On 1/17/12 10:31 AM, Ken wrote:
> On Jan 17, 2012, at 10:52 AM, Paul Christensen wrote:
>> With top loading for verticals, or end-loading for dipoles, super
>> high efficiency can be maintained. I would start by reading
>> Sevick's work as a general analysis of shortened antennas, and also
>> study how and why a radiator's feedpoint Z, I, and E changes with
>> radiator length.
> THERE I agree with you because the high current portion (e.g. center
> portion of a dipole, or the "effective" quarter wave portion of a
> vertical) is used as a radiator. End loading works well because the
> high current section of a half wave is still the radiator), but just
> using the ends of a dipole (e.g. center loading,etc.) is not nearly
> as effective.
> Shortening a half wave dipole, without end loading, significantly
> reduces it's efficiency.
Not exactly. What end loading does is make the feedpoint impedance
closer to your desired resistive value.
Think of this... a 1/4 wavelength long dipole is going to look very much
like a capacitor. If you were matching at the feed, you'd add an
inductor. Now, say I wanted to avoid an inductor, and instead, what I
used was a capacitor at the end of 1/4wavelength line (so the line
transforms the C into L). That would work about the same (ignoring losses).
Well, I could just as easily hang that capacitor at the end of the
antenna element, which just so happens to be 1/4 wavelength long.
There's a whole lot of handwaving and there's some technically incorrect
stuff I just did (end loading isn't really a C at the end of a 1/4 wave
line).. but the idea is sound... leaving aside the change in current
distribution along the radiator, how you get the feedpoint Z to be
reasonable can be done a lot of ways.
You can also make a folded dipole, which transforms a 75 ohm dipole to a
300 ohm one. Basically, the two wires in the dipole form a transformer,
and one that hopefully has cost, loss and structural benefits over an
alternative scheme of a transformer with 1:2 turns ratio at the feedpoint.
> A 70' dipole used on 80m without end loading is much less efficient
> of a radiator than the same antenna with end loading. And the
> simplest way to end load such an antenna is to add about 30' of wire
> on each end ;-)
> Ken WA8JXM _______________________________________________
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