Tuesday, July 28, 2015

Building a new antenna at N6QW ~ Part III

More Front End Engineering

In Parts I and II, I covered looking at some of the basics about where I would place the new beam support mast and the all important dimensional clearances needed to raise and operate the beam antenna. In parallel with that activity I had been "noodling" the three very important components of the antenna system which I will describe here.
 
  1. The mast selection was based principally on cost which hopefully I will not rue the selection I made. A roof mounted mast was ruled out in large part because of the implications of boring holes into the roof of my "California Cracker Box" home. A roof installation most likely would involve a structural engineering review and very likely some internal construction to support the mast, beam and rotator. That also has implications for earthquake safety as we are located in an area of active earthquakes. A permit and engineering review by the local building department would be required if this option was selected. Thus a ground mounted mast was an obvious choice. The Rohn H950 Mast must be guyed with the option of an installation with a house bracket plus some guying which is my plan. The H950 once fitted to the concrete base and affixed to the house bracket is raised section by section until it is at its full height of about 34 feet. The mast itself weighs about 34 pounds, the beam about 20 pounds and rotator about 7 pounds. To this one must add in the weight of the coaxial cable pus the rotator cable which I figure is about another 10 pounds. Since the lower section of the mast is affixed to the base and attached to the house bracket its weight van be subtracted since it is not being lifted vertically. Our initial weight calculation was 34 + 20 + 7 + 10 = 71 pounds. Removing the lower section which I estimate to weigh 10 pounds leaves us with 61 pounds to raise to the 34 foot level. But the force required to lift the assembly vertically is progressive. In raising the first section (which is the top) we have the weight of the mast section, the beam, the rotator and some of the cables. So that comes to 27 (beam & rotator) + 6 (mast section) + 4 (cables) = 37 pounds. The next section lift adds the weight of the section plus cables which I estimate is about 8 pounds thus 45 pounds and the same goes for the remaining two sections which now is 53 pounds and 61 pounds respectively. Raising and lowering the mast is not something you want to do daily. I best start with my body building exercises straight away.
  2. The choice of antenna was again a cost factor. Forty five years ago I bought a HyGain TH3 MK3 antenna and I think it was about $200 which has now tripled in price. Thirty five years ago I bought a two element beam (HyGain TH2MK3) and it was less than $200. That antenna has doubled in price. While I was on a walk in my neighbor hood I spotted a new beam installation just around the corner from my home. It was a Mosley Mini32 which is a 2 element light weight beam weighing in at about 9 pounds. Given its small footprint and light weight it does have some compromises on a lower max power level, and the Front to Back ratio. But it certainly looked attractive which cause me to look at other Mosley offerings. My final selection is a custom (non-standard) offering called the MP-32. This beam uses the driven element from the TA-32 which can handle the legal limit and the director form the TA-32 Jr. antenna thus reducing the weight. The gain on 20 Meters is about 3 DB and the F/B ratio is 20 DB. I do have a homebrew legal limit amplifier which is on 20 Meters and thus was interested in having the higher power rating. Why not buy the TA-32? Because it weighs more like 26 pounds. Since it is a custom build I have about a month wait before it will be built and shipped. That is perfect as it will provide me a window to make all of the other installation.
  3. The rotator is the next key element. The wind load is about 5 square feet and so that eliminates some of the less expensive rotators. If you step up to the larger more familiar rotators you are now at around $700 not including the cable. My research led me to the Yaesu G450 which is rated at 10 square feet and called a light duty rotator. I figure that means it is really good for 5 sq. ft. without too many problems. But the eham reviews have not been so kind with specific comments that the rotator is OK but the control box fails. The Yaesu with all of the hardware you really need to make it work (called accessories), costs about $500 with the cable. I figure if worst comes to worst it might be possible to homebrew a replacement control box using an Arduino microcontroller.
To get a better feel of the H950 Rohn Mast and clevis assembly, the next series of photos show those features. The 2X4 is just a shop aid in the drilling of the 5/8"  base bolt holes. The Clevis will be bolted to a section of 1x4 and attached to the concrete forms to keep the bolts aligned during the pouring and curing of the concrete. When the concrete is cured the 1X4 will be removed (as well as the forms) and then the mast will be bolted to the clevis assembly. One the mast is in place and mounted to the house bracket there will be some final shimming to assure a vertical and properly aligned mast.

Rohn 34 Foot Mast with Homebrew Clevis Assembly

The Clevis Assembly

Clevis Assembly

More of the Clevis

Alignment is Critical to the Install

Guy Rings for Upper Sections

Close up of the Clevis

Oblique View of the Clevis