Topband: Modeling W8JI and Hi-Z 8 Circles

[Dan Maguire via Topband]

Based on some user feedback, the 8-circle models mentioned earlier have been modified with an additional variable to control the azimuth direction of the main lobe.  That lets you change the direction just by changing (or doing a sweep on) a single variable.  So you can easily:  1) see how the azimuth patterns overlap as the array is steered in 45° steps, and 2) compare patterns against other models which may be fixed in a particular direction.

For case 1, here's how the "Hi-Z type" (8 elements in use) 8-circle array patterns would overlap as you turn the knob on the control box.  This is for a 200 ft diameter array with ±106° phasing, 20° TOA for the patterns.  Compass rose angles are shown in parenthesis.

http://ac6la.com/adhoc/8circle7.png

For case 2, this is how a "W8JI type" (aka BroadSide/End-Fire or BSEF, 4 elements in use) 8-circle array with 0.604 wl broadside spacing and 125° phasing compares against a 4-square receiving array with 0.125 wl element spacing and Crossfire feeding.  Both arrays use the same basic element, the top hat model rxvrhat.ez from Tom's page.  At a 20° TOA the 4-square has a gain of -22.43 dBi compared to -10.95 dBi for the 8-circle.  (But the RDF is only about 1 dB lower for a diameter of 94 ft compared to 348 ft for the 8-circle.)  In order to get an accurate comparison of the pattern *shapes* on a common polar chart, the *gain* of the 4-square has been normalized to that of the 8-circle by reducing the value of the swamping resistors.  Original gain below on left, normalized gain below on right. 

http://ac6la.com/adhoc/8circle8.png

The 4-square can be fed either as Crossfire or BroadSide/End-Fire using exactly the same phasing lines, a neat idea picked up from IV3PRK.  In the patterns below, the swamping resistors are back at the normal values (to give 75+j0 at the feedpoint for a standalone element).  With Crossfire feeding the gain at 20° TOA is down 7.45 dB compared to BSEF feeding but the RDF is 2.1 dB better, below left.  Note that a design criteria for IV3PRK was good rejection at backside 45°, below right.

http://ac6la.com/adhoc/8circle9.png

For more details on this dual-feed 4-square see the "Receiving Antennas" portion of the IV3PRK site and follow the links on the left side of the page.

http://www.iv3prk.it/rx-antennas.htm 

Finally, thank you Jim Brown for mentioning the 2014 Contest University W3LPL "Receiving Antennas" presentation.  Lots of good info there.  Here's the direct link.

http://www.contestuniversity.com/attachments/W3LPL_Receiving_Antennas_2014.pptx

These are the revised 8-square models with the additional variable to change the pattern direction.

http://ac6la.com/adhoc/W8JI_8_Circle__TopHat.weq
http://ac6la.com/adhoc/W8JI_8_Circle__AL-24.weq
http://ac6la.com/adhoc/Hi-Z_8_Circle__TopHat.weq
http://ac6la.com/adhoc/Hi-Z_8_Circle__AL-24.weq

And here's the 4-squareRX model with a variable that switches between Crossfire feed and BSEF feed.

http://ac6la.com/adhoc/4SquareRX__TopHat.weq

Dan, AC6LA