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An Easy Build GO BOX Site Antenna

from Scott Reaser, K6TAR on August 12, 2017
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An Easy Build GO BOX Site Antenna
Scott Reaser, K6TAR

I describe a VHF/UHF site antenna that you can build using low cost parts found at hardware and big box home improvement stores. So what is a site antenna? I show a completed example in Figure 1 (Erected Site Antenna). It is made to pack in a GO BOX that supports three phases of emergency communications. These are:

a) Handheld with a duckie antenna

b) Temporary mobile operation with a small base mag-mount vertical

c) Support of semi-fixed positions for data collection and hand off to public service authorities, and to be an intake point for shelter messaging, etc.

The site antenna is my application solution for c). The same mag-mount used for b) is also my vertical element for the site antenna. I provide a “flag” on the support mast as a place to clip on the handheld and its speaker-mic. This ground plane antenna with a little height gives me much better radiation efficiency compared to a handheld duckie. It gets out.

I use ¾ inch Schedule 40 PVC pipe and various fittings to make this antenna. The major parts are the base assembly, mast, and a steel platform for the mag-mount. The parts you will need are:

(2) 10-foot lengths of ¾ inch Schedule 40 PVC pipe

(3) slip caps

(2) tees

(3) 45 degree elbows

(3) couplings

(1) slip plug

(1) ¼ -20 carriage bolt 1 inch

(2) ¼ -20 carriage bolt 2 inch

(3) ¼ -20 wing nuts

(1) 5 inch joist tie plate, Simpson TP-15

A piece of masonite or thin ( ¼ inch or so) plywood that is big enough to hang the handheld and speaker mic.

PVC solvent cement. (check that what you have has not dried out).

A length of solid house wire, or whatever you might have, to make the ground plane radials.

The pipe is an interference fit into the couplings, etc. Nominal pipe outside diameter is 1.05 inches, and the fitting inside diameter is 1.04 inches. The solvent cement softens the pipe for a few seconds allowing you to push the pieces together. You must act quickly to get the alignment you want.

I use alternating solvent-joined and dry slip-together joints to make the site antenna. To make the dry slip joints you need to dress down the insertion part of the pipe by about 5 thousands. Figure 2 (Pipe Diameter Reduction) shows the size step. You work the size down until the pipe lightly slips into the fitting. Not exactly orthodox, but I used very light pressure on the side of a grinding wheel to shave down the parts in short order. Please do use eye protection with power tools.

I make the site antenna leg tripod base from tees and 45-degree elbows. A completed base is shown in Figure 3 (Completed Base Unit). The topmost 45 elbow shown is lined-up with the free port of adjacent tee. The next tee is at right angles to the first one. Short lengths of pipe join these parts. All the joints are made with PVC cement.

The remaining two 45 elbows mount two of the support legs. These legs are equal length at 3 feet. Cap feet are glued on one end, and the other end is dressed down. You now slightly start the 45 elbows on to the base assembly and slip in the two legs. Rotate the elbows so that the legs are about 45 degrees from vertical. One mast element with a glued-on coupling on one end and dressed down on the other can be made up and slipped into the free tee port. Make this part about 33 inches long (more on this later). Now make minor adjustments to your initial elbow insertion position to make the mast vertical in a lateral sense. Mark your settings as Shown in Figure 3 (Completed Base Unit). The elbows can now be mounted with PVC glue using these marking guides.

The remaining leg is about 29 inches long. It is shorter because it only angles downward-outward instead of also splaying out horizontally. You can size about the length you need with a yard stick, and then make a part a bit longer. A final length adjustment on the dressed-down end completes trimming the mast to vertical.

