Posted by
Ron D'Eau Claire-2 on
Jan 05, 2006; 7:18pm
URL: http://elecraft.85.s1.nabble.com/T1-Antenna-Considerations-tp385235p385243.html
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
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