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Multiple path transmission,
diffraction around obstacles, absorption by foliage, and reflection from moving
objects have always been challenges to the wireless system designer and/or user.
Whether it concerns communications between a WiFi router and a notebook
computer, a cellphone and a tower, an FM radio with a broadcast station, or deep
space probe with an earth station, all of the aforementioned mechanisms must be
dealt with to some degree. Although in a different way, even transmissions
within a waveguide or coaxial cable deal with those same issues - reflections
and the resulting standing waves have the same effect as multipath in terms of
vectorially additive versions of the same original signal. Signal degradation
issues can usually be overcome when all components are performing within
specifications, by having knowledge potential causes, and then assessing the
situation at hand. Of course an insufficient signal power from the transmitter,
too-high Friis-determined atmospheric path
loss, and inadequate receiver sensitivity under ideal conditions, will
always result in an unresolvable problem.
U.H.F. Fringe Installations
 By Walter H. Buchsbaum
Television Consultant
Radio & Television News
Down-to-earth hints on how to squeeze every possible db of signal out of u. h.
f. in the difficult fringe areas.
After the first rash of new local u.h.f. installations, many service technicians
find that additional customers are available, often in much greater numbers, in
the u.h.f. fringe areas. This type of area differs from the familiar v.h.f. fringe
area in many ways and requires a somewhat different approach and new service procedures.
Fig. 1 - Some of the causes for weak u.h.f. reception. (A) Hills
and obstructions do not diffract as much signal at u.h.f., as at v.h.f.; (B) reflections
are more troublesome at u.h.f., especially from relatively small sized structures;
(C) trees and foliage absorb signal energy at u.h.f. leaving less for the antenna
and set. See discussion in text.
Fig. 2 - Directions in which an antenna should be tested before
the installation is definitely made for u.h.f. receiver.
Fig. 3 - Various steps between the antenna and set in a typical
installation showing the major sources of signal loss and their causes in each
section.
In most cases, the u.h.f. fringe area is much closer to the transmitter than its
v.h.f. counterpart. Most locations more than 50 miles from any u.h.f. station will
receive only weak signals and many spots much closer have reception troubles due
to obstacles and reflections. All the disturbing aspects of v.h.f. wave propagation,
such as multiple path transmission, diffraction around obstacles, absorption by
foliage, and reflection from moving objects, occur in u.h.f., but their damage is
greater. A good example is the well known diffraction phenomenon, where an intervening
hill blocks TV waves, but enough energy is bent around the crest of the hill to
permit some kind of reception behind it. The amount of energy diffracted in u.h.f.
is less than half of that normally expected from a v.h.f. station. To make matters
worse, trees near the top of the hill will absorb much more of the diffracted u.h.f.
signal than of any v.h.f. signal.
Fig. 1 illustrates some of the causes of weak u.h.f. signals and although they
are also present in v.h.f. fringe areas, their effects are much more marked in the
u.h.f. TV band.
This article describes the various factors in a typical u.h.f. fringe installation
which can be adjusted for better reception. When the signal at the receiver is so
weak that "snow" appears, every change which will improve the signal strength is
worthwhile. Getting every possible db of signal requires careful consideration of
all the factors which can cause losses and the major portion of this article is
devoted to pointing out these "signal sinks" and methods of avoiding them. Starting
at the antenna we shall show how each portion of the installation can be designed
for minimum loss and optimum signal.
Antenna Installation
The popular argument as to which antenna is best will probably never be settled,
but for practical reasons, gain and directivity are the most important characteristics
in the fringe area.
Bandwidth, cost, and ease of installation are certainly secondary when a single
station is received weakly and the acquisition of a new customer is at stake. For
the typical fringe installation, therefore, a yagi antenna as in Fig. 2, a corner
reflector, or a rhombic is recommended. Above all, when using a yagi, be sure that
the antenna is cut for the channel to be received.
In planning the installation it is well to remember that, optimizing all other
factors, the actual signal which the antenna picks up represents the best possible
reception. Therefore the type of antenna, location, mounting, and orientation can
be considered as at least 50% of the job. This should be pointed out to the customer
and the resulting installation charges can be based on the signal strength with
which the technician must work.
Recent advertising stresses the fact that some antennas use gold or some other
type of plating, special dielectric spacers, or air dielectric to operate well in
the u.h.f. fringe area. The boon of gold or silver plating is somewhat dubious since
the surface resistivity of the antenna elements is normally quite low compared to
the characteristic impedance. However, such plating does help to prevent rusting
and deterioration.
