More Problems - More Questions and Answers
February 1967 QST Article
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).
February 1967 QST
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 (if any) are hereby acknowledged.
See all available
vintage QST articles.
Beginner and Novice
More Problems - More Questions and
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 any band.
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.
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
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!
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
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
"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
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.
"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?"
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.
"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?"
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.
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,