This page describes my homebrew 80/40 meter trap vertical. The Hex Array has 6 of these antenna elements, as identical as I could make them. The trap is a coaxial cable trap, made from RG-213 cable. The overall height of the antenna is approximately 48.6 feet. The radiation resistance on 80 meters is approximately 25 Ohms, and 35 Ohms on 40 meters. The 2:1 SWR bandwidth on 80 meters is 250 KHz, and the entire 40 meter band is covered.
Electrically, the antenna is nothing special. For me, the value of the vertical is in its mechanical properties. The vertical has a single guy point, and can be raised and lowered by myself for adjustment and maintenance.
My design goals for this vertical came from my experiences, good and bad, with other verticals that I have used over the years.
My first vertical was a Butternut HF6V. With a height of 26 feet, it was no problem to raise up and down by simply grabbing it with two hands, and placing it over the iron pipe driven into the ground as a mount. It could be taken from horizontal to vertical with very little effort. The Butternut is not normally guyed, although the manual states that guying it will greatly add to it's ability to withstand high winds.
My next vertical was a homebrew 160 meter vertical, which is approximately 72 feet tall. The lower 36 feet is made from wood, and is topped with a large inductor providing center loading. The vertical is topped with a 35 foot long whip made from aluminum tubing. The base of the antenna is hinged, and I buried a companion wooden base several feet into the ground. The antenna is taken from horizontal to vertical by walking it up. This turned out to be very difficult, even for two people. As the antenna starts to rise, you walk closer and closer to the base. This creates a leveraged situation where the downward force due to the antenna becomes larger and larger as you walk in. Since this is such a pain, the antenna has been vertical for a number of years, which is not a problem because it's working just fine (having said that, it will probably fall over).
One good feature of this antenna is that it is guyed at a single point right at the top of the wood and bottom of the whip. The wood section is very rigid and needs only top support. By definition, a whip is only attached at the bottom (you don't guy a whip). Three guy ropes run down to metal anchors screwed into the ground. The whip is made from telescoping 12 foot long sections of 1.25", 1.125", and 1.00" aluminum tubing.
My last vertical antenna experience relevant to the Hex Array came from the MFJ-1792 antenna. This is a top-loaded 80/40 meter vertical which replaced my Butternut once I installed a tower with a beam, and no longer needed a vertical to operate above 40 meters. This antenna is an excellent performer, and ruggedly built. It comes with a hinged based, and is also walked up into position. While it is not hard to raise it vertical, the weight of the top loading coil and top-hat causes the antenna to bend quite a bit as it rises majestically into the sky.
These experiences suggested that it was indeed possible to build a vertical which is at least as tall as an 80 meter vertical needs to be which can be walked into position, and guyed at a single point. But, the materials and dimensions might be critical if one wanted to avoid an antenna which is either too heavy to raise with a single person, or, so wimpy that it bends and collapses under its own weight.
I've mentioned on a number of pages that I believe that the MFJ-1792 80/40 meter antenna is very good performer. It's full-size on 40 meters, and top loaded on 80 meters. Top loading will increase the radiation resistance on 80 meters to close to the theoretical maximum without making the antenna physically taller. A higher radiation resistance, all things being equal, will lead to a higher efficiency.
There are a few reasons why I decided to create my own 80/40 meter vertical based upon a coaxial cable trap and a whip above the trap..
By using larger diameter tubing on all parts of the antenna, the bandwidth of the antenna should improve. The 2:1 SWR bandwidth for this vertical is about 250 KHz. The MFJ-1792 bandwidth was approximately 100 KHz. This is on 80 meters, on 40 meter, the whole band is easily covered by both antennas.
The homebrew antenna should have a higher radiation resistance, and be more efficient than the MFJ. I admit that the improvement is small, but lots of small improvements can sometimes lead to a good result.
When used on 160 meters (as a receiving antenna), which is a secondary design goal, taller seems as it would be better.
It's a little different. Two paths diverged into a wood..... you get the idea.
It's probably true that the homebrew vertical is a little less expensive than the MFJ-1792, but not by much. To the down side, the homebrew vertical was a lot more work.
