BETTER THAN A RUBBER DUCKY, Duckie.
If you're sitting in the dentist's chair and a satellite pass is imminent,
even the LA2QAA
pocket beam is oversized (especially if the dentist is in the process of
wrenching out a stubborn root) and a rubber ducky isn't worth a quack (duckie)
... unless of course your callsign is PH7PCF ... (Aldert Kwak).
Nota Bena.
(It's his SWAN song......LA2QAA).
(Or his Cygnet- ature tune......GM1SXX).
Taking into account the above - therefore, the logical halfway point between
a 2 element beam and a rubber ducky (duckie) is a halfwave dipole. A 1/2
wave dipole has a little gain over a ducky (duckie) and it's quite easy to
match.
As we all know (don't we?) a dipole is balanced whereas 99.9% of all
transmitter outputs and 100% of coaxial cable are unbalanced, therefore,
ideally, for maximum power transfer - and to stop currents raisin: (!) a
ruckus (radiation from the transmission line - no matter how short) ideally,
you need a BALUN. - (BALanced/UNbalanced).
Currents on even a 'short' transmission line can cause havoc to poorly
shielded VFO's.
A 'RESONANT' halfwave dipole will have an impedance of aproximately 73 ohms
and to match it to the 50 ohm characteristic impedance you can lengthen or
shorten the split driven elements to introduce either inductive or
capacitive reactance.
To directly match to 50 ohm the elements should be 0.52 wavelength and the
'simple' formula for finding a halfwavelength is 145.5/f where f is the
frequency in megacycles - oops !!!, sorry, I mean hertz ... the answer being
in measured in meters (or centimeters at VHF).
A suitable CURRENT balun is usually a few turns of the feedline on a ferrite
ring, alternatively the feedline can be passed through a two ferrite sleeves
over the feedline an 'electric' quarter wavelength from the feedpoint.
An 'electric' 1/4 wave is 71.3 (in meters) x the velocity factor of the
cable /f.
Experiment with the length of the elements to acheave a perfect match (don't
settle for anything less than 1:1( even though that 1.5:1 is every! bit as
good as 1:1) - the 'difference' is unnoticable ... but *DON'T* be satisfied
with 4:1 ... (just KIDDING!).
Too short a dipole requires a bit of inductive reactance, a dipole too long
requires capacitive reactance. The intelligentcia will soon figure out that
the 'resonant' 75 ohm impedance dipole can be suitably matched to the
frequency being used by adjusting
the length of the elements ... the loss of power due to impedance mismatch
will *NOT* be enough to melt a couple of feet of RG-58CU.
Take it from me - you *DON'T* want to be using RG-214 or Heliax for portable
satellite operations ... unless you resemble Hoss Cartwright of 'Bonanza'
fame.
Obviously a dipole isn't going to have the gain of an 'Arrow' (or an Elk, if
you're into Moose) but there are situations when more elements are just too
cumbersome (i.e. hospital operating theatres) ... or ... you just don't want
to look like a 'candidate' for Macclesfield. (Google it).
Nia Bena.
LA2QAA is from near Macclesfield ... (which explains a lot......GM1SXX).
73 John. <la2qaa@amsat.org>
PS.
I *AM* aware that if you want to split hairs the formuli 'could' be tweaked
... but the article is aimed at the potential newcomer to satellite
operating - as opposed to the know-it-all theoretician.
J.A.H.