I construct two more mast sections to make up the full set of PVC parts. The set is shown in Figure 4 (Site Mast Parts). One part is 33 inches long. I make two of the mast lengths shorter so that the end pieces are not stacked when the disassembled mast is stowed in the GO BOX mast-parts foam cutout. I also mount a handheld rest, or “flag” on one of these pieces. The positioning works well for either operating standing, or from working a table-chair emergency data station setup. You have a convenient place to park the speaker-mic. I mount the flag on one of these mast sections with the pair of 2-inch ¼-20 carriage bolts. I use this structurally overkill size so that you can find dropped parts, and also be able to assemble the antenna with little or no light. I show the installation in Figure 5 (Handheld Mounting Flag). The mast is drilled ¼ inch for the carriage bolts. I find a small pilot drilling helps get the bigger hole started through the center of the pipe.

I made things light topside by going with three radials and a small lightweight pad for the mag-mount. I make this pad from half of a Simpson TP-15 tie plate. Three radials mount to a common ¼-20 x 1 inch carriage bolt and wing nut. A Lasco-brand slip plug mates into the top coupling. It has the required flat top for the tie plate part. The radials, the cap and the half of a TP-15 plate use one ¼-20 x inch carriage bolt. This puts the radials at the center of the mast and the mag-mount a bit off to one side.

The Lasco plug also first requires the same diameter reduction as you have done with the PVC pipe free ends. I drilled the cap ¼ inch and used a long carriage bolt, washer and nut to make a mandrel to hang onto the part as shown in Figure 6 (Plug Work Mandrel). You need the hole anyway.

I have made the radials 21 inches long. I put loops on both ends. I tin the connection end. I found the best match on 2 meters with the radials horizontal when I used a mono-band mag-mount. Interestingly, I found a 30 degree droop worked best with a duo-band mag-mount on 2 meters. I tried single length, 21 inch, radials on 70cm as well as radials with 21 inch and 7 inch segments. I do not find any difference. The 21 inch radials are operating as ¾ wave length elements on 70cm, and they do well. I get under 1:1.2 SWR with the radials set with 30 degrees droop (not critical).

The mag-mount antennas with RG-174U feed line are marketed stateside with a range of coax connector styles. You can also find like items on E-bay under the manufacture’s Nagoya brand at an attractive price.

I have packed all these antenna parts, handheld, speaker-mic, batteries, charger, and mobile power cord into a GO BOX that looks on the pricey side, but is actually $40 on E-bay. I believe the intended case use is for rifles, but it makes a great GO BOX (See Figure 7, GO BOX) that packs well, and is easy to carry.

Member Comments:
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An Easy Build GO BOX Site Antenna  
by KA3AUD on August 12, 2017 Mail this to a friend!
Great idea. Nice story.
 
An Easy Build GO BOX Site Antenna  
by KJ4DGE on August 12, 2017 Mail this to a friend!
Great Ideas, good article!



 
An Easy Build GO BOX Site Antenna  
by K5TED on August 13, 2017 Mail this to a friend!
Good show, OM.
 
An Easy Build GO BOX Site Antenna  
by KE5PQR on August 14, 2017 Mail this to a friend!
Nice! You might consider modifying the ends of the legs to allow for staking them down, and/or adding eye-bolts near the top to secure a few guy-lines. Because emergencies and wind often go hand-in-hand.
 
An Easy Build GO BOX Site Antenna  
by KT4EP on August 15, 2017 Mail this to a friend!
I like that "plate" mounted on the pole so you can clip the radio to it... ditto on the guy lines/wind/stake it down. Good club project? Thanks for the post.
 
RE: An Easy Build GO BOX Site Antenna  
by SWMAN on August 15, 2017 Mail this to a friend!
Nice and Neat !
 
An Easy Build GO BOX Site Antenna  
by W8CWM on August 15, 2017 Mail this to a friend!
The easiest way to anchor in the wind is fasten a line to the center of the leg spreader bracket and run down to a stake or a suitable weight. That will hold all three legs with one connection.
 
An Easy Build GO BOX Site Antenna  
by KC0W on September 4, 2017 Mail this to a friend!
A real Plumbers Special!!!
 
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