Tuesday, April 23, 2013

Basic Site Planning

I have slowed my antenna design plans a little, which is why I've had less to say on the topic recently. The reason is that I am somewhat constrained with what I will use to mount the antenna. Well, not only that but also a variety of related tasks such as building my new shack and dealing with non-radio matters.

The shack-building has gone slow because of the warming weather. I'd rather be outdoors than mucking with drywall in the basement. However I must finish that space since I need to move out of the spare upstairs bedroom. I also have no intention of broaching the walls on the second floor to bring in coax. On the plus side the basement area I'm using was designed for a shack from the start, including a couple of 240 VAC outlets for the amplifiers I no longer have nor intend to buy.

Getting back to mounted antennas at some height, my plans have changed. I was never comfortable with the idea of using the roof as a support -- although it is a good spot -- because it's simply too visible (see picture in this post). There might also be some friction with the city for much the same reason. The policy on this matter was new to me since I've been out of the loop for so many years.

Since I also have no intention of installing a tower (at least not for the foreseeable future) my options are more constrained. The diagram below shows my major options.

The drawing is not to scale or accurate (my house has a more complicated layout). It is only intended to show relevant structures in approximate positions. The direction we're looking is west.

Option A is rejected, as I've said. In addition to the visibility issue, especially if I put up delta loops, there is a lesser concern with guying the mast. I do have a plan for that which would suffice nicely for a fibreglass mast plus wire loop, since the wind load would be low. However a yagi is out of the question! I am more concerned that the structure would appear fragile to others even if it is in fact very robust.

Option B is my second choice for where to mount the antennas for 20, 15 and 10 (and maybe 17) meters. This would be either a mast or short "TV" type tower secured to the house below the roof line and anchored to the ground without using any concrete. The antennas themselves would stand clear of the roof although the bottom of any delta loop would be below the roof apex. The eaves are about 6 meters above grade and the roof apex is about 8 meters. The roof pitch is not nearly as steep as shown. The aluminum eaves trough that runs along the south roof line is my present antenna.

There are important considerations when it comes to attached a mast or tower to a building. I do know how I would go about it but will not get into that here. It would make a good subject for a future post. One advantage I have is that my house was custom built so I not only know the structural details, I also have many photographs of the raw frame that I took during construction, including the exact spots where the brackets would be placed.

The base and bracket position has been selected so that the mast/tower does not block any windows (important!). The base will be close to the basement shack. I have a spot selected to open the wall to install a conduit for the coax cables that is convenient, secure and out of the way of accidental encounters.

Options C and D are for masts, or short towers plus masts for 40 and 30 meters. Since I concluded earlier that I am favouring delta loops which come close to the ground (~3 meters up) their supports must be placed in the away from the house and deck. My property is deep (200') but relatively narrow (50'). Fortunately it is long in the north-south direction (350°-170°) so it is easy enough to position a loop broadside to Europe. I did this very thing in the 1980s.

What has changed is the vegetation. I drew the two trees nearest to the C and D to their approximately correct heights. Both were much shorter in 1992. Even though both were decapitated in the 1998 ice storm they have since grown well past those heights. Neither is robust enough to support an antenna but the lower trunks are ideal supports for guy lines. They are each close to the property edges (east and west sides) and almost exactly opposite each other. The space in between is a septic field, which is not the place for a tower base. My old tower was close to D so even the concrete had to be broken out of the bedrock to meet DOE regulations for the new septic system.

Option C would place the north end of the delta loops somewhat close to the house but still well out of the reach of eager fingers. The guy line for the south end of the loop would go a tree or something else farther to the south.

Option D places the loops further away from the house, which is desirable. The problem is that there are two other, larger trees to the south of D (not shown in the diagram). Their trunks are just outside my property lines and they branch out over my property so that they nearly touch at a height of about 10 meters. The squirrels love the arrangement (which didn't exist 20 years ago), using it to practice their acrobatics as they bravely jump the space from one flimsy outer branch to another. They almost always make it.

For a ham these trees are a problem. They are large and old, regularly shedding debris that would surely impact any delta loop or inverted vee that dared to traverse that space. There is also a real risk that one of the trees will come down in future years. Option D is out.

So that's the tentative plan: high band antennas at B and low band antennas at C. The choice for high band antennas is still in flux for a variety of reasons. I'll get to those reasons in a coming post. Regardless of how it goes I will make sure to write about the 20-15-10 delta loop array I've designed. I just don't yet know if I'll build it.

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