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Dxing from small garden (page 2)

1. Antennas (cont.)

So what about the low bands (160,80,40)? If you want to put out a decent signal on these bands from a small garden there is really only one choice, some kind of vertical antenna unless of course you can get a dipole more than 1/2 wavelength above ground (feasible for 40m!). There are plenty of commercial and homemade designs to choose from but the golden rule for any vertical (particularly ground mounted 1/4 wave verticals) is to spend some time constructing an effective earth system.

Good ground system essential

If you go out and spend heaps of money on a fancy vertical and then stick one copper pole in the ground as an earth (like some people I know!), you really are pissing in the wind, unless of course you live in a salt marsh or by the sea! Many new licencees will often fall into this trap, they buy a vertical and provide an inadequate ground or radial system. Yes of course you may work some nice DX on the higher bands particularly during sunspot maxima, but with a poor ground system don't expect to work DX on the lower bands at ANY time of the sunspot cycle.

Most verticals require the ground to make up the missing 1/4 wave of a dipole, and good ground conductivity is therefore a must. You can improve ground conductivity by adding radials. Ideally you should have between 90 to 120 radials of 1/4 wave in length and these can be bent to fit the shape of the garden if necessary. At a bare minimum you need to get around 30 radials buried just under the surface extending out from the antenna although . The fewer radials you have the shorter they should be (but not less than 1/8 wave). A 1/4 wave vertical with 16 radials of 1/8 wavelength will have an efficiency of around 50%.

If you can, interconnect all the radials like a spiders web. You can also use chickenwire (not made of steel) to achieve a similar effect; in fact 1/4 wavelength of chickenwire extended out in all directions from the base of the antenna would provide the best practical earth (other than sea water!).

The alternative to burying radials is to elevate the feed point to around 15 feet and add four 1/4 wave radials per band (I have achieved reasonable results with 3 radials per band and a feedpoint 10 feet above the ground). Note that elevated radials will be shorter than those buried in the ground or laying on top of it, indeed the higher the feed point the shorter the radials will become. Use of a grid dip oscillator is useful in this case. You may also find that the length of the radiator will differ.

Some commercial verticals (usually an electrical 1/2 wave) claim not to need radials or an earth. Whilst you might get the thing to tune without radials or an earth, the efficiency will be compromised without them.

Horizontal or vertical?

In some circumstances you may find that a vertical will perform poorly even with an effective ground system. This is due to poor earth conductivity in the 'far field' away from the antenna. What happens here is that downgoing waves launched from the antenna are reflected from the ground and combine with direct waves (radiated at angles above the horizon). If the reflected waves are out of phase (which is what happens with poor earth conductivity) there will be cancellation and loss of signal. There is little that the city dweller can do about ground conductivity in the far field and in this case a horizontal antenna may give better performance. In such a case go for an inverted V configuration and try to get the feed point as high as possible and the ends of the dipole as far away from the ground as you can.

If you want the dogs danglies on setting up a vertical for the low bands check out 'Low Band DXing' by ON4UN published by the ARRL.

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