The trap on this vertical is a coaxial cable trap which is resonant at 7.150 MHz. The trap is made from RG-213 coax, and wound on a 4" form. It is described in detail on another page. A trap is a parallel resonant circuit which has a high impedance at it's resonant frequency. This high impedance is designed to act as a switch, which effectively terminates the antenna at the point of the trap at the trap frequency. The trap will be located at what amounts to the top of a 1/4 wavelength 40 meter vertical, so that the whip above the trap is not used on 40 meters. Parallel resonant circuits have the property that below the resonant frequency they act as an inductor (positive reactance). Above the trap frequency, the trap appears as a capacitor (negative reactance). This is the opposite sense of a series resonant circuit.
On 80 meters, the trap will appear as an inductor, creating what amounts to a center-loaded vertical. We can be rather confident that the overall height of the antenna on 80 meters will be less than the height of an unloaded vertical due to the effect of the trap.
|(A): Trap Vertical; (B): Antenna at trap frequency; (C): Antenna at lower frequency|
The antenna A in the diagram is the schematic of a two-band trap vertical. The portion above the trap is the whip. At the trap resonant frequency, the antenna behaves as shown in B. The high impedance of the trap isolates and effectively removes the whip from the antenna. In this case, it would be a good idea to make the length of the section below the trap a useful length for the trap frequency. In my array, and many applications, it's 1/4 wavelength. As you lower the frequency below the trap resonant frequency, the antenna appears as in schematic C. The trap is equivalent to an inductor, and the whip performs its function - extending the antenna. Due to the inductor, the electrical length of the vertical will be longer than the physical length of the antenna. The inductor makes the antenna act longer than it is.
Given this general design it's possible to sketch out the approximate dimensions of the vertical. A 40 meter 1/4 wavelength conductor is considered to be around 32 feet long. An 80 meter 1/4 wavelength conductor is approximately 66 feet long. These dimensions refer to the length of an antenna made of thin wire. When the diameter of the element increases, the length needed to achieve resonance decreases. On 40 meters, we expect that the location of the trap, the top of the 40 meter section, will be somewhat less than 32 feet. Estimating the 80 meter height is a little more complicated, due to the effect of the trap. We know that it will be less than another 32 foot long section. My initial estimate was 1/2 to 2/3 of the full size, or between 16 and 21 feet.
Getting a handle on these lengths becomes important when it's time to start to order aluminum tubing. I didn't want to order some parts and then find out that I was way off in what I needed. That would waste time and money.
Adjusting the length of the vertical below the trap will influence the resonant frequency on both the 40 and 80 meter bands. Adjusting the whip above the trap, however, will only influence the 80 meter resonant frequency. In practice, the 40 meter resonance point is adjusted first. Once this is set, the whip is adjusted to set the resonant frequency on 80 meters. This is the standard rule of trap verticals and trap dipole adjustments - you begin adjusting on the highest band, and then work down in frequency.
Aluminum tubing is typically sold in lengths of 6 and 12 feet. I was planning on ordering my tubing from Texas Towers. Since the tubing would be shipped via UPS, the uncut lengths I would be working with would be 6 feet. The 0.058" wall thickness tubing has the property that it telescopes very well between adjacent sizes. For example, the 1/2 inch tubing slides smoothly into 5/8 inch tubing, which slides right into 3/4 inch tuning, which slides right into 7/8 inch tubing, and on and on. Each standard section size differs by 1/8" (0.125") in diameter.
If there was going to be a single guy point, then the antenna below the guy point would need to be rigid and self-supporting, implying a larger diameter. The larger diameter tubing would also be needed to support the trap, and the whip above the trap. The trap will be located approximately 32 feet from the bottom of the vertical. It seems to make sense that the guy point would need to be near the trap. A little wrinkle to take into account is that the adjustment of the vertical length should not move the guy point. If it did move the guy point, then the implication would be that adjusting for resonance would also require adjusting the length of the guy lines. This is clearly a very bad idea. The guy line length should be fixed, and resonance adjustments should be independent and relatively easy to perform. The bottom line is that the guy point should be under the trap, at a fixed length point from the base. The trap should be able to be adjusted for length, through a telescoping slip joint. While the tubing below the trap is of a larger diameter, the whip portion will be mounted on top of the trap, and that is of a much smaller diameter. The whip will be around 20 feet long.
