Hf Longwire Antenna

[Mike Fowler]

LongWire / End Fed Wire Antenna Includes:

End fed / long wire antenna Multiple wavelength long wire End fed half wave antenna W3EDP antenna Random wire antenna Terminated long wire antenna V beam antenna (bidirectional) Unidirectional V beam antenna End fed wire antennas are one of the simplest antennas to construct and erect. These antennas have been used for many years and provide a particularly adaptable form of antenna, especially for newcomers to radio communication.

Strictly speaking a long wire antenna is just that - a very long piece of wire - many wavelengths long. However the term is widely used to describe a random length of wire used to receive and often transmit signals on the HF or short wave bands.

The end fed wire antenna simply consists of a length of wire that is as high and reasonably long as possible. The wire is then connected to the transmitting and receiving radio communication station.

The antenna tuner is placed between the transmitter or receiver and the antenna wire. If a tuner is not used, then the impedance of the antenna will not match that of the receiver or transmitter input and this will result in lower efficiency. Also the transmitter may have trouble matching to this, and this may result in lower power output, or even damage to the transmitter output.

A further requirement is that it is necessary to have a good radio earth - this is not the protective means earth, but an earth connection against which the long wire antenna or end fed wire antenna can work.

It is often thought that an end fed wire should be a quarter wavelength long on the major band of use. An end fed wire of this length presents a low impedance at the feed point and this enables it to be matched very easily.

However the current flowing in the antenna gives rise to the radiation. Looking at the current distribution in a quarter wavelength antenna it is found that the current rises as the distance increases from the end, and reaches a maximum a quarter wavelength from the end. In other words a quarter wavelength antenna has its current maximum, and hence the maximum radiation at the feed point.

As a result the quarter wavelength antenna has its maximum radiation, and also pick up at the feed point. Typically the feed point will be next to the radio: transmitter, transceiver, or receiver and this results in local pick-up of noise etc and this can mask the wanted signals. Also for transmitters, lots of radiation occurs near the radio and this can cause interference to many other problems with electrical and electronic items. Even if the antenna is not a quarter wavelength long, some pick up and radiation will occur near the radio and this is not good.

The antenna wire should be hard drawn. As copper is very soft and ductile, normal copper wire tends to stretch very quickly, and what may have been a taut antenna wire, can soon sag if the correct wire is not used.

Another component that is required (but not essential) for a long wire or end fed wire antenna is the insulators. One is needed for each end of the wire, or if it is bent, anywhere that the wire comes into contact with a rope, etc.

As would be expected, antenna insulators are made from highly insulating material. They are also ribbed so that there is more surface length across which any leakage current must pass. This ensures that they have the highest possible level of insulation.

End fed wire antennas can work very well under many circumstances. They have some disadvantages, but with careful installation, they can work well and be particularly versatile offering multi-band operation.

I was thinking about creating either a simple longwire (10m) or a dipole (2x5-7m). It seems like I wont be able to get hold of any ready made baluns/chokes, or any ferrite cores/toroids to make a balun myself by that time. Seems like no one here is selling stuff like that.

Since I wont be able to balance the antenna, would a good option be to use two coax cables as feed lines? Using the center core of the coax for both legs of the dipole? And then connecting the shield of each coax to earth together with one of the cores?

For a receiving antenna, you do not need a balun. You will experience more noise from whatever equipment is located at the radio end of the feed line, and the antenna pattern and impedance will be less predictable because the feed like will to some degree be acting as part of the antenna, but it will still work. Even transmitting antennas can be constructed without baluns and are often used this way in low-power portable operation.

This design does not need a balun, though it might benefit from impedance matching (again, not critical for receiving), which would be more precisely an "unun" (both the antenna and the coax are unbalanced devices in this case).

