Our Portable Ham Shack - Part 4

Miscellaneous Projects

A Stable Little Table

Since the trailer's dinette is used for a bed at night, it quickly became apparent that unless I planned to participate in Field Day by myself that the table would not be available for night time operation. I was starting to search the Internet for ideas when I was given a set of old folding bar stools. Uncomfortable to sit on, the folding legs would work for building a small folding table that was high enough that I could sit on my bunk bed and have my computer at a comfortable level. I just happened to have one more piece of 3/4" plywood in the shed which would serve as a top. While it may look a bit strange, it is stable and serves its purpose well. It is also small enough so as not to block access to and from the trailer's single door. How big is it? I don't know. I've never measured it.

Aiming Device For Solar Panels

The November 2008 issue of QST Magazine included an article titled "Some Thoughts on Solar Trackers". The idea was to build a simple device to help the solar panel user take advantage of as much solar power as possible in a situation where the panels can be moved. This is most important when camping in the winter months, and also for communications events such as the ARRL Field Day or emergency situations when battery power might suffer significantly from high use.

The device has only two parts -- a piece of wood for a base and a piece of 1/4" wooden dowel. The wooden base needs to be large enough for a 2-1/8" circle to be drawn on it, and the dowel is 2" in length plus the thickness of the base.

The authors advise using a student compass to draw the circle, and a drill press to drill the hole exactly in the middle of it. Do they make student compasses anymore? I didn't find any among my office supplies, and Walmart didn't have one. And, I didn't have a drill press. I can feel another "Plan B" coming on!

Searching around the house, I came up with a vitamin bottle with a 2-1/8" cap, and the cap had a dimple in the center. I used the cap for a guide when drawing my circle, and drilled a small hole in the dimple to mark the center. A hand power drill was used to make the 1/4" hole in a piece of flakeboard siding, carefully starting with a small bit and increasing the size until the 1/4" hole was achieved. A bit of trimming on the hole with a pocket knife brought the dowel within close proximity to pointing straight up. I'm sure the authors, with their careful trigonometry calculations would flinch at this, but what I came up with is "close" to their calibration and the panels were aimed a whole lot closer than they would have been without the "solar trackers".

A piece of trim board wider than the thickness of the base board was tacked onto one side of each tracker. This made it easy to clip the tracker onto the metal solar panel frame with two small PVC spring clamps. One clamp would suffice, but two assure it isn't going to move. The edges and back of the base, as well as the dowel, were sealed with a stain containing polyurethane.

When the tracker is aligned with the dowel pointing directly at the sun, it shows no shadow within the circle. As the sun moves, a shadow appears. When the shadow touches the circle, the panel's position is off by 25 degrees. But that's not bad! An aiming error of 25% still gives an effectiveness of 90%. Keeping the shadow within the circle guarantees a minimum effectiveness of 90% and a higher average effectiveness.

A Jumpered Dipole Antenna

My original plan for a portable HF antenna for the trailer was to do something different and build a jumpered dipole. Then I decided I didn't have time nor the energy to make one before Field Day 2009 and bought a G5RV Jr instead. Upon reading the instructions that came with the G5RV Jr, however, I found that any metal near the wire feed line would detune the antenna. It further specifically stated not to use it with a metal antenna pole, which I would have on the trailer. Yes, I could make do with it by taking additional PVC pipes for masting, but why go to all that trouble? It was time to go back to "Plan A".

A jumpered dipole offers low loss (assuming negligible resistance across the jumpers), coax feed, and in most conditions an uncompromised bandwidth and a standard dipole pattern without a tuner. The major disadvantage is that you need to lower the antenna each time you switch bands. This can be difficult at night. It may not a good choice for rapid or frequent band changes, but for a portable antenna used as an Inverted "V" for Field Day, it should work fine. I already had the set-up built for lowering the center of the "V" from the mast.

