The Ubiquitous Ground Mounted Vertical - Add A Phased Clone and Start Enjoying Big Time Signal Reports!

PART I

By Tom Sowden K0GKD

www.k0gkd.com

I get on the air with my friend (and famous fisherman) Orrin (K9KEJ) from time to time, and we also talk on Skype. Orrin’s signal, whether on 40M or 20M, always amazes me – it is consistently strong and in the S9 plus area! He runs a Pro-III with an amp that probably is puffing smoke into a ground mounted vertical antenna (by ZeroFive Antennas) out of Buffalo Grove, Illinois – a Chicago suburb. He routinely gets 10 – 15 db over on 20 meters from Honk Kong, Singapore, and all over the Far East. It is just an amazing fact – vertical antennas play well if they are properly designed!

Vertical with radials laid out on the ground

There is no question that dipole antennas work well for most hams. However, consider that to get good angles of radiation with wire dipoles they need to be up high – at least a quarter wavelength; or better yet a half wavelength. This becomes an increasing problem as the frequency of the antenna goes down. At 80 meters one half a wavelength is 40 meters, or over 131 feet! Mounting the antenna to this height is a bit of a stretch from a practical standpoint – not that it cannot be accomplished.

Enter the vertical antenna alternative idea and the problem goes away – providing of course you do it right! To get the performance out of a vertical you have to understand what is going on. In a manner of speaking the antenna is no different then a half wave dipole turned on its end, with one element missing. The antenna is often called a “monopole”, or in the old days a Marconi. The missing element is the “ground” element, and it plays a major role in the radiation characteristics needed. Most of us have to deal with this or the antenna just won’t perform very well. There are exceptions where the ground conductivity is exceptionally good. The ocean would be the perfect “other half” of the antenna if this were on option. In other words mounting a quarter wavelength element vertically on a boat parked in the ocean would be the perfect scenario. Other then a slight mismatch at the feed point this antenna always plays exceptionally well and is impossible to beat for low angle radiation and overall performance.

The best of all locations

Every time I hear a “ham” bad mouthing vertical antennas I think to myself they just don’t “get it”, or maybe from lack of understanding they take the criticism of others at face value. In some instance these criticisms are valid even with the best-designed verticals. The key to good vertical performance has more to do with ground conditions, and ground conductivity, then the antenna per se. I tease Orrin of having a copper mine in his backyard as to my knowledge he does not have one radial laid down. There have been volumes written on the subject, and there are overall parameters that define land characteristics that are informative as to the “general” level of conductivity.

Antenna gurus will tell you that verticals have two fields – near and far. The former is also called the Fresnel zone. Depending on many factors it is about one to two wavelengths away from the antenna. To some degree it is all about the “far field” radiation effect that one needs in order to get the best performance from a well-designed vertical antenna. Without good soil conductivity there is very little we can do to make the antenna work well in the “far field”. The “far field” is removed from the immediate vicinity of the antenna, and is where the radiated signal comes together and forms its pattern.

While we cannot control the ground conditions in the “far field” we can manage the “near field” affect. Lets assume for the moment that most of us have acceptable ground conditions and the “far field” will not be a major issue. If you want to get a rough idea of your areas’ conductivity the ARRL antenna book has a section on this subject which indicates different areas of the county and their likely ground conditions. The FCC also publishes a map of the US with estimates of ground conductivity (http://www.fcc.gov/mb/audio/m3/). I downloaded the map for the area around Eastern Kansas and Western Missouri just to see how our soil stacked up.

The maps indicate that the ground conductivity in my area (Kansas City) is fairly good at 15 to 30 millimhos per meter. I guess 30 millimhos is okay as far as mother earth is concerned by pales in comparison to seawater is 5,000 millimhos per meter!

With a ground mounted vertical antenna the two main factors that determine loss of transmitted or received signals are the ground characteristics and the antenna height. Both of these are manageable, to some degree, with the good designs. The latter issue is primarily one of length. Short antennas, that is shorter then one-quarter wavelength, need loading coils to stretch them out to the resonate frequency. These coils tend to dissipate significant amounts of the output power through heat, and the antenna efficiency suffers. Coupled with a bad ground system one can lose a goodly amount of signal output. Mobile rigs are an example of these lossy designs by necessity, as their short antennas and poor ground systems just don’t perform well unless there is a lot of power being used.

