I am considering improving my 2m vertical dipole by replacing the top section of mounting pole with non-conducting tube. Will this have a positive effect on gain? I am theorising that the downward element of the dipole propagation will be affected by the present aluminium pole which is 30cm away from it - at the very least the mounting pole would presently appear to be blocking some 5 degrees of the compass. Can anyone offer their experience, and if this is a worthwhile improvement then should the insulating part of the mounting pole stretch even further down than the bottom of the lower dipole element?
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1$\begingroup$ re: blocking: no, that doesn't happen like that. "blocking" is an idea from ray optics, and that only applies to electromagnetic waves fro the case that 1. objects are much larger than wavelengths, and 2. objects do not directly couple into the emitter. And both conditions aren't fulfilled here :) So, yeah, you're right that a conductor parallel to a branch of a dipole will couple into it. Antenna designs actually exploit that – Ever seen a Yagi-Uda antenna? Exactly! These are aluminium rods, in parallel with a driving dipole. This gives the Yagi-Uda it's defining gain! So, the question is: $\endgroup$– Marcus MüllerCommented Jan 10 at 11:40
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$\begingroup$ "positive effect on gain": Gain in which direction? You have a "director" in a distance of roughly 1/7 wavelength, which, probably gives the whole antenna a gain slightly perpendicular to the plane spanned by antenna and pole. It will also detune the antenna, but that might or might not be an issue. $\endgroup$– Marcus MüllerCommented Jan 10 at 11:43
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1$\begingroup$ How is the dipole supported at the moment? Where does the cable from the dipole run? Can you draw a diagram (perhaps edit the question, use the Circuit Diagram feature) $\endgroup$– tomnexusCommented Jan 10 at 19:28
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$\begingroup$ @MarcusMüller A director that probably is not properly offset from the driven element nor the proper length. $\endgroup$– Mike WatersCommented Jan 11 at 21:27
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$\begingroup$ Marcus is absolutely correct about blocking. $\endgroup$– Mike WatersCommented Jan 11 at 21:29
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should the insulating part of the mounting pole stretch even further down than the bottom of the lower dipole element?
Yes it should.
The way it is now will affect the gain, pattern, and impedance.
Now if your bottom element is hollow and the coax runs through it so that the center conductor is connected to the top element and the shield is connected near it to the top of the bottom element (elements insulated from each other), then you have a real problem!
That is called a Coaxial Dipole.
In such a case, the mast must be a tall insulator.
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1$\begingroup$ I never thought about that, but how are such dipoles fed? $\endgroup$ Commented Jan 10 at 15:05
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1$\begingroup$ A related question: given that the stated effects exist... are they material? Is the sweet juice of efficiency gain worth the squeeze of cost and effort? $\endgroup$– webmarcCommented Jan 11 at 19:28
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$\begingroup$ @MarcusMüller In the center. The user's choice of feeder. $\endgroup$ Commented Jan 11 at 21:22
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2$\begingroup$ Re feeding: I think Marcus's point is that if there's interaction with a mast, then there's similar interaction with the feed line regardless of mast material (assuming that the feed line is run up the mast). To effectively address the putative concern, if the feed line is kept the same, the support off the mast should be extended to move the dipole away... and then the question becomes does changing the top mast material even matter? $\endgroup$– webmarcCommented Jan 11 at 21:46
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1$\begingroup$ Exactly, both your edit reminding me of the coax dipole as well as webmarc's explanation of my under-specific question hit the nail on its proverbial head! $\endgroup$ Commented Jan 11 at 21:59