This might make a balanced line; I'm not sure. But for it to do anything useful you would also need a balun at the receiver end. Otherwise, the receiver itself connects the shield side of its antenna port to (its) ground, so one of your two coax lines would be just acting as a wire tied to ground on the outside of the shield, so it has no effect, or at least the line is not balanced any more because the two sides are attached to unbalanced things.

Like Kevin Reid says, you could be fine without a balun, especially at a mountain cabin where there there may be nothing to generate noise. For a receiving application, proper attention to the feed arrangement keeps the feedline from becoming part of the antenna, and since the feedline is usually near computers, switch-mode power supplies, and other sources of noise inside, this can be a huge improvement. However if there's no electricity besides a battery and your radio at the cabin, then none of these noise sources won't be around.

But if you did want to make a balun, there are options that don't require any sort of ferrite core. Many depend on a quarter-wavelength transmission line, but you want a broadband antenna so let's exclude those.

Sometimes folded dipoles are made from ladder line since it provides two parallel conductors. The two conductors are shorted at the ends, and one of the conductors is cut in the middle to make the feedpoint, labeled A and B. It could then feed like an ordinary dipole, either with a balanced feedline, or with coax and a balun.

But notice the center of the other wire is ground: the voltages on the left and right are always equal but opposite, just what you want from a coax shield. If you could somehow connect the shield there, but then also connect to the feedpoint on the other side...

Solution: build the folded dipole from metal tubing, with a tee at the ground point where the feedline can enter, then pass inside the antenna to the feedpoint on the other side. If 10 meters of tubing isn't what you had in mind, then just use coax, which is already a tube!

The connections you need to make for such a construction are drawn in blue. Note that it's the shields of the coax that make the folded dipole, while the feedline runs inside the left half of the antenna to connect at the feedpoint at the top. The center conductor on the right half does nothing: cut it off and ensure it can't come in contact with the other parts.

Long wire or random wire antennas are very simple antennas. They can come close to half wave antennas in efficiency, although efficiency decreases as they are made very long or installed closer to earth. Like every antenna that exists, random or long wires have advantages and disadvantages.

Because the radiating area is often brought into or near the operating position, longwires often create RF interference to consumer goods or RF in the operating room. The easy installation part comes from generally needing only two supports, and not having a heavy feed line hanging from mid-span like a dipole. The long expensive feed cable normally associated with a doublet or dipole is not needed, the antenna wire itself serving as a "feed line".

This is the feed impedance and SWR of an end fed 80 meter half wave with an ideal 49:1 transformer and good stable counterpoise or ground. The black dots represent exact harmonics of the 3.57 MHz fundamental resonance. The actual resonance, 49:1 transformed SWR at resonance, and the antenna impedance at resonance are shown:

The antenna itself works just as well as any other wire of similar height and length. Any or all problems are in the counterpoise and feed system. The difficult problems associated with random wire or long wire antennas are caused by ground currents and radiation from the single wire feeder.

End-fed antennas, or antennas with the single wire feeder brought into the shack, come with a little misconception. One commonly repeated myth or "theory" is that half-wave antennas, being resonant, do not require a counterpoise, or that some magical length of antenna will prevent RF in the shack. This does not mean the antenna will be worthless and not make contacts, it simply means something else replaces the missing counterpoise area and we also bring RF fields right into the shack. The feed line, as well as everything connected to and surrounding the single-wire feed line and counterpoise, becomes part of the radiating system. This creates three potential problems:

Since we often do not have a baseline for noise, unnecessary additional noise will often go unnoticed. The remaining two issues are more likely to be noticed, but only if we run enough power to cause RF burns, power supply shutdown, or other forms of RFI.

Transmitter power levels, feed line length and routing, and the susceptibility of equipment to RF problems greatly influence things we most likely notice. This is why some people (usually with QRP power levels) swear by end-fed half-waves, while others (usually with higher power) avoid end-fed antennas. The reasonfor that is simple,end-fed half waveshave common mode feed line current problems affecting their performance, and these common mode currents cause inconsistencyinuser satisfaction.

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