Note: If you would like a copy of any of the circuit diagrams
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c/o Dave Miller NZ9E
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Niles, IL 60714-3108
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Ham To Ham #30 - March 1998
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 2 of that series was presented last month, part 3 follows:
Lightning protection - what your mother never told you! - Part 3
Tower Considerations
We continue this ongoing series on lightning protection with some tips on ham radio tower installations. Be sure to read the two previous parts in this series in the January and February issues of 73 Magazine as well, they contain information essential to achieve a complete understanding of the material. You can read the entire text immediately, by calling up the Special Bulletin, "Protection to Keep You Communicating", at PolyPhaser's home page on the world wide web at: http://www.polyphaser.com/. Past Ham To Ham columns can be accessed 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
Now on to this month's topic. The first rule ... don't use a nonconductive structure for an antenna support! Conductive towers or metal poles should be used for mounting antennas high into the air. If the tower or pole has sliding contacts (telescoping, crank-up or push-up), the joints should be jumpered using short sections of copper strap attached with appropriate clamps (such as PolyPhaser's TK clamps ... see Figure 1). Normally, self-supported and guyed towers do not require jumpers across their joints. Joint compound should also be used so that rain water will not drip onto the galvanized surface of the support structure.
Guyed towers are better from a lightning protection perspective, if the guy anchors are grounded properly. Because the anchors are located away from the tower base, some of the strike energy will traverse the guy wires to ground, even though these guy wires may have relatively high inductance. This is okay, because the more the strike energy is divided, the less energy the
equipment indoors may see, and that, after all is our end-objective.
Dissimilar Metals
Copper should never touch galvanized metal without proper joint protection. Water shedding from copper contains ions That will wash away the galvanized (zinc) tower covering. Stainless steel can be used as a buffer (transition) material, however, stainless steel is not a particularly good electrical conductor. If stainless steel is used as a buffer between copper and galvanized metals, the surface area of the contact should be large and the stainless steel itself should be thin. Joint compound (available from a number of sources, including PolyPhaser) should be used to augment the
connection, so that water will not be allowed to form a "bridge" between the dissimilar metals.
Magnetic Energy
Lightning produces a very large magnetic field as would be expected with its typically 18,000 ampere pulse. This magnetic field will tend to inductively couple into all nearby conductive materials. There are two ways to minimize the amount of magnetic coupling:
1.) Carefully shielding all sensitive equipment.
2.) Placing some distance between the equipment and the likely strike location.
A galvanized steel sheet may also help (when used as a magnetic shield) to attenuate the lightning's magnetic field pulse (usually by about 10dB). This steel sheet should be at least 30 gauge (0.016 inch thick) and should be bonded to the system ground. The shield must also be broadside to the direction from which the pulse will emminate.
Distance can sometimes be effectively used to limit magnetic field coupling. The strength of any magnetic field diminishes at the rate of one over the distance squared. Since a moderately high tower is more likely to be struck than any other nearby structure, the placement of the tower with respect to your equipment room warrants significant consideration. Factors to consider include the magnetic energy that will likely radiate from the tower, and the benefit of distance in terms of the inductive loss provided by the length of the orthogonally run coax; this added inductance of the coax line will help buffer any energy entering the equipment area. So this is one time when
inductive loss is desirable. Do not, however, add loops to your coaxial line, since those loops may act as a transformer (depending upon their orientation), actually capturing more of the magnetic filed energy and ultimately bringing it right into your ham shack!
Additionally, extra distance to the shack will provide more time for the tower ground system to absorb and dissipate the strike's energy, resulting in less energy heading toward your equipment. These factors indicate that a separation between the tower and the operating equipment of greater
than 20 feet appears reasonable. For towers already located closer than this, it may be necessary to utilize some form of shielding (as mentioned previously) to minimize the magnetically induced energy.
Antenna Location
A ground-mounted vertical antenna is similar to a ground-mounted tower. Both should have a substantial and low impedance connection to the station ground system. However, if the antenna or tower is mounted on a roof, the inductance inherent in the vertically-run conductors to the ground system can be significant. Voltages of several hundred thousand volts could be present.
To reduce the inductance in these ground conductors, increase the surface area of the conductors (wider copper straps) as well as the number of conductors. For the roof-mounted antennas and towers, the multiple down-conductors can be spread over the roof and can then be brought down
to ground in multiple locations. This will require that the ground system to encircle the building (also called a perimeter ground) as shown in Figure 2.
