Varicap Tuned Active Balanced Loop Receiving Antenna Head Amplifier PCB Built And Tested.
Designed
by G6KSN, this is a remote varicap tuned balanced loop active receiving
antenna PCB. Typical frequency ratio is approximately 11:1 over 2 remotely swichable
bands so will cover 500khz to 5.8MHz for example. The frequency range
depends on the number of turns on
the loop and its diameter. Great for Medium wave
DXing, Top Band, 80 and 60 Metres.
The listing is for the 2 PCB's only (built and tested).
Ideal
for use in urban areas where noise is a problem. The loop antenna is
bi-directional with a sharp null that can be used to minimise
interference or null out unwanted stations. Also ideal for use with
SDR's, as the head end is tuned, overload is less of an issue. I use
mine with an RSPdx.
You
will need a regulated 12 volt DC power supply at 150mA max (not supplied). I recommend using RG174 coaxial cable and a common
mode choke on both ends of the coax to help with common mode noise. 75
mix toroids work well at these frequecies and you can get around 12
turns on a FT-87-75 Fair-Rite toroid when using RG174. You will also need 2 further cables to connect up the tuning voltage and the band switching signals. These do not need to be filtered or screened as the signals are both high impedance and filtered on board. These 2 connections do not induce common mode signals onto the antenna due to the way the circuitry has been designed. Any 2 core cable can be used for these connections.
The
Control board supplies power, tuning voltage and a band switching voltage to the remote antenna. There are coarse and fine tuning controls as well as a band switch and a 10dB attenuator switch on the control board.
I have one of these antennas in my garden and it uses a 4 turn
70cm loop. The tuning range is 500 to 2500 kHz. on the lower band and
1425kHz to 5800kHz on the upper band. It is very important to
adequately space the turns of the loop because stray capacitance will
degrade the upper frequency limit and lower the gain on the high band. A
spacing of at least 1 inch per turn is suggested. You may have to
experiment a little with the size of the loop to get the exact frequency
range that you need as this will vary to some extent depending on the
diameter of wire used and the construction of the loop.
Any
cable can be used to make the loop but thicker cable is better as it
will have less resistance and hence the 'Q' will be higher. The loop is
series tuned to resonance with 2 banks of 10 varicap diodes. This is
followed by a matching isolation transformer which also helps with
common mode signals. The 2 stage amplifier is fully balanced which is
converted to an unbalanced signal to feed down the coaxial cable to the
receiver.
There is provision to add additional varicap diodes to lower the frequency of the lower range (the upper range is not changed). When fully populated the bottom range goes down by almost a factor of 2 and increases the coverage down to approximately 250kHz (great for NDB hunting) with a 4 turn 70cm loop whilst still allowing reception of top band, 80 metres and the 60 Metre Bands
Remember
this listing is for the built and tested pcb's only. You will need to
fabricate a loop antenna and supply your own coax, chokes and 12 volt DC power supply. You will also need
a waterproof box to house the pcb. I do not recommend using this type
of antenna indoors. Unless you live in a very quiet location with no QRM
you will be disappointed! Put it outdoors! Antenna height doesn't seem
to be an issue. I have one mounted about 3 feet off the ground and it works great!
Thanks for looking!
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