Martin wrote: "...another antenna I have used is a 66ft length of wire at 15feet, fed in the centre with 300 ohm twinlead.
In this case, the driven half of the wire is still 33ft long - but I get really low SWR om 20m and have worked DX with it, So why does that work? Is the feedline doing something to help me out here? Or is a dipole just a completely different scenario altogether?.." ---------------------- You are describing a classic "doublet" antenna, Martin (a dipole is, by definition, exactly 1/2 wave long and may be fed anywhere: center, off-center or at the end. The "dipole" or "two poles" refer to the two electrostatic poles set up at the ends of a radiator when it is exactly 1/2 wavelength long). In your doublet both sides radiate, as you observed. Feedline losses are relatively low because it is of a relatively high impedance (compared to common coaxial lines), which reduces the SWR the feeders may experience under extreme conditions. For highest efficiency, an impedance in the 400-600 ohm range is often used. Another disadvantage of twin-lead is that its electrical characteristics may change significantly with moisture, snow and ice. True open wire with virtually all air dielectric is much more stable in that regard. In this case the feedline acts as an impedance transformer. In another post I explained how the famous Zeppelin antenna behaves with a 1/4 wave feed line transforming the very high impedance at the end of the 1/2 wave wire to a low impedance at the rig. The same thing happens here, although exactly what the impedances being 'seen' by the rig are change dramatically from band to band. Most hams simply add or, if possible prune, some feeder to find a length their matching network (antenna "tuner") can handle on all the bands they want to use. As the antenna is made shorter than 1/2 wavelength, the impedance at the center drops very quickly. However, if the missing length is made up in the length of the feed line (e.g. a 33 foot radiator and a 33 foot feed line will be very close to 1/2 wave long on 80 meters) the effect at the transmitter end will be very small. However, since radiation from the feedline of a center fed doublet is minimal if balanced feed is used, the field strength of such an antenna drops as the radiator part becomes shorter and shorter. It's not too bad as long as the radiator is at least 1/4 wavelength long. If I recall correctly, such an antenna is only about 1 dB (1/6 of a typical "S" unit) lower than a half wave radiator. So a 66 foot long doublet will do a very good job down as low as 3.5 MHz, especially if the feed line is at least 33 feet long. The other issue is height above ground. A horizontal antenna's pattern is controlled a great deal by the height above ground in wavelengths. A horizontal antenna about 1/2 wave above ground is FB for DX with lobes at fairly low angles for DX that are as much as 6 dB - a whole S-unit - stronger than you'd get from, say, a good vertical, thanks to the reflection from the ground helping the signal. As the antenna gets closer to the ground the maximum radiation lobe moves more and more vertically until, at about 0.2 wavelengths above the ground the lobe points straight up. That's not much good for DX but it's great for short skip contacts out to about 1000 miles as the radiation straight up is scattered and reflected off of the ionosphere. Hams setting up antennas specifically for working other stations out to about 1000 miles often purposely put them fairly close to the ground for just that reason. Below 0.2 wavelengths, the main lobe continues to point straight up but grows weaker as the ground losses increase. So your 15 foot high 66 foot long antenna was a FB 40 meter short-skip antenna and it probably got out quite well for skip out to about 1000 miles on 80. On 20 you start working more DX with 15 and 10 meters being excellent DX bands for it. On those bands it is high enough for good low-angle lobes and at that length the pattern breaks up into multiple lobes that show significant gain over a half-wave radiator. When speaking of lobes, keep in mind that the radiation from an antenna is never zero in any direction. There's always some radiation in every direction. The lobes only indicate that some directions are favored more than others. And, as the QRP and QRPpers constantly prove, miniscule signals can work the world under the right directions. That's why even a low antenna like yours can, at the right times, work DX on 40 or even 80 meters. Ron AC7AC _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
