Contacter l'auteur / Contact the author

Recherche dans ce site / Search in this site

 

 

Wire antennas for listeners

3D pattern of a vertical antenna (omnidirectional).

Radiation pattern of a vertical : omnidirectional around its axis.

Designs of antennas (II)

Before determining the place at which you want to install your antenna, we have still to explain what length should have an antenna to work properly, at what heigth is it necessary to raise it above ground, and for what reasons we must apply these rules.

Whatever say books, you will soon discover that in the field there is no two receiving antennas that perform the same way. Reasons are numerous. Take the simple case of the dipole, one of the oldest design of antenna.

Here the ham has used the roof of his building covered with asphalt where he installed a half-length, 31m long multi-band dipole attached to supporting walls, there this is also an half-length dipole but it is installed in zig-zag across the orchard at 4m high due to the presence of trees, to another ham the dipole is tight 8m high across the garden, and to this last ham his dipole is installed in inverted V against a large tree.

Each configuration produces a different radiation pattern, here altered by trees, there by the ground or near obstacles. All these installations will allow you to hear any far DX rising above the noise level but in a few occasions, one of another station will capture with more or less ease the weak signal from a small polynesian island or will capture some more QRM. Why ? 

There are so many factors to consider that it is virtually impossible to modelize this configuration, even if any material can be electrically represented by a specific combination of capacitors and selfs, including a car on which you installed a whip atenna. The first reason is that the theory of antenna is either non perfect nor accurate. As such a great part of the tuning requests experimentation, one of the good sides of our hobby. 

To build a model of your future antenna and evaluate its performances, theoretically you should take into account not only its technical specifications (resistance, length, characteristic impedance of the feed line, etc), but also the solar indices (K- and A- index) at the time you make your test, the time of day,  the shape of your aerial lobes, the soil conductivity, the loss in the feed line, the SWR, etc.

We can say that this evaluation is not accessible to amateurs although you can already taken into account the solar flux, the lobes of your antenna and SWR value. Most of the time these values are available on the Internet. But all these data must be entered in one single antenna model and no software can perform such calculations. So we have to experiment, to test various solutions until to find the best compromise while knowing that the optimized theoretical model requests unusual infrastructures : full length aerial, wide open field, height above ground of 1/2l or l, low loss feeder, etc. But using these tools you will discover that your "compromise" performs quite well for an amateur installation... Here are some example of installations.

Software to design your antenna:

EZNEC from W7EL - MultiNEC from AC6LA

The length of your antenna

To fix our ideas, take the case of a wire antenna, one of the cheapest aerial offering excellent performances and used by thousands amateurs worldwide. By the length of your wire antenna we understand the complete physical length of your wire from one end to another, knowing that most long wires are constituted of a simple copper or steel wire, sometimes linked together to reach the expected length. Most are linear but your can fold it in inverted-L or inverted-V, in the vertical or horizontal plane for convenience. Note however that the physical length of your wire can be different from its electrical resonant length really used in operation.

The length of your antenna must be determined by the working frequency (a VHF antenna is 10 times shorter than a HF antenna) and the design selected (a vertical loop is far different than a long wire...). Usually you should work with an harmonic antenna system, I mean an antenna cut at 1/4l (the working wavelength divided by 4), at 1/2l if you have much place or l if it is designed as a loop or nl for a Beverage (see below). To listen to the HF bands for example a common length is about 20m or as long as your roof or your garden permits. This length should be resonant on most ham bands from 160 to 10m.

Note that to listen at the VHF bands you do not need a long wire. A simple VHF Yagi of 5 elements at a few dozen euros or even a ground plane vertical 2m high attached on a small mast will allow you to capture most regional stations.

The height of your antenna

The next problem is to find a location where you can place your wire, and mainly at what height and how to attach it ! As explained in other pages dealing with the Basics of antennas, to get an optimum response on HF bands (say centered on the 20-m band) your antenna must be placed at a height of 20m for DXing, while 3-4 m high is enough to work locally.

A large height is not mandatory but it will improve the antenna radiation pattern, specially the takeoff angle of its main lobe. This is very usefull in order that your receiving antenna can "pick up" easier weak signals arriving low over the horizon and passing close by your antenna. 

radiation pattern of an inverted V antenna. Document http://www.antenna.be

Radiation pattern of a 2x 10m long Inverted V. This pattern is as important for receiving as it is for transmitting.

Of course for listening purposes the ground effect is not as important as for transmitting where the amateur needs to work in excellent conditions.

Check then both the length of your free space and the maximum accessible height at which you can place your antenna. You want maybe to place it 10m high, but how can you practically reach such a height ? Check also the underground to know how deep you could burry the concrete base of your optional mast or pylon.

