PA8W Amateur Radio

Wil, PA8W,  E-mail: PA8W@upcmail.nl           
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The Fixed Site Array (120-180MHz)

Revised 190310

The fixed site array consists of 4 halve 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 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:

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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 0,7 dB of directivity is still remaining, which is superb.

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I also modelled the array versions of pretty elaborate designs, using preamps on each element:



The preamps 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 directivity was over 6dB in two directions.
On other frequencies it gets far worse still!

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 would work fine:




Good Array Dimensions for a 145MHz version, suitable from 118MHz to 172MHz:
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.

I recently re-designed the construction of the 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 combiner pcb is glued in and all wires attached.

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.

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 VHF antenna array.
For the RDF41 and RDF42 stick to this schematic.

Note that for the conventional V2,3 doppler the 1k resistors may be changed into 1uH inductors.

Right-Mouse-Click on the drawing, and choose "Save as" to save the drawing to your map or desktop.
The resolution will be doubled compared to the drawing on this page. 


Summary
Of course, the array may be scaled up or down for other frequency bands.
With the above technology and dimensions, good results will be achieved not only on the designing frequency, but also over a considerable frequency span up to 20% above and 20% below the designing frequency. 

73, Wil.


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