Posted by
Geoffrey Mackenzie-Kennedy on
Jan 04, 2008; 10:55am
URL: http://elecraft.85.s1.nabble.com/dipole-antenna-efficiency-tp458071p458120.html
Shaun Oliver <
[hidden email]>
> if I were to make a dipole, would coiling it round a piece of plastic
> piping decrease the efficiency of the thing? I intend on making one for 40
> meters, one for 80 meters, and one for 10 meters. I've worked out that1
> meter of piping will suffice for each leg of each antenna.
> thanks in advance.
> shaun
Cookie, K5EWJ wrote:
> Shaun, it will decrease the efficiency drastically. A
> two meter long 80 meter dipole will be about 10% as
> efficient as one 40 meters long at the same height.
> You
> can't cheat the laws of physics, they work the same
> even if no body is watching. In
> addition, the antenna you are considering will have an
> impedance of only an ohm or two and be almost
> impossible to match.
-----------------------------------------------------------------------------------------------
While Cookie's comments are true, there is a useful type of short antenna
known as a Normal Mode Helix, which if built, tuned and matched correctly
can work as well as a full size dipole at the same height - although its
VSWR (say 2:1 for example) bandwidth is narrower than that of a full size
dipole. It consists of wire wound around a fibreglass or plastic rod / pipe
with the turns spaced, not closewound. If plastic pipe is used it must have
good RF 'Insulating' characteristics i.e if a piece 'cooks' when placed in a
microwave oven, when your XYL is not looking, do not use that material. For
80m and 40m a pipe diameter of 2 1/2 inches is about right, for 10m 3/4 inch
or thereabouts. The total length of wire in a NM Helix 'dipole' is
approximately one wavelength, equations do exist which could be used to
calculate the length but I have yet to find an equation which is exact
enough to eliminate the need to tweak. The length of pipe for a 40m NM
Helical 'dipole' is close to 6 metres, the length could be reduced by
increasing the pipe's diameter but there is a limit beyond which the
radiation pattern suffers. Very short and effective NM Helix antennas have
been built using additional end loading such as hats, but their drive point
impedance is very low and difficult to match efficiently and the VSWR
bandwidth becomes narrower.
The 'Rubber Duck' antenna sometimes used on mobile phones is one type of NM
Helix antenna.
73,
Geoff
GM4ESD
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