  Wire 1 can be higher then 1 foot above the ground. It is set to that during simulation to keep wire 2 as short as possible. Mine ended up about 3 feet on the antenna installation.

The ground mat is cattle panel (Photo 1), sometimes called hog panel. I use AWG #4  welded galvanized wire panels that are 16 ft by 4 feet wide. (They're really 52 inches wide.) The mesh is about 4x6 inches for the ones I used. So long as the longest space between wires is less then .01 wavelength the mat will work. I use 4 of these panels to form a 16 x 17 foot ground plane for my 5-BTV vertical and it works well 80 through 10 meters even though .01 wavelength on 10 is about 4 inches.

Photo 1

The following table shows the simulation result for 4 different sized ground mats.

   Mat dimensions             Gain dBi         Elevation angle deg.  
     8 x 16 (128 sq ft)         2.31                46
   16 x 16 (256 sq ft)         3.72                49
   16 x 32 (512 sq ft)         3.53                43
   32 x 32  (1024 sq ft)      2.84                40

                              Table 1

Notice that bigger is not better as far as gain but does effect elevation angle in most cases. However, in the real world I suspect that ground differences will have a greater effect than mat dimensions on gain or elevation angle as long as the mat does not get too small. I would not recommend anything smaller then 256 sq ft over average soil. The gain drops off rapidly as the mat gets small then 256 sq ft.

I use the 16 x 32 mat and the on air performance is somewhat better than the vertical discussed in my Feb 2013 post. My mat lays on the surface, not buried. I've not tried burying a mat in any of my on air tests so I cannot comment to the effect of that except to say that the life of the mat will decrease with burial. I only put the mat down after the last lawn mowing in the fall and take it up again in spring. The 16 x 32 shape is dictated by my installation space. It is the overall area that counts. I built a 40 meter vertical where I use a plus shaped mat made of two pieces of 4 x 16 foot welded chicken wire and it worked great. So you can make your mat nearly any shape so long as you connected near the geometric center to preserve the antenna's radiation symmetry.

Ground Mat Construction Notes - I connect the panels together in three places along the 16 foot side and one place on the 4 foot side with split bolts (Photo 2, 3, 4) from the local electrical supply or Lowe's. These are used to make ground connections and come in several sized. I also recommend putting copper bearing anti-seize on the bolts if you intend to do as I do and take up the mat each spring. Loctite 39650 Copper Anti-seize comes in 8 oz bottles and is less then $10. It will last you nearly a lifetime. (I use it on all my ground connections and in extremely small quantities on coax connectors, power plugs, etc. This stuff is very conductive so don't let it get on the insulation between conductors.) I also use one of these to connect the loading coil to the mat and a small one on the loading coil for an adjusting tap.

Photo 2

Photo 3

Photo 4

Loading Coil Construction Notes -  The loading coil (Photo 5) is wound from bare AWG #14 on 18 inches of 1 1/2 PVC which is 1.9 inches outside diameter . Wind 16 turns over 16 inches of the form. As you wind on the 4th turn attached a small split bolt on the wire and finish winding the coil. The split bolt is where you will attach the tap for the coax center conductor. The split bolt will slide along the wire allowing you to adjust the tap. I prefer this method rather then solder and unsolder.

In the final installation, I used a SO239 connector with 15 inches of  wire soldered to the center pin and 2 inches soldered to the ground. This gave me enough wire to reach from the tap on the coil and solder the ground side to the bottom of the loading coil. As you can see in Photo 5 the bottom of the loading coil is then connected to the ground mat with a split bolt.

Note:  Photo 5 is of the first loading coil I build. I later went with 16 turns on subsequent antenna. It proved easier to locate the 50 Ohm matching point.

When you reach the final adjustment carefully apply anti-seize to the wire then slide it into the split bolt. It is a good idea to put some anti-seize to the thread of the split bolt in case you need to remove it later.

Photo 5

Bringing the antenna into resonance is fairly easy.  If you have an antenna analyzer use it, else an SWR meter will do the job as well. Adjust the length of the antenna to the center of the part of 160 you wish to operate the most. Don't worry about SWR value initially, just look for the frequency where the SWR is lowest. If it is close to the desired operating frequency, then adjust the tap on the coil for lowest SWR. You will find that moving the tap will change the lowest frequency some. Usually it is not enough to matter, but the two adjustments are somewhat interactive.

This antenna is a viable compromise. I've had many hours of fun QSOs and contesting with mine. Hope you give it a try and enjoy 160.

* - The red lines on the drawing of the antenna are artifacts left from my EZNEC simulator. They are not part of the antenna.