Alimentada en un extremo (EFHW)

Las antena alimentadas en un extremo se describen en https://sites.google.com/site/ea7ahg/antena/tipos-de-antenas/antenas/alimentada-en-extremo, pero quiero presentar un diseño monobanda para la banda de 40m, en V invertida, de W8HDU. Este tipo de antena monobanda se puede diseñar y construir para cualquier banda.

Recomiendo, igualmente, la lectura de de la página Web http://ea3ghs.qrp.cat/estudio-endfeed-banda17m.html de EA3GHS, para aclarar conceptos sobre las antenas alimentadas por un extremo.

En el foro de URE EA2HO indicaba lo siguiente:

"Esta antena, al parecer funciona razonablemente bien en cualquier posición, colgada de un árbol.

Tengo algunas dudas para construir el acoplador con un sencillo medidor de ROE. Después de leer el trabajo de AA5TB me surgen dudas sobre el circuito más sensible del medidor de ROE.

No se si escoger el circuito propuesto por el propio Stephen o el de WA5BDU."

Para la construcción de este tipo de antenas para QRP se puede leer el manual

Hendricks 40m – 15m SOTA Halfwave Tuner

en http://www.qrpkits.com/files/sota_assembly_050312.pdf

También recomiendo la lectura de http://www.qrpproject.de/Media/pdf/MultibandfuchsEnglish.pdf

Basic End Fed Vee

Autor: Frederick R. Vobbe, W8HDU (http://hf-antenna.com/009/)

An End Fed Inverted Monoband Vee has a low impedance point around 1/4 wave away from ground, and the multiples of 1/4 wave on the feed side.

This antenna does require a tuner, but the tradeoff is that it's often easy to construct. One application of this antenna is by using a tower as the top support, and making the guy wires the antenna. Be sure to account for the physical pressures of the guy tension in the fabrication of this antenna.

In Figure 1 we see a typical End Fed Inverted Monoband Vee. Note that the ground portion of the feed point is 1/4 wave, while the "hot" side is multiple 1/4 wavelengths up and over the support.

figure 1

Let's suppose we're designing a End Fed Inverted Monoband Vee for 40 meter use. Our center support will allow us to have at least 50 feet of elevation. We know from the Pythagorean Theorem that the hypotenuse of a right triangle can be easily calculated from the lengths of the sides. The hypotenuse is the longest side of a right triangle. So we just need to make sure we have a wire where the two longest sides (added together) plus insulators equals a distance of approximately 1 wave, or (4) 1/4 wavelengths. See Figure 2 below.

figure 2

We know that a 40 meter antenna, cut to 7.200 MHz, would be 41.66 meters or 136.657 feet for full wave. Let's assume that we make this antenna "perfect" by having the 1/2 wave spot being the apex of our support. That means that we'll have 1/2 wave on one side of our support, and the other 1/2 wave on the opposite side. So our antenna will look like figure 3. The red dots are the insulators. Black is the wires. Blue is our support. And green is ground.

Picture of what our antenna will look like

figure 3

So now that we have the basics, let's get building!

To make this easy get about 175 feet of wire. Cut off 150 feet of #12 wire (assuming that we're building for the 40 meter band). Why 150? Because our antenna will be 136.657 feet full wave (for 40 meters) and we'll need to wind the end of the wire around some insulators. When we are done we can "tweak" the antenna by shortening the wire at the ground end by a few inches. The other 25 feet will be used to support the antenna at the ground ends, (gray closest to ground), and for jumpers.

Next, find the center of the 150 foot wire, which is 75 feet. Cut at this point. Next, take one of the 75 foot pieces and cut it exactly in half again. You should now have (2) pieces 37.5 feet long, and on that is 75 feet long.

You will need six insulators. Unless you are running extreme power, you can use plastic "egg" insulators. If you are running over 500 watts of power I would use the glass or porcelain insulators. If you need the plastic ones, I have them for sale. I would appreciate your business.

Take each piece of wire and attach them to an insulator. If you are using insulated wire, you can wind the wire with insulation through the insulator. However, we will need to bare at least 2" of the end. Make sure that your wire does not slip through the insulator. Personally, I use Belden 8000, or I have used #10 THHN. But I'll remove some of the insulation an wind the loose end around section goint to the insulator and put some solder on it to keep it from slipping.

Next, on one end of a 1/4 wave wire, and on one end of your 1/2 wave wire, attach some of the excess wire which will attach to the ground supports. You will also need a small amount of wire between the insulators separating the 1/4 wave sections. Add about 6" of wire at the feed point to attach to your coax. After you install this, seal the wires attaching to, and the exposed dielectric of the coax with RTV or Silicone caulk. You don't want water ingressing into your coax.

At the top of the support you'll tie the 1/2 and 1/4 wave sections to the support via the insulator, but don't forget to jumper on the hot side from the 1/2 to the 1/4 wave section to make your connection. Your antenna should loook something like figure 4 below.

figure 4

The ratiation pattern of this antenna looks like what is seen in figure 5.

figure 5

You'll note that there is a lot of energy going UP! However, there is some signal going out to the sides. This antenna tends to be generally omnidirectional in the horizonal plane, but does have some artifacts due to basically being a dipole. The ratiation pattern of this antenna looks like what is seen in figure 5.

