"Lee" wrote -

i`m currently
playing with a helical on 40 and am thinking of making a dipole with
a
matching helical, i can get that in my postage stamp garden ......

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A back-to-back horizontal helical dipole is a good proposition.
Radiating efficiency is what matters with physically small antennas.
The radiating pattern is whatever you happen to get.

A 1/2-wave helical resonant dipole is considerably more efficient than
a 1/4-wave vertical simply because of the absence of ground connection
losses.

A low dipole does have a small ground loss due to its proximity to
ground. Ideally its height should be at least equal to its overall
length but half that is a satisfactory compromise.

A horizontal helical dipole is the same as a centre-loaded wire with a
very long loading coil. It is easier to match to a feedline than a
vertical and there is a choice of type of line and line impedance.

Either coax or balanced line can be used. Make the loading coil
continuous through the antenna centre and enclose the centre with a
link coupling to the feedline. The number of turns on the coupling
link is dependent on line impedance.

The antenna itself is electrically isolated from the line and remains
nicely balanced about ground.

The greater the diameter of the loading coil former, a rigid plastic
pipe, and the thicker the wire, the higher the efficiency. For 100
watts transmitters, close wound turns of thick enamelled wire are
usually OK.

Resonant frequency is adjusted by pruning the length of the wire or
rods which extend beyond the ends of the coil. If essential due to
space limitations, the end wires can be allowed to fall down towards
the ground but not allowed to swing about in the wind.

Short loaded antennas are essentially single frequency, very narrow
bandwidth devices.

Download program MIDLOAD from website below. It may be of assistance
or at least provide you with something to think about.

In postage-stamp backyards, size-for-size, Magloops are the most
efficient of all very small antennas and can cover several adjacent
bands, but are deceptively difficult/easy/expensive to construct
depending on your ingenuity. Download program MAGLOOP4.
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Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
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