This active Antenna has been doing its job in my loft for well over 10 years. l’'ve seen anumber of designs over the years, and this is the simplest, deriving from experiment. When | recently saw a highly sophisticated-looking commercial unit, | felt it was time to ‘go public’.
Active antennas rely on a combination of an antenna element (Such as a dipole, monopole, orloop) and anamplifier, which is the ‘active’ part. The antennaelementis non-resonant, and tends to be physically small. They have broad operating bandwidths, so don’t need to be tuned. In comparison, a resonant antenna would need tuner adjustments to cover the whole HF and lower VHF spectrum. So, the attraction of active antennas is convenience.
It is only fair to point out that some people dislike them, and there are pitfalls, which | shall point out. If you want a really excellent receiving antenna for all the HF amateur and broadcast bands, and have masses of space, why not put up a Beverage or rhombic antenna? If, as in my case, that’s out of the question, then consider an active antenna and, better still, try building your own! This one can be put together in a few hours and covers 160 to 4 metres.
Active antennas rely on a combination of an antenna element (Such as a dipole, monopole, orloop) and anamplifier, which is the ‘active’ part. The antennaelementis non-resonant, and tends to be physically small. They have broad operating bandwidths, so don’t need to be tuned. In comparison, a resonant antenna would need tuner adjustments to cover the whole HF and lower VHF spectrum. So, the attraction of active antennas is convenience.
It is only fair to point out that some people dislike them, and there are pitfalls, which | shall point out. If you want a really excellent receiving antenna for all the HF amateur and broadcast bands, and have masses of space, why not put up a Beverage or rhombic antenna? If, as in my case, that’s out of the question, then consider an active antenna and, better still, try building your own! This one can be put together in a few hours and covers 160 to 4 metres.
DESIGN CONSIDERATIONS
THE CHOICE of a small antenna element (less than a tenth of a wavelength or so) is between the dipole and monopole, which respond to the electric field component of the radio wave; and the loop, which responds to the magnetic field component. A broadband active loop antenna is still on my list of things to try.
My first homebrew active antenna was a dipole, and was quite successful. The main thing it taught me was thatit’s nota good idea to have too much gain. It is natural to conclude that, as a short antenna picks up a smaller signal than a resonant dipole, the gain must be made up in the amplifier. Being a broadband device, the amplifier is subjected to the entire HF radio spectrum including powerful broadcast transmitters. What tends to happen in practice is that it distorts, generating intermodulation products. These appear to the receiver as additional signals and, though giving the impression of a ‘lively’ receiving system, are entirely unwanted. An attenuator between the active antenna and receiver is of no use at all, if the distortion has already happened in the active antenna.
It occurred to me to try asingle wire monopole, which made for a simpler amplifier. This worked and has been in use ever since.
My first homebrew active antenna was a dipole, and was quite successful. The main thing it taught me was thatit’s nota good idea to have too much gain. It is natural to conclude that, as a short antenna picks up a smaller signal than a resonant dipole, the gain must be made up in the amplifier. Being a broadband device, the amplifier is subjected to the entire HF radio spectrum including powerful broadcast transmitters. What tends to happen in practice is that it distorts, generating intermodulation products. These appear to the receiver as additional signals and, though giving the impression of a ‘lively’ receiving system, are entirely unwanted. An attenuator between the active antenna and receiver is of no use at all, if the distortion has already happened in the active antenna.
It occurred to me to try asingle wire monopole, which made for a simpler amplifier. This worked and has been in use ever since.
CIRCUIT DESCRIPTION
THE AMPLIFIER, shown in Fig 1, is a source-follower circuit designed around Tr1, aJ310 FET (field-effect transistor). This has to present a high impedance to the small monopole, otherwise signal voltage is lost, and then deliver the signal to the receiver input, commonly a 50 Ohm impedance.
The FET has an output impedance in
the region of 50 to 100 Ohm , which means
that, if the FET source fed the receiver
50 Ohm input, more than half the signal voltage would appear across the FET, and
less than half would be delivered to the
receiver. That's where the transformer
T1, in the source circuit, comes in. I used
a quadrifilarwinding to give a 4:1 voltage
step down ratio. This gives the source
follower an overall gain of almost 1/5 in
voltage (-14dB when the ratio of gate
voltage to output voltage is expressed in
decibels).
The benefit of doing this is that the FET
has much less work to do. The action of
the transformer makes the impedance
presented to the FET source bigger by a
factor of 42 = 16 times, which is 800 Ohm.
The result is improved linearity. Locations differ, but I have never known the
active antenna produce unwanted signals. You may be concerned that this low
gain would produce a rather 'deaf' receiving system but, from experience, comparing it to a transmitting dipole and tuner,
you won't miss much, if anything. The
internally-generated noise is very low,
and the background noise in most of the
HF spectrum is high.
Power to the active antenna is fed via
the coaxial cable, and the supply is
injected via choke RFC1 housed in the
power-feed unit near the receiver. R4 is
included to limit the current in the event
of an accidental short-circuit. An LED in
series with the supply indicates that current is being
drawn, and protects against inadvertent supply polarity reversal.
