Contents How the PA0RDT Mini Whip
Works The PA0RDT Mini Whip
This is a
picture of the PA0RDT mini whip, an active antenna for the VLF and shortwave
bands, together with its power feed unit: The whole antenna is smaller than a ballpoint pen! How can anybody
believe this toy can replace a dipole of full length?! But
yeeees, if we think of the wavelength in the VLF segment and even in the
lower shortwave bands – is there a little chance that this antenna works and
solves our problems with the landlord being not very enthusiastic about our
nice wire entanglements? Let us
look at active antennas and how they work. Fig. 1: The PA0RDT antenna together with its power feed unit to
the right How Active Antennas Work
Assume a full-sized antenna
picking up a signal and external noise feeding both to a preamplifier stage.
The signal to noise ratio (SNR) is determined by the external noise as the
noise added by the preamplifier is of much lower level (see Fig. 2). Fig. 2: A full-sized antenna picking up a signal and external
noise Now think of the antenna to be
shortened to a whip of, say, 2 m length and the preamplifier being the
amplifier of an active antenna. Because of its reduced length, the antenna
achieves a much lower signal strength. But – and this is crucial – the
external noise is on a much lower level, too. So the signal to noise ratio
(SNR) remains the same (see Fig. 3). With both
antennas, the full-sized one of Fig. 2 and the shortened one of Fig. 3 you can achieve the same
sensitivity [1]! Fig. 3: An active antenna picking up a signal and external noise For sure,
an essential condition for this statement is that both antennas are in the
same environment (outside the house). Do not think of the active antenna to
be placed beneath your plasma tv because it is so small and then blame it to
be of lower performance! Now we will go a step further to
the point, where the active antenna becomes too short (see Fig. 4). When
shortening the antenna again, the level of external noise and of the signal
is decreasing more. At that point, the amplifier’s noise level becomes higher
than that of the external noise. Now the signal to noise ratio (SNR) is
lowered and small signals that could be detected with a longer antenna are
now buried in noise [1]. Fig. 4: An active antenna, too short, reducing sensitivity How the PA0RDT Mini Whip
Works
Roelof,
PA0RDT, uses a J310 JFET transistor as first amplifier stage of his active
antenna [2]. This semiconductor is designed for VHF/UHF
amplifiers and adds only low noise to the signal picked up from the antenna.
The external noise at 30 MHz
(upper shortwave band) exceeds 20 dB and at 100 kHz (VLF) the external noise
reaches a magnitude of 90 dB [1] in relationship to
the thermal noise at standard temperature conditions. With this
low noise semiconductor being the first amplifier stage, Roelof could shorten
his antenna to an extent where the external noise still gives a higher
voltage than the internal noise produced by the UHF transistor [3]. At that point, the antenna is still performing as
depicted in Fig. 3. Roelof has run lots of
experiments to prove that the length of the (shortened) antenna does not
affect the ratio between the wanted signal and the external noise, in other
words: has no impact on the sensitivity of the antenna. Read Roelof's full report on his
experimental results. Compared
to other active antennas, the PA0RDT Mini Whip is very small. This is because
it has a copper plate in place of a rod acting as antenna. As any other
shortened (active) antenna, the PA0RDT Mini Whip is a capacity coupled to the
electromagnetic field. The electromagnetic field doesn't care if this
capacity is formed as a whip or as a copper plate - it works in both cases.
;-) When you
place this tiny antenna outside the house (!), it will perform like a
full-sized antenna for the VLF and shortwave bands. Take advantage of the
electric shielding a building provides. Local noise, generated by electric
and electronic components belonging to modern life, is attenuated markedly by
walls [1, 3] So put
your active antenna out, out, out of the house as high as you can! Get
it attached to a broomstick and fix it in the attic window! For more
information on the PA0RDT Mini Whip, write an email to Roelof Bakker, PA0RDT, roelof+++ndb.demon.nl [1] ITU-R:
Radio Noise; Recommendation ITU-R P.372; P Series; Radiowave Propagation [2] Roelof Bakker, PA0RDT: The PA0RDT-Mini-Whip [3] Roelof Bakker, PA0RDT: The PA0RDT-Mini-Whip,
an active receiving antenna for 10 kHz to 20 MHz [4] Pieter-Tjerk de Boer, PA3FWM: Fundamentals of the MiniWhip antenna |