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Thursday, 22 October 2015

Antennamabob Swap


Having played with the antenna I installed back here  for a while now, I have decided to go back to the Cushcraft MA5B that I had previously.

I have found that the Q52 antenna had little or no front to back, furthermore there was a switch on the control box that was supposed to "remove" the reflector to make the two element antenna into a dipole, whilst this altered the SWR very slightly, I found no difference whatsoever to signal strength from the front or the back of the antenna. Extended tests in both dipole and beam mode showed no signal strength difference from the front or the back of the antenna. Tom, N6BT, for reasons only be-known to him, chooses to ignore my email requests for technical support - so I haven't been able to get any. I did climb a ladder and conferm that I could hear the relays going click in both the director and reflector when the band switch was altered on the control box, I also double checked the control box to antenna wiring - not sure what else I can do to test the setup.

More recently, 500W of CW to the Q52 antenna has caused the relay box on the driven element to fail. Investigation shows that there are signs of arcing on the element connections - this antenna has proved to be a very expensive mistake!

I have also been having some concerns recently over the amount of weight on the mast and it's ability (or inability) to hold everything. I've therefore designed myself a new mast arrangement, there is no telescoping in this design, but it's much stronger and sturdier (and heavier) than the previous arrangement.

Here we are in the midst of the swap:

The Base section of the mast is 2.5 inch, thick walled box Aluminium, and that is then topped with a 48.3 mm OD stainless steel tube to the rotator cage. The stainless tube drops into the box section with a 2 meter overlap - the two items are bolted together in opposing angles so it can't move or turn. The rotator cage is made from galvanized steel and was manufactured by Tennamast. The stub mast is currently a 48.4 mm OD steel scaffold pole, but I have ordered a 3 meter long aluminium replacement to try and reduce the weight a little.

The mast all pivots and is lifted into the vertical using a winch and steel cable.

Once I have the replacement stub mast I will put back the VHF antennas and all will be well. The rotator is a replacement and is the G-1000 from Yaesu.

Here are the SWR curves from the MA5B as installed:

local conditions.

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