 Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
from
Popular Electronics,
published October 1954 - April 1985. All copyrights are hereby acknowledged.
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Few homeowners in the era
of television antennas on the roof had any knowledge at all about how the antenna
and twin lead transmission line system worked. Even those who were familiar with
it only knew the basics like keeping the transmission line away from metallic objects
and properly terminating the ends. I have seen photographs from servicemen of
twin lead laying in aluminum guttering and along the top of chain link fence
rail, and amazingly, the TV set still received a fairly good picture. That must
have been in areas with exceptionally strong signals. This article in a 1970 issue of Popular Electronics
magazine
described a method for optimizing the antenna and transmission line in terms of
impedance matching and using very low loss open ladder line to optimize signal strength
to the receiver. It is exactly the subject (received signal strength) I recently lamented about being often
ignored when discussing aspects of antennas and transmission lines.
No Snow in June: Match Your TV Antenna to Receiver for Best Possible
Picture
 By George Monser
Good TV reception is not obtained by accident;
it is carefully sought for and designed into your antenna system. You can get the
best antenna and lead-in cable money can buy, but if the antenna is not impedance-matched
to the cable and/or the cable is not matched to the TV receiver, you might just
as well be using outdated "rabbit ears." This is especially true for color TV reception
- and not just in the "fringe" reception areas.
Everything in your TV receiving system must be just perfect, and the only way
you can make sure that it is is to do the job right - the first time. But do not
think that you have to be a TV antenna / transmission line expert to set up a receiving
system. With the help of the information provided in this article, you can set up
the best possible antenna system.
The Loss Factor. Nothing is perfect. No matter whether it is
an automobile engine or an electronic circuit, every system suffers from some type
of loss which reduces its efficiency. While you cannot completely eliminate receiving
system losses (known as signal attenuation), you can limit them to an acceptable
level.
Fig. 1 - Transformers are cut to specific lengths for individual
channels or for multichannel band.
To demonstrate how loss becomes a critical design factor, consider a 300-ohm
folded dipole antenna (tuned or cut to any TV channel) connected to a length of
300-ohm twin-lead cable. Very little loss would occur between the antenna and cable
for the channel to which the antenna is tuned. But for all other channels in the
TV band, the loss might be as high as 3-4 dB; and over the complete band, an average
loss of 2 dB would be typical, enough to cancel the characteristic 2-dB gain of
the folded dipole (favorably oriented) antenna.
Now, consider a resonant 300-ohm folded dipole, reflector, and several director
array (representative of most commercial TV receiving antennas). An estimated 2-dB
loss would occur at the antenna/cable connection due to the lowering of the dipole's
impedance. (The effect of placing a reflector and directors in close proximity to
the folded dipole is to lower the 300-ohm characteristic impedance of the dipole
to about 70-100 ohms). But since this antenna array provides 6-10 dB of gain, a
2-dB loss, severe in our first case, can usually be acceptable, particularly in
good reception areas.
For both cases cited above, the cable lead-in loss, assuming about 40' of twin-lead
at VHF, amounts to between 0.6 and 1 dB. Hence, the total loss in antenna signal
strength is 3 dB. This means that only 50% of the antenna signal power would be
delivered to the TV receiver.
Fig. 2 - Insulator spacers support gradual tapers when matching
600-ohm open line to 300-ohm cable.
Reducing the losses. The choice of improving the antenna-to-transmission
line match basically involves inserting an impedance-matching transformer between
antenna and line. The drawing in Fig. 1 illustrates the makeup of one type
of transformer you can use. It is easy to fabricate and consists of two lengths
of 300-ohm twin-lead cable.
The decision of whether to fabricate your own transformer as opposed to buying
one that is commercially made should depend on the end results. Tests made with
both types show that at the 70-MHz frequency of channel 4, the commercial ferrite-core
balun lowers the signal level by about 2 dB, while the quarter-wave, twin-lead homebrew
transformer improves the signal level by 1.5 dB.
