Many folks have asked me about the PLPV so here goes:
Like many of you I try to find the time to do as much QRPing as possible given the schedules of work and family. Although I have been a ham on and off since 1957 I never have been active during a sunspot cycle. I was bound and determined to be active this cycle and find a way to maximize my operating time. The only way possible for me to increase my operating time was at lunch time near work. Since like many of you I work in an urban environment I am faced with lots of concrete. My one restriction was not to have a BIG antenna on the car permanently. Bob Edwards W4ED showed the North Georgia (NOGA) QRP group a portable setup for his SLA vertical that showed promise but it did require a car and the antenna had to be adjacent to the car. He used a frame/stand that was held steady by a car tire resting on the antenna support frame. I was looking for something a little more flexible. Something that could be operated from a car, in the event of rain or bad weather but, also could be operated from anywhere it could be hauled. I like to bike ride and trail hike so I was looking for a method of setting up an antenna that was independent of trees and other local supports.
The design criteria:
Using the idea of the SLA-type collapsible fishing pole as a vertical support I wondered how could I achieve the other criteria: self-supporting, quick setup/band change times and no tuner.
I came on the idea of a foldable wooden support structure that would hold the fishing pole erect and stable in reasonable winds. The fishing pole selected from a local Bass fishing store was a multi-section Carpie Pole made of black plastic that collapsed down to less than 4 ft and extended is almost 20 ft. I think any of the pole brands that have been mentioned on the various SLA articles would work.
I initially built a wooden foldable base that worked but was too heavy. At Home Depot I found some 1X1X36 inch aluminum angle that looked promising. Using four pieces I formed the base. Using a fifth piece I cut it in pieces and made a square base frame 3X3 inch and hinged the four 3ft pieces to the small square base. The remaining 2 ft piece I bolted to the small base (not hinged). I made a wooden Tee to screw the four base legs together so the base and legs were fairly rigid and self supporting. The 2ft section was vertical and the resulting structure looked like a four radial GP sitting on the ground. The fishing pole was cable tied to the 2 ft angle so the vertical when extended reached about 20ft. With the wooden Tee unscrewed from the four legs the legs fold towards the fishing pole. When then pole is collapsed and the legs are folded the entire structure is about 4ft tall and about 6-8 inches in diameter. It can easily be picked up with one hand. Its very light.
I had read that verticals with a limited number of above ground radials being more effective than verticals with a similar number of radials on the ground so I decided to have the base of the vertical be about 1.5 to 2 ft off the ground and that any radials would be slightly sloping downward towards to ground at the outer ends. I decided on initially trying two radials on opposite sides of the base.
The antenna would be coax fed with 50 ohm RG-58 and coax sections would be available to cover varying distances from 20 ft to 70 ft from the base of the antenna to the transceiver. This arrangement would accommodate a variety of setup conditions with the shortest amount of coax.
The no tuner requirement meant pretuning the antenna for the bands of interest. Initially I tried the multi-wire parallel configuration with individual wires cut to each band but the interaction seems to be too tough to solve. So I cut a 1/4 wave length for the highest band of interest (10 mtrs). One wire for the vertical section and two for the radials and attached them to the coax at the base in the classic inverted Y configuration. The vertical wire connected to the inter-conductor of the coax and one end of the radial wires to the outer shield of the coax. To decouple the coax feedline from the antenna I made a RF choke by wrapping about six turns 6 inches in diameter of coax and tapping them together. With my trusty MFJ SWR analyzer I checked the arrangement for freq vs min SWR. On all bands tried I could get a low SWR but using the book formula length where a quarter wave (ft) equals 468/freq(in Mhz)/2. The problem I noticed was that the frequency at the low SWR point was always lower than expected by the formula. But by careful pruning of the radials and the vertical section I could get the antenna to have a low SWR < 1.5 to 1 in the frequency range of interest. I then calculated and added sections to all three elements to get to the next lower band and prune for SWR in the new frequency band of interest. I have done this for three bands to date (20,15,10). So you have one, second or three little wire segments depending on the band. How do you keep them together, yet apart, for the unneeded segments. My wife's sewing box had some light weight elastic band tape (looks like stretchy string) that is used for making or mending expandable clothing. I took a four inch section and attached the end or the wire segments by simply knotting the wire and the elastic leaving about one inch of wire over the end of the knotted area. To the wire ends I crimped on some small quick disconnects (from RS). When completed the connected wire segments can be pulled taut and the segments will be apart a few inches. To give the horizontal radials a little extra support I taped the segments to a piece of light weight nylon rope. The vertical is clipped at the very top of the extended pole to an section of elastic connected to the last wire segment. When fully extended the segments are stretched a small amount. To change from band to band you simply collapse the pole and connect the needed segments and re-extend the pole. The radials are similarly connected. Since the all the segments are already inplace the operation takes less than a minute. The pole can handle a 1/4 wave length vertical wire down through the 20 mtr band (i.e. 16.6ft). The accomplish the remaining bands (40 and 30 mtrs) you need to form a loading coil and an additional wire segment above the coil. These can be made in a quick disconnect manner previously described. The 40 and 30 mtr segments should be made separately to simplify the antenna. If these lower bands are rarely used I would keep them aside and not complicate the vertical or radial segments. But if operation is desired routinely you can have them in place just like the wire segments for the higher bands.
The results of the antenna have been better than expected. QRPTTF and ARS Bumblebee events have proven its effectiveness. More importantly I can go out at lunch and catch some QRP action.
Sam Billingsley AE4GX Atlanta, GA e-mail: firstname.lastname@example.org
|Figure 1. Gear in trunk|
|Figure 2. Parts on ground|
|Figure 3. Detail of base of stand|
|Figure 4. Brace and stand|
|Figure 5. Brace with 3 legs attached|
|Figure 6. Brace with three legs in position|
|Figure 7. Brace and stand in position|
|Figure 8. Antenna connected to stand via cable tie|
|Figure 9. Ant and radials deployed|
|Figure 10. Detail look at antenna wire segments quick disconnect|