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More Problems - More Questions and Answers
February 1967 QST Article

February 1967 QST

February 1967 QST Cover - RF CafeTable of Contents

These articles are scanned and OCRed from old editions of the ARRL's QST magazine. Here is a list of the QST articles I have already posted. All copyrights are hereby acknowledged.

Here is a good old fashion Q & A session on Ham radio topics... with the emphasis on 'old.' QST magazine published a couple of these columns in the 1960s, and this is the second in the series. I didn't read anything that wouldn't be applicable today, especially if you have some vintage gear. As with most such articles, there is something to be learned by just about anyone who deals with electronics, especially in the RF realm. One particularly interesting part is where the author, in response to a question about building and tuning your own radio, states, "Too many beginners are concerned about making 'Chinese Copies' of [manufactured] equipment described, even down to the same placement of nuts and bolts." Little did he know then that nearly every piece of equipment purchased new by Amateurs nowadays would actually be made in China and not in the USA. ... Heavy sigh (this link goes to a Mork & Mindy video clip).

Beginner and Novice

More Problems - More Questions and Answers

By Lewis G. McCoy, W1ICP

The answers to some of these questions may prove helpful to you, both in time and trouble. It's useful information, so read on.
Some time back, we had a question-and-answer article1 on problems that are common to many newcomers. Since then, a new batch of questions has been collected, and the answers may be of help to hams with similar problems.

Q.  "I am using an 18-foot vertical antenna, ground-mounted on my front lawn where it is subject to being touched or contacted by children.

How much of a safety heard is this if a person touches it while I am transmitting? What is the danger, if any, at 75-watts input against what it might be at one kilowatt, and are some bands less of a hazard than others?"

Transmitter lowpass filter - RF CafeA.  First and foremost, any antenna or feed line should be installed so as to be protected from any physical contact. The antenna or feed line can present a very serious hazard. There is always radio-frequency energy present on an antenna when transmitting. While r.f. is rarely present in a degree that could be considered lethal, even at a kilowatt level, very severe r.f. burns can occur.

For the benefit of the newcomer to amateur radio, r.f. voltages will not give you an ordinary electrical shock but will cause a penetrating burn or actual "cooking" of the flesh. Such a burn is slow to heal. The greater the power the larger the amount of r.f. energy present on the antenna. However, it is possible to get a nasty r.f. burn even when running relatively low power, such as 50-watts input, and on any band.

Still another hazard is that of getting an electrical shock from the antenna. Fig. 1 shows the typical pi-network tank circuit which is common these days. C1 is the blocking capacitor which permits r.f. to flow from the plate of the tube to the pi-network tank circuit and thence to the antenna. C1 prevents the plus-B voltage from being on the antenna circuit. If C1 should short out - and it is not a rare occurrence for capacitors to short - the plus-B voltage will flow out the feed line to the antenna. Anyone coming in contact with the antenna could be electrocuted.

One method of protecting against this is to install an r.f. choke between the output lead and ground, as suggested at RFC1 in Fig. 1. If the blocking capacitor shorts, the plus-B will be shorted to ground via the choke and the fuse in the transmitter power supply will blow.

Build a fence around your antenna or put the antenna up in the air where no one can touch it. If you are doing antenna work yourself, be sure the power is completely off in the rig. If you have insurance, such as the home-owner's type, be sure to read the fine print. Certain restrictions on amateur stations are laid down by the National Electrical Code and your insurance could be void if you don't comply with the code. Your local library or building inspector's office will have a copy of the wiring code so it would be a good idea to check. (Along the same lines, although not pertinent to the question, if you have a tower and think you have it insured, you had better check to make sure. In many homeowners' policies a separate rider is required for adequate coverage.)

Q.  "None of the articles on transmatches I have read in QST, the Handbook or elsewhere, have ever mentioned shielding of such circuits. Photos invariably show a chassis and a front panel but no signs of an enclosure behind the panel. This is surprising since stress is usually placed on shielding transmitters, especially around the output circuits. Do transmatches require shielding or don't they?"

A.  The answer is no, they don't. It is true that transmitters require extensive shielding to prevent radiation of harmonics that could cause TVI. Once the transmitter is shielded there is only one way for harmonics to get out, and that is via the antenna terminal. Any harmonics coming out of the transmitter should be attenuated by use of a low-pass filter. In such a case, it will make no difference if the transmatch is shielded. There are no harmonics reaching the transmatch so there is no point in unnecessary shielding. If you have harmonics that could cause TVI they must be kept enclosed within the transmitter and suppressed with a filter.

Protect your antenna against accidental contact - RF CafeOn the other hand, a transmatch is useful in suppressing the lower-frequency harmonics - those that can cause problems by interfering with other services than television. Because the transmatch is a selective circuit tuned to your operating frequency, it will present a load to the transmitter that is optimum for the operating frequency but is not optimum for the harmonics. Therefore, harmonics will be attenuated through the transmatch. But in either case, low-frequency or TVI harmonics, the transmatch doesn't require shielding.

Q.  "My rig was working OK, as I was making contacts. Now, all of a sudden, I can't work out at all. Can you tell me what is wrong or what to check? The rig loads up into a light bulb OK and the antenna seems to be the same."

A.  There are several possibilities in such a case. First and most important is to make sure that you are transmitting and listening on the same band. Believe it or not, many Novices think they are tuning up on the right band when actually the output is on another. You can spend a lot of frustrating time by calling on 40 while listening on 80!

One method of checking is to use a wavemeter to make sure that you are on the right band. A recent QST article2 had the construction details for a simple wavemeter and s.w.r./output-indicator combination, which leads us to another point.

