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From
Longwire to Yagi Longwire Every amateur radio working on HF knows this aerial : take a very long copper or steel wire, connect one end to the transceiver and attach the other end to a distant support. It becomes a longwire mainly used by amateurs (licensed as well as listeners) who do not want to invest much money in their aerial and who are not DX addicts, although a very long wire well tuned works fine in DXing too. This wire can be a simple electric wire 1 mm thick (#14 or #16 AWG) the longer as possible and not necessary tight in straight line. The far end can be attached to an insulator or simpler in making a simple knot around a tree, a pole or a fence. Of course to work in good conditions, to match impedances, get a low SWR, transmit all the input power to the antenna and without RF radiation from the "feed line", additional accessories are necessary between the antenna and the transmitter.
Installed in a few minutes, a longwire requests no special size or adjustment. It radiates best on any frequency for which its overall length is equal or greater to 1/2l. So a 20m long wire will work perfectly to transmit from the 40 to the 10m band with a low SWR. However, as a longwire displays usually a very high impedance, mainly when short (including windom and other shortened wire antennas), like a dipole you need to reduce the extremely high impedance to a range that can be matched with your antenna tuner (external A.T.U. or built-in in your transceiver). Recall to novices or casual amateurs that in this matter, even if your built-in antenna tuner shows a very low SWR, it can be sky high to the antenna. To get confirmation, you must use an external SWR-meter, if possible connected close to the antenna feed point. In this way you will know the true values of forward and reflected powers and your SWR. Without matching the antenna impedance with the one of the transceiver, don't be surprised if you get a very high SWR at the antenna and unable to work DX stations over a few thousands kilometers : you are losing most of your output power in heat with high currents flowing in your system. Avoid to work this way or you might damage your transceiver and be injured.
So solve this problem you need a matching section between your antenna and your transceiver. You can use an open-wire feed line with a 4:1 balun and a coaxial to get both a 50-ohm impedance at the transceiver antenna terminal and a very low SWR close to 1:1 on any band. Now your antenna will radiate all the output power, 100 W PEP instead of 50 or maybe only 20 W; two or five more power is worth well some tuning, Hi ! This solution requesting some accessories and changing somewhat the original longwire design, another method is to use an MTFT magnetic balun like the one display at right. This accessory is a resistance transformer with a ratio of 10:1; you will get no more problem to reduce the extremely high impedance of the wire to a value close to the one supported by your antenna tuner. This magnetic balun allows to get a SWR better than 1.5:1 on all bands. At last, there is the solution of connecting several wires of different lengths (at least 6m long) to the same antenna terminal. The resulting antenna impedance should be reduced because all impedances will be added in parallel. Whatever the solution used to match the impedances, this simple wire gives astounding results. Here is an example that surprised my correspondents by the quality of the audio (mainly due to the transceiver Kenwood TS-570D) and the strength of the signal (performance shared between the transceiver and the antenna). I built a longwire using a roll of 15m of ordinary stranded electrical wire (about 1mm thick) protected with a PVC jacket. Living in the country, on top of hills, 250m over Meuse valley, I tight this simple wire outdoor at 4 m high over the garden, passing over a tree and attaching one end to an appletree branch, the other side to an MTFT balun connected to a short coax length linked to my transceiver antenna terminal. The fire angle was approximatively NE-SW with a main lobe at 90° due to its low height over the ground. Beleive me or not, used in SSB with 50 W PEP, less than 30 W out (average reading), I worked with ease all European countries and even some more ! In CW this wire could work the world !
Used during one week in summer 2004 this modest antenna worked fine taking into account its setup and the poor propagation conditions. I worked stations from Ireland to European Russia and Turkey to Portugal, so up to 2200 km away at daytime with a signal between 56 and 59+. Leaving Europe was another affair but I was able to reach the Canary islands, Kazakhstan (3000 km) and Asiatic Russia (5000 km) using sometimes the grayline. But there is no secret. Due to its too short length and low output power, in SSB this "longwire" was unable to reach Canada, Middle East, Central Africa or Asia for example, but at this time the ionosphere was not in good shape what affected also its performance, but doesn't explain all. Indeed, with the same low power, if you can install a longer wire (say 40 m long), place it 0.7-1 l high and properly tuned on each band, this longwire can compete against a dipole or a vertical and even give better results if it is very long (> 5-10l). The best proof, using a 40m longwire at the first calls I reached Algeria, Canada and French Guyana (7500 km) with signals up to 58. This confirms two points. The first and essential, that feeding correctly your antenna improves much its performance, allowing it to radiate correctly into space all the transmitter power. Second, extending the length of a longwire or a dipole antenna and placing it higher over the ground improves much its radiation pattern, allowing DX activities. In fact the longer the longwire, the sharper the lobes, with an optimal length that can exceed 10l (200m to work on 20 meters !). What can we say about the other designs ? Most if not all longwires operate like a transmission line terminated in a open circuit; they are not terminated with a non-inductive resistance. Such longwires are also named "standing wave antennas".
