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1-8 of 8 messages
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End-fed half wavelength antenna
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by AG4DG on September 23, 2004
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Many people have praised this antenna, saying that the high radiation resistance makes minimizing ground losses less important. I'm not familiar with it.
What else is needed in addition to the 1/2 wavelength of wire? How long does the radial need to be? And putting MFJ's random wire tuner at the feedpoint help match it so as to minimize the otherwise gargantuan SWR and feedline losses?
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RE: End-fed half wavelength antenna
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by N4ZOU on September 23, 2004
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With a 1/2-wave antenna you do not need a radial system under it. You're simply feeding a dipole antenna at the end and not in the middle as normal. You do need at least a ground rod for electrical safety. Feeding the dipole at the end where the impedance is very high requires an L network tuner with large high voltage/current inductor and capacitor. These are expensive! If you make the antenna element longer with an odd wavelength like 3/4-wave then the impedance will be about the same as feeding it in the middle like a normal 1/2-wave dipole. Then the L network (random wire) tuner is not pushed very hard and still no radial system will be required. The other way to feed the end-fed dipole is to connect a 1/4-wave length of ladder line to it and the 4:1 Balun in a normal antenna tuner. Only one side of the end of the ladder line is connected to the end-fed wire. This is called a ZEP antenna. If you make it for 80 meters it will work as a multi-band antenna on the higher HF bands.
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RE: End-fed half wavelength antenna
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by W8JI on September 23, 2004
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For every ampere into the antenna, an equal amount of current has to come from some sort of countepoise. Since the impedance is somewhat high, that current is small but it can cause problems.
So you DO need some sort of ground system, or the RF will go places you don't want it to go. It just doen't need to be as major as with an odd 1/4 wl end fed antenna.
If you look at:
http://www.w8ji.com/verticals_and_baluns.htm
You'll see some stuff that applies.
The MFJ ground tuner really doesn't help much. It won't turn a bad ground into a good one. It really won't do much more than a regular tuner for this particular application.
End-fed antennas work OK, but I generally try to avoid them myself. They very often (but not always) cause headaches with RF in the shack, because they require current from a ground connection to work. They also bring radiation into the shack, which may be OK at low power.
73 Tom
73 Tom
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RE: End-fed half wavelength antenna
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by K3AN on September 23, 2004
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The antenna at my last QTH was a 130 foot end-fed wire, made from the so-called "stealth" insulated antenna wire, and installed using the slingshot method. The first 40 or so feet was more-or-less vertical, working its way up into the treetops. The remaining portion was a flattop lying in (on) the treetops. I matched it with a SGC-230 that was attached to a tree trunk, about two feet off the ground. Ground was a single four foot rod, with about three feet of braid between it and the SGC. I could operate all bands 80-10, including the WARC bands. The SGC tended to "hunt" for a match on one of the WARC bands, but I don't remember which one. Overall, I was very satisfied with the antenna's performance. Try your end-fed half wave idea. I think you'll like it.
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RE: End-fed half wavelength antenna
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by AA4PB on September 23, 2004
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If you have a sufficient RF ground, an end fed 1/2 wave antenna will work exactly like a dipole - no less, no more. They can be difficult to match with the typical tuner because the impedance is so high.
You'll see a 1/2 wave end feed antenna advertised that has a built-in matching transformer and is fed with coax. They advertise no ground needed. I assure you that the coax shield is serving as the counterpoise for that antenna. This means that its effectiveness depends on how you route the coax and there is the potential for bring the RF right back into the shack.
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RE: End-fed half wavelength antenna
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by AG4DG on September 23, 2004
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How can anyone say that an end-fed antenna needs no ground plane? If operating an end-fed antenna with no ground causes the feedline to radiate (because the feedline becomes the other half of the antenna), then the end-fed antenna really does need a ground plane (to maximize ground current) and a current balun/unun (to minimize the common mode current on the coax). Or maybe I'm thinking too much about 1/4 wavelength monopoles, which I'm more familiar with.
The tuner I referred to wasn't the ground tuner but the MFJ-16010, the random wire L-network tuner. I don't think a ground tuner is really necessary when improving the ground plane and using chokes/baluns/ununs would take care of the common mode current.
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RE: End-fed half wavelength antenna
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by KT8K on September 24, 2004
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Ground current is only minimized when the wire is near a half wavelength. As far as I know, an inverted L or other long wire configuration DEFINITELY needs a ground plane (radial system) to work its best on any frequencies where it is not near a half wave, and the ground plane will help it some even when it IS a half wave.
The auto-tuner at the feedpoint makes for a great antenna, in my opinion, but with just a ground rod you will definitely not get all the performance you could -- I think K3AN would be very pleased with the improvement if he added radials. (See http://www.eham.net/articles/4148, Steve Katz' WB2WIK/6 article on the importance of radials for vertical antennas, which are end fed ...)
Good reception & 73 de kt8k - Tim
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RE: End-fed half wavelength antenna
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by WB6BYU on September 24, 2004
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I often use an end-fed half-wave wire with an "L" network
tuner and it seems to do well. Because the current is
relatively low at the feedpoint, ground resistance has
less effect on overall efficiency, which is why many
articles will say it doesn't need a ground plane.
As a practical matter I've used both a Zepp feed and
a tuner with no ground radials, and they do make contacts. That doesn't mean it is good engineering
design, or that I haven't had problems with RF in the
shack on occasion. And I haven't burned up a tuner yet,
either, but I rarely crank my rig up to the full 100 watts.
A quarter wave vertical needs a good set of ground raidals
because they have to have a low impedance to match the
antenna. A much simpler system works with a high
impedance antenna, since 50 ohms of ground resistance
will make an insignificant difference for a 2000 ohm
antenna. A ground rod, single radial wire, or just a
chunk of metal is often sufficient, and generally has
a low enough impedance to reduce the current on the
outside of the coax to a tolerable level.
I'd much rather have a beam, or a big horizontal loop.
But if that is the best you can manage in your situation,
then it usually can be made to work reasonably well.
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