VSWR, as seen in figure 6, might be somewhat of a problem without a tuner. The feed point is rather high, (207.4+j15.28 ohms), and has a return loss of 3.00 dB. The usable gain is only .559 (a 2.53 dB loss), but what I have found is that its radiation pattern sometimes snags stations closer to me than the standard skip zone exhibited on a standard dipole.

figure 6

To counter this, there are two solutions. One is a balun, and the other is a matching network. I've used a 4:1 balun with success, but I've found that on solid state rigs (which are somewhat intolerant to any VSWR issue), the better solution is a tuning network. The way I have designed the network is to run my 50 ohm line to a Pi tank, and then use laddar line to reach the feed point. A simple Pi tuner looks like what you see in figure 7.

figure 7

Your network will be different for various bands. But you can also multiple tap the coil for various band segments. For example, you could tap the coil for best match on 7.050, and then have a second tap for 7.200, and have a relay select one or the other taps.

Frederick R. Vobbe, W8HDU

Otras versiones de antena EndFed consisten en construir una EFHWA o Antena de Media Onda alimentada en un Extremo, muy utilizada en la modalidad QRP por lo sencillo de construir, llevar e instalar en salidas para activaciones. Igualmente, se puede instalar de forma fija en un QTH.

Teóricamente tiene una ganancia de entre 4 a 6 dbi, con la que se puede trabajar varias bandas sin necesidad de acoplador, radiales o contraantena, puediéndose de forma vertical empleando para ello una caña de fibra de vídrio de las de pescar, en L, en sloper, en horizontal..etc,. Por ejemplo, con un alambre de 20´35m se puede trabajar las bandas de 80, 40m, 20, 15m y 10m, siendo necesaria una bobina de 110 uH construida sobre un tubo de pvc de 19mm de diámetro enrollando 260 vueltas de hilo de cobre esmaltado de 1 mm y un trozo de radiante de 2,39 metros; siendo posible utilizar cable de instalación eléctrica de 1,5 mm, como se puede observar en la imagen siguiente

Lo interesante es no colocar los extremos cerca del suelo, para que no varíe su impedancia de 2000 a 3000 Ohm, por lo cual, es necesario construir un transformador de impedancias 49:1 con un toroide FT240-43 para QRP o con dos apilados para potencias superiores, tal y como se puede observar en la imagen siguiente.

Por seguridad, el condensador debería soportar 5KV aunque también se puede construir con cable coaxial RG58, sabiendo que su capacidad nominal está en torno a 101 pf/metro, deberíamos emplear un metro de este cable coaxial enrollado dentro de la caja que albergará el trnasformador.

También puede emplearse un transformador 64:1 como se observa en

https://www.nonstopsystems.com/radio/img-ant/antenna-mltibnd-EF-config-b.jpg

de modo que el transformador está conectado a tierra o emplear una configuración con cable de contrapeso en el transformador

https://www.nonstopsystems.com/radio/img-ant/antenna-mltibnd-EF-config-d.jpg

También se puede añadir a la línea de alimentación un balun de corriente 1:1 (choque).

https://www.nonstopsystems.com/radio/img-ant/antenna-mltibnd-EF-config-e.jpg

Por otro lado, decir que esta antena también puede construirse con un acoplador en su punto de alimentación tal y como se puede observar en la imagen

Esta es una variación de la antena EndFed original de W3EDP

Las últimas 4 imágenes han sido tomadas de (N4SPP) https://www.nonstopsystems.com/radio/frank_radio_antenna_multiband_end-fed.htm, siendo recomendable la lectura de esa página Web para aclarar ideas, ampliar conocimeinto y tener una orientación a la hora de construir el transformador; contactar através del correop electrónico frank@nonstopsystems.com con FRANK M DOERENBERG.

En a URL https://www.qsl.net/dk7zb/Wire-Antennas/w3edp.htm se describe esta antena y se dice:

"The original EDP antenna was an end end-fed wire matched with a parallel LC-circuit ("Fuchs-Circuit") and a counterpoise wire.. The description [1] was in the QST 1936. "

Más teoría sobre el transformador para este tipo de antena la encontramos en https://squashpractice.com/2021/06/23/performance-of-491-ferrite-core-transformers/

LINKS:

COMMON 40-10 MTR OR HALF-SIZE 80-10 CONFIGURATION

6-40 MTRCONFIGURATION BY EARCHI

https://radioaficion.com/cms/antenas-end-fed-multibanda-de-35-30-mhz/

https://ea7eer.com/2017/09/07/antena-end-fed-para-10-15-20-40-y-80-m/

"Lambda/2 no Counterpoise: Fuchs Antenna matching unit", DL-QRP-AG, 6 pp. [pdf]

EARC END FED 6-40 Meter Multiband HF Antenna. [pdf]

QRPGuys Portable Multi-Band End Fed Antenna 40/30/20 [schematic and build instructions], QRPGuys Portable No Tune End Fed Half Wave Antenna 80-10m ].

"End fed matching – PA3HHO design review ", (VK1OD) blog. [pdf]

"Multiband PAR End-fedz nabouwen" (in Dutch), 2011. [pdf]

"Multiband end fed antennas 3.5 - 30 MHz", (PD7MAA), 2012. [pdf]