|PA8W Amateur Radio||
|The Homebase Preamped Amplitude Array (400-470MHz)|
switchable directional pattern antenna array for UHF is
This array is suitable (using the amplitude comparison method) for hunting narrowband and wideband signals in the UHF band.
Of course also digital and analog voice transmissions can be hunted.
The center conductor plus the 4 drooping radials are the actual antenna elements, with a MMIC preamp in its feedpoint.
I use a bias-T to feed the preamp over its coax.
The antenna is surrounded by 4 dipole type reflectors that can be switched on and off.
A selected reflector changes the omnidirectional pattern of the central antenna into a directional pattern with a front to back ratio of around 11 or 12 dB.
This directional pattern is "rotated" simply by switching the reflectors on and off so that at least one reflector is active all of the time.
This is done in a fast pace: 2000 steps per second = 500 cycles per second.
This fast rotating directional pattern modulates the incoming radio signal in amplitude,
so an AM recever hooked up to this antenna produces a massive 500Hz tone.
(In a pseudo doppler RDF we would call this a doppler tone but in this case this really is the wrong terminology)
The RDF compares the phase of this AM tone with the phase of the reflector commutation to calculate the Angle Of Arrival of the incoming signal.
As this array is meant for homebase applications, we have no metal car roof as a ground plane.
Therefore the actual antenna is built as a sleek Ground Plane antenna,
with the 4 drooping radials acting as a ground plane.
The 4 reflectors are built like dipoles which can be electronically closed and opened in the center.
In a opened state, the reflector breaks up in two 1/4 wave conductors which are too small to have an impact on the radiation pattern of the actual antenna.
When closed, a reflector is a conductor of a little more than 1/2 wavelength and thus it acts as a reflector.
The achieved (measured) front to back ratio is approx. 12dB.
That is about 5dB gain at the frontside, and 7dB suppression at the rear.
So when no reflector is closed, we have a simple preamped GP antenna.
The measured figures are very close to the calculated ones. I only noticed a downward shift in tuning due to the heat shrink wire covering, which was expected.
So I reduced element length to get the desired 400-470MHz with uniform pattern properties.
So stick to the sizes I state below, and heat shrink the wires like I did.
focus on the building process:
We start off with the 4 switcher PCB's:
On the left is the control wire that feeds the 1N4148 diode (or PIN) via two 470ohm resistors.
So when say 10V of control voltage is applied, the diode current will be close to 10mA and so the diode will be conducting and closing the gap between the two reflector halfs.
These reflector halfs are soldered to the pads on the right side of the PCB.
The black ring on the diode should be the bottom side of the reflector.
switcher PCB is put in an oversized piece of PVC 3/4" tubing.
Then both reflector halfs are soldered in place and covered in heat shrink tubing.
And finally I sealed the end opening plus the hole where the top-reflector-half enters.
I never seal the bottom element to have a tiny opening where condensed water can escape.
Additionally I drill an extra 1,5mm hole in the botton of the tubing and hub for the same reason.
|The center hub, made of PVC tubing.|
|A mast fix is added, plus the 4 ground plane elements enter the hub.|
|The ground plane wires are soldered into a piece of Printed Circuit Board that doubles as a washer to clamp down on the tube arms.|
the radiator is prepared and soldered in a hole in the side of
a brass spacer
The M4 bolt is fastened to the center pad of the preamp PCB.
The tiny black thing on the PCB is the actual preamp.
Any MMIC will do fine here.
|Now the preamp PCB is soldered onto the PCB material holding the ground plane wires.|
picture shows the complete array just before the
wiring is done and the top cap over the center hub is glued in place.
I used 2mm steel wire to make all elements:
The reflectors are made of 2 pcs of 19cm. (8pcs in total)
The ground plane is made of 4 pcs of 17,6cm.
The radiator is a piece of 16cm long.
All elements are covered in heat shrink tube after they were soldered in place.
The distance radiator-to-reflectors is 10cm.
The actual ground plane is rotated 45 degrees so these elements are at maximum clearance distance from the lower reflector elements.
When installing the array, it's best to use a glassfiber, wooden or plastic mast top, and put a ferrite clamp over both the coax and the control cable, at approx. 10 - 16 cm below the hub.
The center antenna doesn't necessarily have to be amplified.
But for UHF the extra gain is more than welcome generally.
This array can scaled to fit other frequencies.
If you multiply all sizes by 3 you will have the size for the VHF band.