November 1957 Radio-Electronics
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Electronics,
published 1930-1988. All copyrights hereby acknowledged.
TVDXing was a very
popular sport in the 1950s through about the 1970s. As the name suggests, it
involved attempting to receive television broadcast stations from as far away as
possible from your location - akin to Short Wave Listeners (SWLers) who used
radios with the same objective. If you were around back when over-the-air TV was
the primary form of broadcast (before or during the early years of cable TV),
late at light you were likely able to pull in stations - especially UHF - that
were sometimes hundreds of miles away. An antenna rotator increased the chances
of doing so. Living near Annapolis, I definitely remember getting among others
TV stations from Philadelphia, PA, and Richmond, VA. At the time I did not know
there was such a thing as TVDXing - it was just a curiosity.
Serious DXers went
to great lengths to claim success, as related in this 1957 issue of
Radio−Electronics magazine. Studying atmospheric propagation tables,
monitoring live reports via Ham radio, erecting high gain antennas, inserting
preamplifiers ahead of the TV set, etc., was in the TVDXer's bags of tricks.
See also TV DX - July
Mac's Radio Service Shop: Television DX - September 1951 Radio &
Tips from a TVDX−er's Notebook - November 1957 Radio−Electronics.
A frequent RF Cafe website visitor, who also happened to be one of those
serious TVDXers back in the day (and a licensed Ham), told me a joke relevant to this article: "A
woman called for a TV repairman and as soon as he took a look at the back of the
set he noticed that the wires from the antenna weren't connected to the antenna
terminal on the TV. He looked up at the woman and said 'I think I know your problem,
these wires aren't even connected.' The woman replied, 'I knew that, but I figured
the signal had come this far I thought it could make that little jump.'"
Tips from a TVDX−er's Notebook
By Robert B. Cooper, Jr.
Types of dx reception and necessary receiving equipment and techniques
The field and hobby of television DX−ing have grown to such proportions
during the past 4 years that the number of DX−ers currently twirling the dials is estimated
in the thousands. For most of them the only contact with other weak−signal chasers
is through the TV DX column appearing in Radio−Electronics on a regular basis (alternate
TV DX picked up in Fresno, California. Top, WBAP−TV, channel
5, Fort Worth, Texas, June, 1956. Center, KFEL−TV, channel 2, Denver, Colorado,
July, 1955. Bottom, CHCT, channel 2, Calgary, Alberta, Canada, June, 1955.
Along with predictions of dx conditions and news of unusual loggings, we
often receive requests for information pertaining to antennas, when to look for
dx, receivers, accurate record keeping and many other phases of full−scale DX−ing. Here are a few
tips for those who are interested in TV DX but may not know how to break into the
Receiving dx stations (those not normally seen at your location) is not
nearly so much a function of the location of the observer and the equipment used
as it is of being at the right place at the right time. In DX−ing circles many make use
of a "timetable" (see table) to give basic information concerning the types of dx
we might expect during various periods of the year. During a year's time seasonal
weather changes and the position of the Earth in relation to the Sun and other heavenly
bodies have pronounced effects on dx conditions.
Types of DX Reception
The most common form of dx is that denoted by the term "sporadic−E skip." Sporadic−E
skip (abbreviated Es) affects the lower television channels (2−6), bringing reception
to your location from stations on an average of 500−1,500 miles distant. Various
forms of Es have been known to provide reception for distances as great as 6,600
miles, although this is exceedingly rare. Es is a result of the television signal
being reflected from a densely ionized layer, called the E layer, of the ionosphere.
This layer exists at a height of 60−70 miles above the Earth. Under normal conditions
the layer's density is very low and very−high−frequency television signals pass
through the layer and on into space, never to return. It is only during periods
of freak ionization conditions that the layer forms (actually forming in spots and
not as a whole layer) and causes low−channel television signals to rebound from
it as if it were a mirror.
Occurring on all TV channels, but most pronounced on high−band (channels 7−13)
vhf and on uhf, is tropospheric (abbreviated trops) bending or conveyance of television
waves to areas beyond the radio horizon via the troposphere. The tropospheric layer
exists immediately adjacent to the Earth, and the majority of our weather occurs
in it. Trops is actually caused by sharp boundaries formed in weather fronts which
act as a duct, carrying the signal to distant points. This form of reception occurs,
for the most part, during the spring, early summer and early fall months of the
year. Distances covered vary from 200−800 miles.