As an added benefit, this multiple down-conductor approach tends to reduce the mutual coupling between down-conductors and provides a low impedance, unsaturated perimeter ground to absorb the conducted surge. The magnetic fields will be divided, and in theory at least, tend to cancel in
the middle of the building (although in practice, the chances of these stray magnetic fields cancelling on anything other than perfect, geometrically-balanced and spaced down-conductor array, are slim to none).
That's all from Roger and Ron Block for this month. Be sure to check back next month for more of their advice on helping to keep your ham station safe from the devistating effects of a lightning strike ... their series will continue here throughout the rest of 1998. As mentioned in the
beginning, you can read the entire text immediately, by calling up the Special Bulletin, "Protection to Keep You Communicating", at PolyPhaser's home page on the world wide web at:
http://www.polyphaser.com/.
Telco Two-fer
Here's a tip that won't break your budget from Tom Siolek N3VUF: "I recently acquired a (Radio Shack (re. trade mark)) HTX- 212, 2-Meter transceiver, which I'm using as a base station to check into my local 2-Meter nets as well as for packet radio. I've found it to be a fine radio for both applications. Yet, since it doesn't have a separate data port for direct connection to a TNC, the user must disconnect the microphone and replace it with a cable going to the station TNC for data work. Because the keypad and up/down scan buttons reside on the microphone, the user ends up losing the ability to change frequencies easily while running packet. For packet node hoppers,
this can be a constant annoyance, since the only available frequency control is the detented main tuning knob, which has proven to be somewhat slow and cumbersome. There's compounded inefficiency if the computer monitor is located some distance away from the radio; it's not practical to run the microphone and its controls over to the remote position. There are
commercially available in-line boxes that will switch between a microphone and TNC cable, but the ones I've seen so far won't accommodate the type of connector used on the HTX-212, plus they come with a rather high cost for a fairly simple function. The microphone jack in this radio is an
RJ-45 type, the same that's used in Ethernet computer networks. There are other 2-Meter transceivers having similar setups today, so users of any of these may well benefit from the following low-cost solution that I've come up with.
To allow me to use the microphone controls while running packet, I simply plug in a standard modular 8-conductor duplex adapter ... directly into the HTX-212. This device will automatically split a single modular RJ-45 jack into dual modular RJ-45 jacks ... just what I want. I then plug the
microphone into one of the jacks, and the TNC into the other. This lets me operate packet, while not relinquishing the ability to directly input frequencies from the HTX-212's microphone mounted keypad. The adapter I used is made by GC Electronics and carries their part number 30-9657. It
cost me a mere $3.32 at my local electronics supply house, a lot less expensive than any other alternative I've seen. You might also be able to find these 8-conductor splitters in computer stores that stock Ethernet 1OBaseT network cabling supplies or via mail-order from some of 73's
advertisers.
One caveat ... the setup works great on packet, but I've noticed that I end up with some 60Hz hum superimposed on my voice signal when I operate on voice FM to transmit (with the TNC cable still plugged into the splitter). This appears to be due to inductive pickup from my station power supply, via the TNC's cable (since it clears up as soon as I disconnect the cable going to the TNC). It's not a huge problem for me, since I only use the RJ-45 splitter during digital operation, but I thought I'd mention it just in case you run into something similar. The inexpensive answer, of course, is to simply unplug the TNC's RJ-45 cable when you wish to operate on voice mode.
Perhaps better isolation between the microphone and TNC circuitry might be the final answer, if you'd like to keep both items permanently connected, but that's the kernel of another, more involved project ... a home-brewed RT-45 based switch box. For right now, the $3.32 solution works well enough."
Moderator's note: The 60HZ hum that Tom noticed on his 2-meter FM voice signal might also be due to a ground loop condition between the TNC, the 2-meter transceiver, the computer, etc. or it
could even be traced to a bit of RF feedback showing up as a hum-like extraneous modulation. Either case would require some "setup specific" troubleshooting, but as Tom states, the easiest answer my be to just unplug the unneeded TNC cable when operating any other mode.