Dielectric losses at the antenna terminals can be considerable, especially if
the dielectric used has a tendency to absorb moisture. One limitation of most antenna
terminal arrangements is that they do not keep dust and moisture out. In this respect,
the antennas which require no dielectric spacers are preferable. Where a dielectric
spacer is used, the antenna terminals can be weatherproofed with special tape. Polyethylene,
polystyrene, or vinyl tape can be used, provided the tape is pressed tight to keep
water and air out and is not used so profusely that its thickness results in impedance
mismatch. No more than two layers should normally be used and never, but never,
use electrical friction tape or any kind of cellophane or paper-based tape.
Having made a good connection, preferably soldered without flux, we are now ready
to hoist the antenna into place for a trial orientation. Fig. 2 shows a simple six-element
yagi and the direction of rotation and motion for optimum performance. Because of
the appearance of so-called "space nodes," the antenna should be moved up and down,
forward and backward as well as rotated to point towards the station. The up and
down motion should be done slowly and extended for at least half a wavelength or
about one foot at the lowest u.h.f. channel. It will be found frequently that maximum
and minimum signal locations exist both in the vertical and horizontal plane, spaced
half a wavelength apart. For the same reason the forward and backward motion is
recommended. In addition to all these maneuvers, it is often helpful to vary the
angle between the antenna beam and the mast, indicated by the 30 degree tilting
at the left of Fig. 2.
It may be necessary to repeat this procedure for several locations until the
strongest signal is obtained. Since foliage absorbs u.h.f. waves to a great extent,
be sure during the winter season that the antenna will not be blocked by trees or
brush when spring comes along.
Once the best location and directional position have been determined, the installation
is made permanent. In addition to the normal rigidity and sturdiness required for
v.h.f. antennas, the u.h.f. fringe installation must be guyed sufficiently to prevent
swaying in the wind. A wind displacement of one foot, while tolerable at v.h.f.
will often cause "picture breathing" or excessive fading on u.h.f. channels.
On one u.h.f. fringe installation, good pictures were obtained at the time of
installation, but when the customer's husband came home the picture fluttered until
dinner time. Then a stable picture was obtained again. When the service technician
came the next day, the fluttering was gone. In the late afternoon fluttering occurred
again until dinner time. The cause for this vanishing flutter was apparently the
reflections coming from a nearby highway where traffic was only substantial during
the morning and evening rush hours. The reflection from passing automobiles was
alternately in phase and out of phase with the main signal, causing flutter. Unfortunately
there was no practical solution possible since the location of the home and the
u.h.f. station required that the antenna be oriented towards the highway.
Transmission Lines
The flat 300-ohm twin-lead used for v.h.f. installations is quite lossy at u.h.f.
and should never be used in weak signal areas. The author measured as much as 10
db loss at 800 mc. in a 50 foot length of flat twin-lead. By comparison, it was
found that the same length of tubular twin-lead had less than 2 db loss.
There are many different kinds of low-loss u.h.f. transmission line on the market
and most of them are quite good. Open-wire line is theoretically the least lossy
and. therefore most suitable for fringe installations. In actual practice, however,
the open-wire line has some distinct drawbacks. Near industrial centers the air
pollution is often quite bad and causes soot and grease deposits on the dielectric
spacers. After a while the sum total of many coated spacers is sufficient to change
the characteristic impedance of the line, resulting in standing wave and dissipation
losses in the transmission line. Completely covered twin-leads do not suffer so
much from dust and dirt because the insulation always keeps the grime away from
the conductors themselves.
More lossy than the transmission line itself are some of the indispensable accessories
such as the standoffs, v.h.f.-u.h.f. couplers, and lightning arresters. The latter,
especially, can be quite a trap for u.h.f. signals. There are basically two types
of arresters on the market, one using resistive elements, the other using condensers.
For u.h.f. the resistive models are not recommended since they cause not only a
poor impedance match, but also unbalance the line. Some of the older style v.h.f.
lightning arresters are unsuitable because their capacities are too large and, in
many instances, the plastic case material has such poor dielectric qualities in
the u.h.f.-TV band that impedance mismatch is considerable. The author checked the
v.s.w.r. of several lines with lightning arresters and found those using "v.h.f.
only" models have more than a 2.5:1 standing wave ratio. The latest u.h.f. or v.h.f.-u.h.f.
models introduce less than 1.5:1 v.s.w.r. Thus, the selection of a good lightning
arrester is another important factor in squeezing every db of gain out of the antenna
in a fringe area.
When a u.h.f. fringe installation also requires a v.h.f. antenna, care should
be taken about using a v.h.f.-u.h.f. antenna coupler. All of the commercially available
crossover networks have some insertion loss at u.h.f. In a fringe area, every db
of signal strength is important. The best solution for fringe locations where both
u.h.f. and v.h.f. antennas are needed is to run separate transmission lines to a
suitable low-loss switch at the receiver. In many instances it is possible to simply
connect each transmission line to its respective tuner input and depend on the u.h.f.
converter or tuner to select the desired signal.
