|PA8W Amateur Radio||
|Some thoughts on Doppler Antenna Arrays|
In a doppler RDF system the Antenna array must be well designed for the frequency band you are going to monitor, and for the specific type of use.
Therefore, we are going to zoom in at some specific applications.
A few design parameters are:
1, The doppler-array should have a clean omnidirectional radiation pattern, so all switched-off elements should be much shorter than 1/2 lambda to prevent them acting like parasytic elements.
2, The array should be sensitive enough to receive weak signals, especially in mobile applications.
3, The array should be reasonably tolerant to strong signals, especially in fixed applications.
Additionally, an RDF antenna has some specific design demands simply because it will be commutated so that only one of 4 antennas is active and the others should virtually disappear to not interfere with the active antenna.
Therefore, important design parameters in both my doppler designs as well as my Amplitude designs are element lenght, low switcher/feedpoint stray capacity and -in case of dipole elements- switched off ground elements.
Now let's look into the specific applications:
A mobile UHF is relatively small in size, so it can be designed as a single ground plate with four whip antennas attached on it.
The size (50x50cm or a little smaller) allows to simply tape it to a car roof in which case the car roof extends the ground surface enhancing the performance of the antennas.
My favourite design uses preamped switchers with about 13dB gain, which is more than welcome on UHF.
On the other hand, excessive gain and the necessary open, unfiltered antenna design will make it a bit sensitive to very strong signals.
13dB gain has proven to be a fair compromise.
A mobile UHF array can be used on VHF as well, but obviously with reduced accuracy and sensitivity.
Here's the design:
For VHF we generally use magnet mounts and passive switchers.
The magnet mounts should be big enough to ensure a good capacitive coupling with the car roof.
Sirio has some very good sizes which can be opened for adding our switching components.
The antennas are 2mm steel whips, ¼ wavelength long, soldered into a PL259 connector. (50cm long for 2m)
Here's the design:
If one would use the preamped switchers in these magnet mounts, one could have an extra set of shorter UHF whips to use the same magnet mounts in a smaller square footprint for UHF.
That way a magnet mount array would be a good compromise suitable for both VHF and UHF, though a bit more of a hassle to put up compared to the ground plate design.
Another option for dual band operation would be:
Fixed dipole arrays are generally passive, since the field strength at the higher antenna position generally is sufficient plus a preamped version may have issues in high-level RF fields.
Also, these preamps may not survive thunderstrikes in the direct vicinity.
So, we use diodes or for switching.
In case where extra gain is desired it is more elegant to have a preamp added in the combiners output, fed over spare wires in the control cable or even fed over coax.
The combiner is prepared for the addition of a ERA3 or similar MMIC preamp.
The same counts for VHF, switching can be done using small signal diodes like the 1N4148.
Preamps are generally not necessary for fixed VHF RDF work.
Here 's the design:
Fixed dual band:
We can’t have a top performance UHF/VHF combination in one single array.
The VHF sections of the array would heavily distort the radiation pattern on UHF.
So we need two separate arrays at a distance of several meters to prevent coupling.
Or, much more elegant, we can stack a UHF array on top of a VHF array.
If we would use a vertical spacing of at least 1 meter between the centers of the arrays the spacing between the VHF and UHF antennas would be about 33cm vertically.
That is sufficient to reduce coupling between both arrays to the point that it can be neglected.
We can even hardwire the antenna control cables so both arrays would “rotate” simultaneously.
Using a double L/C series network, the output of both arrays can be coupled into the same coax cable running down to the shack.
Here 's the design: