Topband: 4-square Rx antenna project

[Luis Mansutti IV3PRK]

Hi Topbanders,



I just got a small lot, one wavelength from the TX antenna, which can fit a 4-square receiving array.

At the moment I'm mostly using a rotatable Flag, several Pennants and a K9AY loop, but for ten years my best Rx antenna has been the "Square-four receiving array" by KD9SV, K9UWA and W7EL design (ARRL Antenna Compendium Vol.3), which I built in 1994.

Its weakness was too critical, too much WX dependent and required a lot of maintenance (vertical dipoles inductive loaded). The innovative design developed by Tom, W8JI, is a great improvement, as confirmed by the success of the DX Engineering products.

But I always built my own antennas, I wanted to make use of the aluminium tubes of the old array, and my only band of interest is 160 meters, so no need to use the short whip active antennas.

With Eznec+5 I modified the original RXvrhat W8JI basic element with my real tube dimensions (9.5 meters high) and I got a small Q improvement (Load 73 R and 293 XL).

The side of the 4-square is 21 m. and with diagonal crossfire feeding I get a beautiful pattern with a great F/B and an RDF of 12 dB.

I calculated that the delay lines indicated by DX Engineering in their instruction manual are 62 deg. to the back and 31 deg. to the middle elements, but my choice will be a little bit shorter (60/30 or 58/29) to maximize the high angle null in order to reduce the EU signals coming from the back.

Such a beautiful azimuth lobe is even too narrow (75 degrees beamwidth), and cannot fully cover all the directions with four switching positions. So I investigated on the possibility to have other 4 directions with an alternative feeding system. I know that the broadside spacing should be over half wavelength, as in the 8 elements circle arrays (and some of my Pennants), but my previous four-square mini array was working well with only such kind of phasing system.

The new main lobe is shifted 45 degrees from the previous one, as desired, the FB is not as deep as with crossfire and also the RDF is about 2 dB lower, but we still have a very good null at 45/50 degrees with some more gain. Using the same phasing lines of the crossfire fed system we can build a nice 8 directions switching array, with the only nuisance being that 7 dB difference gain between the alternate side and diagonal positions.

In any case the output signal is quite good (-15dB and -22dB) compared to the much lower gain of a single Flag (-30dB), end-fire Pennants (-40dB) or Waller Flag (-54dB) with less problems and easier common noise handling.

The whole project seems too fine and too easy..do I miss something ? 

I wrote a document with lot of tables and plots, and I will send the PDF file (or the Eznec files) to anybody interested.

Now I have a couple of specific questions to the wisdom on this Reflector:

1)      Why the output Xfmr of all these kind of arrays is always an autotransformer and not an "easier one" with separate windings like that for Pennants and Beverages? 

2)      So the shield of the coax to the receiver is connected to the ground of the switching delay box, but is a good idea to have tied there together also the elements radial system? (ON4UN Low-Band Dxing, pag. 7-27)

3)      In any case all my feedlines to the shack are RG213 coax. So going from 18.75 to 50 ohms needs a 1.63 turns ratio. I calculated 5 turns on a binocular BN73-202 with a tap on the 3rd turn: is that ok?

4)      To tune out the reactance at the base of the verticals I need a 25 uH inductance. I have two stuff available: FT50-61 toroids and some 4 inch rods of 33 material; both have a low AL and require around 20 turns. At the frequency of 2 MHz  the #61 material has u' = 200 and #33 material has u' = 750. Considering that with the larger winding on the rod it's easier to get a precise tuning, which is the preferred material?

Thanks to all who had the patience to read and I hope to get some good advice as usual.

 

Best 73

Luis IV3PRK