Shortened w3edp
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I have gotten confused again about the w3edp antenna.
Some things say it's somewhere bewteen 84 to 89 feet long with a 17 foot counterpoise....one post said it's a total of 84 feet, and of that 17 feet are counterpoise.....the g3ycc website says 84 feet long and different lengths of counterpoise based on band and then there are folks like myself that used 85 feet with 1/4 wave counterpoises for all bands worked. I can no longer avail myself of a convenient telephone pole across the street from the front of my house....and in the back I have very little space. So, if one were to attempt to make a short w3edp, what might the length of the radiator and the counterpoise be? I remember seeing a post about various lengths that would make effective non-resonat end fed wires....but I can't find it Thanks Thom www.baltimorehon.com/ Home of the Baltimore Lexicon www.tlchost.net/hosting/ Web Hosting as low as 3.49/month _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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Hi Tom,
>From "Practical Wire Antennas", by John D Heys, B3BDQ, page 33: "The W3EDP antenna, when examined (Fig 33), looks something like an end-fed Zepp with a 68ft (20.7m) top and a 17ft (5.2m) feedline. The feeder is rather unusual in that the wires which make it up need not be parallel." I'd describe it as an 85 foot wire (a "68ft top and a 17ft feedline"), and you either use/don't use a separate 17 foot wire as a "counterpoise" (the other half of the feedline). This "counterpoise" wire is used on 40m, 20m, and 15m. It is not used on 80m and 10m. If you can give up 80m, you can make the antenna shorter. Hope this is the information you were looking for. 73, ed - k9ew On 5/18/07, Thom LaCosta <[hidden email]> wrote: > I have gotten confused again about the w3edp antenna. > > Some things say it's somewhere bewteen 84 to 89 feet long with a 17 foot > counterpoise....one post said it's a total of 84 feet, and of that 17 feet are > counterpoise.....the g3ycc website says 84 feet long and different lengths > of counterpoise based on band and then there are folks like myself that > used 85 feet with 1/4 wave counterpoises for all bands worked. > > I can no longer avail myself of a convenient telephone pole across the street > from the front of my house....and in the back I have very little space. > > So, if one were to attempt to make a short w3edp, what might the length of the > radiator and the counterpoise be? > > I remember seeing a post about various lengths that would make effective > non-resonat end fed wires....but I can't find it > > Thanks > > Thom > > www.baltimorehon.com/ Home of the Baltimore Lexicon > www.tlchost.net/hosting/ Web Hosting as low as 3.49/month > _______________________________________________ > Elecraft mailing list > Post to: [hidden email] > You must be a subscriber to post to the list. > Subscriber Info (Addr. Change, sub, unsub etc.): > http://mailman.qth.net/mailman/listinfo/elecraft > > Help: http://mailman.qth.net/subscribers.htm > Elecraft web page: http://www.elecraft.com > Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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In reply to this post by Thom LaCosta
I've looked at the W3EDP myself and have seen the conflicting numbers. When
faced with such a dilemma, it's useful to go back to the fundamentals that apply no matter whose name (or call) is attached to the antenna. The W3EDP an end-fed wire. Nothing more. That can make for a very efficient antenna. The trick is to ensure that most of the RF is dissipated in the "radiation resistance" of the radiator and not in other things. One of those 'other things' that wastes RF is in the earth connection. It's common for a Ham installation to have an earth connection in the range of several hundred ohms. That resistance is in series with the "radiation resistance" of the radiator (and the resistance of the radiator wire, which should be quite low in an antenna in the lengths we're talking about). For example, a 1/4 wave radiator will have a radiation resistance in the range of 35 ohms. Let's assume the ground connection is 300 ohms. Those two resistances are in series for the RF: 335 ohms. Let's say you're running about 13 watts output into a 1/4 wave radiator (35 ohms) against a typical ground with 300 ohms resistance. You'd find the RF current at about 0.2 Amperes. Ohm's law shows us that the ground resistance (300 ohms) will dissipate about 12 watts while the radiator gets to dissipate only 1.4 watts! Not very good efficiency: 90% of the RF is wasted in the ground connection. One solution is to improve the ground connection. The other is to raise the radiation resistance of the antenna. For example, suppose you have an end fed half wave wire. It'll show you a resistance of perhaps 3000 ohms. Now if you run about 12 watts of RF power it'll to produce 0.06 amps of RF flowing in the total 3300 ohms resistance (still assuming the 300 ohm ground resistance). Now ohm's law shows us that the antenna will radiate 10.8 watts while the ground connection will consume only 1 watt! That's an efficiency of over 90%, very spectacular in the real world of antennas! The problem is that most modern antenna tuners can't handle an impedance of 3000 ohms. One problem is the voltage. The voltage gets into the hundreds of volts RMS at QRP levels and jumps into the thousands of volts at 100 watts. It takes a huge box like the venerable E.F. Johnson "Matchboxes" or my home brew link-coupled tuner to handle such voltages. So the trick is to find a balance between the highest impedance your tuner can match while providing the best ground handy. For a given matching network the "trick" is to get as close as possible to a voltage loop (antenna 1/2, 1, 2, 3, etc wavelengths long) that will still allow your ATU to find a match. The next step is to add a decent RF ground - either a counterpoise or radial system. But the ground isn't nearly so important if your radiator is presenting a very high impedance to the ATU. The W3EDP in one person's compromise, but there's nothing "magic" about the numbers. Ron AC7AC -----Original Message----- From: [hidden email] [mailto:[hidden email]] On Behalf Of Thom LaCosta I have gotten confused again about the w3edp antenna. Some things say it's somewhere bewteen 84 to 89 feet long with a 17 foot counterpoise....one post said it's a total of 84 feet, and of that 17 feet are counterpoise.....the g3ycc website says 84 feet long and different lengths of counterpoise based on band and then there are folks like myself that used 85 feet with 1/4 wave counterpoises for all bands worked. I can no longer avail myself of a convenient telephone pole across the street from the front of my house....and in the back I have very little space. So, if one were to attempt to make a short w3edp, what might the length of the radiator and the counterpoise be? I remember seeing a post about various lengths that would make effective non-resonat end fed wires....but I can't find it Thanks Thom _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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