If i mount my vertical 10 m 4 element yagi directly on a metal pole so that the pole is between the elements and so messes up the radiation pattern and input impedance, if i put lots of type 43 material ferrite rings around the pole will that 'hide' the pole from the antenna in an RF sense ?
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2 Answers
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W8JI's article on Detuning Towers shows how to add a resonated loop to a conductive structure in order to make it "vanish" electrically:
When the capacitor is tuned to resonate at the frequency of interest, the loop comprising sections A and B carries maximum current, analogous to a tank circuit. Because the currents in A and B are equal but flow in opposite directions, the radiation from the two sections cancels. And, just like a tank circuit, the resonated A-B loop acts as a high impedance that breaks up the continuous conductor comprising sections C, A and D into smaller conductors section C and section D. By properly positioning, sizing and tuning the loop, you can use your vertically-oriented antenna on a conductive vertical support.
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For the whole pole they will be too heavy.
I made lots of LPDAs with feed cables in the middle. They had about 50 beads on the cables, each of them about 35x30 mm, with a 12 mm hole. This sort of bead has about 100 Ohm impedance, at HF you might get a bit more and so need fewer beads.
For a 50 mm pipe an equivalent bead would have a 120 mm OD, 150 mm long, and weigh several kg. You would need many of them.
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$\begingroup$ But ignoring the weight would it work ? $\endgroup$– AndrewCommented Oct 11, 2019 at 8:54
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$\begingroup$ Yes certainly. Big cores like that won't be cheap either. You would be a lot better off using smaller ferries on the legs of a lattice structure, rather than a solid pole. But the best, cheapest, lightest, is to use a fibreglass pipe and beads on the coax only. $\endgroup$– tomnexusCommented Oct 11, 2019 at 10:45