EFHW антени (крайно захранени полувънови антени)
#1
https://www.hamradio.me/antennas/simulat...tenna.html
http://dl2kq.de/ant/3-65.htm
http://dl2kq.de/ant/kniga/411.htm
http://hamfest.w7yrc.org/wp-content/uplo...slides.pdf
https://vu2nsb.com/antenna/wire-antennas...w-antenna/


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.pdf   end_fed_half_wave_antenna.pdf (Размер: 487.72 KB / Сваляния: 564)
.pdf   EFHWslides.pdf (Размер: 2.6 MB / Сваляния: 553)
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#2
Прилагам любопитен анализ на напрегнатостта на полето на дипол и ендфед антена.

https://www.hamradio.me/antennas/electri...ation.html

В статията е показано, че напрегнатостта на магнитното поле и при двете антени има еднакво разпределение. При ендфед антената има леко понижение от 1.6 dB (по сметка на автора), което той отнася към използвания трансформатор.


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#3
Добавям един линк и ще си позволя да копирам последните няколко абзаца от статията на W8JI

https://www.w8ji.com/2end-fed_1_2_wave_m...20feed.htm

Цитат:So there we have it. Matching loss in the worse transformer system I measured is around 4 dB. If the antenna was 4700 ohms j0, more power would be consumed in the tank circuit feeding the antenna than is actually applied to the radiating part of the antenna system! Of the power applied to the antenna, a significant portion will be distributed in the antenna feed cables, rig, and everything connected to the rig including the operator and station wiring.

On the other hand we could build a dipole, and feed it with RG-174 cable. We would eliminate about 4 dB of  matching circuit losses, and a few dB of power lost in common mode losses. How much RG-174 could we use to equal the end-fed losses? At 50 MHz we could use about 100-feet of RG-174 and break even with end feed using the system I constructed, a small T50-2 toroid with link coupling resonated by  a typical polyethylene insulated "broadcast tuning" capacitor. At 7 MHz it's a no-brainer. I'll take the RG-174 dipole feed every time! My reasons are mostly the convenience and repeatability of the feed system. 

Another possible alternative is a 1/4 wl 450-ohm Q section or stub. TLA estimates loss as .205dB on 7 MHz with that system. 
In my experience it's far easier to have repeatable results with a stub, rather than small inconsistently manufactured lumped networks. Even large tuners using transmitting components become inconsistent in loss measurements at impedance extremes. Losses that would cause big smoke at high power can go unnoticed at low power levels..    

End-fed half waves are sometimes inefficient or troublesome feed systems. At low power we might never  notice efficiency problems or common mode current problems. Of course at high power very few shortcuts can be tolerated.
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