A form of DX−ing theoretically possible, during the next few years at least,
is F2 skip. Like the E layer, the F2 layer of the ionosphere is often capable of
reflecting television signals back to Earth during years of peak sunspot activity.
The channels influenced most often by this form of skip are the lowest (2−4). The
distances covered by F2 are enough to stagger even the most imaginative mind, beginning
at 2,200 miles and working upward in steps of 2,000 miles at a time. This form of
DX−ing promises to be extra interesting as it will provide us with the opportunity
to view programs originating in other countries and even other continents!
Lastly, we have what is commonly known as
the DX−ers' form of dx. As sensitive receiver design developed to usable levels
and antennas with higher gain were put on the market, alert observers began to notice
a form of burst reception. Short bursts of reception could be obtained for seconds
at a time during periods when no signs of dx were present. It was decided that these
bursts were a result of meteorites entering the E layer of the ionosphere and burning
up there. As the friction causes them to burn, they leave a trail of ionized gas.
Thus, like regular Es, meteor bursts (or MS) could provide reception from stations
500−1,500 miles distant. As meteorites are entering the E layer at all times (though
in varying degrees of .intensity), we are able to log distant stations whenever
we wish. The only hitch in the scheme is the short duration of the bursts. It usually
takes many bursts to identify the program material and a few more to identify the
source. Lots of patience and a good deal of practical experience really payoff with
this form of DX−ing.
Keeping an accurate log is a very important part of DX−ing. As DX−ers, we are
able to provide scientists with a great deal of information not obtainable in any
other way. TV DX is definitely freakish in nature and is therefore subject to explanation.
However, as with many other occurrences in nature, information is needed before
detailed study may begin. Radio−Electronics is providing without cost through the
TV DX column specially prepared forms on which you may report your dx observations.
When your reports are combined with those from other sections of the country, scientists
get a broad view of dx conditions over the country as a whole for any given date.
An accurate log is also important when you wish to write to stations requesting
letters of verification for your reception. It is always best to include information
as taken from the receiver screen or audio, referring to local advertisements, etc.
This will help the station in verifying your report. Many DX−ers have verification
letters from 100 or more stations.
XEW TV, channel 2, Mexico City, Mexico, seen in Temple, Tex.,
March, 1955. Courtesy Richard Lowry, Temple, Tex.
During the past 2 to 3 years the old−timers in the dx game have become
increasingly aware of the fact that the newer television receivers are sadly
lacking in many dx essentials. In fact, with the current drive to use
multi−purpose tubes, etc. for receiver compactness individual set gain and
stability have deteriorated measurably. Therefore, DX−ers usually recommend that newcomers in the game purchase a sensitive
receiver having a cascode front end (cascode rf stage). Keeping your receiver in
good electrical working order is very important. If you do a moderate amount of
DX−ing, change the rf amplifier tube every few months. Other important tubes
such as the detectors, if amplifiers, rectifiers, etc. should all be tested
frequently and perhaps changed completely every year. Remember, the only
difference between the receiver you use for DX−ing and the one your neighbor uses for normal home viewing
is the shape you keep yours in.
Boosters are also frequently mentioned. Remember this simple rule: If your receiver
is of late−model design, with a cascode type rf amplifier stage, boosters are of
very little value. Receivers using pentode and triode rf amplifier stages will benefit
greatly from a cascode booster stage.
When we approach the question of the correct antenna for DX−ing purposes (or
just plain deep−fringe reception), we encounter a controversy. Some DX−ers
prefer large−screen reflector arrays with dipoles situated in front for signal
pickup. Others lean toward the various forms of Yagi antennas while still others
like the collinear style. To provide optimum gain on each channel with a single
antenna is an engineering feat yet to be accomplished. However, several
compromises may be made to give fair to good gain across the spectrum and still
allow the use of a single antenna for all−channel vhf operation. What is wanted
is as much gain as it is possible to get, high front−to−back ratio and good
directivity. Many DX−ers
use 5−· or 10−element Yagis for channel 2 and then some form of all−channel for
the rest. The reason for this is that both Es and F2 work up from the lowest channels.