From my other vertical antennas, I had a sense that I wanted the diameter of the lower (under the trap) portion of the antenna to be approximately 2" in diameter. The whip, on the other hand, could have a maximum diameter of around 1". The 2" diameter approximation came from my experience with the MFJ, which is approximately 1.5" in diameter. I knew I wanted to have a larger diameter to reduce the bending in the lower section. A larger diameter should also increase the antenna bandwidth. The 1" whip diameter came from lessons learned with the 160 meter vertical whip. There, an initial size of 1.25" was sufficient for a 35 foot tall whip. Since I would need a shorter whip in this case, smaller tubing should also work.
The trap has important electrical and mechanical functions. The electrical function and design is described on another page. The mechanical requirement is that it must mate with a larger diameter aluminum tube on the lower side, and a smaller diameter tube on the upper side. My source of fiberglass is Max-Gain Systems. In checking out their standard sizes for round tubing, I found that the tubes have 1/8" wall thickness, and fortunately step in sizes of 1/4". This means that you can telescope fiberglass and aluminum.
I picked a tube size of 1.5" for the basic trap mounting tube. It would telescope into a 1.625" aluminum tube. As that tube telescopes into wider sections going towards the base, the diameter will reach the 2" goal.
The whip will telescope into the fiberglass. With a 1.5" outer diameter fiberglass tube, the internal diameter is 1.25". This is a little larger than needed in this application. My solution was to insert a second fiberglass tube into the first, creating a tube with a thickness of 0.25". The whip would then start with a 1" diameter section of aluminum.
The trap is built over a 2 foot long section of fiberglass tubing. A 1.25" diameter piece is inserted into a 1.5" diameter piece. The aluminum tubing which fits over the fiberglass is 1.625" in outer diameter. The whip telescopes into the fiberglass on the other end, and is 1" in diameter. The overlap between fiberglass and aluminum will be 6". This leaves a 1 foot section of nothing but fiberglass, and the trap is centered on this region.
The whole vertical is based upon telescoping aluminum and fiberglass. Joint overlap is usually 6". When a joint is fixed, it is pinned with a 1/4" stainless steel bolt and locknut. The bolt is placed halfway from the ends (3"). When a joint needs to slip, usually for resonance adjustment, the end of the larger tube is given 4 lengthwise slits made on a bandsaw with a metal cutting blade. The slits are approximately 2" long. Stainless steel hose clamps compress the end onto the inserted tube, and provide a friction fit. Below the trap I use a pair of hose clamps per junction.
I applied an anti-oxidation compound to the overlapping metal junctions before assembly.
My target diameter for the lower section was 2". On the top end, we know that the tubing must telescope down to 1.625" to accept the trap. That will take 3 intermediate diameters - 1.875", 1.75", then 1.625". I decided to use three 6 foot sections of 2" tubing for the lower part of the section. This would provide 18 feet of length at my target diameter, and have room above it for the intermediate tubes. Rather than telescoping into smaller and smaller sections, I cut coupler tubes from a 6 foot section of 2.125" aluminum tubing. These couplers are 1 foot long. The remaining 4 feet of 2.125" tubing was put at the very bottom of the antenna.
After adjusting for joint overlap, and assuming uncut 6 foot sections, the length of this arrangement is approximately 38.5 feet. This is much longer than the estimated maximum of 32 feet. This meant that I had at least 6 feet of excess tubing in the section below the trap. I decided to reclaim that by initially cutting the 1.75" and 1.625" tubing in half, and using 3 foot sections. Since I had to build 6 verticals, I could reuse the second half of these tubes in other verticals.
Here is a diagram of the vertical below the trap.
|Lower Section Diagram (not to scale)|
The base of the vertical is on the left. The first section is 2.125" in diameter. Two 1 foot long coupler sections were cut off this tube, leaving a 4 foot (48") base section. The couplers are used at the next two transitions between 2" diameter sections. At the top end of the 2" section is a stainless steel eyebolt, which is described later on this page. Above the 2" tubes, the sections begin to taper. The last section, 1.625" in diameter, is adjustable in length, and sets the resonance point on 40 meters.
The top of the 1.75" section holds the guy ring, which is the mounting point for the three guy lines. The ring is described in a later section.
The length of the 40 meter section, 29.2 feet, was several feet less than the often quoted length for a thin wire. This was due to the large diameter of the 40 meter section. A good article describing this effect is Vertical Antennas: New Design and Construction Data found in the ARRL Antenna Compendium, Volume 2. The ON4UN book also talks about the effect.
