Wednesday, July 31, 2013

Site-B Mast

The next step in the development of my "antenna farm" is putting up another antenna support. This allows for some experimentation, including comparisons against the my tri-band dipole, the so-called TH1vn. In my site plan, the small tower holding the TH1vn is at Site C, however that is only for the summer. That tower is the lower part of the support structure for low-band antennas. In fact you can see the reflection of that antenna in the photo below.

Site B is for a house-bracketed tower or mast. Not wanting to make a large investment in time and effort I used what I had. In this case that was the part of the 22' Schedule 40 galvanized water pipe that I did not use for the tower's house anchor and cable conduit. Since that took less that 3' I had a perfectly suitable mast of a little more than 19', or almost 6 meters. The pipe formerly served as a rotatable mast for stacked yagis on my previous tower.

Six meters isn't very tall, only enough to poke above the roof by about 15 cm (6"). Since my experiments involve vertically-polarized loops I want to keep the metal mast and eaves trough at least 1 meter from the antenna bottom. In other words, it is the perfect height. The mast is positioned close to the eaves trough (for roof access) and not directly in front of any window (aesthetics!).

The 4' fibreglass sections can be stacked to the desired height. The smaller-diameter end of the fibreglass mast fits nicely inside the pipe with two strips of duct tape (!) as a shim. The ID of the nominal 1-½" pipe is approximately 1.6". The OD of the fibreglass sections is 1.75".

To attach the mast to the house I used L-brackets that I found in my junk box. I don't know what tower they were originally desired for but they are thick and strong. Lag bolts (⅜" x 4" and 6") secure the brackets to the frame. Commonly available 0.074" x 1½" galvanized steel angle stock (pre-drilled) and U-bolts connect the two brackets to the mast.

The biggest challenge was locating suitable frame wood for the lag bolts. In my case these were exterior wall studs (2x6) or the dual top plates between the studs and the rafters (pre-engineered trusses).
Never bracket a tower to siding, fascia or wall sheathing. Always attach to a structural part of the frame. Even then it is important that the selected part of the house frame and the bracket are able to support the load, both static and wind load. Call for professional help if this is outside of your ability. Don't gamble.
Since my house is custom built I have all the blueprints and, importantly, pictures of most of the interior structure just before the walls were insulated. A couple of pilot holes missed, but a minor course correction always located the desired wood.

The pipe is supported on two pieces of framing wood that I pulled from my scrap pile. They are nailed together and the lower piece is staked to the ground. There is a circular cut-out in the upper piece to hold the pipe in place. Strategically drilled holes permit water to drain from both the inside and outside of the pipe.

The Schedule 40 pipe is not really recommended for masts since it is relatively heavy and not overly rigid. However it is easy to find and inexpensive, and it is strong. In its present application it also is heavy enough to repel accidental (or deliberate) bumps. The only significant problem with the pipe is that its low rigidity means it can oscillate when strong winds hit large antennas or masts. I will guy the upper fibreglass mast to reduce this effect, which is needed in any case for added wind resistance.

All this said, the pipe and my improvised house bracket suit my immediate need. In time, if I intend to permanently install antennas at this site, I can substitute a small tower or a different mast. The angle iron brackets can also be replaced with something stronger, even if not strictly necessary.

Next on the agenda is an experimental delta loop for one of the high bands. I want to test the performance I can expect from a vertically-polarized loop at approximately the same height as a dipole. That's another reason why I installed my summer TH1vn antenna. I need to get this all sorted out in August before the painters arrive to completely strip and renew the finish on the cedar siding. The mast may have to be temporarily removed for that work.

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