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I have two pieces of antenna wire which are roughly 10 m long, with banana plugs at the ends, which I bring to hang in the trees when operating portable. I also bring a 49:1 unun and hopes that my longish coax serves as enough of a counterpoise. For working 20 m, there are two mainstream ways to use these.

The first and simplest is to us one of the wire segments as an endfed halfwave. Accounting for the reflecting ground, this is fairly omnidirectional with some emphasis in the broadside direction.

The second, which I like, is to arrange the two segments in series to a longer endfed antenna and use the first harmonic. Because the two halves of such an antenna are out of phase, the broadside radiation is suppressed, and more of the radiating power will be along the axis of the antenna. (Of course, another reason I use this is to quickly be able to go on 40 m!)

Both these configurations are resonant with SWR < 1:1.5.

By deploying the second configuration, I can easily switch between the two by lowering the antenna and disconnecting the two halves at the centre. Thus, I at least imagine I get some difference in directionality, though I haven't verified this by any measurements.

There could be a third option though, which I haven't been able to find described anywhere:

What if I would leave the segments as above, but feed them both at the same point in the middle? Edit: To clarify, I electrically connect the two segments, and then connect both of them to the same (electrical) point of my transformer, pointing 180° away from each other.

Provided I adjust the segments to be resonant, would I thus get the two halves oscillating in phase, and thus amplify the broadside radiation? (Bonus question: Could I perhaps even angle the two segments somewhat to get even more directionality?)

On the one hand, I could imagine that a full-wavelength piece of wire doesn't like to resonate with two halves in phase as much as it does doing this with the halves out of phase. On the other hand, I know I can drive a skip rope holding the middle with one hand if the ends are stationary, and a skip rope is one of my naive imagined models for efhw's. On the third hand, doing that with a skip rope is a delicate manoeuvre, and you need to take care so that the sides don't interfere with each other.

I messed around with this a bit on my last activation, but I didn't manage to get the SWR low enough to really compare it to the others. This could either be because it's impossible, or just that the adjustments are more delicate than when you feed near the end. I don't have a good home qth where I can put up wires, but I intend to try again next time I'm out. That is, unless there is some good case for why it would be futile and my precious hours in the sun could instead be spent chasing DX instead :)

This is not a duplicate of Multi-wire EFHW?, as the main question there is about how parallel endfed antennas can be resonant. I'm interested in the particular case of two equal segments, and their radiation pattern.

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    $\begingroup$ There are two ways to answer this question. 1) Model and simulate it 2) build it and test it. $\endgroup$
    – user10489
    Mar 25 at 1:43
  • $\begingroup$ Does the antenna you describe here have a single feedpoint? Because your comments on @hobbs-kc2g's answer seem to be asking a different question about two antennas, each with a separate feedpoint (which should probably be asked in a new question). $\endgroup$
    – Mike Waters
    Apr 1 at 13:07
  • $\begingroup$ @MikeWaters The antenna I ask about has a single feedpoint, that feeds two endfed segments going out in opposite directions. In other words, this means it's not a feedpoint like that of a dipole, but the tow sides of the antenna are instead electrically connected. I brought up the two separately fed segments because hobbs claims my proposed antenna does not radiate at all, and I can't imagine how this could happen, even if you could feed the two halves independently. $\endgroup$
    – EdvinW
    Apr 1 at 19:59

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Depends on what you mean by "feed them both at the same point in the middle".

If you're connecting the two wires to opposite sides of a voltage transformer, then you have a (slightly unusual version of) 1-wavelength doublet. The behavior will be substantially the same as your 1-wavelength end-fed, and the pattern will also be the same (but if you operate it on 40m it becomes a half-wave dipole fed in the middle, with the impedance you would expect from that... whereas if you feed from the end, you get "end-fed" behavior on both 40 and 20).

But if you're talking about connecting two wires going in opposite directions both to one side of the transformer then what you get is an "antenna" that doesn't radiate at all because the two elements cancel each other out entirely. Any radiation you get would be either from imperfect cancellation due to asymmetry in the antenna, or feedline radiation (the feedline is the real antenna, the "antenna" is a capacitance hat, and your radio chassis capacitively coupling to ground forms the counterpoise, such as it is). That would explain your inability to find an SWR match. In short, this is a bad antenna, and not a way to get extra gain.