The bottom of the trap has a diameter of 1.5". This telescopes into a 1.625" aluminum tube. The overlap is 6". A stainless steel bolt and locknut pins the junction. Above the trap, the whip telescopes into the fiberglass tube. This junction has a similar 6" overlap and bolt and locknut pin. Here is a picture of the trap section while a vertical is lowered to the ground. Please click on the picture for a larger view.
The electrical connections coming out of the trap are made with #12 insulated wire. These wires extend over the aluminum tubing where they are attached to the tubing with a stainless steel hose clamp and an end ring terminal which is held to the tube by a self-tapping screw. I originally spray painted the trap with a light blue color to help the trap blend into the sky, and to reduce the effect of the sun's UV radiation on the plastic and fiberglass parts. Over the last several years the paint has started to flake off.
The whip begins with 1" aluminum tubing which telescopes into smaller and smaller sections until the target length is reached. Each tube is 6 feet long, and will have a 6" overlap. Three tubes will provide a length of approximately 17 feet due to the overlap. This is a few feet less than the expected length. A fourth section of tubing should be enough to achieve the needed length, and it's length can be adjusted to set the resonance point on 80 meters.
On the top of the highest section I pressed on a rubber cap to keep water out of the vertical. Caps in various sizes are available at a good hardware store. They are typically used at the bottom of wooden legs, but they work well at the tops of antennas. An alternative could be a rubber stopper.
Whip sections are held together with slits and stainless steel hose clamps. Here is a diagram of the whip.
|Whip Diagram (not to scale)|
The top tubing section started off at the full 6 foot length. After the target resonance point was experimentally determined, I cut off much of the length so that only a foot or so was inside the next section. This reduced the weight at the top of the whip, but I still have enough length to allow the resonant frequency to be set at any point in the 80 meter band.
I needed a method to attach 3 guy lines to the vertical below the trap. Antennas usually have 3 or 4 guying directions. In this case, I used 3 guy lines because of the way that the number 3 interacts with the 6 vertical elements. This is described on the site preparation page. I knew that I would be using the UV-resistant Dacron rope that I find works very well. It's commonly available in three sizes (3/32", 3/16", 5/16"). I have used all of the sizes, and even after many years, I've noticed no deterioration or stretching. I usually obtain my supply from the Radioware Bookstore. In this application, the smallest diameter is sufficient.
While there are many ways to attach a guy line to a round vertical antenna element, the best scheme that I have found was used on the MFJ-1792 antenna. MFJ provided a round plate with holes drilled for either 3 or 4 mounting points. The plate has a center hole that fits around a certain tube diameter, and will sit on the shoulder of the next largest diameter. The plate is supported by that shoulder, and is otherwise not attached to the antenna. This means that the guy ring can freely rotate around the antenna, self-aligning with the guy line directions. This keeps the guy lines from wanting to twist the antenna, which would happen if all three guys did not point at the exact center of the antenna. The guy ring sits at the transition between two telescoping sections.
The outer edge of the ring is folded back on itself, in the form of a relatively large circle. The mounting holes are located at the inner radius of that circular profile loop. A guy line attaches around that circle, which means that the line will be pulled against a curve, as opposed to being bent sharply over an edge. The odds that the guy will fail because the loop going through the mounting hole will be sliced by a sharp edge are greatly reduced. The additional metal also means that it is going to be very hard to pull a hole open on the plate. It's a very good design, and I wish I understood how they fabricated it.
I contacted MFJ, and they were willing to sell me several rings ala carte.
Here are a picture of the rings, and a ring installed on an element. Please click on a picture for a larger view.
The standard center hole diameter is 1.25", and I had to drill out my rings to accommodate the larger tubing diameter.
The guy ring is located at the telescoping joint that slips to allow resonance adjustment. In the right picture you can see the slits cut along the tube that allow the tube to be compressed by a pair of stainless steel hose clamps. A pair of clamps are used for redundancy. The guy ring does not change height, but the smaller tubing section can be moved in and out for adjustment.
I use the classic bowline knot to form a loop through the hole and around the curve. The plastic cable ties are there to reduce movement of the line ends in high winds. While I don't believe that a bowline would work loose due to a flailing end, why take the chance?