As to the “ground half” of the vertical, without good conductivity surrounding the antenna a lot of not so good things happen. The effects are critical as the ground collects the antenna return currents and basically acts as the other half of the “monopole”. You can somewhat visualize the radiation pattern coming off of the vertical element. Picture a fountain in a park where the water sprays out in a 360-degree pattern. It usually is collected and returned to the spray nozzle. If the collecting basin is not wide enough, or its integrity is poor, the water it is lost to the surrounding environment – the ground. The same principal can happen with vertical RF energy. The antenna just won’t work without a good ground, unless like Orrin (K9KEJ), you have the equivalent of a copper mine in your backyard or you are setting out in the middle of the ocean. When we are able to close in on “a perfect ground” there is no phase shift of the reflected vertically polarized wave at the reflection point.

Every time I read one of the better amateur antenna books I get a headache with the terms, difficult equations, graphs and blabber. (I had the same problem in college – please can’t someone put this is simple terms – I am kind of lazy!) Anyway a lot of the information is important and needs to be understood. One such confusing term is the pseudo Brewster angle – oh boy! To put this in laymen’s terms this is the angle at which the radiated signal is -6db down from the “real” ground (not your ground). In other words you want your angle of radiation well above this number for the antenna to perform. Since verticals have low angles of radiation this can be a big problem in some areas. There are complicated formulas to calculate the angle but all we really need to know is that it goes down with good ground conductivity. In the case of salt water the angle is nearly zero so the low angle RF emanating from your vertical will be well above the Brewster and you will dodge the –6db bullet or worse!

Okay, what can we do if your low ground conductivity is responsible for raising the pseudo Brewster angle to a level that will keep your vertical from playing well? Assuming the antenna is at least a quarter wavelength in height the Freznel zone (near field) is probably no further out from the antenna then one or two full wavelengths. (Note: this zone could vary dramatically due to different conditions, angles of radiation, and other factors). At 40 meters this “might” be about 130 to 260 feet. Blindfold your XYL and quickly lay out a circle of chicken wire, or whatever you can buy on the cheap, with a radius of 135 feet. Now you are ready to put the radials on top and perhaps you will not have to worry so much about the pseudo Brewster angle – just the XYL when you take the blindfold off. Practical – no, workable – maybe! It may not improve your far field issue but it will help the antenna play better. There are hams living in the desert southwest that have done this very thing successfully.

I asked Alan Christman (K3LC) his take on this issue. He responded with the following note:

This is a good idea regardless of your soil conductivity. By getting the antenna up and away from the ground and using “tuned” radials you take the “lossy” ground out of the equation. The idea is that most of the antenna return currents have a less resistive return path compared to the ground. A 40 meter antenna can be raised about eight feet, and two “tuned” quarter wavelength radials will work and be the equivalent of 60 or more on a ground mounted vertical. Try to keep them opposed or 180 degrees apart. They need to be completely isolated from the ground. You will have to play around with their length, as a better match may be closer to .2 wavelengths. Sloping them down about 30 degrees will raise the feed point impedance to more nearly match your 50-ohm coax. You can also make up a matching section (see later text) of coax and leave them horizontal, which will likely allow the antenna to function more efficiently - the further away from the ground the better! You will have to locate them so they cannot be inadvertently touched by people or animals to avoid ‘rf” burns.

I like mono-band antennas for a lot of reasons that will become more apparent later, especially if you want to gain the advantage of adding a second or parasitic clone. When you are designing for 40 meters and up the issues are not as complicated as for the lower bands – 80 meters and 160 meters. Self-supporting antennas at these higher frequencies are fairly easy to put up without guy wires or other external support mechanisms. There are multiple methods that one can employ to get the height necessary for a quarter wavelength, and the ARRL Antenna Book3 is full of examples, including those using top loading coils and “capacitive hats” to electrically shorten the antennas.

The simple approach would be to use steel pipe that tapers up in three or four sections. You will need about 33-34’ for your 40-meter version, and it will require a supporting bracket that will keep it insulated. A well-lacquered fence post sunk down four or so feet into the ground should provide a good support. Use special nylon bolts to secure the lower section to the above ground post with PVC spacers and it will be adequately insulated.

Of course, commercial aluminum antennas are available. Some of these are very well built and come with all of the mounting hardware required to get you up and running. I especially like the ZeroFive antennas, hand built by N9ZV (Tom). A 40-meter self-supporting model can be purchased for about $300 (plus freight) and are well worth the money (www.zerofive-antennas.com).

