2 Element Parasitic Array 
for the 30m Band
 
 
The VB302 is a 2 element top loaded parasitic array tuned for resonance on the 30m band (10.1Mhz – 10.150Mhz). The antenna is very simple to construct in its basic form and can be strung out between two suitable supports just like a conventional dipole or G5RV.   The measurements are critical, especially the distance between the ends of the two elements (33cm exactly including insulators), if it’s not exact the front-to-back ratio is affected detrimentally.   Both elements are exactly the same size however one element is electrically lengthened by a 0.6 micro Henry coil at it’s base to produce the reflector part of the antenna. The coil dimensions are as follows :           Turns         6         Length       4.4cm         Diameter    3.1cm   The performance of the antenna is dependant on the quality of the ground system, thus you will need to put plenty of effort into the ground system ensuring good radial layout, perfect connections electrically and mechanically and good weather proofing.   The 0.6 micro Henry coil is connected to the antenna inside a water tight plastic box at the base of the element (left side in diagram below), as is the SO239 socket on the radiating element.   Make sure that the two sets of radials below the two elements are both electrically and mechanically connected.   I’ve found that it’s better to use un-insulated wire for the antenna, however I have had excellent results using a “Flex-Weave” type wire, however remember that since it’s insulated you will have to shorten the lengths by as much as 5% due to the “electrical lengthening” caused by the outer insulation.   Below is a simple diagram of the antenna’s overall dimensions.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
The antenna gives a gain of 3.93dBi at 25° with a front to back ratio of 25dB to 46dB. 
Using either 2.5mm wire or “Flex-weave” type wire this antenna will easily handle 1KW PEP.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
It is also possible to make the basic VB302 into what I call the VB302s, or “Switchable” model. This will allow instant 180° switch in radiation direction, ideal for beaming North/South or East/West.   The switching is accomplished by using standard 12v automotive relays with a current handling capacity of in excess of 35A. With contacts of this size I have found that the relays easily handle the RF presented to them during operation, and being 12v they can easily be run from the shack 12v PSU.   A simple wiring diagram for the relay switching system is below. Note that the relays must be housed inside the water-tight plastic boxes at the base of the elements along with the 0.6 micro Henry coils. Since either element can be either the radiating element or the reflector element this model requires 2 x 0.6 micro Henry coils and a switched coaxial link between the two elements.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
The relay wiring diagram showing the two 0.6 micro Henry coils. For the system to function correctly ensure there is a connection between the outer of the coaxial cable and the ground line and the GND terminal on the 12v DC PSU.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Note: All calculations made using “Average” ground conductivity figures with “Copper” wire “Loss” taken into account.
 
 
30m Band Parasitic Array
Sunday, 8 July 2007