In a doppler Radio Direction Finder system the Antenna Array must be well designed for the frequency band you are going to monitor, and for the specific type of use.

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.

Other important design parameters in both my doppler designs are element length, low switcher/feedpoint stray capacity and -in case of dipole elements- switched off bottom elements.


Generally, a doppler array has 4 quarter wave whips spaced 1/4 wavelength apart, or half wave dipoles spaced 1/4 wavelength apart.
Allthough performance and accuracy will be best at the design frequency, the array can be used up to 1.5x the design frequency (re-calibration will be necessary!)
Below the design frequency there's no hard limit: The sensitivity and accuracy will decline slowly with lowering frequency.
In all cases: When used far from the design frequency, check calibration first!

Now let's look into the specific applications:

Mobile VHF+UHF:
A mobile UHF array 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 18dB 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.
18dB gain has proven to be a fair compromise.
A mobile UHF array can be used on VHF as well, especially with longer (50cm) which -surprisingly- work fairly well on UHF as well!
Here's the design:
https://radiodirectionfinding.nl/MobileUHFArray.html

Mobile VHF:
For best performance on 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 models in good sizes (100mm and more) which can be opened for adding our switching components.
The antennas we use are 2mm steel whips, ¼ wavelength long, soldered into a PL259 connector. (50cm long for 2m)
Here's the design:
https://radiodirectionfinding.nl/MobileArray.html

Fixed UHF:
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 directly at the antenna feedpoints may not survive thunderstrikes in the direct vicinity. (Had that once...)
So, we use (pin-) diodes for switching.
In those cases 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.

Fixed VHF:
The same counts for VHF, switching can be done using PIN diodes or small signal diodes like the 1N4148.
Preamps are  generally not necessary for fixed VHF RDF work.
Here's the design:
https://radiodirectionfinding.nl/HomebaseArray.html

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:
https://radiodirectionfinding.nl/DualArray.html


Cheers, PA8W.