Well Hidden Mono Band Vertical

The best way to get the “other half” of the antenna to work is to put out radials, and lots of them. Usually copper wire with UV type insulation works the best. The length and number of the radials is a subject of which there has been volumes written. A good rule of thumb is 40 to 60 radials evenly spaced around the base of the antenna cut to about 16 or so feet for 40 meters - 1/8 wavelenght. The “electrical wavelength” or ground radials behave differently as they are placed on or near the ground. The velocity factor (the speed at which the RF travels over the wire) goes way down compared to what it would be in free space. This is good news, as the electrical wavelength in most situations becomes about 50% percent of the physical “one-quarter” wavelengths. In other words you can save on wire costs by shortening the length of the radials. If you think about what is happening with most of the high Rf currents taking place at the base of the vertical, they do not need to go out very far to find a return path to ground! Shorter is better from a practical standpoint!


A commercial Zerofive Antenna insulated mount

Regardless it is a lot of wire to put down and can play havoc with your lawn mower. I like to use number 14 insulated copper wire for mine. There are several tricks to get the radials placed so they will not interfere with the regular lawn cuttings, and hide them from the wife and neighbors. First you will need a mounting plate that will allow the individual radials to be secured to the base or ground side of your vertical. There are a number of commercial sources available, or Home Depot will provide a reasonable alternative. A number six or eight piece of copper wire made into a 36-inch circle with the ends connected will work just fine. You can either solder the radials to the copper wire or use mounting connectors. It is usually a good idea to lay a few out around the antenna perimeter to get a good idea of what the finished result will look like. On the commercial side I like Lance Johnson engineering for their good prices (http://www.lancejohnsonengineering.com).

Once they are all laid out on the ground you can use a hoe or a “weed eater” to cut a path along the straight line for each radial going away from the antenna. Just move each radial while you get the “path” cut. Once they trail is ready stretch the radial out and secure it in several places with galvanized fence “u-nails” so the grass will grow over them in time. You will get the idea as you go along and the lawn mower will not pull them out until the grass has a chance to entwine them securely. Within a week or two they will start to disappear.

Disappear – maybe?

If you get lazy and decide that 60 radials is too many start out with a lesser number, like 30, and see how the antenna performs. Sometime I hear of hams putting down thousands of feet (or pounds) of radials that literally blanket the area around the base of the antenna. While more is better there is a point of diminishing returns. In a study by Al Christman1 he found with average soil conductivity the peak gain varied from -1.49db to +1.4db using nine radials on the low side, and 100 on the high end. While we all want the higher gain the middle range of the study indicated that 40 radials moved the gain into positive territory. Is adding the additional 60 radials worth the cost and effort to get less then 1db of gain? I guess it depends on who cuts your lawn!

With 40 to 60 radials your feed-point resistance is likely to be somewhere around 25 to 35 ohms – not so good for your 50-ohm coax as your SWR will be a tad high. Okay, no problem. Just feed it with a matching section made up of two 50-ohm sections of coax in parallel one-quarter “electrical” wavelength at the operating frequency. The drive source will see 50 ohms at the matching section connection, and the SWR should be around 1:1. Different combinations of 50 and 75-ohm coax at one-quarter wavelengths give you options to match about any feed point resistance. For example 2-75 ohm pieces equals 37.5 ohms, a 75 and a 50 ohm equals 30 ohms, two 75’s and a 50 ohm coax work out to 21.5 ohms: and so forth. The efficiency of the antenna will be markedly better with this type of matching then with a quick fix using an antenna tuner!

Be aware that the “electrical length” and the “physical length” of the coax are different. The fast way to figure the electrical wavelength of coax is to look up the velocity factor from the manufacturer - normally it is around 0.85. You can measure out 90 degrees (1/4 wavelength) at the operating frequency and multiply by .85 to get the approximate “electrical length”. However, this is not the best and most accurate method. It would be much better to use an antenna analyzer to get the exact electrical length or consult the ARRL Antenna handbook3 for other methods.

Okay, now you are ready to go with your single vertical. Try it out and various times of the day and see what kind of reports you are getting. Keep in mind that with the low angle of radiation you are going to get the best results further out then with a dipole type antenna with a higher angle of radiation. If you find that it is not up to what you expect add more radials, or deal with the far zone, until you see an improvement.

Once you have a good feel for the antenna and it is working to your best estimates you may be a happy camper and satisfied to work around the planet enjoying good signal reports. The other day I worked a container ship that plies between Hawaii and the mainland running three watts into a vertical antenna – he was S9. Clearly the ocean and vertical antennas go together!

I highly recommend buying ON4UN’s Low-Band Dxing2 and the ARRL Antenna Book3. They are great references you will use over and over as you seek knowledge of antenna ideas and concepts. Part II of this article explores the world of adding a clone or second vertical and making them dance with phasing the elements. Stay tuned.