Some rocky location do not allow to burry over 30 or 50 cm depth. Positive side, rock will help you stabilizing your pylon. Note that a light mast (e.g. a pole or a light tower 15m high) does not require so heavy foundations, even if it supports a 3-element beam made of stainless steel.

You can drive your pole or your light tower 20 cm deep only to avoid that it slips. Then secure it with four guy wires attached at each third of the heigth to four guy anchors. 

If you place your antenna on a roof without tower, you can also take advantage of concrete blocks to build a heavy base on which will be attached the pole secured with guy wires. 

In all cases, if the pole or tower is made of galvanized tubing, your accessories made of stainless steel or weatherproof (coated or painted), and guy wires protected with PVC, such a solution can resist to bad weather years long, even decades.

Then you have still to find a way to reach the required height with or without ladder... Several solutions exist like the telescopic mast equiped with a winch, the lift, the pulley attached on top of the mast, etc. My solution was to use a fishing weight to throw the end of my dipole across trees, 8 m high ! 

Of course your installation will work if you place your wire antenna 2 m high across two distant trees, even if it must run on the roof of your veranda, but do not expect miracles ! If you cannot reach high heights, try to place at least one part of the wire - not necessary the middle - a few meters higher than the other segment to improve the reception and directivity. If you don't listen any station over 1000 km away change your configuration : double the size of your wire antenna or see if an inverted-V (L) or a loop should not be the best compromize. Using an inverted V, the highest point should be attached to a wall, on top of a tree or on the chimney.

At last do not forget that an antenna is coupled to a feed line, usually made of a coaxial connected to the antenna terminal of your receiver. To get the best results, both ends must be tuned (using an external balun or an antenna tuner) to match impedances with the 50 ohms that should "see" your receiver.

But there is also another effect to take into account. Placed very low over the ground (0.2l) the radiation resistance of an antenna will be very low and, for a same antenna, its variation is accompanied by a change in the radiation pattern of the antenna, and thus its gain. Of course in emitting, the current flowing in the antenna will change proportionally to the field strenght and it is inversely proportional to the square root of the radiation resistance : for a given antenna and power that means that the lower the radiation resistance, the higher the field strenght. This effect is applied successfully to increase the gain of antennas using multi-element directive arrays (Yagis, etc).

The Beverage

An aperiodic dipole R20T-M available in 20, 40 or 100m m long ended with a non-inductive terminator (resistance). It works like a Beverage.

By design, a long wire receives much less QRM than a vertical : a 20m long wire tight 4m high only in Belgium is able to pick up the signal from stations transmitting from Senegal, U.S.A, Canada, Island, Israel, Oman, India, Thailand, Japan, etc. However the receiving conditions using a too short long wire (< 20m long) are lesser to the ones of a dipole 31m long placed 25% higher.

If you want a better receiving using a long wire, as explained in the page devoted to basics of antennas, the best results are recorded tightening the wire at a height close to 1.0l in order to preserve the radiation pattern (in lowering the takeoff angle). Of course any antenna should be associated to a quality transceiver, mainly the receive module if you want to extract signals out of the noise and QRM.

At last, a long wire can be a "gold ear" if you extend its length to 1 or 2l and place it at low height (3-4m high). It becomes a Beverage, named after the amateur Harold Beverage, W2BML, who invented this design in 1919.

Azimuthal radiation patterns of a Beverage 2l long tight 3.5m high.

Due to its conditions of use, the Beverage is not a transmission antenna but rather a directional wire antenna for listening purposes, usually used on low bands, where its length can exceed 300m (for the 160m band). 

If the end is terminated with a 500-ohm non-inductive resistance, the signal does not reflect back to the receiver, and the antenna becomes as directive as a true Yagi, with a rear lobe more than 20 dB down from the front lobe. Placed 3.5m high, in these conditions a 2l Beverage used on 1.83 MHz (thus 320m long) shows a maximum gain of 8.52 dBi in the horizontal plane and up to 6.57 dBi in the vertical plane at about 20 of elevation. Compared to a Yagi this solution becomes very competitive. Its only drawback is to not be rotative.

I personally use such a configuration also on 20 to 10m bands when the receiving conditions are degraded and when the QRN for example become severe. In this case it seems to be more sensitive to weak signals than a dipole or a Yagi. This improved sensitivity has been confirmed by the ARRL technical staff too.

However, as very few amateurs have large fields in their property, another alternative is to build an horizontal loop of this size (25m on each side) or simpler, to set up an full or half-length dipole like the G5RV (see next page) which performance is really appreciated even for DXing when it is placed at good height.