Here is a picture of the guy ring and trap on a raised vertical. Please click on the picture for a larger view.
It was indeed possible to walk-up the antenna that has been described. I don't believe that the flex in the antenna was a structural problem, although it looked excessive. The energy required to walk up the antenna was much less than what was needed on the 160 meter vertical, but I could not see myself raising and lowering each element many times while adjusting the resonance points.
After I walked up the antenna a more important question emerged - so now what do I do? The answer seems obvious - attach the three guy lines to their mounting points on the ground. But I'm standing at the base of an antenna rising almost 50 feet above me anchored on a single pin down near the ground. Can I really let it go and start running to the three points and attaching guy ropes before it falls over? I don't think so. Perhaps with a second person this would work, but not by myself.
The solution to all of these issues, the flexing, the walking up effort, and the single person operation was to add what I called the raising fixture. This would be a vertical post temporarily mounted next to the antenna, with a pulley at the top which could be used to raise and lower the antenna, and hold it vertical while the guys were not attached. How tall should the raising fixture be? What should it be made from? I happened to have a number of 2" X 6" pressure treated boards left over from a backyard deck project. These planks were 20 feet long. I had used them on other antenna projects. I decided to use one of these boards for the raising fixture. This means that the temporary vertical support point would be approximately 20 feet off of the ground. I would need to put an anchor point on the vertical around 20 feet from the bottom which would be pulled up to the top of the raising fixture.
While the antenna was in between horizontal and vertical, it would be trying to pull the raising fixture over rather than remain vertical. This was solved by adding a backstay line near the pulley at the top of the raising fixture. The backstay line would run diagonally down to the ground, in the opposite direction of the antenna movement. It counteracts the force of the antenna.
Trying to get a 20 foot long 2" X 6" board to stand on end would require some support. I decided to use a pressure treated 4" X 4" post as the mounting base for the antenna. Each post was approximately 5 feet long. I used a posthole digger to sink each post approximately 3 feet into the ground, leaving two feet exposed above ground. The antenna base would be hinged to one side of the post, and I mounted a pair of studs on the opposite side of the post to hold the raising fixture. Large flat washers and wing nuts are used to hold the raising fixture against the post while the antenna is being raised and lowered.
The post is also used to mount an outdoor-rated plastic box which can hold any circuitry needed at the base of the antenna. Here are a few pictures. Please click on a picture for a larger view.
The left picture shows the studs which hold the raising fixture coming off of the left of the post. The plastic box on the front side of the post is used to hold any components which are located at the base of the vertical. The box has a hole drilled in the bottom which allows the feed line to enter the box. The ground system is attached to studs mounted on the bottom. The box contains a small PC board with an SO-239 female jack. It is the junction point between the coax, the ground system, and the wire that goes to the aluminum tubing.
There is ample room in the box for additional components such as relays and inductors which could be used to resonant the antenna at other frequencies, such as 160 meters, or the 80 meter CW segment. The box is designed for outdoor electrical use, and was obtained from Home Depot. The lid has a rubber gasket which helps keep water out of the box. The box is mounted on a small piece of plywood which is screwed to the pressure treated post.
The middle picture shows the back side of the box, as well as the element mount. The wire that feeds the antenna is run through a hold drilled in a rubber stopper. The stopper is pushed into a hole in the box. This scheme was devised to bring out the wire while not letting in water, but also allowing disassembly and replacement. The aluminum tubing terminates in a PVC collar. A stainless steel bolt runs through the tubing and the PVC, and serves as the attachment point for the feed wire. A second hole is drilled at a right angle to the first hole, and is the hinge pin mount for the vertical. The hinge pin is run through two two oak arms which are let into the sides of the post. All of the wooden pieces were given several coats of stain for weather protection. All of the hardware is stainless steel, and 1/4" in diameter. The oak arms keep the aluminum tubing from touching the post. The spacing distance is approximately 1/2". The vertical pivots on the hinge pin. The standoff from the post must be large enough so that when the antenna tips horizontal the bottom of the PVC collar will not run into the post. The bottom of the aluminum tubing is about 6" off of the ground. In general, this distance should be kept small. I chose this distance so that the antenna would be above most of our winter snowfalls. We can certainly have snow which is much deeper than 6", but the lowest few inches tend to be the dense and wet layers, and I didn't want to have the vertical in long term direct contact with that much moisture.
