Monday, July 27, 2015

Building a new antenna at N6QW ~ Part II

The all important "Engineering" of the project.

I learned a long time ago that the "front end engineering" of a project is the critical success factor whether it is building a new radio or installing an antenna. I have coined a term that I call "noodling" where much time is spent researching the background information and where the alternative analyses are evaluated. When the mast is 1/2 way up in the air, that is not the time to think about will this assembly support the antenna or will it come crashing to the ground. Haste does make waste and you must never forget that sage advice when engaging in a new project.
When I installed the current  fiberglass antenna mast I created a plot plan of my house, where I assessed the available real estate and possible site locations of the support mast. Surprise! I found that my options were really limited when considering the length of the Extended Double Zepp, the attachment point to the eaves and choosing the best direction for working DX. That plot plan analysis is shown below and for the most part the same location of the new support mast would serve the beam installation.
The critical factors for the beam install is rotational clearance as the element lengths are in the range 28 feet long and that the eave support is readily accessible. The current concrete base for the fiberglass pole is somewhat diminutive and is unsuitable for the beam mast and so a new base must be designed. The third photo in the series explains it all and the new base will be about three times that size.   Since the beam will be rotational the best direction to point the beam is merely a matter of spinning the rotator to that direction. By the way whether I am designing circuit boards or new antennas the old fashion "quad pad" graph paper I find is an invaluable tool to visualize the project and this is where I started. In Part I, I mentioned that my current QTH is a postage stamp sized lot -- the noodling sketch surely affirms that statement.

So far we have "noodled" several pieces of critical information for the beam installation:
  1. The current location of the fiberglass mast is suitable for the beam mast as we satisfy the rotational clearance requirement and there is access to the eave so that the lower part of the mast will be anchored to the house structure using a homebrew house bracket similar (but more robust) to the fiberglass bracket.
  2. The concrete base must be larger in size since the clevis assembly will be affixed to the concrete base using 5/8 Inch by 12 inch long galvanized anchor bolts. The new base will be a cube of about 1.5 feet in each direction. The current base is much smaller and contains a stub piece of pipe where the fiberglass bottom section simply slips onto the stub.
There is a chimney quite near the current mast which is shown below and the mast location in relation to this chimney has several implications. The chimney is 17 feet high and thus installing the beam on the mast presents some "clearance" issues which are best thought of now rather than later. The beam boom is about 6 feet long which means the boom to mast bracket is located at about midpoint thus about 3 feet either side of the mast. A critical dimension is the measure of the distance of the boom as it extends past the chimney. A further critical dimension is that the beam MUST be higher than 17 feet for a full rotation. The second photo below shows the current mast and gives a rough feel of these dimensional considerations.

The discussion so far should give weight to the level of analysis involved to effect a successful installation. Stay tuned. Pete N6QW