Transmission line standoffs and supports are another potential signal trap. This
is especially true in most fringe installations where the transmission line is quite
long and is supported in many spots. The ideal standoff, or at least the best possible,
is the type consisting entirely of polystyrene or some other low-loss dielectric.
For rigidity, however, as well as for mounting purposes, steel is usually required.
The most popular types of standoffs use a metal loop with a polyethylene grommet
inside which supports the transmission line. At u.h.f., the metal loop together
with the dielectric forms a condenser which effectively shunts the two wires of
the transmission line. This capacity is quite small and by itself has little effect
on the transmission line characteristic. When a great number of standoffs is used,
however, their total capacity may seriously upset the impedance of the line. This
is especially true when the standoffs are placed at equal distances resembling multiples
of 1/2 wavelength at the channel being received. In this event considerable losses,
as much as 6 db for 12 standoffs, can be encountered.
For u.h.f. fringe installations, therefore, it is recommended that as few standoffs
as possible be used while still maintaining good mechanical support for the line
and, whenever possible, use standoffs without metal rings. Where a large number
of standoffs is essential, try to space them at different distances from each other
to avoid multiple 1/2 wave spacing effects.
Needless to say, such poor v.h.f. practices as simply laying the transmission
line on top of a metal roof, dropping sections into gutters, or taping it to metal
supports are even more detrimental at u.h.f.
All the measures discussed so far are aimed at avoiding signal losses, but there
is one important step, the addition of a booster amplifier, that can actually increase
the signal amplitude.
U.H.F. Boosters
In v.h.f. fringe installations the use of a booster is now almost standard practice
and many different boosters are available. However, such u.h.f.-r.f. amplifiers
are complex and u.h.f. boosters are still somewhat of a problem. The limiting factor
in any booster is its noise figure as compared to the noise figure of the receiver.
If the u.h.f. tuner or converter has a noise figure of 12 db at the channel used
and the booster has a 16 db noise figure, then the addition of the booster will
actually result in a snowier picture.
Most continuous tuners have a noise figure of 14 db at optimized channels, and
24 db at poorly tracked channels. Turret tuners have a noise figure of 12 db at
optimized channels, and 18 db at average alignment. Converter strips for turret
tuners result in a 16 to 24 db noise figure with little chance of optimizing at
any channel.
Most u.h.f. boosters have a noise figure of 11 to 15 db, with a gain of 7 to
13 db.
We can see from this data that the greatest advantage of the booster is an increase
in gain. In other words, where the over-all receiver gain is low, a booster will
help. Where it is possible to get a snowy picture with plenty of contrast, the over-all
gain is sufficient and only a better noise figure can improve reception. In many
cases for both continuous and turret-type tuners or converters it is possible to
optimize at least one channel so that the noise figure is about as good as that
of a booster. The real utility of the booster comes with the use of u.h.f. conversion
strips in v.h.f. tuners. In these cases, the noise figure of the booster is superior
and the over-all gain of the receiver is usually sufficient, so that the booster
can really improve reception.
In addition to the currently available u.h.f. boosters, new models will soon
appear having noise figures below 8 db and stage gains of 10 and 12 db. This will
be possible by the development of inexpensive u.h.f.-r.f. amplifiers. Improved versions
of the 6AF4 and 6AJ4 are now in pilot production at RCA and G-E and these tubes
will operate satisfactorily over the u.h.f. band, in grounded-grid circuits, and
in regular 7-pin miniature sockets. These tubes will, of course, be used in most
new v.h.f.-u.h.f.. tuners, but their application in u.h.f. boosters will help overcome
the limitations of many older u.h.f. converters and tuners.
Optimizing the Receiver
The final destination of the u.h.f. signal is the input circuit of the TV receiver
and at that point further improvements can be made to get the best possible picture
from a weak u.h.f. signal. First of all, the various approved and time tested v.h.f.
fringe receiver modifications should be tried. These include reducing a.g.c. bias,
adding an a.g.c. delay circuit, realigning the i.f. section for a narrower response
curve and similar measures. In receivers where double conversion is used, be sure
the v.h.f. channel acting as first i.f. for the u.h.f. signal is tuned up for optimum
response. It is possible to adjust v.h.f. input and mixer networks to less than
6 db noise figure if a cascode circuit is used and less than 9 db for a pentode
r.f. amplifier. Checking the bandpass and over-all gain of the v.h.f. section will
make the optimizing of the u.h.f. portion much easier.
