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 Capacitance to earth of a FAN dipole. 
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Post Capacitance to earth of a FAN dipole.
When using a fan dipole for the 80m, 40m and 20m bands the aerial wire, not in use, will have "distributed element" capacitance to earth and the other wires. While not at resonance, do these wires have a detrimental effect ? Apologies if this question has already been asked/answered. Cliff.


Wed Aug 29, 2012 6:28 pm
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 Capacitance to earth of a FAN dipole. 
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Post Re: Capacitance to earth of a FAN dipole.
I'm not sure what "distributed element capacitance" is, but the answer to your question is "No"; the existence of the unused dipoles doesn't have any detrimental effect.

Did you have something in mind?

Steve G3TXQ


Wed Aug 29, 2012 7:02 pm
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 Capacitance to earth of a FAN dipole. 
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Post Re: Capacitance to earth of a FAN dipole.
Hello Steve, distributed elements as in lumped components, a way to explain the spread of C and L along an Ae or transmission line.
Anything in mind ? Yes, it's that low 80m dipole again. It's part of a fan arangement and I'm consious of the C to earth as it's low. And would it work better without the other wires. Cliff.


Wed Aug 29, 2012 8:02 pm
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 Capacitance to earth of a FAN dipole. 
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Post Re: Capacitance to earth of a FAN dipole.
Try working out the capacitance of a wire with such a small diameter, spaced so far from the ground plane; you'll see it is tiny.

But that isn't really the issue: the "unused" dipoles can only have an effect if they flow significant current compared to the "in-use" dipole. Compare their feedpoint impedances and I think you'll find those currents are small.

Or look at it another way: if you took a low 80m monoband dipole and increased its wire diameter - thereby increasing this "distributed capacitance to ground" - do you think you it would be more efficient, less efficient, or the same?

73,
Steve G3TXQ


Wed Aug 29, 2012 8:12 pm
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 Capacitance to earth of a FAN dipole. 
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Post Re: Capacitance to earth of a FAN dipole.
What about interaction from being close to something else metal?

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Wed Aug 29, 2012 8:44 pm
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 Capacitance to earth of a FAN dipole. 
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Post Re: Capacitance to earth of a FAN dipole.
The question was whether the "unused" elements of a fan dipole can have a negative effect on the overall performance of the antenna because of capacitive effects - I can't visualise a realistic scenario where that would be the case.

I don't see how nearby metallic objects would have an effect, via the unused elements, that wouldn't exist anyway directly on the "in-use" element. Of course, if you have metal structures that are close to the "unused" elements but distant from the "in-use" element, there might be an effect, but that doesn't sound very realistic for a typical fan dipole.

73,
Steve G3TXQ


Wed Aug 29, 2012 8:59 pm
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Post Re: Capacitance to earth of a FAN dipole.
G3TXQ wrote:
The question was whether the "unused" elements of a fan dipole can have a negative effect on the overall performance of the antenna because of capacitive effects - I can't visualise a realistic scenario where that would be the case.

I don't see how nearby metallic objects would have an effect, via the unused elements, that wouldn't exist anyway directly on the "in-use" element. Of course, if you have metal structures that are close to the "unused" elements but distant from the "in-use" element, there might be an effect, but that doesn't sound very realistic for a typical fan dipole.

73,
Steve G3TXQ


I meant if the interaction from being so close to the other elements on the basis they're also metal has a baring on the 80m dipole length as opposed to an 80m dipole free from such close proximity?

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Wed Aug 29, 2012 9:06 pm
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 Capacitance to earth of a FAN dipole. 
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Post Re: Capacitance to earth of a FAN dipole.
Sure, the presence of the "unused" elements has an effect on the tuning of the "in-use" element, but I wouldn't categorize that as a detrimental effect.

Only if you start postulating extreme geometries do you begin to see detrimental effects: for example an 80m element at 20ft connected in parallel with a 40m element strung below it much closer to ground may well show increased loss over the 80m element on its own. For more-typical situations where the wire spacing is small compared to the distance from ground there is negligible detrimental effect.

73,
Steve G3TXQ


Wed Aug 29, 2012 9:43 pm
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 Capacitance to earth of a FAN dipole. 
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Post Re: Capacitance to earth of a FAN dipole.
G3TXQ wrote:
Sure, the presence of the "unused" elements has an effect on the tuning of the "in-use" element, but I wouldn't categorize that as a detrimental effect.

