PA8W's
Radio Direction Finding Technology
The
fixed site VHF dipole array (120-180MHz)
The fixed site array consists of 4 half wave vertical dipoles in a
square configuration.
The spacing between two adjacent elements should be around 0.25
wavelength.
This is a good compromise giving a fair amount of doppler signal in
most FM receivers.
Wider spacings will yield a louder doppler signal but this may go
beyond the maximum deviation that some narrow band FM receivers can
cope with.
I've been modelling my array using MMANA, and doing that I found a few
things that obviously were missed in other amateur designs.
For best performance, only the active element may be accepting current
from the incoming signal.
All other elements need to be virtually non-existent to prevent
parasitic behaviour and therefore severe distortion of the pattern.
This can not be achieved by simply switching the top elements, which is
common practice in existing designs.
The bottom leg of the element connected to the attached coax shield
will be picking up current heavily, as MMANA simulations showed.
So, it proved to be necessary to switch the four bottom elements as
well.
The below picture shows the current distribution in this new approach:
Note that in this simulation there are ferrite beads modelled half way
of the coax length, which turned out to be not necessary at all.
On the contrary: leaving them out will give some further improvement.
However, it is obvious that the switched off elements are really dead
now. A big improvement compared to the simple switching method I
started with.
This is also clear if we look at the pattern:
Only 0,6 dB of directivity is still remaining at the design frequency,
which is superb.
And the pattern stays very neat over a wide frequency range.
-
Compare that to a pretty elaborate design of another Dutch radio
amateur:
His preamps are always on and connected to the antennas and the preamp
outputs are switched on and off to the output coax,
so the preamps are always loading the non-active elements.
MMANA showed massive interaction between the elements and
omnidirectional pattern was poor at the design frequency 145MHz.
On higher frequencies it gets far worse still! Only at much lower
frequencies the pattern becomes fairly omni.
One more case proving that a higher component count does not
necessarily mean better performance...
Furthermore, the coax and control cable should be taped flat to the
metal tubing to avoid them picking up RF.
Additionally I have a few clamp-on ferrites in the first meter running
down from the central combiner box.
One every 40cm as in below picture would work fine.
Good Array Dimensions for a 145MHz version, suitable from 118MHz to
180MHz:
Element length: 1m, (2x 50cm)
Element distance to centre: 35cm.
Element to element spacing: 50cm.
Make sure that all 4 antennas are absolutely identical, and use equal
lengths of coax to feed them.
The UHF 430MHz versions on the second picture:
Element length: 34cm, (2x 17cm)
Element distance to centre: 12cm.
Element to element spacing: 17cm.
(These sturdy UHF arrays are constructed with the valuable help of
Gerton, PG0G)
_
I recently re-designed the construction of the VHF array, to make it
more sturdy and to improve the looks:
Above the new housing for the combiner, out of 125mm PVC tubing. The
arms are made out of 32mm PVC tubing, one running clean through,
the other two arms are cut in a way that they fit the first arm, so
they can be glued firmly together using PVC glue.
A metal top plate is glued on and bolted down. On the other side, the
bolt runs through the metal L-profile as well.
In this way a simple but very solid frame can be constructed. The four
arms still need holes to be drilled for the coax pieces to the
dipoles at the arm ends.
The center PCB (combiner) is glued in place using silicone sealant.
The yellow line points at the spot where the outgoing coax core should
be soldered.
Note that there's room for a MMIC there in case the combiner is used in
a passive array.
The red lines indicate the spots where the antenna coaxes should be
soldered.
Of course all coax shields are soldered to the nearest ground surface.
The blue lines point at the pads where the antenna control lines should
be soldered.
Don't forget to connect the ground of this PCB to the ground of the RDF
(the center pin of the 5 pole DIN)
So, using a 8 wire network cable, 4 wires are used for controlling the
4 antennas and the 4 remaining wires are used as ground connection.
The combiner pcb measures 42x42mm = 1.64 inch
square.
This is the new small PCB for all 4 array arms, with normal 1N4148
diodes, a small inductor and a 1 Meg resistor to bleed off any static
electricity on the doublet.
For the RDF41 and RDF42, the inductor is to be replaced by a 1k
resistor.
The dipole VHF switchers fit into a tube of 25mm inside diameter.
The UHF versions fit into a tube of 13mm inside diameter.
Here's the same PCB now looking at the copper side, just to show the
way the whips are soldered on.
The two 50cm whips are running into the tube through tight fit holes
and soldered to the small PCB.
The whips are made from 2 mm steel, and covered in heat shrink tube,
so they are well protected against the the environment.
This is the schematic of the combiner.
For the RDF41/42/43 stick to this schematic.
Note that simple 1N4148 diodes may be used.
For UHF the inductors may be decreased to 470nH.
Cheers, PA8W.