Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
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. This article
in a 1970 issue of Popular Electronics 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is not matched to the TV receiver, you might just as well be using outdated "rabbit ears."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
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 August 14, 2017