# Tag Info

11

It's very difficult to predict the impedance of an end-fed wire, other than to say it's high. Usually it's determined empirically. You are looking for a theoretical formulation. Consider, the feedpoint is a voltage source which makes a difference in electric potential between to things. The end of the dipole, and...what? simulate this circuit – ...

8

An end-fed half-wave operates on a principle that, for lack of a better name, I will call "voodoo". If you look at the standing wave on a half-wave element in isolation, you see that the center is the point of highest current and lowest voltage (therefore lowest impedance), and the ends are the points of highest voltage and zero current (therefore infinite ...

7

A random longwire is just that, a random length. There is no optimum length, by definition. Usually people use random longwires for multi-band operation. Since the design frequency is "a lot of them", the strategy of a random longwire is more or less to give up on making the wire suit any particular frequency, and just deal with the mismatch with a ...

4

An end fed wire may not necessarily work BETTER for reception, but it should be effective and it is much simpler to put up. You might pick up a bit more hash noise from local RFI sources with an ungrounded configuration though, due to common mode currents induced on the outer shield of the feedline. Feeding the wire through an isolation transformer, and ...

4

I have used an End-Fed Long-Wire antenna for many years. I have used it on 80M, 40M, 20M with 1:1 SWR... I did NOT try for the shortest long-wire possible. I did use a Long length which is several Odd Quarter-Wave-Lengths of my target bands 40M and 20M. It is 186 feet long, up 17 ft, in the shape of a letter "Z" and controlled by a Antenna Tuner and ...

4

Wikipedia: Monopole Antenna The monopole always has another set of conductors to which the second wire from source is connected. This second coundutor may be earth, or a metal object commonly known as "ground plane". The ground plane turns the monopole into a virtual dipole with length each limb of virtual dipole equal to length of monopole. Thus the ...

4

End fed antennas have become quite popular in recent years, primarily due to their convenient installation conditions. Many people subjectively report good results from the antenna. But how the antenna works is not often well understood. The end fed antenna often utilizes an unun at the feedpoint of the antenna. This is done because the feedpoint impedance ...

4

The EFHW (end fed half wave) needs to be classified as a specific case of the class of end fed wire antennas. The name is even a bit of a misnomer since the EFHW is often operated on multiple bands and therefore is no longer a half wave antenna. And due to the implementation details, the antenna is in fact often not even a half wave at its design frequency. ...

4

First you don't necessarily have to do anything. The SWR is below 3 for the bands and most transceivers can handle that OK. If you want to try to get the resonate points inside the bands you will need to lengthen the wire. it is usually a good idea to start with too much wire and trim the length to bring the resonance down, but in your case it look like the ...

3

Half wave end-fed has appr. 5k impedance, you need a 49:1 transformer to match 50Ohms cable. As the impedance of the antenna is very high a small counterpoise of only 1/10 of a wavelenght does the trick. Do use some form of coax-choke at the antenna-end of your coax to stop RF current flowing in the coax-outside. Usually some 10 turns of coax with a ...

3

An end fed, 1/2 wavelength antenna will have a relatively high impedance of 5k ohms or more. So a 9:1, or greater, ratio of balun may reduce the impedance sufficiently to help your tuner get a match and help avoid a voltage flash-over in your tuner. However, there is another issue that may require some planning. When using an end fed antenna, the current on ...

3

I cannot speak to the off-center type dipole feeds that use the 1:9 (impedance) transformers, but for the truly end-fed dipoles fed with something like a 1:49 impedance ratio, the answer is, it depends. I performed some simulations of an end-fed dipole system with an autotransformer and an additional wire representing the currents along the feed line. I did ...

3

No, it is not needed, but it can improve the antenna system. An EFHW is simply a half-wave length of wire and a matching device, and that's it. Counterpoises are commonly added to shunt common mode reflections when operating outside of the wire's resonant frequency to avoid RFI issues ("RF in the shack"). An RF ground (defined here as either very ...

3

It may be simpler to think of the matching network this way: The high-impedance antenna (in the example, R1 = $3500\Omega$) is connected to a parallel-resonant network consisting of an inductor (L1+L2: note the coupling indicated by the "k1" statement) in parallel with a capacitor (C1). L1 is, in reality, a "tap" on a single inductor comprising (L1+L2). As ...

3

Eventually I managed to make the antenna work on more than one band. I used a modeller (CocoaNEC) to approximately determine the impedance on each band of interest for my antenna configuration (inverted-V on a 10m long fishing rod). The impedance was about 2450 Ohm. Thus I rewinded the transformer to 1:49. Also I used a 1:1 balun (8 turns of RG58 on FT240-31 ...

