My experiments with raising a portable antenna mast continued this weekend. Building on the lessons of my failed first attempt, I went back to the drawing board. After a few delays in getting out to the field (thanks to early winter sunsets and busy schedule), I finally achieved success in raising an effective 20 meter dipole 30 feet into the air at a nearby school park.
My primary concern in the redesign was finding a solid base for the mast. In my first attempt, I used a tent stake in the ground as my anchor point. As you recall, the mast broke free during a failed raise and I sprained my thumb.
This time, I followed a mast design that Lew Thompson (W5IFQ) built for TX Army MARS which has been used frequently during Field Day. I purchased a swiveling base from TMastCo. It’s made of composite (plastic) wood material and has anchor holes, four 12″ nails, and a swivel that fits the army surplus mast sections.
I used 8 aluminum sections for a mast of 30′ instead of the previously unwieldy 10 sections. The reduction of 7′ represents a negligible ERP loss according to Lew, so I went with it. Besides, 10 sections would’ve needed a second level of guys, so it’s far too much work for a temporary mast.
Another challenge was to correctly calculate the length of the guy ropes and anchor locations so I can pre-tie the guys and raise the mast solo. I built a spreadsheet to calculate distances for the anchors and length of the guys varied by the number of mast sections in use. I factored in things like the length of the D-rings on the guy rope, the height of the base, dimensions of the anchor point, and the dimensions of the Mastrant sectional bracket at the apex of the mast.
This time, I nailed it. In the first raise attempt, with hard hat on my head, I successfully stood the mast up and was able to lean it against two of the guys while I pulled the third guy outward to attach it to its anchor.
It took me about an hour from arrival to:
- survey the site for flatness, safety clearance, and soil stability
- drag out the gear from the car
- drive the 3 anchors and swivel base
- assemble the mast and attach the apex bracket, pulley, and vertical rope
- measure and tie the guy ropes — used an adjustable Midshipman’s Knot at the bottom and a static Bowline Knot at the top
- raise the mast and attach the 3rd guy
- adjust the guys to make the mast vertically plumb
- string out the dipole with 1:1 balun and RG58 feedline and raise it
- spread the dipole into inverted-V with friction knots on the anchors
- assemble the radio gear and call CQ
On a technical front, the 20m dipole that I’ve used successfully between trees had a significantly lower resonance due to the aluminum mast. It became too long. The most resonant point was 13.4MHz.
|13.4 MHz||1.1:1||40 Î©|
|14.0 MHz||2.1:1||45 Î©|
|14.1 MHz||2.4:1||40 Î©|
|14.2 MHz||2.5:1||35 Î©|
|14.3 MHz||2.6:1||34 Î©|
Just terrible. Thankfully I had an antenna tuner. If I had time, I would’ve shortened the dipole for tunerless operation. But at least it was able to hear lots of things very clearly, specifically a handful of the NCDXF propagation beacons in this hemisphere that I’d not heard before on any of my antennas.
Since it’s Christmas season, I did have a number of looky-loos in the school park coming by to ask what I was doing. From their point of view, it looked like I was raising a Christmas tree similar to what we have in Zilker Park (which is a moontower with a cone of lights). When I told them it was a ham radio tower, they were surprised and asked interesting questions. Glad for the opportunity to teach.
Unfortunately, the whole enterprise was a womp-womp moment. Despite all my preparations and the success of the raise, what critically stopped my ability to log any QSOs was the fact that my Yaesu FT-857 radio was failing to put out any appreciable power on SSB with the hand mic. Even with yelling into the mic on USB, all I could get was a paltry 9W, although my output was set to 100W. My CW key put out full power. But sideband voice with the hand mic was dead mute. No changes with mic or compressor level would fix it. I’ll have to debug it later. Womp. Womp.
Also, my 12V battery was undercharged, even though I charged it recently. So I kept getting beeps from my West Mountain RigRunner (which has an undervoltage warning) when I keyed down. My voice transmissions were also coupling with the RigRunner’s alert beeper and making shrill rattles on vocal peaks. Womp. Womp.
By then, the afternoon was growing long, the sun was coming out, the park was filling up with families and hyperactive kids enjoying holiday R&R, and I needed refreshments. It took about 40 minutes to take everything down and head back home.
Now that I’ve done most of the measurement and assembly, I can do it a little faster next time. The guy ropes are still marked with the correct length for 8 sections, I left the knots intact when I rolled up the ropes, and I left the apex bracket loosely assembled so I don’t have to mess with the bolts next time.
I now have a functional portable mast and I know how to use it. It’s just another tool in the toolbox; practice for when I’ll need it most. Future experiments may involve fiberglass sections, fan dipoles, delta loops, and vertical antennas with ground planes…once I get the radio to work right. Hi-Hi.
73 de KG5RHR.