Posted by
Don Wilhelm-4 on
Dec 21, 2010; 1:27am
URL: http://elecraft.85.s1.nabble.com/Inverted-L-was-OT-Vertical-antenna-tp5852957p5854481.html
Dave,
If you read the radial tuning procedure in Low-Band DXing, you will find
that it "all comes out in the wash". Yes, your statements are correct,
and in fact that is how I did mine. Although I did not resonate my
vertical monopole against any ground plane - I simply cut the lengths to
what was stated in the article, and then cut each radial to resonate
with that particular length of the "monopole". If the result was
slightly "off-center fed", so be it, the impedance and the resonance
point obtained were a good match for 50 ohm coax, and I left it at that.
OK, so the ideal impedance of a vertical antenna is 32 ohms - I got
closer to 50 ohms which indicates a 15 ohm loss in signal efficiency.
It matches my feedline nicely, and I an willing to accept the the 3%
loss in efficiency that represents.
The ground conductivity in my area is not the greatest, so I have
accepted the logical consequences of that fact.
73,
Don W3FPR
On 12/20/2010 8:06 PM, David Gilbert wrote:
> "I personally would not consider elevated radials that are non-resonant"
>
>
> I guess I'm having a difficult time with that comment. You can have a
> resonant system without the elevated radials themselves being
> "resonant" ... i.e, any two of them acting like a resonant dipole. If
> the elevated radials are shorter than a 1/4 wavelength, all that is
> required is that the vertical section be a little longer than a 1/4
> wavelength to compensate. If the radials are longer than a 1/4
> wavelength, the vertical section needs to be shorter than a 1/4
> wavelength for resonance.
>
> Elevated radials are kind of like the lower half of a vertical 1/2
> wavelength dipole except that it is "fanned out" for symmetry, and if
> the radials are longer or shorter than half of a 1/2 wavelength dipole
> the feedpoint simply behaves similarly to an off-center fed dipole. You
> can prove this to yourself with EZNEC ... start with a vertical section
> longer than 1/4 wavelength and see what length radials you need to have
> a resonant feedpoint. Then do the same thing with different lengths of
> vertical sections and see the effect on radiation pattern. I can tell
> you that it isn't a direct function of radial resonance.
>
> I could be wrong, but I don't see any need at all for the radials to be
> "resonant" on their own. You want system resonance and generally
> speaking, for radiation effectiveness you want as long a vertical
> section as you can manage. You might want to choose a vertical section
> somewhat longer than a 1/4 wavelength and radials correspondingly
> shorter than a 1/4 wavelength in order to take advantage of that
> "off-center feed" aspect I mentioned in order to get a 50 ohm match.
>
> The only situation where resonant radials might be required is if you're
> trying to decouple whatever is on the other side of the radials. In
> that case, the radials act like a choke to keep current on the feedpoint
> side of the resonant radials. You can see that effect with EZNEC as well.
>
> 73,
> Dave AB7E
>
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