Shown on the circuit diagram is a
power feed for a receiver. This is for
the case where the
receiver and antenna can share
the same power
supply. You may
choose to omit it.
The frequency
response is shown
in Fig 2, and is
nominally flat to
within 1dB to
60MHz and within
2dB to 100MHz
CONSTRUCTION
TRANSFORMER T1 requires some care
in construction, and is described in some
detail, starting with the quadrifilar wire
itself. This would probably be a labourintensive and expensive item to produce
commercially, and is where the amateur's craft skills come into their own.
Take four strands of 0.2mm diameter
(35/36 SWG) enamelled copper wire,
length approximately 300mm for each
strand. Placing the wires side-by-side,
clamp one end and, pulling the wires
taut, fix the free end in the chuck of a
hand drill. Turn the drill to twist the
strands together. There is no need to
twist too tightly, a few twists per centimetre being adequate.
The core should be a high-permeability (greater than 100) ferrite toroid, 10 to
15mm in diameter. The purpose of the
core is to produce a sufficiently high
inductance to avoid gain roll-off at low
frequencies, and given a high enough
permeability, a wide variety of types, still
to be found at rallies, should be suitable.
If you are buying new, Table 1 shows the
types that should be suitable. Supplier
contact details are given at the end of the
article. Between them, they should be
able to source all items needed for construction.
Wind seven or eight turns of the
quadrifilar wire on the core. (Each time
the wire passes through the core counts
as one turn.) The photograph shows
how this has been done on a T37 -61
core. To secure the winding, the core
has been dipped in polyurethane varnish
and left to dry.
The individual wires need to be separated and the windings identified. Each
wire end should be stripped of its insula tion. An easy way to do this is to hold the
wire end in a blob of solder on the end of
a soldering iron for a few seconds. Make
sure you do this in a well-ventilated area
and avoid inhaling the fumes or getting
them in your eyes.
The ends of each winding can then be
identified with a multimeter or continuity
tester. I found it useful to markthewindings
with short strips of insulation stripped
from ribbon cable, and slid overthe wires
as shown in the same photograph. If you
do this with three windings, the fourth can
be left plain.
Naming the windings arbitrarily 1 to 4,
take the end of winding 1, and twist
together with the start of winding 2. The
end of 2 is then twisted with the start of
3, and so on. Twist fairly close to the
toroid, and make electrical connection
using the soldering iron, as described
above, observing the precautions. The
transformer is now complete. Check for
electrical continuity through the whole
transformer by measuring across the
un-paired wires.
Once the transformer is done, the rest
of the construction is straightforward.
Start with R2 as 47 Ohm or 68 Ohm . It may need
to be changed on test. The photograph shows my loft unit built into a
diecast box, with a couple of solder tags
for earthing to the box. Alternatively, the
circuit can be built above a small piece
of plain copper-clad board, which can
then be fitted inside a weatherproof enclosure if outdoor mounting is required.
The enclosure itself can be plastic - it is
an antenna after all!
Make sure you select the correct tap on
transformer T1 for the output, and take
care to prevent the unused taps from
shorting to any other part of the circuit.
THE POWER FEED
AS THE PHOTOGRAPH (right) illustrates,
I built my power feed unit in a small plastic
box. The choke isa single winding of around
20 turns on another high permeability toroid,
which can be the same type as that used
for the transformer. A metal enclosure would
make sense for the power feed unit, since
it will be near the receiver and possibly also
domestic interference sources. If using a
plastic housing, link the coaxial sockets
with coaxial cable: I used some RG178.
Keep the braid 'tails' short to avoid unwanted pickup.
TESTING AND
COMMISSIONING
CHECK CAREFULLY forwiring errors. For
bench testing, the powerfeed and antenna
units can be linked with a short coaxial
cable. Having ensured that its voltage and
polarity are correct, connect the power
supply and check that the LED is lit. Measure the voltage across R2 and divide this by
its resistance to find the current, or measure the supply current directly. This should
be in the region of 10 to 20mA. I selected R2
for a current of around 15mA.
Ifthe power feed output is now connected
to a receiver, a small amount of additional
hiss should be heard. Nothing should be
heard until a short wire (1 metre or less) is
placed on the antenna input. Signals should
be heard on the HF bands, given suitable
propagation conditions, or perhaps television or PC monitor timebase harmonics.
The antenna unit should be installed as
high and as far away from local sources of
interference as practicable. Mine is at the
apex of the loft, with an antenna wire of
around 1 metre length, suspended from a
hook in the highest beam. Avoid the temptation to increase the wire length excessively in order to increase the signal. This
brings the risk of distortion, and departure
from a flat gain with frequency.
from a flat gain with frequency.
POSITIVE - EARTH VERSION
WITH A NEGATIVE supply and positive
earth, a couple of components can be omitted. This is shown in Fig 3, below.
However, note that, while this is fine on its
own, it must not be connected to a receiver with a negative earth, because this
shorts out the supply
=* by Ian Braithwaite, G4COL *=
