Ham To Ham #34 - July 1998

Where You're Input is Always Welcomed!

73's Ham To Ham column c/o Dave Miller, NZ9E 7462 Lawler Avenue Niles, IL 60714-3108 USA E-mail: dmiller14@juno.com

Moderator's note: Roger and Ron Block of PolyPhaser Corporation have put together a well written series of tips and suggestions on how we can effectively protect our ham radio stations from the effects of a lightning strike. Part 6 of that series appeared last month, part 7 follows:

Lightning protection - what your mother never told you! - Part 7

Antenna Support Last month, we talked about the special considerations needed for high-rise bulding antenna installations, but even so called ground-mounted vertical antennas require the same type of earthing as do other antenna structures for lightning protection. If operating on 160 or 80 meters, the lightning ground should also make a great counterpoise for your 1/4-wave ground-mounted vertical! Remember, a vertical's impedance is half that of a dipole's (about 35 ohms for a full sized 1/4 wave). This means that the better the ground plane, the worse the VSWR match will appear to a standard 50 ohm coax cable. As a result, a vertical with a poor ground plane may actually give a better match, and as the ground plane is improved, the match will worsen. But don't be fooled. A good ground plane is critical. The better the ground plane for RF, the better the earthing for lightning as well (assuming that the RF ground plane is actually in the ground). You're far better off with a good, low inductance underground earthing system, than with a slightly better VSWR reading. Correct any slight mismatch with an appropriate tuning unit instead. Consider running two 75 ohm coax feed lines in parallel; this can often be the best match for a ground-mounted vertical (75 divided by 2 = 37.5 ohms).

In terms of antenna supports, avoid using trees or wooden poles to support wire antennas. If one of these supports is already in place, then install two paralleled vertical copper straps from the top of the support to ground. The straps will ground a VHF/UHF antenna and divide the strike currents with the coax cable. By using two straps, the inductance in such a ground run is minimized. Also, place the straps on opposite sides of the pole to reduce mutual inductance. The coax line should run down between (and well clear of) the straps.

If a long wire (fed by a transmatch) and a wooden pole (as a support for the antenna wire) are being used, the grounded straps should extend higher than the pole to intercept a strike or to divert energy to ground if the wire is struck. Additonally, place a high voltage gas tube between the long wire and the ground straps or by make a mechanical an air gap between the wire and the ground straps. A gas tube is not adversely affected by temperature, humidity, pollution, wind or other environmental forces, while the air gap is greatly affected. It may be difficult to calculate the voltages present at the gas tube or air gap point, because they will change as bands are changed. As a rule of thumb however, at about 7kV, an air gap would be 0.1 75" at sea level with 50 percent humidity. The break-over voltage goes up with higher elevation and/or increased humidity (surprisingly, humid air is actually less dense than dryer air). Gas tubes can also be connected in series for a higher turn-on voltage, however, the turn-on times are also additive. Another gas tube assembly may be added closer to the match box for additional safety.

For dipole antennas using baluns, use the one gas tube across the balun (one lead of the gas tube to each of the dipole wires) and one tube from each side of the balun (where it connects to the dipole's wires) to its ground straps. This will help to protect the balun from a strike to the dipole wires and greater strike energy will be diverted to the ground before reaching the equipment.

Power/Telco Entrance The story of complete protection for a ham shack covers not only strikes to the tower but also high voltages on the utility lines coming into your shack. By using single point grounding, ham equipment will survive a hit to its tower. If the outside (tower/perimeter) ground system has a low impedance, most of the strike energy will be dispersed into the ground and little energy will enter the shack. If the ground conductivity has deteriorated over time, the ground system can only absorb a limited amount of energy before becoming saturated. In 90 percent of all earth-bound lightning strikes, a traffic jam of electrons will be forced into the tower. If the electrons can not disperse in a reasonable time frame, the back up pressure (voltage) will find or create another path. The ground system, if too small in area, will cause more energy to traverse the cables and other lines into the shack. These last two sentences are very important, please read them once more. The I/O protectors can keep the voltage levels between the single point ground and the signal line(s) at survivable limits, but the energy is only diverted elsewhere. This "elsewhere" might onto the house telephone and power lines, leaving other appliances at risk. When the ground system is saturated, the energy can even actually come up from the (utility) ground system (and go through a television receiver for example), in an effort to leave the area by way of the cable television drop. Similar problems can also exist with satellite dish installations.

The best way to protect the rest of your house from these occurrences, is to provide protection at the power and telephone service entrance(s). The utility ground rod is used by both the power neutral and the telephone protector installed by the phone company. By placing a power mains protector, and a secondary telephone line protector at this location, the entire house will be protected. The cable television or outside antenna coax should be redirected, and a coaxial protector installed at this same point. The cable company installed protector is usually just a grounding block that will earth only the outside shield of their coax; it does nothing for the center conductor energy, which can carry as much of the surge as the outside shield. Install your own protector to eliminate the problem.

