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As was said previously, all end fed antennas are monopoles with displacement current flowing from the antenna element to earth. Unlike dipoles where the displacement currents flow from element to element.
So without an adequate radial system to reduce the ground loss all end Feds regardless of length are only about 10% efficient. There are good reasons why 45 years ago when coax became readily available we switched from endfed monopoles to dipoles. Ray W8LYJ Sent from my iPad ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
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Ray,
I think that is 'stretching' the facts a bit. A halfwave dipole is a halfwave dipole whether fed at the center or at an end or somewhere in between. The radiation pattern and efficiency is the same. The problem is that an end fed resonant dipole does need a little bit of a counterpoise (0.05 wavelength) in order to be able to feed it properly. 73, Don W3FPR On 2/11/2017 4:07 PM, Gmail wrote: > As was said previously, all end fed antennas are monopoles with displacement current flowing from the antenna element to earth. Unlike dipoles where the displacement currents flow from element to element. > So without an adequate radial system to reduce the ground loss all end Feds regardless of length are only about 10% efficient. > There are good reasons why 45 years ago when coax became readily available we switched from endfed monopoles to dipoles. ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
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The next time I experiment with end fed antennas, I want to try
a matching transformer at the end of the antenna wire of about a 50:1 matching ratio. That way I will have an impedance in the feed line of about 50 ohms and minimize coax loss. Now if I am connecting the antenna directly to binding posts on the side of the KX(2/3), the loss won't matter. But if I am using a length of RG174 for a light weight feed line, the lower loss will be very nice. Adding Don's recommended 0.05 wave length "counterpose" in the binding post scenario sounds useful. With a coax feed, the coax should perform that function. Now, where am I all wet? 73 Bill AE6JV --------------------------------------------------------------------------- Bill Frantz | "I wish there was a knob on the TV to turn up the 408-356-8506 | intelligence. There's a knob called "brightness", but www.pwpconsult.com | it doesn't work. -- Gallagher ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
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You're close, Bill. The short counterpoise should go at the antenna end
of the feedline by the transformer, not at the radio. It probably won't make much difference, though, as the outside of the coax will serve as a counterpoise. Depending on the design of the transformer, its inductance and stray capacitance may have some effect. The inductance will affect the lower frequencies, where it appears in parallel with the antenna. You can compensate for that by making the antenna a bit longer. On the higher bands, the stray capacitance may require the antenna to be slightly shorter than a half wave multiple. (I hope I got that straight.) In any case, if you can't match it, experiment with the wire length and see if you can find one length that will work on all the bands you need. I'd be interested in hearing the results. A single EFHW (multiple) and transformer that can be used with an internal ATU alone on 80-10 meters would be very attractive. (Or even up to 15 or 20 meters.) Note that a 50:1 transformer has a turns ratio of about 7:1. 73, Scott K9MA On 2/11/2017 16:52, Bill Frantz wrote: > The next time I experiment with end fed antennas, I want to try a > matching transformer at the end of the antenna wire of about a 50:1 > matching ratio. That way I will have an impedance in the feed line of > about 50 ohms and minimize coax loss. Now if I am connecting the > antenna directly to binding posts on the side of the KX(2/3), the loss > won't matter. But if I am using a length of RG174 for a light weight > feed line, the lower loss will be very nice. > > Adding Don's recommended 0.05 wave length "counterpose" in the binding > post scenario sounds useful. With a coax feed, the coax should perform > that function. > > Now, where am I all wet? -- Scott K9MA [hidden email] ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
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In reply to this post by Don Wilhelm
No, end fed halfwaves are not dipoles. They are monopoles. Take a look at the ARRL antenna book, LaPort, Johnson or any of the classic antenna books. There is a reason why all the AM broadcast Stations with halfwave end fed antennas have radial systems that consist of 120 radials that are 0.4 wavelengths long. Look at the licenses for KRLD, WIP etc. they can be found on fccinfo.com. I don't think KRLD would have spent over 200,000$ replacing their ground system If end fed half wave antennas were dipoles and did not need a radial system to be efficient radiators.
The best way to picture an antenna (ARRL Antenna book) is as a capacitor with displacement current flowing between the plates. With a dipole each of the plates is a antenna element. With a monopole, one plate is the radiator and the other plate is ground. Since even good ground is a poor conductor, a large resistor has been inserted in the circuit. The radial system acts to reduce that loss resistance and increase the power radiated. Ray W8LYJ Sent from my iPad > On Feb 11, 2017, at 16:46, Don Wilhelm <[hidden email]> wrote: > > Ray, > > I think that is 'stretching' the facts a bit. > A halfwave dipole is a halfwave dipole whether fed at the center or at an end or somewhere in between. > The radiation pattern and efficiency is the same. > The problem is that an end fed resonant dipole does need a little bit of a counterpoise (0.05 wavelength) in order to be able to feed it properly. > > 73, > Don W3FPR > >> On 2/11/2017 4:07 PM, Gmail wrote: >> As was said previously, all end fed antennas are monopoles with displacement current flowing from the antenna element to earth. Unlike dipoles where the displacement currents flow from element to element. >> So without an adequate radial system to reduce the ground loss all end Feds regardless of length are only about 10% efficient. >> There are good reasons why 45 years ago when coax became readily available we switched from endfed monopoles to dipoles. Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
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