3

I own an O-Day 272LE sloop and have been operating HF on it for the past several years, including serious Field Day operation from along Delaware Bay and Rehoboth Bay in Delaware. The stainless-steel cockpit railing makes a good mount for vertical antennas on the stern and bow. This allows for maximum separation from the boat's rigging. For serious FD ...

3

Model the 1/2 wave center fed antenna using the wire size, altitude, and all location attributes to determine the center fed impedance. divide 600 by the 73 ohms or whatever you calculate the center fed impedance to be then multiply the answer by 600 --- you're home. Example: $600/73 = 8.22 \times 600 = 4900$ ohms. This is the original and correct ...

3

A random long wire is just that, random. While large loops at the end or bending the wire might change the resonant frequency, a random wire isn't (unless by chance) resonant anyway. So loops and bends just make a different random wire antenna which is neither better nor worse. The shape of the antenna isn't going to affect efficiency directly. Think about ...

3

For illustration, here's a 9:1 unun construction using 3 wires from M0UKD: Since the A and B windings are connected together in series, there's no compelling reason you couldn't replace them with one longer wire that has the same number of turns. For an ideal transformer only the number of turns would matter, and how those turns are arranged would be ...

3

Interesting. Assume half-wave resonant antenna. The feed point in the middle (just a series source, or a transformer) results in 70 Ohm real impedance. When the antenna height is lower a match to 50 Ohm is sometimes possible. When moving the source insertion point in the direction of one of the ends the impedance grows up to beyond 1500 Ohm (depending on the ...

2

Any "realistic" end fed wire antenna has a counterpoise which may merely be the ground or actual wire conductors on the ground or some variation. The best way to determine R,X of the feed point (assuming that is what you want) is to compute them using NEC2. NEC2 is very easy to use and computing the R,X input is also very easy to do as it is one of the ...

2

Transformers are inductors. They just happen to have mutual inductance with another inductor. Like inductors, they also have self-capacitance, and so a resonant frequency. They also have loss, and consequently some maximum power handling ability which if exceeded causes the transformer to fail by overheating. As such, any parameter which is relevant to an ...

2

The length doesn't matter much. If you make it the right length, then it will present a good match to your feedline, but if you have a tuner, and either place it near the antenna or use a low-loss feedline, then that doesn't matter. It's also possible to get really unlucky and pick a wire length that's outside of your tuner's range. What these lengths are ...

2

I have used both 107 and 71 ft lengths, but didn't see much difference as my old KW At-230 tuner tuned both just fine. I fed them through a 9:1 balun and 50 Ω coax with 25 ft of coax back under wire, possibly acting as counterpoise, but I'm not sure about that. Everything is well grounded in my shack. It worked pretty good on 40 & 20m.

2

Below is a more elaborate analysis of an EFHW, with a length of vertical coaxial cable attached at its feedpoint. It includes the effects of radiation from the "unchoked" r-f current that could flow along the outer surface of the outer conductor of the coaxial cable. Other details are shown in the Comment block at the lower left section of the graphic.

2

I'm really not interested in putting insulators in these cables or trying to load them as antennas That's a shame, because the mast and the cables probably make an excellent antenna. In fact unless you go through some trouble to get the antenna far away from the mast, or oriented orthogonally, the mast and cables are going to end up being a significant part ...

2

If at all possible, the feed line should go through a panel that is connected by a short cable, or directly connected, to a ground rod. A lightning arrester is a good idea also. All earth grounds should be connected, including the ground rod connected to the service panel, the ground rod connected to the coax shield, and your station ground. In the US, ...

2

The radiation pattern of an antenna laying on the ground can be easily approximated for the transmit case: it's roughly equivalent to a dummy load. Unless of course you are in Antarcitca: ice is transparent enough to HF that it's basically invisible. But I don't think there are any retirement centers in Antarctica. For a receive application, you might hear ...

2

It looks to be a well done experiment. We see a sinusoidal current distribution in either case, and the current tapers off to (approximately) zero at the ends. But this is what we'd expect of any standing wave on a wire of this length. Where its fed doesn't matter. The problem with end-fed dipoles was never that they are impossible, but that they are ...

2

There's confusion about counterpoise because there's two kinds of end fed antennas. Both are multi-band, but different approaches to achieve this. The first kind is deliberately non-resonant, uses a 9:1 unun and requires a tuner. It may require a good tuner if you want to work the low bands on a short wire. Some suggest a counterpoise of .05 or .1. ...

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