A longwire 1l long placed 1l high shows a front-to-back ratio of about 3 dB, thus 0.6 dB more than a 1/2l dipole placed at the same height. In practice both azimuthal and elevation radiation patterns of a longwire placed 20m high and working on the 20m band show from 2 to 8 lobes if we respectively extend its length from 1/2l to 8l. A 20m long wire (1l) placed 1l high, shows two lobes at 15 and 45° of elevation as displayed at right. Its azimuthal radiation pattern shows a double-8 shape (35°, 140°, 220 and 320°) where a dipole of the same length shows a simple ¥ (90 and 270°). In this configuration the advantage is already to the longwire over the dipole. A 100m long wire (5l) placed 1l high shows a main lobe at 10° of elevation plus 4 narrower ones at approximatively 40, 50, 70 and 80°. Its horizontal pattern shows up to 16 secondaries lobes where a dipole shows always a simple ¥. Inverted-L, V-beam and rhombic The gain of a longwire can be optimized by tilting it of a few tens of degrees or bending one segment at 90°. However, the benefit can be nulled and even negative due to the proximity of the ground. Several variantes of the straight longwire can be thus successfully tested, depending on your free space. An inverted-L (G) performs rather well if the vertical section can be 7 m or more long, and away from the building wall. Complementary, the combination of horizontal and vertical polarizations increase its sensitivity to DX signals as polarization from far emitters signals can vary considerably from minute to minute according propagation and the way that radio waves are reflected through ionospheric layers and cross the geomagnetic field.
To get more gain and directivity you can also combine several longwires together, using parallel wiring, a V-shape (V-beam) or a horizontal diamond shape (rhombic) to increase the intensity of some major lobes and cancel others. Using for example 2 legs 1l long, tight 1l high and forming an angle of 75°, you increase the two opposite horizontal lobes (at 90 and 270°) at decrease of about 15 dB the intensity of the two perpendicular lobes (at 0 and 180°). If legs are terminated with a 500 to 800-ohm non-inductive resistance, there are no more standing waves, and you get a very directive V-beam which displays about 2 dB more gain than a 1/2l dipole and only 4 dB below a 4-element Yagi. But remember that placing a longwire (or even a beam) too close to the ground (say below 1/2l or 10m high) will drastically impact its radiation patterns on both E and H planes. Recall that the usual feeding method of a V-beam is using a quater-wave matching section. A open-wire line connected to a balun or a MTFT magnetic balun with optionally an antenna tuner are as many alternative. NB.
We will review the Beverage when we will discuss about wire
antennas for listeners as due to its conditions of use it is
first of all dedicated to listening purposes. Indoor
aerial As
explained in another page dealing with wire
antennas for listeners, if you have no place left to install a
long wire antenna
outdoors you can
tight it in a restricted loft space, e.g.
under the roof, in the attic or in an elevated spare room. As such the wire is very sensitive to artificial
noises and if you experiment severe QRM replace the lower part of the aerial
with a coaxial cable which braid is connected to the ground.
Practically using a 50-ohm RG-58 coax (insulation black colored) the
grounding can change the QRM from S-9 to S-4 or even less, allowing
weak signals to be heard. In smaller rooms up to 16 m2 you can strung a similar aerial around the room at ceiling level, at the condition to use at least 12 m of wire, the longer the best. However do not expect miracles, mainly in SSB You might work stations in bordering countries and will be able to ragchewing in regional QSOs but not more. The main reasons for which indoor antennas do not work well are multiples : - The antenna is placed at a low height above ground in terms of HF wavelengths - The antenna is usually small in terms of HF wavelengths - The antenna works close to sources of RFI, it can couple to electrical wiring and other gutter and besides lossy dielectric materials like masonry, plaster and wood. - The antenna cannot be rotated. Among its advantages list the next facts: - The antenna is invisible from the outside (like stealth !) - The antenna is out of weathering effects (wind, rain, ice, etc) - The antenna can be made of low resistance material as it only needs to support its own weight - The supporting structure is in place - The investment is reduced compared to an outdoors installation that requires a mast, fixings and often foundations - The antenna is accessible all time what simplifies its tuning.
Some amateurs installed a shortened HF beam in their attic too. As we saw in the pages devoted to portable installations, several 3-element beams offer indeed a turning radius less than 3 m (e.g. Cushcraft MA5B, Butternut HF5B, TGM MQ-36SR). However, set up a beam, even short, in an attic or in a spare room is like oblige a fish to live out of the water ; this is unthinkable. Not only a beam is bulky but most of the time the parasitic elements (the reflector and directors) will couple to the house wiring or the roof structure will cause dielectric loading of the parasitic elements which, for once, deserve well their name; they will generate QRM. This is mainly the case if the room in which you install the antenna but also the near rooms contain conductors near a quarter wave in length or longer at the working frequency (5m to work on the 20m band). Unfortunately I bet that your house is full of electrical wires and plumbing measuring at least 5m long. You will experiment losses and your antenna will radiate less or no energy at all into space. In these conditions an external antenna tuner can help you to fine tune the impedance of your system. So the best alternative should be to install an horizontal or vertical loop antenna (see next page). With a diameter that does not exceed 1m in HF, this solution has been experimented successfully by several amateurs. Kim Stenson, W4KVS, for example related his experience on the 6-m band in the pages of QST in March 2004. He concludes that the M2 HO 6-m loop set up in the attic has given him good signals but his short Cushcraft 6-m beam, also installed in his attic for a while, pulled in the weakest signals better. Of course all these considerations are mainly valid for a transmission antenna. For receiving all these solutions work very well There is only one constraint installing your emission antenna indoors, this is the danger of RF radiation to not disregard. This is mainly the case using a single-wire feed line that is not well balanced and creates a potential radiation hazard, with a short Yagi that concentrates all the energy in a narrow beam, and with a magnetic loop that generates a strong magnetic field. Here are some useful recommendations.
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