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1-7 of 7 messages
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RE: end fed dipole
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by NZ5L on February 10, 2003
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There was an article on this in an issue of CQ Magazine last year - I don't remember which, but it was in the "Antennas" column. Perhaps your library keeps the back issues? (If I find it, I'll let you know).
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RE: end fed dipole
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by K5LXP on February 10, 2003
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The next question is, "why"? The impedance would be very high, you wouldn't feed this with a piece of coax. What's the goal?
Mark K5LXP
Albuquerque, NM
k5lxp@arrl.net
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RE: end fed dipole
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by WB2WIK on February 10, 2003
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An end-fed 1/2-wave antenna is usually just referred to as an end-fed wire, not a dipole. Dipole implies two identical elements fed at their joining point, which would be the center.
However, it is possible to build a 1/2-wave dipole that is end-fed. Normally, the way to do this is to make a "coaxial" antenna, where you simply expose the center conductor and dielectric (unshielded) for 1/4-wavelength, then pull back the braid over itself for 1/4-wavelength. At 146 MHz, this is very easy to do and is a popular vertical antenna design. At 40 meters, it is a lot more difficult, but possible to accomplish.
Now, what you have is a feedline that is already directly connected to one end of a center-fed dipole, with the dipole radiating elements made of (1) the cable's center conductor and (2) the cable's outer conductor. The coaxial feedline runs up inside the cable's outer conductor radiating element for the last 1/4-wavelength.
It works, and it works just like a center-fed dipole does. It's just fed at one end. It still has the standard 1/2-wave dipole feedpoint impedance, in the 50 Ohm range (thereabouts).
I've used this most successfully with an air-wound RF Choke type balun employed at the point where the coax feedline meets the open end of the folded-back braid radiator. For that, I usually use 6-9 turns of the coax itself wound as a solenoid choke with about a 4" diameter winding dimension, and the turns of coax taped tightly to hold it in the form of a coil.
WB2WIK/6
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RE: end fed dipole
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by AC5E on February 10, 2003
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Steve's Coaxial Dipole is easy to build and works well. But end feeding a half wave section of wire does not.
Without getting into technicalities, a quarter wave section of wire appears to be a series tuned circuit followed by the radiation resistance of the wire. That keeps the feedpoint impedance of the wire within the range of practical feedlines.
But a half wave radiator appears to be a parallel tuned circuit feeding the radiation resistance of the wire. So the feedpoint impedance is several thousand ohms. The lowest I know of was more than 3,000 ohms, the highest more than 40,000 ohms. That is much higher than the impedance of any practical transmission line. So you have a horrendous feedpoint mismatch, high VSWR, and extreme feedline losses. EVEN with open wire line.
Now, you can match the monster at the feedpoint. A tuned autotransformer arrangement works pretty well.
The last one I built for 40 Meters used 9 turns of quarter inch copper tubing, a 150 pf 10 KV transmitting variable, bottom end to ground, top to the radiator, and fed at about 2/3rd turn from the bottom. It worked quite well, but the cap would arc at around 500 watts. And it was a pain walking out in the weather to retune when we wanted to go from 7.050 to 7.200.
73 Pete Allen AC5E
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RE: end fed dipole
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by WB6BYU on February 10, 2003
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I've done this a number of times. It is much easier
if you are feeding it against ground instead of against
free space.
That is not to say that any of these methods are
optimum or ideal. You sometimes have to deal with RF
in the shack or similar effects of RF flowing on the
outside of the coax. But all have been used to load
up such antennas and make contacts.
1) parallel-tuned circuit at the base, as Pete
discribed. If you don't have a handy ground, you
likely will get current flowing back down the outside
of the coax - ferrite beads can help. Since I don't
run more than 50 to 100 watts, I've never had the
capacitors arc.
2) L-network, usually a series coil with parallel
capacitor on the antenna end. Again, it helps to have
some sort of ground or radials to give any remaining
RF somewhere to go besides down the coax braid. While
the parts for the parallel tuned circuit are less
critical (they just need to be able to be tuned to
resonance), both values of the L network should be a
specific value to match the load. In my experinece,
I have found that the parallel tuned network covers
a wider range of frequencies when the capacitor is
adjusted, but the L gives a wider bandwidth at a single
setting.
3) Zepp feed. The traditional method was to use a
quarter wave of open wire line with the antenna hooked
to ONE conductor at the open end. This gave a low
impedance in the shack that was suitable for link
coupling. You can also just run whatever length you
need to a tuner, though the load may be radical enough
to have high losses in the common "200pf" tuners with
a ferrite balun. (If you use a quarter wave shorted
stub and tap the feedline onto it, you get the
traditional J-pole.
Again, none of these are ideal. But each has been
used by probably thousands of hams at various times.
Your mileage may vary.
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RE: end fed dipole
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by NZ5L on February 17, 2003
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Check out Dave Ingram's (K4TWJ) article on p. 74 of January, 2002 CQ Magazine on end-fed dipoles. The idea is feasible, but you will need a total of 3/4 wavelength linear space.
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