That's Roger's and Ron's presentation for this month. If you'd like to see the original, unabridged version of this series, you can contact PolyPhaser Corporation, Customer Service Department, 2225 Park Place, P.O. Box 9000, Minden, NV 89423-9000 and ask for their Special Bulletin, "Protection to Keep You Communicating" (copyright 1995). You can also pay a visit to PolyPhaser's home pages on the world wide web at: http://www.polyphaser.com/. PolyPhaser's web site also supports text downloads of the original material that's going to be condensed here, plus other related texts on the subject. The PolyPhaser Tech Line telephone BBS at (702)782-6728 is also available to interested readers. The communications parameters are: Data bits - 8, Parity - None, Stop bits - 1, Baud rate - 300 to 14400. If you are dialing in for the first time, the Tech Line requests your name, address and telephone number. You will also need to create a password. Once you've logged-on, just follow the menus to navigate around the Bulletin Board. The Ham To Ham column will continue this series on protecting your ham station from the destructive effects of a lightning strike with part 8 coming up next month.

Follow the pattern! Here are some interesting notes from Dick Warren W7TIO, regarding the addition of radials to a 1/2-wave amateur VHF (or UHF) transmitting/receiving antenna as described in the Ham To Ham column for November 1997. In that column, M. Marcel Chapleau VE2GMZ of Quebec Canada described a VHF/UHF J-pole antenna to which he added 2-meter and 70-cm radials to achieve better results. In the moderator's notes, I commented that the use of radials wasn't normally called for with 1/2-wave antenna designs, since a 1/2-wave antenna is a complete minimum antenna unto itself (no phantom ground plane was needed). In the following text, however, Dick Warren brings up some interesting points gleaned from his years of experience in the commercial 2-way radio installation and maintenance field ... points well worth remembering.

"Over the years, I've worked on many commercial 'base station' installations, and while many hams feel that a half-wave vertical doesn't need a set of radials because it's a 'complete' antenna unto itself, it is commercial practice to put ground radials on every vertical collinear gain antenna installation (of those not made up of folded dipoles as explained more fully below).

Most hams know that the horizonal ground radials under a 1/4-wave vertical are needed to provide a 'mirror' or 'image' antenna, thus making a 'complete' 1/2-wave radiator, as well as to support the driven element itself. The down side is that these horizontal radials also put the radiation angle way above 45 degress (referenced to the horizon). An antenna used for HF skip work may beneift from this high angle of radiation, but at VHF/UHF frequencies, it wastes a goodly part of the RF into the atmosphere, by not putting it at the horizon where it's needed. Drooping radials, however, provide the same 'image' antenna, but also lower the radiation angle nearer to 45 degrees. The drooping radials also raise the 50 ohm feed-point impedance to nearer the 72 ohm expected 1/2-wave feed impedance.

Sometimes you'll see a repeater/base station antenna mounted up-side-down, on a very high point such as a mountain top. Mounting it up-side-down inverts the high angle of radiation and puts the energy down into the desired coverage area in the valley below.

In the case of the ever-popular 5/8-wave vertical, the 5/8-long radiator brings the energy back down nearer the horizon, and additionally narrows the beam-width. This concentrates the signal and produces the typical gain figures attributed to 5/8-wave radiators. The coil provides impedance matching to the 52 ohm feed point, by presenting a simulated 3/4-wave feed, and does not effect the radiation angle. This antenna still needs ground radials, however, and is used in some commercial 450MHz limited-range installations.

Other commercial installations use vertically-stacked, vertically-polarized 'folded dipole' type antennas .... and these need no radials. However, those commercial antennas that look like a stretched-out ground plane, some 20 feet long at 150MHz. and perhaps 6 feet long at 450MHz. do have radials, under what is a multi-half-wave collinear gain antenna ... ground radials are always put under this variety. An antenna of this type can produce omnidirectional gains of 10db, almost like having a 10 element beam in all directions!

The next time that you pass a professional communications installation, be it a commercial or governmental facility, stop and study some of the antenna design options used in this service. See if you can pinpoint the designs I've referred to here, it likely that you'll likely see these techniques put into practice. They work well, since these installations are engineered for the coverage needed as well as for dependability. People's lives often depend upon them!"

Go figure! Here's a tip from Doug Mckibben KB0IMG for determining how to easily and quickly get the sloping elements of an HF inverted "V" antenna at a true 45 degree angle for optimum performance: "It's been known for years that the optimum angle for the elements of an inverted 'V' should be 45 degrees from vertical, but most antenna books don't explain how you can easily determine when you've achieved that 45 degree angle. The answer can be found by using basic Geometry, but in case you've forgotten (or would rather forget) that high school experience, I've laid out the basics here, limiting the explanation to only what's needed for the job at hand.

