Thread: Antenna design
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Old 23-Sep-2015, 6:18 PM   #7
Retired A/V Tech
Join Date: Aug 2012
Location: S.E. VA
Posts: 2,538
The yagi antenna has a bandwidth that is much less than a bowtie antenna, because the yagi is a high "Q" antenna as compared to the bowtie antenna which has a low "Q," as defined by the max allowable SWR over the required bandwidth. The bowtie antenna is a member of the collinear array family, which you will read about in the ARRL Antenna Book.

Here you see the collinear array with straight wire elements. The >< shape "whiskers" make the bowtie antenna even more broadbanded.

When designing a Yagi the element length and diameter must be carefully controlled for max gain at the desired design frequency. Each element must be resonant at a certain frequency. The director elements are shorter than the driven element, and the reflector element is longer. Larger diameter elements must be shorter for the same resonant frequency, and larger diameter elements make the yagi more broadbanded to cover a wider frequency range.

Computer modeling will give you the optimum element length, diameter, and element spacing for a yagi; much better than trial-and-error.
Please comment on a hypothetical antenna setup that may tie in my questions ( or maybe not)
Is there a hypothetical tvfool report for your hypothetical antenna?

A multi-folded dipole is configured with a horizontal length corresponding to the current hi-VHF director length that then drops vertically at 90 degrees for a 3 inch distance similar to "closing the gap" on 8 bay mods for UHF. A one inch gap is at each fold of the dipole. The array ends in a standard balun with a contiguous multifolded dipole with no phasing.
I'm having a very hard time picturing your hypothetical antenna in my mind. Can you show us a sketch or diagram? Is this a yagi or bowtie antenna? What is a multi-folded dipole? Something like this?

Can a dipole be folded infinitely (within reason) , optimize surface area for signal capture and gain, and be effective for both UHF and VHF?
No, when folded you reduce capture area, reduce the sections that are horizontal to match polarization, reduce gain and are less likely to work on VHF and UHF, if I understand your question.

Are you thinking of a fractal antenna?

or are you thinking of one antenna for UHF and VHF?
Attached Images
File Type: jpg 4BayCurrentFlow.jpg (42.8 KB, 8208 views)
File Type: jpg FD Ant Folded Ends.JPG (40.0 KB, 3985 views)
If you can not measure it, you can not improve it.
Lord Kelvin, 1883

Last edited by rabbit73; 23-Sep-2015 at 7:18 PM.
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