Without such an indicator it is possible for a rig to be tuned up in what appears a normal manner when, in fact, there is no power going to the antenna. You could have an "open" or "short" in the feed line and not know it, if you have no means of checking. The simple device mentioned above contains a reflectometer which monitors both the standing-wave ratio and the output. If something goes wrong in the antenna or feeders, the s.w.r. should show a change from the normal reading. On the other hand, if something happens in the transmitter, this would show up as a change in the output, as indicated by the meter in the device. We highly recommend the use of a combination instrument that shows which band you are tuned to, the match in the system, and the output, because by simple elimination you can quickly pinpoint your troubles.

Q.  "I recently passed my amateur exam. I have been a CBer and I was wondering if it is possible to use my CB beam on an amateur band. It is a 3-element job with the elements in a vertical plane."

A.  While this isn't a common question, the answer is worth passing along to hams who may have an opportunity to pick up used (or new) CB antennas at a bargain price.

The CB frequency assignments are very close to the amateur 28-Mc. band, being centered around 27 Mc. As one goes higher in frequency, a half-wave antenna becomes shorter. In this case, 27 Mc. is lower in frequency than the 28-Mc. band so any CB antenna can be shortened to work on 28 Mc. The formula for figuring the dimensions of a 28-Mc. beam is quite simple.

First decide on the portion of the band in which you plan to do the most operating. Let's say it is around 28.6 Mc. Divide 28.6 in to 468 to give you the length in feet of the driven element. The director should be made 5 percent shorter than the driven element and the reflector 6 percent longer. The spacing between elements does not need to be changed but it is a good idea to make the elements horizontal to the earth. Most fixed stations on 28 Mc. use horizontally-polarized antennas and you'll probably get better results if you mount your antenna horizontally.

Q.  "I would like to build the Super-Duper 75-Watter described in...QST. Please send me step-by-step wiring information and pictorial drawings of the unit. I am afraid to attempt to build the rig with only the schematic."

Ham radio antenna not working - RF CafeA.  Variations of this request keep popping up. Unless specified in an article, we have no pictorials, layout drawings, or step-by-step information. If you want to do that kind of construction it is better to buy a kit. Kits usually come complete with pictorials and step-by-step instructions.

Here is the important point: A Novice must acquire a certain amount of radio knowledge if he wants to stay in amateur radio. In order to pass the FCC exams he must acquire some "know-how" in radio circuitry. One of the best ways of acquiring this knowledge is by building a piece of gear, trouble-shooting it, and getting it working. Too many beginners are concerned about making "Chinese Copies" of equipment described, even down to the same placement of nuts and bolts. Don't be afraid to change the layout. Don't be afraid to make substitutions. And above all, don't be afraid to experiment. The worst that can happen is a burned-out component or a blown fuse. The important thing is that you will learn.

Q.  "I have an s.w.r. bridge and its dial is calibrated in watts along with s.w.r. I am completely confused because I am reading 70 watts output with 30 watts reflected, and I am only running 60 watts input. What goes on...will the 30 watts coming back damage my rig?"

A.  No, the 30 watts won't damage the rig. This is a complicated thing to explain because a certain amount of knowledge is required about reactance, phase, and transmission-line and antenna theory. The subject is treated in detail in "The A.R.R.L. Antenna Book."

However, here's a simple analogy: If you go into a radio store to buy a four-dollar capacitor and hand the clerk a ten-dollar bill, you get six dollars back. The six dollars coming back doesn't hurt your pocketbook. It just hasn't been spent. The same thing is true of the reflected power in the above question. If you subtract the 30 watts from the 70 watts you'll find the remainder is 40 watts. This is the total power the transmitter is putting out and is actually going to the antenna to be radiated.

Still another way to look at it is that with 60 watts input it would be pretty darn difficult to get 100 watts output (70 forward, 30 reflected). The actual output is the difference between the two, 40 watts.

Q.  "I am building the two-band receiver described in last month's QST and am having a difficult time locating all the necessary parts. None of the local radio stores seem to stock parts. Where do you suggest I look?"

A.  Without a doubt this question (or a variation of it) is the most common one. At one time, radio stores carried a fairly complete line of parts, or would order them for you. It has become increasingly difficult to obtain components, at least on a local basis. Not only do the stores not stock components, but the manufacturers who make the parts a ham would want have become less numerous.

Hams: Don't be afraid to substitute parts - RF CafeIf you like building, you will almost certainly have to order parts by mail. Although some distributors still stock a fairly wide range of components, no one distributor is likely to have everything you need. It is suggested you write to the large mail-order houses and obtain their catalogs. Take a current issue of QST and go through the index of advertisers and write those that have catalogs or flyers available. You can, of course, write to those closest to you, but if you want a complete "availability file" it is a good idea to write to them all. Some of these concerns have two catalogs, a general type and another larger one for industrial users. Also, nearly all manufacturers of amateur equipment and components have catalogs and usually, they are happy to send them on request. If you cannot find a component in a distributor's catalog you can look it up in the manufacturer's catalog and write and find out who sells the item. In some cases, you'll find the manufacturer will sell direct.

We might add that at the ARRL, in any projects that are staff constructed, every effort is made to use parts that are readily available. However, even then it may be that a certain part is not easily obtained. That's why it is a good idea to have a stock of catalogs on hand. Also, as we stated in the previous question and answer, don't be afraid to substitute components.

1:  McCoy, "Is One of These Your Problem?", QST, May, 1966.

2:  McCoy, "The Wavebridge," QST, July, 1966.

Posted  3/10/2013

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