What happens to a vertical Yagi when you mount it on a vertical metal pole?

Question

How much difference does a metal pole make when it's sitting in the middle of all the lower elements of my beautiful new highly optimized to the max YO.EXE 10 m yagi ?

I get it that the closeness of the pole will mess up the impedance a bit.

But the elements are still there in the right place with the right length constructively adding to and destructively subtracting from the incoming signals (for example) and after a few tests it seems that a metal pole makes less difference that you might think.

Does it just affect the radiation pattern in the elevation plane ? Can anyone tell me if actually a metal pole which only goes up to the boom lowers the angle of radiation .. imagine that ! a rare case of owning an antenna cake and eating it at the same time :o

Answer 1

Mounting a vertically-polarized Yagi on a vertical conductive mast usually screws up the pattern. You either have to use a non-conductive mast, or offset them from it with a horizontal boom.

40 years ago, Cushcraft sold an 11 element beam designed for 146 MHz. Not realizing this, I (and countless other hams) mounted it on top of a steel mast. That caused so many extra lobes that it made a better omnidirectional antenna than my ground plane! It still had a useful forward lobe, but those that mounted it as described above had a much cleaner pattern.

Answer 2

You definitely don't want to mount a long metal pole through a Yagi. My rule of thumb is that it at best cuts off the Yagi at that point, perhaps worse.

You have three options to avoid this.

1. Mount the Yagi at its back, perhaps supporting the front with a non-conductive rope. The mast can get quite close to the reflector, 1/8 wave is ok.

2. Offset it from the mast with a bracket that's perpendicular to the plane of the Yagi. Exact length required is hard to say, but at least half a wavelength, maybe more. The Yagi is a slow-wave structure, its capture area at the front is large, perhaps 1 wavelength square for a 10 dBi antenna. Halfway along the boom the sensitive region is smaller.

3. Use a non-conductive pole. I used many filament wound fiberglass poles for this purpose. Again, you probably want a half-wavelength clearance from the boom, but a third of a wavelength will work. The coax of course can't go in the tube, so it has to hang down from the very back, somewhat behind the director.

It is possible to put the coax in the pole if it's adequately loaded with ferrites. At 30 MHz I'd use a string of about 50 beads, 1.5" long, 1/2" dia, 1/4" hole, made of material 31, big enough for RG223 or RG58 to fit. You could also use three or four larger cores of 61 material, 1 m apart, wound with 6 turns of coax.

Finally, as you're only looking for a narrow band, you might be able to design some sort of quarter wave trap, several of them in series, on the coax or even on the metal pole. This would need to be carefully designed in simulation, and I've never tried it in this application.

Answer 3

The best way of understanding what happens is to think in terms of currents on the mast as well as on the boom tube. The impedance on the boom tube varies along the tube. So does the impedance of the vertical mast. If both are low you can get high currents and an associated high amount of radiation from the mast as well as from the boom tube. Isolating the boom tube from the mast with a low capacitance while having a good current balun (ferrite cores) will make the current on the boom tube very low, but the free mast tube may pick up a large current because it can couple efficiently to the strong vertical electric field of the yagi. The best solution I know of is to put a half-wave horizontal tube at the point where the mast tube connects to the boom tube. That tube would create a low impedance point on the boom where it connects to the vertical mast. Then, 0.75 wl vertically down the mast, place another 0.25 wl long tube that is parallel to the yagi boom tube. That would create another low impedance point on the vertical mast. As a consequence the impedance at the top of the mast tube will be very high and the mis-match to the boom tube will be maximized. The current on the vertical mast would be very low so there would not be any radiation and no interaction with the yagi. For long yagis on 144 MHz I have seen degradations in the order of a few tenths of a dB. These things can be simulated. The elements of the yagi can be in metallic contact with the boom tube or they can be isolated. That may have a big influence on the impedances at various places along the boom tube. With this kind of solution there is no need for ferrites, just run the cable tight to the boom and mast tubes or maybe even inside. These things can be simulated!!