I like the idea of having a full half-wave dipole for the 40, 20 and 10 meter bands, and with the MFJ-949E antenna tuner I use for portable operation, the antenna will also work on 30, 17, 15, and 12 meters for occasional DXing if/when the HF bands improve. The antenna I chose to build was patterned after one shown in the November 1996 QST magazine. I found the plan to be very close to what I had envisioned when I contemplated building the jumpered dipole earlier.

Materials List:

  • 1:1 current balun (optional, but I had one available) or center insulator

  • 75 feet of #14 AWG stranded copper wire, insulated
  • 4 pieces 1/2" ID PVC pipe, 3" long for insulators
  • 2 pieces 1/2" ID PVC pipe, 5" long for end insulators
  • #8 bolts x 1-1/2" (2" would be better)
  • #8 flat washers
  • #8 lock washers
  • #8 nuts
  • #8 wing nuts
  • 3/4" wide x 1/8" thick aluminum (thinner metal will do, but this is what I had)
  • Wire cutters
  • Pliers
  • Screwdriver
  • Drill and bits
  • Metal file
  • Hacksaw
  • Soldering iron or gun
  • Solder

Making the insulator/jumper assembly

Before laying out wire for the antenna, I made the insulator/jumper assemblies. There are several was to make a jumper, and most will be lighter in weight than what I made. I even thought of using Anderson Power Pole connectors on wires. They certainly would be light weight, but possibly difficult to connect in the dark.

The original plan called for pieces of flat Plexiglass for insulators. I didn't have any on hand, but I did have 1/2" inside diameter PVC water pipe. Four 3" pieces were cut from it and drilled for #8 bolts.

After pushing the bolts through the PVC pipe and fastening them with nuts, I marked the aluminum for drilling. After drilling the two holes for the bolts, I used a drill, hacksaw and file for finishing the notches.

 No flat washer was used on the outside of the notched end between the jumper bar and wing nut. The #8 washers are so thin that it would take a lot of time to fiddle with getting the metal bar between them when lengthening the antenna. It would be almost impossible in the dark. This way, all that needs to be done is the turn the jumper assembly until the bolts are upright, and the single washer on the inner side of the metal bar drops into place against the regular nut.

To change to a lower band, let down the antenna, loosen the wing nut on the unnotched end slightly, loosen the wing nut on the notched end of the metal bar, turn the assembly so the bolts are pointed upward, flip the notched bar onto the bolt, and tighten both wing nuts. It's easy, even in the dark. Make the same adjustment to the other half of the antenna, and then raise the antenna back into position. To go higher in frequency, "unlock" the appropriate notched end on each leg of the antenna.

Your first reaction might be, "Wow! I would have to lower both sides of the dipole to change bands." You would be correct. That's why I have chosen to use the antenna in the Inverted "V" configuration.

I checked to see what the length of each antenna leg (connector included) should be, then started out with additional wire on each leg. It is easier to cut wire off then to add it. An MFJ-249B antenna analyzer was used to determine the length of each individual dipole section, starting with the band highest in frequency and moving outward to the lower bands.

If you are unsure what frequency to cut the antenna for on a given band, cut it to cover the SSB portion you intend to work. You can always dangle a short piece of wire off the end of the section in use to bring the antenna to resonance on the lower frequencies. If you want to expend the extra effort, you could make complete additional sections of wire for the lower frequencies to substitute when necessary. I'm not that much of a purist.

We did not have time to test the antenna with a rig, however the antenna analyzer showed full band coverage on 10 meters at 2:1 SWR across the entire band; coverage of the entire 20 meter band using 14.180 for the center frequency and both band edges at 2:1 SWR; and all but 8 KHz of the 40 meter band with band edges showing an SWR at 2:1.

Used with the tuner, the antenna performed very well on 40 and 20 meters during the 2009 Field Day event in June.

Things I would do differently next time? I am not a fan of 14 gauge stranded wire. It is too flexible and tangles easily. I would use solid wire, or at least 12 AWG stranded wire. I would also use a flat base for the insulator/jumpers rather than PVC pipe. It is difficult to keep the bolts tight on a rounded surface. Using 2" rather than 1-1/2" bolts with the PVC pipe insulators would provide more usable threads when changing the jumpers.

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