Towertalk
[Top] [All Lists]

## Re: [TowerTalk] Dipole fed with balanced line?

 To: towertalk@contesting.com Re: [TowerTalk] Dipole fed with balanced line? Jim Lux Tue, 17 Jan 2012 11:00:14 -0800 mailto:towertalk@contesting.com>
 ```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. > > > Paul, > > 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 _______________________________________________ > > > > _______________________________________________ TowerTalk mailing > list TowerTalk@contesting.com > http://lists.contesting.com/mailman/listinfo/towertalk > _______________________________________________ _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
 Current Thread Re: [TowerTalk] Dipole fed with balanced line?, (continued) Re: [TowerTalk] Dipole fed with balanced line?, Al Kozakiewicz Re: [TowerTalk] Dipole fed with balanced line?, Steve Hunt Re: [TowerTalk] Dipole fed with balanced line?, Gene Fuller Re: [TowerTalk] Dipole fed with balanced line?, Steve Hunt Re: [TowerTalk] Dipole fed with balanced line?, Gene Fuller Re: [TowerTalk] Dipole fed with balanced line?, Jim Lux Re: [TowerTalk] Dipole fed with balanced line?, Ken Re: [TowerTalk] Dipole fed with balanced line?, Jim Lux Re: [TowerTalk] Dipole fed with balanced line?, Ken Re: [TowerTalk] Dipole fed with balanced line?, Steve Hunt Re: [TowerTalk] Dipole fed with balanced line?, Jim Lux <= Re: [TowerTalk] Dipole fed with balanced line?, Jim Miller Re: [TowerTalk] Dipole fed with balanced line?, Ken Re: [TowerTalk] Dipole fed with balanced line?, David Gilbert Re: [TowerTalk] Dipole fed with balanced line?, Eddy Swynar Re: [TowerTalk] Dipole fed with balanced line?, Mike Re: [TowerTalk] Dipole fed with balanced line?, Gene Fuller Re: [TowerTalk] Dipole fed with balanced line?, Hank Garretson Re: [TowerTalk] Dipole fed with balanced line?, Gene Fuller Re: [TowerTalk] Dipole fed with balanced line?, Geoffrey Way