| This is about using an End Fed antenna
on 80 metres with a Paraset. What is an end fed? Well it is quite simply
an antenna wire fed by the transmitter at one of its ends the other end hanging
free. By the way, the end fed antenna cannot work properly unless the transmitter
chassis is earthed. So when using an end fed make sure you have a good earth
or fit a counterpoise. A counterpoise is simply a length of wire laid on
the ground and connected to the earth connection on the Paraset. Since the
band in question is 80 metres the counterpoise will be shorter than the resonant
length for this band as it is laying on the ground and so has a high capacity
to earth. It is not connected to earth just laid on the ground. After
experimenting with different lengths it has been found that 33 feet of
counterpoise makes the Paraset with its end fed antenna perform
well.
The antenna circuit of a Paraset consists
of a variable capacitance C and a fixed inductance L and this arrangement
can bring an end fed antenna to resonance by adjusting the variable capacitor.
See the diagram. The resonant point is clearly shown by the antenna current
light bulb glowing fairly brightly as one adjusts this variable
capacitor.
The fixed inductance L performs two functions
in a Paraset. Firstly it is used as a coupling coil allowing the transmitter
tank to transfer energy to the antenna circuit. Secondly it lengthens the
antenna wire electrically and this together with the variable capacitor allows
the Paraset to resonate various lengths of wire on the 80m hamband band.
So what lengths of end fed antenna can
be tuned resonance? Test reveal that the shortest end fed that can
be tuned for maximum antenna current is 40 feet. At this antenna length
the antenna tuning capacitor is fully meshed. The longest antenna length
that can be tuned for maximum antenna current was found to be 82 feet. At
this length the variable capacitor is nearly fully unmeshed.
From this then we can conclude the Paraset
will load any end fed on 80m that falls between 40 and 82 feet in length.
Most radiation of the signal occurs at the end closest the transmitter and
so it makes sense to get the antenna going vertical as soon as leaving the
Paraset! The far end can be horizontal if need be.
It is also worth experimentation with
the length of the counterpoise.
Above all have fun with antennas...try
some others....like a dipole....now that is a good antenna! |
2009