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  • $\begingroup$ I intended the second case, connecting them to the same side of the transformer. I fail to see why they would cancel each other out though. Would this also be the case for two stacked dipoles fed in phase, right next to each other? If not, how are the two different? $\endgroup$
    – EdvinW
    Mar 25 at 7:02
  • $\begingroup$ Also, if I would point the ends in the same direction, this would electrically correspond to a single efhw. As I start to split them apart in an increasing angle, something would happen, not sure what, but they would still radiate. Are you saying that this something would lead to the "elements" would move from amplifying each other at 0° to increasingly cancelling each other until the angle between them reaches 180°? $\endgroup$
    – EdvinW
    Mar 25 at 7:19
  • $\begingroup$ @EdvinW in a regular dipole the two halves have opposite current moving in opposite directions. -1 * -1 = +1 so the two halves add in phase and contribute to radiation. In your setup the two halves have same current moving in opposite direction. +1 * -1 = -1 so they cancel each other neatly. Two stacked dipoles in phase would be same current in same direction, +1 * +1 = +1. $\endgroup$ Mar 25 at 13:31
  • $\begingroup$ @EdvinW cancellation isn't always bad — every gain antenna has partial cancellation in some direction, which you can kind of think of as letting it store up energy and then let it go in the favored direction. But you came up with the perfect symmetrical case of cancellation in every direction. $\endgroup$ Mar 25 at 13:33
  • $\begingroup$ The currents will be in opposite directions, cancelling perpendicular to the array. but the current nodes will be at the center of each half wave, which are a half wave apart, so will be in phase in the parallel/end-fire direction. There will also be radiation associated with the feed line, radiating in who knows what direction, depending on length, choke placement, and topology. $\endgroup$
    – hotpaw2
    Mar 25 at 16:56
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I've been doing simulations, i.e. wetware simulations, i.e. drawing things on a paper and grunting at them. I believe I've solved the puzzle.

When you are transmitting through an antenna, all you're really doing is making current go back and forth at your transmitting frequency. This creates radio waves. The waves bounce at the end of the wire, and you can exploit this by choosing the the length of the antenna in relation to your desired frequency to get a resonance.

Up to high school physics, a length of wire is resonant if its length is an integer multiple of a half wavelength. In such an antenna, a standing wave can form, where the current is constantly zero at the boundary between each half wavelength segment of the antenna, and moves back and forth between these nodes. The direction of the current is reversed at each segment, in the sense that, at any given time, the segments will have currents (+, -, +, -, ...) or (-, +, -, +, ...).

For a more "visual" description, focus on any fixed segment $s$, and let's say current is flowing to the right. It moves happily along until it reaches the end of the segment, where one of two things happen. Either, it will find the right end of the antenna, or it will slam into another wave of current moving to the left; recall that the direction is flipped at every section. In any case, the current will bounce back¹ and start moving to the left until it reaches the other end of the segment ant the same thing happens again.

As is hinted by the name "end-fed antenna", the feed-point is designed to sit at the end of the antenna. It makes sure that the waves of current bounce, and provides large voltages in phase with the current waves to push them even higher, a bit like pushing a swing every time it reaches the end. An end-fed half wave has only one half-wave segment. "Using the harmonics of an end-fed antenna" simply means that we divide it up into more than one half-wave segments, and make sure our feed-point pushes and pulls at the right moments to get them resonating with the corresponding frequency.

Now we're ready to get to the antenna at hand: a wire one full wavelength long!

It has two of the half-wave segments I mentioned above. If we don't worry about any feeding points, but just imagine it resonating in free space, the current will move back and forth between the centre and the ends; the waves both go outwards, bounce at the ends, move inwards, bounce at each other and the cycle repeats.

Now the main question can be phrased as: What happens if we move the feed-point of a full-wavelength end-fed antenna from the end to the middle?

Formulating the theory from the point of view I've chosen to do in the previous paragraphs, the answer should be rather clear: Nothing happens! In a resonating wire, the current and voltages behave in the same way between segments as it does at the end points, so a feed line that manages to excite the antenna from the end should be able to perform the same function at any of the nodes along the antenna where the current is 0 and the voltage is maximised.

Answer: The radiation pattern obtained obtained from feeding the full wavelength antenna in the middle is thus the same as that obtained from feeding it at the end!


EDIT: I've now tried this setup with a borrowed antenna analyser! Though it didn't result in any data about directionality, I can now at least onfirm that I can get descent SWR (better than 1:1.5) feeding a full wave antenna in the middle. It seems to be resonant on a frequency slightly lower than you get when feeding at the end, but I suppose this is natural as you double the end effects both at the feedpoint and at the ends.


¹) In the second case, you could also say that the waves pass through one another, cancelling each other out in the middle. You could argue which description is more accurate, but the two really look the same so I will not go into that.

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