Monday, May 4, 2009

R8 testing (analysis) and further prep

The R8 is now close to being ready to raise on the tower.

Adam Farson (VA7OJ) visited this week to give a quick test with his mobile HF rig and power meter. This was while the R8 was lying on its side and without radials. The results demonstrated some sort of resonance on all bands except 40m and 30m. The conclusion was that this was probably the best one could get with the R8 in that condition.

This weekend I endeavoured to perform a proper test having borrowed the club's MFJ Antenna Analyser. This involved adding the capacitance hat radials at the base of the R8, which having a diameter of over 2m certainly changes the handling of the antenna. Initially, I was at a loss as to where I could rig up a 9m long antenna with a 2m diameter effective base. Then I spied our 'wheelable', water ballasted basketball net. This was a perfect prop to lean the antenna up against, and bungie it relatively securely at some height, while the radials were added and for testing with the analyser.

Testing in this condition revealed that all the bands were now displaying some sort of resonance, though the 40m band in particular was resonant at a much lower frequency than it was supposedly tuned for. It turns out that this band is the most sensitive to ground proximity (and the antenna was sitting on the ground at this point). The theory was that raising the antenna a good 8' or so off the ground would raise the resonant frequencies somewhat and flatten the VSWR curves. Indeed, the manual for the R8 makes it abundantly clear that the antenna will not perform in close proximity to the ground. Luckily I had a spare 2" mast, so the plan was to jack the antenna up another 9'-10' and re-analyse.

Getting the R8 down from being strapped to the basketball net was relatively straightforward, though now, the base had to be propped up on a ladder to allow for the long radials. Equally easy was attaching the mast. The fun came with raising the antenna again with its extra 10' of length. The entire, top heavy R8 is now springing around 10 or 11 meters higher than you can reach to steady it. Similarly, getting this whole caboodle safely into the vertical from a horizontal start is somewhat hairy, and the thing was creaking and 'pinging' under its own weight as it was slowly raised by brute force to the vertical by me and my 17 year old daughter. We used the bottom of the garage wall to anchor the base (which otherwise would be tending to 'lift' until the antenna is beyond 45 degrees), and then had to walk the whole thing in the vertical to where it could once again be strapped to the basketball net. Walking something of that height is something best done very slowly and gingerly - a foot or so at a time, and in as coordinated a way as possible. It's tough to keep antenna and mast vertical while simultaneously moving the base. Looking up to judge the vertical doesn't work too well as you can see the antenna bouncing around, but have very little context (just the sky and a few trees in peripheral vision). Looking down at the last meter or so of mast close to the ground is better, relying on feel and the occasional glance upward to help 'rebalance' the antenna after every shift in position along the ground.

To cut a long story short, the antenna was eventually re-bungied to the basketball net, and retested with the analyser. The resonances were much improved, though still a little low in 40m and 10m compared to the tuned and expected results. The VSWR curves seemed flatter on most of the bands in which anything of a difference was noticeable over the band range. There might be some room for a little tweaking, though there might also still be a little improvement still with the R8 properly installed well over 8 foot above the apex of the roof.

I will make some final checks of the lengths of the radiators/tuning elements at 40m and 10m particularly, and may tweak just a little by shortening for these bands.

The next step (besides this checking) is to prepare the coax and attach it at the antenna's matching box, with appropriate weather proofing of the connection with silicone grease, electrical tape and "coax-seal". After this, the R8 can be attached to the tower mast and rigged for raising with ropes.

Here's a picture of the R8 on top of the 10' mast, bungied to the basketball net.

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