Clip-lead-clutter reducer
Here's a simple, but clever, suggestion from Peter Bergman N0BLX: "Often after building or repairing some electronic gadget, it's nice to know exactly how much current the item is drawing. Using the meter on the bench power supply itself (if your supply even has one), isn't always the full answer to the question. Power supply meters aren't usually all that accurate and you're generally stuck with just one "overall" range, which may not be the right one for your needs. The meter on the power supply also records all of the current being supplied to all of the devices that may be connected to at the time, again, perhaps not what you'd like to see specifically. The usual
approach that most of us take at this point, it to grab our collection of alligator-tipped test leads and start lashing our multimeter into the circuit. The result is clip-lead-clutter, and often unintentional
shorts or dangerously exposed wires scattered around our work area. The cure for this form of clip-lead-clutter is amazingly simple. Take a look at Figure 3 and you'll see what I mean.
All that you need to do to avoid the clutter is to install one of those handy 5-way binding posts on your power supply's front panel ... the type of binding post that allows you to hookup several wires to it at the same time, safely. It's just a 'dummy' binding post, so you don't have to connect it to
anything internally in the supply! It's basically just a 'splicing point' for one of your multimeter's test leads, and the lead that will feed voltage to the equipment or circuit under test. It's the simple and safe way to put your multimeter in series with the device who's current you'd like to check, and Figure 3 shows the concept graphically. When you don't want the multimeter in series with the bench supply, just connect a short from the power supply's negative post to the dummy post and you're back to the old days!
I hope that this idea finds as much use on your ham test bench as it has on mine."
Moderator's note: I'll have to admit that this solution to clip-lead-clutter never crossed my mind ... great suggestion Peter!
Family (of) Ties
Back in the October 1996 Ham To Ham column, I described one possible method of making your own cable coil ties from standard Velcro (reg. trade mark) sew-on strip material, available at most fabric and variety stores. Charlie Smith KE4OZN recently sent me a sample of a find that he came across for pre-fabricated cable ties, usng an even better material ... double sided Velcro. Here's how it works: "I recently ran into a great product that I use almost every day and know that other hams would do the same if they knew about it. They're one-piece cable ties using the Velcro loop & hook principle, but with an interesting twist. Grip Strips (reg. trade mark) as they're called, are made from strips of double-sided Velcro, ie., the top side is the 'fuzzy' or 'looped' side, the opposite side is the 'pinchy' or 'hooked' side. The material has this double-sided feature throughout it's
entire length (see Figure 4). The can be wound around a cable coil or power tool cord, and then attached to itself, to hold the cord in a neat coil for storage or transport. Grip Strips, made by GB Electrical, Inc., 6101 N. Baker Road, Milwaukee, WI 53209, come in 3 lengths (8", 11" and 15"
long) and in 3 colors (red, green and black) for different sizes of coils and to quickly identify different cables. The 11" size (an all around handy size) in red, is designated as 45-V11RD. They're available in the electrical supply departments of many hardware stores and larger home centers (such as Home Depot (reg. trade mark)) for about $2.50 to $3.00, depending upon their
length, for a package of 5. That's less than $1 each, which I feel is well worth the expenditure, since they should last at least as long as the cable itself ... maybe longer! Grip Strips have a widened design at one end, which also has a cutout slot for self-storing the tie right on the cabling,
but I prefer to utilize a small (4") plastic cable tie or two to make sure that the Grip Strip stays with a particular cable or power tool permanently. The strips can easily be cut to a smaller size, so you might want keep a stock of the longest onces as a norm. I think that once you've tried these little gems, that you'll 'stick' with them (ouch)!"
Murphy's Corollary: Whatever it is that you want to do, you must always do something else first that you don't want to do.
The Ham To Ham column is here to provide a forum for your ideas, even if they may be, well, a little different. Different can often be very creative, and those are the type of ideas that I really enjoy receiving and including on these pages. Don't worry about your writing skills, just include as much
detail as you can and I'll put it together in the style of the column. Let's hear from you!
As always, many thanks to those who've contributed to this months column, including:
Roger Block, President
PolyPhaser Corporation
2225 Park Place
P.O. Box 9000
Minden, NV 89423-9000
Tom Siolek, N3VUF
32 Westwood Park Drive
Havertown, PA 19083
e-mail: n3vuf@aol. com
Peter A. Bergman N0BLX
3517 Estate Drive SW
Brainerd, MN 56401
Charlie Smith KE4OZN
12900 Meadow Creek Lane
Apt. 204
Pineville, NC 28134
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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