Shortened antennas generally are most effective if the loads or matching
networks used to match the shortened elements are designed and employed
to minimize IR losses. A good approach to shortening a dipole is to add
capacitance to each end. This can take the form of zig-zagging the end
of each element or of adding multiple wires perpendicular to the element
(just like the capacitance hat for a vertical, only it is horizontal).
A very effective capacitance hat is an array of spokes, perpendicular
to the dipole element and connected by a perimeter wire. Any of the NEC
based antenna modeling applications model this easily and accurately.
Losses are low with this technique because current is low near the ends
of the dipole elements. It radiates well because the high current
portion of each dipole element is not compromised. The pattern is good,
also. Like any dipole near the ground (on 160M, most practical dipoles
are near the ground in terms of wavelength), the pattern is nearly
circular and the radiation angle is high. This is sometimes called a
Near Vertical Incidence Skywave (NVIS) antenna; it isn't the best for
DX, but it eliminates skip. It should be quite reliable to 300-500
miles distance and will sometimes work well beyond that. This is true
of any horizontal antenna placed at 1/4 wavelength or less above ground.
It is easy to match a short dipole with an L network at the feedpoint,
but the high current there makes it important to use very high quality
components to keep losses low. Similarly, adding either inductance or
traps (which appear as inductance at frequencies below their resonant
frequency) in the elements makes it easy to match the dipole to the
feedline, but the Q of the inductors used must be high to minimize losses.
Can you support either full circle capacitance hats or half circular
hats at the ends of your dipole? If so, let me know what length fits
best in your space and I'll run some models on EZNEC to find candidate
dimensions for the hats.
73 de WOØW
John von Gonten wrote:
>Well since I sent it to the wrong address the first trip let us hope I
>got it right this round.
>
>
>Has anyone dealt with or know what kind of performance
>Can be had from W9INN's shortened dipoles??
>
>I am looking for a way to get something in the air
>To use for 160M shows to have a 71' space-saver and I am
>Trying to figure out if it would perform well, or acceptably
>
>I am dealing with limited space, so I have to kinda squeeze and
>compromise a bit At least until I can move to a location with more room
>and more possibilities
>For Antenna placement
>
>Thanks in Advance
>
>------------------------------------------------------------------------
>| John von Gonten - N5BSD | ERROR 406: file corrupt:
>
>| Packet: | config.earth --- reboot
>universe?
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>
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>| WEB2 : HTTP://n5bsd.dyndns.ws |
>
>------------------------------------------------------------------------
>
> CH3
> |
> N
> / \
> N----C C==O
> || || |
> || || |
> CH C N--CH3
> \ / \ /
> N C
> | ||
> CH3 O
>Can you name this molecule that we all love??
>
>
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