|
  |
|
About RF Cafe
Copyright: 1996 - 2024 Web BSEE - KB3UON RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling
2 MB. Its primary purpose was to provide me with ready access to commonly needed
formulas and reference material while performing my work as an RF system and circuit
design engineer. The World Wide Web (Internet) was largely an unknown entity at
the time and bandwidth was a scarce commodity. Dial-up modems blazed along at 14.4 kbps
while tying up your telephone line, and a nice lady's voice announced "You've Got
Mail" when a new message arrived... All trademarks, copyrights, patents, and other rights of ownership to images
and text used on the RF Cafe website are hereby acknowl My Hobby Website: |
 |
||
Mac's Radio Service Shop: A Windy Subject |
||
It is a pretty good bet that most multi-element TV aerials you find on rooftops and even on ancient towers were decommissioned years ago. They have been replaced either with cable (whether via CATV or Internet) or satellite dishes. A few hold-outs still use them for local over-the-air broadcast stations and/or even FM radio reception. There was a time, though, that photographs taken looking across a vast expanse of house roofs showing an endless array of antennas and guy wires was a sign of 'modern' living. Most were erected by Harry Homeowner types or minimally qualified service technicians, and were well-known for toppling, twisting, bending or un-aligning when stiff winds were imposed upon them. This story-lesson from the March 1953 edition "Mac's Radio Service Shop," a regular feature in Radio & Television News magazine, provides a bit of analysis on causes of failure due to improper guying and why many people's "One Hoss Shay" of an installation failed despite their best efforts. A Windy Subject By John T. Frye
Barney stormed into the service shop in his usual going-to-a-fire manner but stopped short as he caught sight of Mac, his employer, sitting at the service bench toying with several soda straws. "So!" the youth said accusingly. "You and Miss Perkins wait until I am away to have a round of sodas sent in, eh? Of all the sneaky, low-down, back-biting, ungrateful, penny-pinch-ing - " "Whoa, Buster, whoa!" Mac interrupted. "Hell apparently hath no fury like that of a glutton who thinks he has missed out on something to eat, but this time you are blowing your top over nothing. These straws are not the debris of secret gorging on the part of Matilda and myself. I was just using them for some experimenting." "What kind of experimenting?" Barney demanded suspiciously. "Sunday I spent several hours driving around looking at the damage done to TV antennas by that big wind we had Saturday night. In several cases I saw damage that puzzled me, and I am using these straws for model masts and towers, employing light thread for guy wires, in an attempt to discover why the antennas were damaged in the way they were." "They really took a licking, didn't they?" "Yes, but considering that some of those gusts were estimated to hit peaks of 80 miles-an-hour, it is surprising there was not more damage. Insurance men tell me they do not think they will have more than one claim in every eight installations covered. In view of the high and elaborate antenna systems that must be used to get a signal in this ultra-fringe area, that percentage is amazingly low." "I think that fifty-mile-an-hour wind we had about a year ago mussed up the signal-sniffers a lot worse." "You're right, and it is interesting to note that the big damage done then was to pipe-mast installations, while this time the pipe jobs came through practically unscathed." "Why do you suppose that was?" "The boys learned then that just guying the top of a twenty-foot section of pipe is simply not enough; so practically every pipe-mast is now guyed both at the top and in the middle. This, when properly done, makes a very sturdy job; and on top of that the pipe-masts seldom go higher than twenty or thirty feet. Tower jobs, on the other hand, are rarely shorter than this and often go up to better than a hundred feet. Then, too, we are likely to rely more on the rigidity of the tower and fail to give it the adequate guying we would give a flexible pipe. This is all well and good in a stiff breeze, but when the wind reaches a gale, as it did here Saturday night, good guying becomes as important to the tower as it does to the pipe." "What happened to the antennas mostly?" "A variety of things. In each case the wind hunted out the weakest point of the installation. In many cases the guy wires either broke or tore loose from their moorings and let the whole business crash to the ground. In other instances a top guy broke while the lower ones held, and then the tower usually broke square off just above or just below the bottom guys. More common were the cases in which the antenna masts bent over or broke off just above the motor, or the cases in which the pipe on which the motor was mounted suffered the same fate. Lots of the conicals lost antenna elements, and I saw one conical in which an insulating block had broken, allowing half of the antenna to fall to the ground. "Few yagis shed their elements, but a great many of them either turned on the mast or turned the mast in the tower clamps so that they ended up pointing the wrong way. Fellows repairing the damage tell of a few cases in which an antenna with a large surface in a vertical plane, supported on top of a high tower, actually twisted the tower itself and did such a good job of wracking the structure that practically every rivet was loosened." "What would you say were the installation mistakes that let all this happen?" "Well, first, let's remember that these 'mistakes' are very easy to spot when looking through the spectacles of hindsight, as we are now; but before we had a chance to see what a strong wind can do, we'd probably have made the same errors. Let's call them 'lessons of experience' rather than mistakes. "The first lesson is: don't try to go too high for the guying area available. When a broadcast station puts up an antenna, it puts it in the middle of several acres of ground so that it can be supported as it should; but a TV owner often tries to go nearly as high and keep all of his guy stations inside his own small city lot. If you ever helped raise a tower, you know that when you are standing close to the base with a guy, you have a heck of a time holding the tower upright in even a slight breeze; but if you back off so the guy makes about a 45 degree angle with the tower, you can hold it easily with one hand. When guys must be anchored close to the base, they are subjected to terrific strain, and a large part of their strength is wasted in a force that tries to telescope the tower endways. "Another fault lies in locating the guys 'conveniently' rather than where they should be. 