As usualy, you did say it better then i can with my restricted english Ron :-)
Have seen your mail after I have written my answer but I thing doubling the knowledge makes it stronger :-) Peter www.qrpproject.de > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] On Behalf Of Ron > D'Eau Claire > Sent: Thursday, January 05, 2006 8:19 PM > To: elecraft@mailman.qth.net > Subject: RE: RE: [Elecraft] T1 Antenna Considerations > > 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: Elecraft@mailman.qth.net > 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 > _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net 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
