http://elecraft.85.s1.nabble.com/T1-Antenna-Considerations-tp385235p385244.html
As usualy, you did say it better then i can with my restricted english Ron
> -----Original Message-----
> From:
[hidden email]
> [mailto:
[hidden email]] On Behalf Of Ron
> D'Eau Claire
> Sent: Thursday, January 05, 2006 8:19 PM
> To:
[hidden email]
> 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:
[hidden email]
> 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>
Subscriber Info (Addr. Change, sub, unsub etc.):