'Make it easier to mow,' 'Make 'em less conspicuous,' and 'Want to keep 'em all on the house' are some of the excuses given for failing to support the tower equally in all directions, and this invariably leads to trouble. Not guying often enough is another error. When the tower manufacturer recommends guying every thirty feet, he means just that; and if you go over thirty feet, he means that you should use two sets of guys even if you do not go on up to sixty feet. I can show you with these straws that if you fail to do this, a big antenna can exert a leverage against the restraint of guy wires affixed only to the top and make the tower buckle in the middle. "Improper anchoring of guys is another common weakness. Antenna towers are usually put up on calm days, and it is hard to imagine then the tugging, yanking strain that will be placed on those guys by a gusty windstorm. Spike nails are certainly not adequate guy-wire anchors, and ordinary lag-screws through the sheeting is little better, for water seeps down around the threads and rots the wood, allowing the screw to pull out. Unless you can drive creosote-covered lag-screws several inches into dry, seasoned wood, it is best to use long eye-bolts that pass through both the edge of the roof and the protective sheeting beneath the rafters - or if the rafter-ends are not boxed in, you can use a board nailed across a couple of them. "Don't fasten all the guys from one corner of the tower to a single anchor of this sort. That piles up the strain on it, and if it does go, the whole tower is left unsupported and is certain to fall. Use a separate anchor for each guy. A stake - either iron or wood - driven into the ground is a poor anchor. When the ground is softened by rains or thawing, a surprisingly long rod can be pulled out easily. Either a pipe set in concrete should be used, or a 'dead man' with a large surface area and considerable weight should be buried in a deep hole and a chain, rod, or heavy cable led from it to the surface for fastening to the guys." "You ought to keep the guys real tight, I suppose." "Not too tight. While there should not be enough slack to let the tower lunge back and forth, if they are made too tight, especially in warm weather, the shortening with cold weather will put unnecessary strain on them and their insulators. Incidentally, of course, you should never use anything but strain insulators of the type that can break without destroying the support of the guy wire. A few owner-erected towers came down because of this obvious mistake. A few cents expended on cable-clamps for fastening the ends of the guy-wires is also very cheap insurance. "Another storm-taught lesson is that the required height should be secured with the tower alone, and the mast should be just long enough to mount the antennas and support them a short distance above the top of the tower. Antenna masts that were too long, that were too thin or made of material with insufficient strength, and that had too much high-wind-resistance antenna stacked on them never had a chance. If a fellow wants to use a half-dozen different antennas, he had better put them on two or more separate masts instead of trying to stack them all on the same one." "How do the insurance companies feel about the situation?" "Naturally, they are not too happy. Most damaged antennas were covered by the 'extended coverage' clauses on either the property or the household goods policies; but there were exceptions to this. For example, all antennas erected on mercantile buildings and many on apartment buildings were not covered in this manner but required separate policies. The only safe thing to do is to check with your insurance agent just as soon as the antenna is erected. One installer I know never leaves a new antenna installation without first impressing on the owner that he should get in touch with his agent at once and find out if the structure is insured. "If there is a single weak spot in an antenna installation," Mac reflected, "a high wind will certainly find it out; so the only thing to do is try to make the job like the fabled One Hoss Shay: without any weaknesses. The tower should be strong enough to support easily the antenna used, and it should be adequately and intelligently guyed. The guy wire used should be of the best, and it should be fastened to anchors that will take more than a breaking pull on the wire. Compression type insulators and cable clamps at all guy wire ends are a must. If a motor is used, it should be mounted on a pipe with a two-inch outside diameter; and this pipe - or the bottom of the mast if a motor is not used - should be fastened in the tower so that it cannot turn. The clamping action of the tower hooks is not enough but should be aided by a strong bolt passing through the pipe and clamped to one of the hooks. "The mast should not be any longer than necessary, and antennas with large wind areas or considerable weight should be mounted as near the bottom of the mast as feasible. Wind-resistance of particular antennas should be considered when choosing a mast." "Sounds to me like the ideal antenna installer ought to be the kind of guy who wears both a belt and suspenders: one who takes absolutely no chances!" was Barney's grinning comment.
Posted May 5, 2015 Mac's Radio Service Shop Episodes on RF Cafe This series of instructive stories was the brainchild of none other than John T. Frye, creator of the Carl and Jerry series that ran in Popular Electronics for many years. Mac's Radio Service Shop began life in Radio & Television News magazine (which itself started as simply Radio News), and then changed its name to Mac's Service Shop after the magazine became Electronics World. "Mac" is electronics repair shop owner Mac McGregor, and Barney Jameson his his eager, if not somewhat naive, technician assistant. "Lessons" are taught in story format with dialogs between Mac and Barney.
|
||
 |
 |
  |
|
Please Support RF Cafe by purchasing my ridiculously low−priced products, all of which I created.
These Are Available for Free |