The array

 

Main antenna is a stack with 4x 6M9KHW by (15m long boom 12.7dBd) with full azimuth, PST71 rotator and elevation control with 36 ” actuator. As a backup antenna I have a single 6el YU7EF design at 30m agl. Phaselines are 1/2” hardline , loss <0.25dB and tuned within 0.02%. The main feed line is a 1- 5/8" coax , loss <0.25dB. Total loss antenna to rig <0,52 dB .Sloping ground gives me a take off angle lower than over flat ground.

Antenna central point is at 110m asl. The antenna H-frame is about 9.1m by 8.4m and made out of welded steel tubing. Weight is >150kg for the frame, and complete structure is >200kg.

The antenna was simulated in EZNEC, taking into account the H-frame. This is the ONLY correct way to evaluate an array. Often one sees a rather big influence of the supporting structure on the array performance. VE7BQH added that even the feedline will influence the pattern, this is very noticable on cross polarised antennas.

Free space plots to compare design are nice but its no real world situation. There are several design that suffer in F/B or sidelobes as soon as you add a H-frame or other close by metalic structures.

The M² is no exception to it, but by carefully designing the supporting frame you can minimize the impact or even improve the pattern somewhat. The metallic supporting frame acts as parasitic elements in the array.

I had first hand experience with a well known design where adding the H-frame to the 'superb yagi' showed hughe impact on the F/B range which dropped almost 7dB. The designer said his antenna where so optimised that it was logical it would degrade somewhat.

Pattern without H-frame over flat ground, (25.77dBi @ 3,2 degrees) due to slooping ground and the H-frame final gain peak is at 2,1 degrees 25,89 dBi. ( my real ground gain starts at 2° anfd goes to negative horizon). And yes a negative horizon (slooping ground) has impact on the lobes.

A word of caution about simulating antennas, above ground they never show any lobes towards earth, weird.

Return loss measurement

Geert (ON4GG) made the phase lines and power splitter in 7/16 DIN and 1/2 hardline. Measured loss on the phase system 0.25dB. Return loss on the splitter terminated with four 50 Ohm Suhner dummy load: -49dB! All phase lines cut and measured on a HP8711 network -analyser. Results are shown below.

 

 

 

 

 

 

 

 

 

 

 

 

 

For fun I remade the measurements on November 28, 2007 with a HP8752C analyser on the actual antennasystem. Span shows 49,9 to 50,4Mhz , maximum swr 1.2 and as seen on first sweep low swr above 50.4 aswell, giving good rain performance.

The SWR minimum is at 50,050MHz with 1.0092 , the return loss is identical to the power splitter with 50ohm loads. The measured return loss is –46.6dB . The actual antenna impedance above 50.3MHz is influencing the SWR curve 1,1915. The 3th picture shows the Smith chart.