Only if you start postulating extreme geometries do you begin to see detrimental effects: for example an 80m element at 20ft connected in parallel with a 40m element strung below it much closer to ground may well show increased loss over the 80m element on its own. For more-typical situations where the wire spacing is small compared to the distance from ground there is negligible detrimental effect.

73,
Steve G3TXQ


Does the proximity to the other metal objects on the same plane (as opposed to dipole elements crossing @ 90 degrees over an antenna boom) have no effect on the radiation pattern?

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Wed Aug 29, 2012 10:03 pm
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 Capacitance to earth of a FAN dipole. 
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Post Re: Capacitance to earth of a FAN dipole.
ei9ju wrote:
Does the proximity to the other metal objects on the same plane (as opposed to dipole elements crossing @ 90 degrees over an antenna boom) have no effect on the radiation pattern?


If you mean the other fan dipole elements, the answer is "no"; well not of any significance.

Picture an 80m dipole in parallel with a 40m dipole, operated on 80m. Because of the low feedpoint impedance of the 80m dipole and the high feedpoint impedance of the 40m dipole, about 26dB less current flows into the 40m dipole. The effect of that small in-phase current flowing an inch or so away from the main current in the 80m dipole is to make the overall dipole look slightly "fatter" across its inner half. That might have a tiny effect on the dipole's Figure-8 shape, but I can't detect it on an EZNEC simulation - the broadside gain difference must be less than 0.01dB.

73,
Steve G3TXQ


Thu Aug 30, 2012 7:39 am
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 Capacitance to earth of a FAN dipole. 
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Post Re: Capacitance to earth of a FAN dipole.
G3TXQ wrote:
ei9ju wrote:
Does the proximity to the other metal objects on the same plane (as opposed to dipole elements crossing @ 90 degrees over an antenna boom) have no effect on the radiation pattern?


If you mean the other fan dipole elements, the answer is "no"; well not of any significance.

Picture an 80m dipole in parallel with a 40m dipole, operated on 80m. Because of the low feedpoint impedance of the 80m dipole and the high feedpoint impedance of the 40m dipole, about 26dB less current flows into the 40m dipole. The effect of that small in-phase current flowing an inch or so away from the main current in the 80m dipole is to make the overall dipole look slightly "fatter" across its inner half. That might have a tiny effect on the dipole's Figure-8 shape, but I can't detect it on an EZNEC simulation - the broadside gain difference must be less than 0.01dB.

73,
Steve G3TXQ


Cheers Steve.
I'm guessing if modeled against a fanned array of multiple elements the effect will become slightly more exaggerated but still not enough for concern?
(Hope you don't mind the list of questions but if you don't ask you don't learn ;) )

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Thu Aug 30, 2012 10:40 am
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Post Re: Capacitance to earth of a FAN dipole.
ei9ju wrote:
I'm guessing if modeled against a fanned array of multiple elements the effect will become slightly more exaggerated but still not enough for concern?

It's interesting! I tried a "fan" of 80m, 40m, 20m and 10m dipoles:

On 80m the broadside gain is still within 0.01dB of a monoband dipole; looking at the current distributions in all the elements, you effectively have an 80m dipole of stepped diameter - fatter at the centre, and stepped tapering towards the tips. So it behaves pretty much like a simple dipole.

On 10m there begins to be a more significant fraction of the current in the other "out-of-use" dipoles, particularly the 80m dipole; that's not because of capacitance effects - it's because the impedance of the 80m dipole is not so very high on 10m. That small current flowing in the 80m dipole makes the 10m dipole appear longer, and in turn that slightly increases the broadside gain - perhaps by up to 0.5dB.

Intermediate bands give results between those two bands.

Of course, none of this would be noticeable in operational use, but it's interesting nevertheless.

ei9ju wrote:
(Hope you don't mind the list of questions but if you don't ask you don't learn ;) )

No problem - I probably learn more researching the answers than the folk asking the questions ;)

73,
Steve G3TXQ


Thu Aug 30, 2012 12:57 pm
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 Capacitance to earth of a FAN dipole. 
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Post Re: Capacitance to earth of a FAN dipole.
Thank you for all of the above.
The effect of distributed C to earth from the unused wires being reduced by the non-resonant feed impedance, great answer. And stepped diameter effect, also understood. '9JU's idea of in-use wire inducing power to un-used wire as reflector/director, interesting but I see the phasing would be wrong. So, also understood. I feel more content with my Ae now. Friend called round earlier and we found a tree on higher ground hidded by a dense bush. So should be able to raise that end soon. Thanks again. Cliff.


Thu Aug 30, 2012 2:39 pm
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