Quality of the installation

Once your wire antenna installed do not forget to test the stability and resistance of your mast as well as wire fixings by ensuring a strong pressure on them. If you attached your long wire or your dipole on top of a guy-wired pole, under gusty wind but not critical conditions (say wind gusts of about 50 km/h), the top shoudn't swing over 5 around its axis or it will bend for ever or even break ! If the top of your antenna swings much more (say over 10-15 around the axe) reinforce your installation and improve the way that your pole is attached (adding guy wires, placing the guy colar closer to the top, placing the guy anchor at a greater distance from the antenna, etc).

Then test the wire itself. The sag (bending) of a wire must be adjusted according the conductor size, quality and weigth, the length of the wire (spanning) and the tension to support. Before attaching both ends of your antenna, attach properly one of its end to its support and make a test in pulling strongly at the other end of your wire; even under your own weight (say 70 kg) your aerial must resists. This is necessary because under bad weather (gusty, grail shower, tempest) the tension on fixings (anchorage points, insulators, etc) and weak points can exceed 100 kg during a few minutes.

So if you build your wire antenna, do not use any type of wire to make it; it must be very resistant to mechanical pressure and quite light. For a 40m long wire or dipole (120 ft) made of an ordinary stranded electric copper cable, with a tension of about 50 kg (100 pounds) the wire is relatively tight with a sag less than 1 m (3 ft).

But if so the wire looks well straight in the air, this tension is at the limit of the copper resistance. Make another test to be convinced. Pull really strongly on both end of a small piece of an ordinary stranded electrical wire that you hold well in hands; you can break some models apart after a few tractions. Worse, if it is made of several shorter wires soldered together and not reinforced with clamps, the soldering will break at the first gust. Check also your knots because under a strong wind, the tension on the wire might undo them if they are not hold tight. 

To prevent these minor accidents, avoid to couple several wires together (dealers sold wires in 100m rol) and made your antenna from a single wire constituted either of a solid copper wire or made of a thin stranded steel cable (e.g. in #14-20 AWG), if possible protected with a PVC jacket and against UV radiations. Such cables will never break under bad weather (tempest, grail, ice...).

Insulators and anchorage points

The role of insulators is to... insulate electric circuits from ground and preserve its properties. If you don't need to attach an indoor wire antenna to insulators, the room and material used being usually dry, for an antenna located outdoor, the ends can be at a high RF potential or accumulate much static compared to the ground. Even if a loss of power has not much importance in receiving (excepting that the pick up radiation pattern will be affected), you must know that any permeable object exposed outdoor and mainly under a wet climate absorbs moisture. And in winter, these accessories might break.

Therefore to work in the best conditions, the insulators at which your antenna will be attached must be specially made to not absorbing the moisture; they are usually made of glazed porcelaine, glass or plastic. As displayed in the next drawing, you can made them yourself with small plates of plexiglass (A) or with a PVC pipe like the ones used in plumbing, in which you will drill two or several holes to attach the ends of your wire. The same insulators can be used to attach the center part of a dipole too.

Some useful accessories to attach a long wire to a mast or a wall. Refer to the text for more information. Item are not at scale. A: insulator, B: clamps, C: turnbuckle, D: pole with guy colar, E: L-bracket anchor bar, F: screw-eye, G: pulley with counterweight, H: L-screw with insulated loop wire. Document T.Lombry.

All fixings and anchorage points must resist to mechanical traction and pressure. First, if you soldered one or more wire segments together, reinforce these weak points with mini-clamps (B). They come in various models and sizing (e.g. like mini U-bolts or rings to screw, the first being more suited in this case). To manage the tension on the wire you can also insert behind the insulators a long spring or a turnbuckle (C).

For a durable installation, if you use a long pole or a telescopic mast, install on top and at mid-height a guy colar (D). To avoid that the pole falls or break down, tight guy wires over a distance of 75% of the height of your pole. At that length for a light installation drive stainless steel L-bracket bars 1m long (E) into the ground. Drive them on 3/4 of their length, preferably maintained in a small concrete base.

If you use a supporting wall drill a hole in the cement or the brick and use a heavy screw-eye (F) on which you will attach your insulator and the wire. As an alternative, if you wire goes down vertically, you can also use a pulley (G) and a sturdy post. Pass your wire through the pulley and attach it to the post, a counterweight attached to the pulley maintaining the wire under tension.

At last if you need to bend one segment of the antenna without cutting the cable itself, you can insert an insulated loop segment (H) at that place and attach it to a support with a L-screw, optionally attached to another screw-eye. In reducing the size of this loop you will tight the wire as much as you need without having to cut it.

Next chapter

How to select your antenna ?

Page 1 - 2 - 3 - 4 -


Back to:

HOME

Copyright & FAQ