In many instances where an external converter is used, the output stage of the
converter, usually a cascode, and the input of the v.h.f. tuner should also be aligned
for best sensitivity and noise figure. The connecting cable should be as short as
possible and its length should be trimmed for best picture. After all i.f. and v.h.f.
adjustments have been checked, the u.h.f. networks can be tuned up. Most u.h.f.
tuners and converters now in use are of the continuous-tuning type. Whether capacity
tuned transmission lines, coaxially tuned lines, shorted rings, or other systems
are employed, the problems of tracking the r.f. network with the mixer and oscillator
networks and of impedance matching the input are always present. In continuously-tuned
devices it is usually possible to get best tracking at the highest and lowest frequency
and some sort of compromise at the center. For a typical u.h.f. fringe installation
only one channel need be optimized and this makes the alignment process easier.
Before adjusting anything it might be well to try different mixer crystals. Most
tuners and converters use either a 1N72 or 1N82 in this circuit and performance
of different crystals can vary considerably. Since the crystal usually is in a clip-type
holder it is a simple matter to switch crystals, but it is also necessary to retune
the mixer trimmer or touch up the r.f. bandpass network. The better equipped service
technician can try improving crystal performance by putting a 1N21B crystal into
the tuner. This is a more expensive crystal, usually used for government work, and
requires a different mounting clip. Its performance is decidedly better than the
1N72 or 1N82. Again retuning is required.
To align the r.f. network of most u.h.f. tuners and converters requires a u.h.f.
sweep generator and oscilloscope and this operation can be performed in the shop,
prior to the installation. Tune the main tuning control for the desired channel,
then check the response curve, leaving the local u.h.f. oscillator on. The oscillator
should produce a pip on the flat portion of the response curve, and the r.f. network
is then tuned for the most even, narrow, and highest bandpass possible. To check
the antenna input circuit, the u.h.f. sweep generator impedance should be 300 ohms
and the coupling network should be adjusted for maximum response curve height.
For detailed instructions on optimizing a turret tuner refer to the September,
1953, issue of Radio & Television News, which gives this data for the Standard
Coil 82-channel tuner. The steps for adjusting the various circuits are the same
as outlined previously, but the range of each adjustment is more limited since the
turret tuning itself permits tracking for many more than the three usual spots.
Many u.h.f. converters and tuners use hi-pass filters at the u.h.f. input and
occasionally these filters are lossy or cause slight mismatch. Where adjustment
is provided, try optimizing it, otherwise it may be possible to bring the u.h.f.
antenna line directly to the r.f. network without going through the hi-pass filter.
Another problem is the mismatch of the antenna line to the receiver input. Try cutting
the transmission line 1/2 inch at a time until the best picture is obtained. Occasionally
it is possible to improve picture performance by folding a 6 inch length of tinfoil
over the transmission line near the receiver and sliding it up and down until the
best picture is observed. Tape the tinfoil onto the line at the best spot.
Fig. 3 shows the sum total of signal losses and the nature of each drop in signal.
In order to get the best possible performance from a u.h.f. fringe installation
it is essential that each of these losses be minimized or compensated. As an example,
the antenna matching losses can be avoided by correct impedance match, i.e., using
the right kind of antenna for the right type of transmission line. The losses due
to the transmission line can often be overcome by the addition of a u.h.f. booster,
provided that the noise figure of the booster is at least as good as that of the
tuner. Standoff losses due to impedance mismatch can be minimized by using as few
standoffs as consistent with mechanical support and avoiding the use of the type
that uses a metal ring around the dielectric.
The type of loss easiest to compensate occurs at the input to the u.h.f. tuner
or converter. To avoid this loss it is essential for almost every fringe installation
that the u.h.f. networks be realigned and tuned up for the weakest channel. This
is especially important for continuously-tuned systems where mis-tracking can introduce
losses of 10 db or more. In addition to tracking, the use of a selected crystal
mixer diode is also recommended and the r.f. input circuit should be adjusted whenever
possible.
As a last step to insuring good u.h.f. reception, weatherproofing the entire
outdoor portion of the installation is essential. Under weatherproofing we include
the guying of the mast to avoid swaying and the resultant "picture breathing," as
well as patching up each hole in the masonry or woodwork with weatherproofing material.
The connection of the antenna to the transmission line should be weatherproofed
as well as possible, with soldered connections wherever feasible. All tape used
should be either polyethylene or vinyl and never should regular friction tape be
employed.
If the reader concludes after this article that a u.h.f. fringe installation
is a complicated and time-consuming job, he is quite right. Competent service technicians
know that such an installation requires a lot of time and equipment and therefore
plan accordingly. As far as the customer is concerned, he should understand that
because he lives in a u.h.f. fringe area the installation will cost him considerably
more than it would in a strong signal location. Usually the entertainment value
of TV in such remote areas is so great that set owners are quite willing to pay
well in order to obtain television reception.
Posted March 26, 2020
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