The right picture shows the bottom of the raising fixture when it is bolted to the post. The black rope runs up the vertical to an eyebolt. A cleat is mounted on the side of the post, and holds the rope. The rope is always attached to the vertical, since it must be in place before the vertical is lowered. When the vertical is lowered, the cleat is used as a tie point for the rope going up to the pulley on top of the raising fixture, and then to the eyebolt on the vertical.
Normally, a vertical stands straight up, held in place by 3 guy ropes. When it's time to work on a vertical, here are the steps.
Go find the raising fixture, which usually is lying on the ground near the array.
Lay the raising fixture on the ground directly behind the post, lying away from the direction that the antenna lowers.
Connect the backstay line from the top of the raising fixture to the metal hook on top of the screw anchor installed for this purpose.
Take the rope which is permanently connected to the screw eye on the vertical at the 21.5 foot level and run the rope through the pulley on the top of the raising fixture.
Lift the raising fixture vertical, and impale it over the studs on the post. Fasten it tight with the washers and wing nuts. This is the only part that is work. Rather than being a heavy piece of wood, the raising fixture should probably be an aluminum tube which is anchored with three guy lines, as opposed to a single backstay.
Pull the rope going through the pulley tight, and fasten it to a cleat on the post. This will provide the only support for the vertical when the guys are removed.
Go to the two outboard guys, and unclip them from their metal anchor hooks. It is not needed to unhook the remaining guy which runs along the direction that the vertical pivots - it is not in the way.
Unhook the rope attached to the cleat. As this rope is slowly paid out, the vertical pivots toward the ground. This is a good time to be wearing gloves.
When the vertical gets to the desired point, the rope is again attached to the cleat. The rope holds the antenna nearly horizontal during adjustments and work.
Work on the antenna. Raising the antenna reverses the steps.
The rope is pulled and the antenna rises vertical.
When the antenna is vertical, the rope is attached to the cleat. The raising fixture is now the sole vertical support for the antenna.
If the work is done, Reconnect the two outboard guy ropes. The antenna is now being supported by the guys and the raising fixture.
The rope connected to the vertical, which runs through the pulley, is laid out on the ground, making sure that there are no knots and that it is not hung up.
The wing nuts holding the raising fixture are removed, and the fixture is taken off of the studs, and placed at the base of the vertical.
I can walk the raising fixture down. Here is where the rope going through the pulley must be able to freely run through the pulley so that the raising fixture does not want to pull down the vertical.
Unhook the backstay line, and put the raising fixture back in the weeds.
While there are a lot of steps, they are fast, and little work. I can raise and lower the antenna in less than a minute, all by myself.
I don't expect any reader to memorize these steps. I wrote them down for my own future use. I will probably not have to adjust a vertical for a few years, and when that time comes, I'll forget some step and have a mess on my hands. Maybe I'll read my own page.
Here are some pictures that go along with the steps. Please click on a picture for a larger view.
These first two pictures show a vertical which is partially lowered. The left picture shows the raising fixture bolted to the base post. The backstay line can be seen at the top, running to the right. The line running from the vertical goes around a pulley at the top of the raising fixture and then runs down to the base of the antenna.
The left picture shows the vertical when it is nearly horizontal, and ready for adjustment. Since my verticals tilt in to the center of the array, they would hit the center post if allowed to become completely horizontal. I tie off the support when the vertical is a few feet off of the ground, and above the center post. It's then very easy to adjust the antenna. The middle picture shows the stainless steel eyebolt which is the anchor point for the raising rope. The rope is 5/16" Dacron rope described earlier on this page. The hole for the eyebolt was not drilled until the antenna was pinned to the base so that it would be vertical as it comes out of the aluminum tubing. The bolt is also drilled at a point where there is a double thickness of tubing.
The right picture shows the antenna when it is vertical and the raising fixture is mounted to the base. The eyebolt is a little higher than the pulley on the raising fixture. The backstay line, which is also 5/16" Dacron, heads off to the right.
Now that I had a completed vertical and a ground system, it was time to make some impedance measurements with the antenna analyzer. A very simple and very incorrect analysis would arrive at a resonant impedance of 36.6 Ohms. That is, 36.6 + j 0 Ohms. This is half of the value of a 1/2 wavelength dipole, and is the often quoted impedance of a 1/4 wavelength vertical. In theory this is correct, but it misses way too many important factors.
