It seems that creating almost cartoonish-looking antenna arrays
for the purpose of signal gain and directivity are usually relegated
to the domains of military and amateur radio practitioners,
but this article from a 1952 edition of Radio & Television
News magazine was done by the
Master Laboratories television antenna company. Successfully
mounting and phasing even two antennas can be challenging, but
in this case four Yagis were arrayed and tuned for operation.
Trying to make the system work over the entire 4 octave band
that is the VHF broadcast realm (54 MHz for channel 2 to 210
MHz for channel 13) would be nearly impossible without extremely
complicated mechanical and electrical design, so the engineers
satisfied themselves with one channel at a time and used an
adjustable spacing scheme to accommodate all 12 channels. Extensive
guying and bracing was required to withstand wind loads. My
guess is this "Supermount" never made it to the production phase.
Stacking Four Z-Matched Yagis
By Harry Greenberg* and Harold Harris†
A new mechanical and electrical system incorporating a simple
mounting arrangement and a new impedance matching harness.
Fig. 1 - The new Channel Master "Supermount"
antenna stacked horizontally. The unit features adjustable structure.
A practical system for stacking four Yagis by extending the
principle of impedance matching which led to the development
of the "Z-Match" Yagi system has been developed by Channel Master
Laboratories. The resulting gains of over 14 db are the highest
yet attained in a practical television antenna.
The problem was of a dual nature because the mechanical problems
were as formidable as the electrical ones. For instance, if
four-bay, half-wave vertical stacking were attempted on Channel
2, the antenna array would require about 25 feet of unguyed
mast. On the other hand, on the high band, the length of masting
required for four bays is about 10 feet, which is a practical
dimension. Therefore, a separate mechanical approach was required
on each band, although the electrical problem of phasing and
matching was the same. On the low band, the four-bay array was
constructed by arranging two stacked Yagis side by side. This
arrangement was made practical by a new adjustable structure
made of heavy welded tubing (Fig. 1). This large mounting framework
was trade named the "Supermount,"
The adjustable cross boom of the"Supermount" provided for
full-wave horizontal spacing between the two stacked arrays.
Since full-wave spacing was utilized, the distance between
the two stacked arrays ran up to 17 feet on Channel 2. The entire
structure was supported at the center. This meant that tremendous
twisting torques were developed under brisk winds. This torsion
was excessive on most commercial television towers because horizontal
stacking narrows the directivity of the antenna and makes it
more sensitive to twisting. The twisting problem was solved
by guying the array at its outer extremities under each stacked
Yagi. Guying at these points also kept the guy wires out of
the field of the antenna. These guy wires, in turn, created
a large downward thrust on the extended booms and this, in turn,
was offset by diagonal braces.
Since all of these operations had to be accomplished while
assembling the entire array from the top of a tower, the entire
structure was designed in two parts so that the spacing between
stacked arrays could be adjusted for each channel and so that
the diagonal brace could be pivoted to fasten with a universal
clamp at any point of the tower. The cross booms are held in
a section of tubing of larger diameter so that the first Yagi
of each pair can be put on, the connecting rods attached, and
then swung into the upper position by rotating the boom. Then
the lower Yagi is put on. The identical process is repeated
for the other pair of Yagis. Before discussing the electrical
considerations in phasing and matching four Yagis, it is interesting
to note that the "Supermount" can be used for mounting stacked
Yagis of two different channels even though they require separate
Fig. 2 - Operating principle of the four
Z-matched Yagi antennas.
In the article, "The Yagi Antenna" published in Radio &
Television News, October 1951, the problem of matching a two-bay
Yagi to 300 ohm line was discussed. A system, subsequently named
the "Z-Match" system, was described. In this system, the single
Yagi uses a three-conductor fold and by wider spaced elements
is designed to accurately match 300 ohm line. The impedance
is dropped to 200 ohms by taking out the center bar in the folded
dipole. The center bars from the two Yagis are then used as
the linear matching transformers and they step the 200 ohm impedance
up to 600 ohms. The two 600 ohm impedances in parallel total
300 ohms and match the line accurately. It is two arrays of
this description which are to be stacked in four bays. From
the following, it will be evident that an accurate two-bay impedance
of 300 ohms is necessary.
Fig. 3 - The "Supermount' antenna stacked
vertically. Because the unit is adjustable. several arrangements
In the case of the high-band Yagi is, the four arrays were
stacked vertically with half-wave spacing between each bay (Fig.
3). On the low band, the two half-wave stacked arrays were spaced
a full wave apart. Since these dimensions made the use of a
quarter-wave transformer impossible, and since half wavelengths
of line do not transform impedances, two 3/4 wavelengths of
line were used. This length has the same impedance transforming
properties as a quarter-wave line. That is, the matching impedance
is equal to the square root of the input impedance multiplied
by the output impedance or
Therefore, the problem resolved itself into the following considerations.
We knew that in order for the feed point of the entire four-bay
array to match 300 ohm line, each two-bay antenna had to present
an impedance of 600 ohms. These two 600 ohm impedances in parallel
equaled 300 ohms which was the required impedance. We also knew
that each two bay "Z-Match" array had an impedance of 300 ohms.
The problem then was to transform the 300 impedance of each
two-bay Yagi to 600 ohms through the 3/4, wave transformer.
Substituting in the formula
get the following:
or Zm = 425 ohms. In other words, a 3/4 wave line
having a characteristic impedance of 425 ohms will tie the two
stacked Yagis into one four-bay array with all impedances matched
to 300 ohms.
The entire system is shown schematically (Fig. 2). Electrically,
the system is the same whether the stacking is vertical or horizontal.
Special 425 ohm harnesses were developed for each band. On the
high band, a self-supporting open-wire system was used. On the
low band, a special wide-spaced 425 ohm ribbon type transmission
line was developed because ordinary open-wire line was difficult
to support on the boom structure.
* Chief Electronic Engineer, Channel Master Corp.
Vice-president, Sales & Engineering, Channel Master Corp.
Posted December 30, 2015