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The birth of the triband Yagi (1946)

It's 1946 and the war is just over. Hams are beginning to get back on the air in earnest. There is talk of a new band being authorized: 15 meters. Since rotatable beams were only used on 14 Megacycles and higher, there had never before been a reason to want a three band HF beam. In addition " ... all-band operation is not associated with the multielement rotatable beam antenna."

This clever - albeit somewhat cumbersome - triband yagi was designed by J. A . McCullough on the theory that 15 meters would eventually be authorized. This fellow was the "Mac" in EIMAC. He must have been sort of the Bob Noyce of his era.

This design uses relays to switch-in various extensions (that are folded around the boom) to make the elements resonant on either 10, 15, or 20 meters. This technique is, in a way, the opposite of the "trap" technique that later became common. Trap antennas have elements cut to a length for the lowest frequency. The traps then isolate portions of each element to make them resonant on higher frequencies. In contrast, this design has elements cut to length for the highest band - 10 meters. Then the relays switch in extensions to make them resonant on lower frequencies. The relays are in those rather large black boxes. The element extensions are made out of 1/4" copper tubing.

McCullough said he was stimulated to build this antenna after he "came into" possession of the "auto-tune" transmitter that was a hit at the Golden Gate Exposition in 1939. Wonder how he arranged that? I suspect that Eitel-McCullough (the earlier name of EIMAC) probably had a hand in building that "auto-tune" transmitter, but I don't have any actual information about that.

This shot is also interesting because we can see the tract houses in the background somewhere on the San Francisco peninsula in 1946. It's a bit more crowded around here nowadays.

Here's a shot of the antenna from the ground. The 50 foot tower was made by Bethlehem Steel and was intended to be used with a 40 foot mast sticking up in the air serving as a lightening rod around oil rigs. The boom is made out of two 20 foot long ladders. The stringers between the antenna elements are 3/4" wood. The elements are isolated from the boom by pieces of Lucite. They had trouble finding an appropriate relay type. So they made their own by modifying a Square D power control relay. They removed the stock armature and fitted a new one using polystyrene as insulating material.

Just A Little SWR Problem

The antenna presented a 35 to 1 SWR. This was not considered a big issue except for the losses in the feedline. McCullough pointed out that there was no chance of using that new "coax" stuff because the losses would be way too high. He decided instead to use tried and true open ladder line. However, in view of the high SWR, he did take some extra steps to reduce the feedline losses. He used 1/2" copper tubing for each side of the line with 4" insulators! To keep all this SWR from bothering the transmitter, he also made the feedline resonant.

Now that's a feedline!

**But it worked**
They were a little short on test setups at the time. They tuned the elements for resonance at roof top level by using a National HRO receiver to find the best rejection off the back of the beam. They quote no figures for front to back ratio but say it is "all that is claimed by the good book." They make note of a few minor lobes and admit that the forward gain is anybody's guess since "no accurate method of comparison with a half wave dipole was available." They closed by noting that "the DX capabilities are comparable to the best arrays found in this area." Loading the tower for 40 and 80 meters An advantage of this tower was that it was only 7" across at the top and 23" across at the bottom. This was slender for the time and led McCullough to use it as a vertical on 40 and 80 meters with the "loose radiators" on top serving as top loading. So this innovative setup produced one antenna system that worked on all the hams bands from 80 thru 10 meters. Not bad, eh?

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