The feedpoint resistance of a resonant vertical is the sum of the radiation resistance and the loss resistance. The loss resistance is usually dominated by the ground loss, but the loss of loading coils or other components of the antenna are also included in the overall loss. Radiation resistance is a function of the length and diameter of the vertical relative to the frequency.
Estimates of ground loss can be difficult to make. The loss will be a function of the number of radials, the length of the radials, and the local ground conditions. Published tables provide loss values for various combinations. While the number and length of radials is a very specific factor, the ground conditions are quite variable. As a result, these tables often are not that useful in a particular installation.
If we knew either the radiation resistance or the loss with some degree of accuracy, we could determine the unknown quantity by simply subtracting the known from the measured impedance. On 40 meters, my verticals had a feed point impedance of approximately 39 Ohms (39 + j 0 Ohms). On 80 meters, the impedance dropped to 33 Ohms. The 40 meter target frequency was 7.150 MHz, and 3.8 MHz on 80 meters. Given the radiation resistance and the loss resistance, we can compute the efficiency of the antenna. The efficiency is:
Efficiency = radiation resistance / (radiation resistance + loss resistance)
In the grand scheme of things, the efficiency will be whatever it is, and I can't do much about it unless I enhance the ground system. Still, it's instructive to try and estimate it.
On 40 meters, the antenna is a true 1/4 wavelength vertical without any loading device. This assumes that the trap at the top truly disconnects the whip from the lower section of the antenna. The antenna should be close to that 36.6 Ohm value. It may be lower, however. In section 6.6, chapter 9, of ON4UN's Low-Band DXing, ON4UN describes his full-size 160 meter vertical, and how he measured a 20 Ohm radiation resistance as opposed to 36.6 Ohms. His explanation was that the 36.6 Ohm value only applied to thin wire elements, and as the element diameter increases, the radiation resistance drops. His figure 9-74 plots radiation resistance versus height to diameter ratio. On 40 meters, the height to diameter ratio of this 40 meter vertical is approximately 180. This translates to a radiation resistance near 27 Ohms. That's quite a range, from 27 to 36.6 Ohms. The loss resistance in those two extremes would vary from 12 to 2.9 Ohms, and the efficiency on 40 meters would range between 69 and 94 percent.
We can also come at the efficiency from the perspective of the ground loss. Table 9-1 of the ON4UN book presents loss resistance as a function of radial length and number of radials over good soil. On 40 meters, I have 60 radials measuring 0.30 wavelength. From the table, the loss resistance would be 6.6 Ohms. This table should be a worst case in my situation since I believe that I have better than good soil, and the 6 verticals have a combined radial system of 360 radials, and nearly 2 miles of wire. In fact, each vertical has only 1/3 of its radial circumference implemented as 40 foot radials. In the other 2/3 of the radial field the ground system merges with the radial systems of the other 5 verticals. It would make sense that this overlap of radial systems would only reduce the ground loss for any individual vertical.
But, if we assume that 6.6 Ohm loss, the efficiency on 40 meters would be near 83 percent.
Perhaps with a bit of optimism in my heart, I would tend to split the difference between 83 and 94 percent efficiency, and call the result near 88 percent. This would make the radiation resistance on 40 meters 34.3 Ohms, and the loss resistance 4.7 Ohms.
What about 80 meters?
Since we have determined a 40 meter loss value, we can use that to estimate the 80 meter loss. According to table 9.1 from the ON4UN book, the 80 meter loss (given the 40 meter loss) is probably between 8 and 10 Ohms. The loss resistance goes up as we lower the frequency since the radials are a smaller wavelength long. Since the 80 meter feedpoint impedance was 33 Ohms, the radiation resistance would be 23 to 25 Ohms on 80 meters. Are these reasonable values?
Turning again to the ON4UN book, equation 9-8 calculates the radiation resistance of a center loaded whip. Applying that formula to my antenna I compute a radiation resistance of 25 Ohms, which is at the upper end of the estimated range. With 25 Ohms of radiation resistance, and 8 Ohms of loss resistance, the efficiency on 80 meters would be 76 percent.