 Lead-in attenuation, the other loss (amounting
to less than 1 dB) can be slightly reduced, but not without considerable effort.
Here, two possibilities exist: transition from the antenna to a home-brew 600-ohm
open-wire lead-in and back to 300 ohms at the TV receiver; or transition from the
antenna to home brew 1"-diameter, 77-ohm coaxial line and back to 300 ohms at the
receiver. Neither of these alternatives will yield a line loss less than 0.3-0.5
dB, which hardly seems worthwhile by itself. However, if a choice were to be made,
it would probably be easier to stay with a balanced line and use 600-ohm open line.
(Fig. 2 illustrates how this can be accomplished with #16 wire and a wire separation
of 4" to yield a line loss of about 0.25 dB/100' at 88 MHz, or less than 0.15 dB
for a typical 40' run. )
You may be wondering when and where it is advantageous to use these methods for
improving signal transfer. As a general rule, they should be employed in "fringe"
reception areas to improve weak TV channel reception. When making your own transformer
or transformers, refer to the Table for the proper quarter-wave transformer lengths
to use for each TV channel in the VHF spectrum. The lengths listed were computed
assuming standard 300-ohm twin-lead cable with a phase factor of 0.84, which is
typical for polyethylene-jacketed twin-lead.
Fig. 3 - Gradual taper matches 300·ohm twin-lead cable to
150-ohm impedance of Pyramidal Antenna.
Now, take three practical examples to show how to improve TV reception. In the
first example, suppose you have a good quality commercial antenna array and wish
to improve reception on Channel 4 by inserting a transformer section between the
antenna and a 300-ohm twin-lead line. Select the transformer length section from
the Table; in this case, 36" is indicated. Cut two pieces of twin-lead cable to
exactly 36" (plus about 1/2"extra at each end). Strip away 1/2" of insulation from
each end of both cables. Then, connect the lengths of twin-lead in parallel with
each other (see Fig. 1).
Insert the transformer section between the antenna and twin-lead lead-in cable.
This should yield an improvement of 1.5 dB in signal strength and a noticeable improvement
in Channel 4 fringe-area reception.
For our second example, suppose you use the same antenna and want the best possible
reception. Rather than running 300-ohm twin-lead cable, try using lower loss 600-ohm
open line. This can be done fairly easily by following the instructions detailed
in Fig. 2. At both the antenna and TV receiver, the line must be tapered gradually
to the 600-ohm spacing of the open line. When completed, the installation should
yield about a 2-dB improvement in signal reception, slightly better than in the
first example.
Fig. 4 - Open line matches two Pyramidal Antennas to 300-ohm
cable. Note half twist in 600-ohm line.
As a final example, assume you are planning to erect the Pyramidal TV/FM Antenna
("Build The 'Pyramidal' TV/FM Antenna," Popular Electronics, July 1969). This antenna's
impedance is about 150 ohms, which means that 300-ohm twin-lead cable is reasonably
ideal to use. However, for the ultimate match, you should insert a tapered section
of line between the antenna connecting terminals and the 300-ohm twin-lead lead-in
cable as shown in Fig. 3 to improve reception by about 0.5 dB.
The added complication of tapering the line in the last example might not be
justified, considering that this antenna has a nearly flat gain characteristic of
10 dB for all VHF TV channels.
Finally, suppose that even 10 dB of gain is not enough to provide quality fringe-area
reception. You could stack two Pyramidal antennas as shown in Fig. 4 to obtain
13 dB overall gain. Here, the individual antenna connecting point impedances can
be tapered to 600 ohms and then paralleled, providing an ideal match to the 300-ohm
twin-lead cable line to the receiver. In the illustration, the center-to-center
spacing between the antennas is 5'. Of course, the antennas could just as easily
be placed side by side to yield the same resultant gain; but erection on a single
mast is usually easier to implement.
Now that you have been apprised of good receiving system basics, you can start
designing your own system. And with the warm weather here, what better time is there
to tackle the job?
Posted March 5, 2024
(updated from original post
on 5/14/2017)
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