Thus, a good antenna system on channel 2 is a big help. If you have a local on channel
2, a similar system for channel 3 would also serve the purpose. Stacking Yagi antenna
arrays pays off for weak−signal reception.
Antenna transmission line is also important. Keep the line in as good physical
condition as possible. Of course, the best line is that with the lowest signal loss:
300−ohm open−wire line does an admirable job and 450−ohm open−wire line is also
very good. A problem here is matching the 450−ohm lead−in with a 300−ohm receiver
input and 300−ohm antenna. If open wire can be used, we suggest the 300−ohm version.
If you have problems with your open−wire line continually shorting to the mast or
tower as you rotate the antenna, try using the 300−ohm tubular uhf line. The loss
is very low and the plastic insulation keeps out moisture, dust, etc. Try to use
just one piece of line from the receiver to the antenna as splices create more signal
Getting your antenna high above ground and he surrounding objects is also of
prime importance. If you live in an area with many power lines, poles, high
trees, etc., raising your antenna above them will really help reception. Giving
the antenna a clear shot at the dx stations is the whole idea in back of raising
it into the heights. Power lines, rooftops, trees and the like all add to the
amount of signal absorbed or reflected before it reaches your antenna. As you
raise it up in the air, your antenna begins to break into signal levels that
have not been decreased by absorption. A height of 70 feet above ground is a
good minimum height for serious DX−ers, although good results can be had down to around 40 feet. A good rule might
be: Raise the antenna as high as guy wire, guying space, pipe, neighborhood policy
and your pocketbook allow.
Rotating the antenna can and does make all the difference in the world. Remember,
you are using a high−gain antenna with its power punch concentrated in the forward
direction. It is designed to reject signals from the rear and sides. Anyone of the
popular antenna rotators is recommended for dx reception from all directions.
When using a large Yagi antenna array be sure that it is at least four boom lengths
,above any surrounding objects like trees and rooftops. This will assure you of
a proper radiation pattern. If you cannot erect a Yagi array so that it is several
boom lengths from any objects at the same height, it would be better to try some
other form of antenna.
Venetian blinds: Horizontal bars move across the screen, alternating black and
white. This is a sign of two (or more) stations on the same channel, the bars being
what is commonly termed a beat note. The blind effect on a channel normally clear
of them is the tipoff that some sort, of dx is trying to appear. Careful orientation
of the antenna will usually bring it in. If you are not able to bring the dx station
through a local or sublocal station, find the antenna heading in which the interference
is strongest and then check the rest of the channels for signs of other dx stations.
Frequent signal bursts on a clear channel: This is a sign of either a meteorite
shower or the formation of a Es cloud. In either case you should stick around for
the next few hours and make frequent channel checks.
Fringe−area stations lose their snow: This is a sure sign that good, trop conditions
are forming. First, determine in which direction the conditions are improving and
then check the channels for weak stations not normally seen. (As fringe−area stations
become of "local quality," dx stations appear with "fringe−area quality.")
If you live very near transmitters, there is not much that can be done for reception
of dx stations on the same channels as the locals. For reception on adjacent channels,
special traps can be used to filter out the overload from your locals, thus leaving
the adjacent channels for dx reception. These adjacent−channel traps can do a lot
for reception of Es signals. If you live 20 miles or so from the actual transmitter
site, you should be able to bring strong E−skip stations in with the help of a folded−dipole
antenna to attenuate the local station. The dipole antenna consists of a length
of 300−ohm line cut to a half−wavelength on the interfering channel. Fed with another
length of lead−in in the usual manner, you have a very effective antenna for cutting
out locals by cancellation. Mount the dipole on a piece of board and experiment
with it at various heights to find the highest you can go before getting into the
extra−strong local signals. By carefully orienting the dipole, you will be able
to "phase" out the local and allow the dx skip station to come through.
From several experiments made in California, we have found the best height above
ground for the dipole is around 20 feet. For this antenna, height above ground is
the all−important factor. If you wish to use a single dipole for the five low channels,
find a compromise length in the vicinity, of channel 4.
For the latest in dx information and predictions of things to come, watch Radio−Electronics'
TV DX column.
Posted December 20, 2021