On Fri, Jan 24, 2003 at 06:51:39AM -0800, Aerofoam wrote:
> A few years ago some of us experimented with adding an equal length lead
> from the ground on the rx and
> created a true dipole on the receiver. Does this negatively alter the
> impedance?
> (the range was significantly better)
>
> Mark Mech
> www.aerofoam.com
Hi Mark,
The dipole was a good idea, might try that one myself >:-))
Being relatively new to R.C. flying I have not had/needed reason to
play with the radio gear... too busy learning to fly off the slope.
Flying is easy... it was mainly a matter of getting to grips with
the landing bit !!!
A very worthwhile advantage of the dipole is to lessen the effects of
inteference. It would also be feasible to run one element down each
wing.
The dipole normally has an impedance of 50/75 ohms at a *specific*
height above ground. Taking into account the fact that model planes
are flying at varying heights then the impedance of the antenna is
going to be varying widely as well, at some heights there will almost
certainly be a good match with the input stages of the receiver.
At other heights there will be a poor match and signal transfer from
antenna to receiver will not be as good. This applies equally to
the shorter 1/4 wave antenna normally used. As I stated above...
the dipole has the advantage of reducing interference.
As usual it is all a matter of compromise.... what will work under the
circumstances.
It is possible to take any length of wire and a decent earth system
and use a few extra components to tune that piece of wire to be
effective on a wide range of frequency bands. However it is not easy
to do that within the confines of a model plane !
When antenna parameters are quoted they are normally only accurate
for an antenna at a particular frequency, at a particular height
and with a particular shape.
A bigger antenna has a better capture aspect (think of the aperture
on a camera bigger=more light). A dipole (which is a 1/2 wavelength)
is better than a 1/4 wavelength antenna.
Sorry for the long message... it is a complex subject. As a radio
ham I only have *some* limited knowledge around it >:-))
Regards,
Reg
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