Since most of us find ourselves limited to fairly low mast heights (because of land-lord or neighborhood constraints), I'll start with an example of a 30 foot mast for a 40-meter inverted 'V'. Looking at Figure 1, the vertical mast is depicted as Side A of the right triangle that will represent one-half of our 40-meter inverted 'V'. Side B of Figure 1 is the straight-line distance from the mast to the first ground anchor for one of the antenna's elements. Using Pythagorean's theory:

A squared plus B squared equals C squared

So to find out what the total value (element length plus extension rope) of Side C will be, simply add the square of 30 (900) plus the square of 30 (900) which gives us 1800. On your calculator, enter 1800 and press the square root button. The answer is 42.43 feet. Since one element of our 40-meter antenna is 33 feet, 9-1/2 feet is left over to reach the ground stake, so the rope extension and egg insulator will be 9-1/2 feet long (plus whatever you feel you'll need to make the knots at each end of the rope. Remember, this is only 1/2 of the inverted 'V', the other side will be identical with the feedpoint at the top of the mast.

If you can only manage 25 feet of mast height, the value of Side C of the right triangle in Figure 1 comes out to be 35.35 feet. This puts the sloping wire end of a 40-meter inverted 'V' (again, 33 feet) only a couple of feet above the ground, but the antenna will still work well.

Now try calculating what the length of Side C would be for a 15-meter inverted 'V' with a mast of 20 feet high yourself. Don't peek! You should have calculated 20 squared (400) plus 20 squared (400). The square root of 800 is 28.28 (feet) and a single element at 15-meters is about 11 feet, so the amount left over for the egg insulator and rope would be about 17-1/2 feet.

While you have the method well in mind, why not come up with a chart showing the element lengths and insulator-rope extensions for all of your favorite bands or band segments. Just to complete Figure 1 for you, however, the various overall lengths of Side C for the 10, 20, 30, 40 and 50 foot marks from the base are:

L1 = 14.14 ft
L2 = 28.28 ft
L3 = 42.43 ft
L4 = 56.57 ft
L5 = 70.71 ft

Just keep in mind that in order to have a true right triangle, with a 45 degree slope on Side C, the lengths of Side A and Side B must be equal. Then squaring Side A and Side B, adding them together, and taking the square root of that total, will give you the length of Side C, which is the antenna element's wire plus any needed insulating extension. The length of the actual wire element is given by the antenna book formula: 468/Frequency in MHz (divided by 2 for just one of the elements)."

Moderator's note: You can use Doug Mckibben's right triangle hint to find the exact 45 degree angle for the radials mentioned by Dick Warren in the piece above here as well. Simply reverse the formula, squaring the length of the VHF antenna's radial, dividing by 2, taking the square root of that figure, and measuring out from the antenna's mast by that amount. The drooping radial should end up at that point for a 45 degree angle.

Murphy's Corollary: Once you open a can of worms, the only way to get them back in is by using a much bigger can!

Many thanks, as always, to our loyal contributors. Remember, I'm always looking for interesting and innovative tips, ideas, suggestions and shortcuts to include on the pages of 73 Magazine within this column. Just jot down your thoughts and send them to the address at the masthead. Those that accepted the offer this month are:

Roger and Ron Block PolyPhaser Corporation 2225 Park Place P.O. Box 9000 Minden, NV 89423-9000 Web site: http://www.polyphaser.com/. Tech Line telephone BBS (702)782-6728

O. Dick Warren W7TIO Certified Electronics Technician PO Box 973 Pleasant Grove, Utah 84062-0973

Douglas R Mckibben KB9IMG 2112 Marion Avenue Mattoon, IL 61938

If you're missing any past columns, you can probably find them at 73's Ham To Ham column home page (with special thanks to Mark Bohnhoff WB9UOM), on the world wide web, at: http://www.rrsta.com/hth

Note: The ideas and suggestions contributed to this column by its readers have not necessarily been tested by the column's moderator nor by the staff of 73 Magazine, and thus no guarantee of operational success is implied. Always use your own best judgment before modifying any electronic item from the original equipment manufacturer's specifications. No responsibility is implied by the moderator or 73 Magazine for any equipment damage or malfunction resulting from information supplied in this column.

Please send any ideas that you would like to see included in this column to 73 Magazine's Ham To Ham column, c/o Dave Miller NZ9E, 7462 Lawler Avenue, Niles, IL 60714-3108, USA. We will make every attempt to respond to all legitimate ideas in a timely manner, but please send any specific questions, on any particular tip, to the originator of the idea, not to this column's moderator nor to 73 Magazine.

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