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Broadbanding w/ Folded Monopole

 
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matsmyth
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Joined: 09 Jul 2015
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PostPosted: Thu Jul 09, 2015 1:59 pm    Post subject: Broadbanding w/ Folded Monopole Reply with quote

Hi All,

I am trying to achieve wide-band performance of a vertical folded monopole at VHF (less than 2:1 VSWR @ 130-170MHz). Supposedly, without the need for an additional matching network, it is possible to achieve good results simply through the geometry of the antenna. I have tried many possible combinations of element diameters, spacing, lengths, and types of connections (e.g. putting an inductor between the elements). Theoretically, there is a way to adjust the antenna mode and transmission line mode of the antenna individually, so that their reactances overlap and cancel. Looking at all of my reactance curves, this has been far from the case for me. I achieved a slight improvement by top loading the antenna elements with small wires, but nothing significant.

Below resonance, the reactance is capactive, and above it is inductive. I'd love to hear any ideas about to achieve reactance cancellation over the entire band.

Thanks in advance.
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curlew
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Joined: 12 Jul 2015
Posts: 2

PostPosted: Mon Jul 13, 2015 8:58 pm    Post subject: Re: Broadbanding w/ Folded Monopole Reply with quote

matsmyth wrote:
Hi All,

I am trying to achieve wide-band performance of a vertical folded monopole at VHF (less than 2:1 VSWR @ 130-170MHz). Supposedly, without the need for an additional matching network, it is possible to achieve good results simply through the geometry of the antenna. I have tried many possible combinations of element diameters, spacing, lengths, and types of connections (e.g. putting an inductor between the elements). Theoretically, there is a way to adjust the antenna mode and transmission line mode of the antenna individually, so that their reactances overlap and cancel. Looking at all of my reactance curves, this has been far from the case for me. I achieved a slight improvement by top loading the antenna elements with small wires, but nothing significant.

Below resonance, the reactance is capactive, and above it is inductive. I'd love to hear any ideas about to achieve reactance cancellation over the entire band.

Thanks in advance.

The basic antenna has a feedpoint impedance of 140 ohms, so it is miles out from what you want. I think a fat (or flat!) monopole would be a better starting point.
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