February 1967 QST
Wax nostalgic about and learn from the history of early electronics. See articles
QST, published December 1915 - present (visit ARRL
for info). All copyrights 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. It is the half-century-ago equivalent of the contemporary "The Doctor Is In" column by the ARRL's Joel Hallas, W1ZR. 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 ...
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?"
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
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.
On 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."
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
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
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
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."
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."
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
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 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.
If 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 September 22, 2019
(updated from original post on 3/10/2013)