While there are some large tolerances and wide swings in the computations, my best guess is that the efficiency of this antenna over the whole array ground system is 88 percent on 40 meters, and 76 percent on 80 meters. This is for a single isolated element over the ground system of all six elements. If I were to use a single vertical as the sole antenna, I would implement the best radial system I could put down, perhaps 60 radials each 0.4 wavelength long on 80 meters. This would raise the 80 meter efficiency over 85 percent, and send the 40 meter efficiency near 100 percent. All of this assumes my soil characteristics. If you are in the sandy desert, or sitting on a large block of granite, your efficiency will drop.
By the way, an efficiency of 80 percent means a signal strength reduction of approximately 1 dB as compared to a 100 percent efficient vertical. A 50 percent efficient vertical loses around 3 dB compared to a 100 percent efficient one.
Could you build a monoband 80 meter vertical using a single guy point and a whip? My best guess is yes.
How tall would a full-size 1/4 wavelength vertical be on 80 meters? The answer for a thin wire is around 66 feet. Since the vertical would be built from tubing, it's length would decrease. The ARRL Antenna Compendium, Volume 5, has an article entitled A 75/80 Meter Full-Size 1/4 Wavelength Vertical by Guy Hamblen, AA7QZ/2. His vertical begins with 1.5" tubing at the base, and telescopes in 12 foot sections. The total height for resonance is 58 feet. His target resonant frequency is the same as mine - 3.800 MHz. The effect of the tubing is to shorten the length of the vertical by 8 feet as compared to a thin wire. With my larger diameter base, 2.125", I would assume that my resonant length would be around 57 feet, which is only about 9 feet more than the length of the trap vertical.
That additional length could be achieved by restoring the full uncut length of the 1.75" and 1.625" sections, and adding a few feet to the whip at the top section. In this case, there is no trap, but the fiberglass tubing is still used to provide a transition between the larger diameter lower section, and the smaller diameter upper section.
The guy ring would move up in the air a few feet, which is probably a good thing. It might be desirable to add a few feet of length to the raising fixture as well.
Why would you want to take the time to remove a trap and add 9 feet of length to create a monoband vertical? I can think of at least two reasons. First, the bandwidth of the vertical on 80 meters would no doubt be larger than the 250 KHz I measured with my implementation. Second, the radiation resistance would increase from 25 Ohms to approximately 34 Ohms, similar to what the trap vertical shows on 40 meters. With my ground system, the efficiency would climb from 76 percent to 81 percent. One can't get too excited about 5 percent improvement. If you are willing to work to add 9 feet of length to the vertical, you should also improve the ground system and cut the loss from 8 Ohms to 4 Ohms. The efficiency would then rise to 90 percent. The lesson here is that having a full-size 1/4 wavelength antenna is not a guarantee that you will have an efficient radiator. Ground loss must always be considered, and minimized. A full-size vertical simply maximizes the radiation resistance.
The verticals have been up for over two years at this point, and there have been no problems. An element can be lowered, adjusted, and raised very quickly and with very little effort. The hard part is moving the raising fixture from vertical to vertical. That's where I tend to break a sweat. Since there is a backstay line to support the force of the antenna, there is no need to have such a heavy fixture. I simply had a number of 20 foot long pressure treated 2" X 6"s available, and they were easy to press into service. I made a second raising fixture where I trimmed down its width, and that did help somewhat with the weight. The fixture could probably be made from aluminum tubing, so long as it was guyed in three directions while being used. The aluminum fixture could be guyed to the same anchors that guy the vertical. This modification would make it trivial for a single person to work on the antennas. My only concern would be the electrical interaction between the antenna and the raising fixture. If this were a problem, perhaps fiberglass could be used.
I did operate a single vertical for a number of months, and my conclusion was that it was a better performer than the MFJ-1792. Certainly the bandwidth was much wider, and it seemed to have a little more gain. This was on 80 meters. On 40 meters, both antennas seemed pretty much the same. This comparison is very difficult to make because the radial system changed between antennas. The MFJ-1792 was over a smaller ground system, which no doubt had more loss. When I operated a single trap vertical, it was over the 60 radial ground system, with all 6 radial systems installed. A good radial system is an essential part of any ground-mounted vertical.
While I have provided dimensions for my own records, if you build an antenna similar to this one, expect there to be variations. Use an antenna analyzer in the field to set the desired resonance points.
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