Friday, 8 November 2019

Another change of the 2m antenna system.

Though the 2 x 9 Element Powabeams I put up last year worked well I really wished I'd installed a system that had elevation and so I've now replaced the Powabeams with a small array 4 x 6 Element LFA from Innovantennas.

Justin also provided the H frame as well as a new boom for the 70cm 12 element LFA I had so it could be rear mounted in the centre of the frame.

While I was buying the antennas I also bought a SPID RAS rotator with Rot2Prog controller but more about that in a later post.


The new array took around five days to put but mainly down to the fact I hadn't ever installed this type of system and so it took a lot longer than it would if I was to get it all done again.   The main time consumer is in the fact I don't have a tilt over mast and so everything has to put up with the aid of an Aluminium tower.   

The array tuned up quite well with each beam within a half a dB of RL.

Top Left      Z 50.6, X 3.7, SWR 1.07, RL 28.6
Top Right     Z 52.6, X 3.2, SWR 1.08, RL 28.1
Bottom Left   Z 53.0, X 2.8, SWR 1.08, RL 28.1
Bottom Right  Z 51.9, X 3.6, SWR 1.08, RL 28.2 

The whole array including coupler and 16m of Ultraflex 10 gave an SWR of 1.1:1

Justin kindly send me a theoretical  plot of the array at 8m high with a spacing of 2.2m horizontal and 2m vertical separation.


Theoretical Azimuth Plot.
You can see from the plot the gain is quoted as 17.36dBi with a F/B of 23dB and beamwidth of 24 degrees.  

I took my own plot using the GB3VHF beacon which is near to me and was about to plot the results out by hand when I did a search for a blank azimuth polar plot and instead came across a great piece of software by Bob Freeth G4HFQ called PolarPlot  this small piece of software saved me a good amount of plotting everything myself and I highly recommended it.

I measured the gain as being 18.48dBi with a F/B of 22dB and a beamwidth of 24 degrees so in good agreement with Justins theoretical calculations.

Azimuth plot as actually seen.

The rear lobe are a bit weird but then there isn't much space between the antennas and surrounding builds, hills, slopes, overhead wires but even so it looks very good.

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