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A while a go i asked a couple of questions on this site about mounting a vertical yagi antenna on a metal pole. One of the answers given to the first question talks about using decoupling stubs to effectively remove RF current from a metal mast.

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

Can I use a 1/4 wave decoupling stub to enable mounting of a vertical yagi on a metal pole?

The answer says to place a half wave horizontal element at the junction between the boom and the mast, then place a quarter wavelength horizontal element down 3/4 of a wavelength along the length of the mast and parallel with the boom. This apparently creates a high impedance at the top of the mast and stops current flow down the mast.

That answer is confusing because when i think about a half wave dipole, the center is a low impedance, which is opposite to what is being suggested.

Can anyone give me some more information on this ? I'm not asking about all the other methods you can use to mount a vertical yagi, i'm asking specifically about using decoupling stubs in this manner to allow the use of a metal pole such that the radiation pattern of the antenna won't be affected by the mast.

I have looked everywhere and cannot find any information on this subject, any help would be very much appreciated.

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A half wave horizontal element at the junction between the boom and the mast with a good electrical contact between the midpoint of the horizontal dipole and the mast will create a low impedance point on the mast.

Then place another half wave element down 3/4 of a wavelength along the length of the mast. You may even place two of them, one parallel to the boom and one perpendicular like the ground plane of a GP antenna. It is essential that the midpoint(s) makes good contact with the mast.

You would have two low impedance points separated by 3/4 of a wavelength. This way you tune the mast to resonance with a very high impedance between the two low impedance points.

One low impedance point becomes a high impedance point as seen from a distance of 1/4+N*1/2 wave lengths.

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  • $\begingroup$ the mast i'm using isn't log enough for 3/4 wave length, If i put half wave elements 1/4 wave length from the dipole then will this work also ? except that the decoupling elements will be close to the vertical dipole ? $\endgroup$
    – Andrew
    Oct 27 at 5:14
  • $\begingroup$ I think 1/4 wave length will work fine, but I am not sure. This can be simulated with NEC. An alternative is to blace a choke 1/2 wavelength down the mast. The choke could be a 1/4 wave sleeve balun. The open end 1/2 wavelength from the boom tube. The sleeve balun could be a couple of rods parallel to the mast and connected to the mast 1/4 wavelength from the open end. In a sleeve balun there is no current flowing around the tube so it does not have to be continous. $\endgroup$
    – sm5bsz
    Oct 27 at 17:14

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