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Stacking Four Z-Matched Yagis
April 1952 Radio & Television News Article

April 1952 Radio & Television News
April 1952 Radio & Television News Cover - RF Cafe[Table of Contents]

These articles are scanned and OCRed from old editions of the Radio & Television News magazine. Here is a list of the Radio & Television News articles I have already posted. All copyrights are hereby acknowledged.

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 Channel 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.

Channel Master "Supermount" antenna stacked horizontally - RF Cafe

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 orientation.

Operating principle of the four Z-matched Yagi antennas - RF Cafe

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.

"Supermount' antenna stacked vertically - RF Cafe

Fig. 3 - The "Supermount' antenna stacked vertically. Because the unit is adjustable. several arrangements are possible.

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 Impedance formula (1) - RF Cafe.

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 Impedance formula (2) - RF Cafe we get the following: Impedance formula (3) - RF Cafe, 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