My most recent antenna project would make Dagwood Bumstead proud, as it resembles some of his plumbing work. The plans are outlined below, should anyone wish to duplicate my effort.
Everything was modeled first in NEC4WIN95 to see if things were on the right track. It took days of trial, error and adjustments to arrive at the final dimensions. There’s probably dozens of similar models out there, but this is my first attempt at NEC modeling.
There are multiple components. The basic part is a 1/4 wave vertical over a 1/4 wave ground plane. Then the “Dagwood” part consists of a phasing section of about 3/8 wave. A 5/8 wave vertical tops it off. The NEC model predicts 7.86 dBi (5.71 dBd) of gain – probably overstated in my opinion, but I hope to do an actual plot in spring.
The SWR analysis provided the following impedance plot:
| At 144.000 Mhz Z = 53.32 – j 20.31 SWR = 1.49
At 144.200 Mhz Z = 53.82 – j 18.88 SWR = 1.45
At 144.400 Mhz Z = 54.33 – j 17.47 SWR = 1.41
At 144.600 Mhz Z = 54.84 – j 16.06 SWR = 1.38
At 144.800 Mhz Z = 55.39 – j 14.62 SWR = 1.34
At 145.000 Mhz Z = 55.93 – j 13.21 SWR = 1.31
At 145.200 Mhz Z = 56.48 – j 11.79 SWR = 1.29
At 145.400 Mhz Z = 57.04 – j 10.39 SWR = 1.26
At 145.600 Mhz Z = 57.63 – j 8.97 SWR = 1.24
At 145.800 Mhz Z = 58.22 – j 7.55 SWR = 1.23
At 146.000 Mhz Z = 58.83 – j 6.15 SWR = 1.22
At 146.200 Mhz Z = 59.45 – j 4.73 SWR = 1.21
At 146.400 Mhz Z = 60.07 – j 3.34 SWR = 1.21
At 146.600 Mhz Z = 60.71 – j 1.93 SWR = 1.22 Resonance
At 146.800 Mhz Z = 61.36 – j 0.52 SWR = 1.23 Resonance
At 147.000 Mhz Z = 62.03 + j 0.88 SWR = 1.24 Resonance
At 147.200 Mhz Z = 62.71 + j 2.29 SWR = 1.26
At 147.400 Mhz Z = 63.40 + j 3.69 SWR = 1.28
At 147.600 Mhz Z = 64.11 + j 5.11 SWR = 1.30
At 147.800 Mhz Z = 64.84 + j 6.52 SWR = 1.33
At 148.000 Mhz Z = 65.58 + j 7.93 SWR = 1.36
The antenna is constructed mainly from plumbing materials. The first vertical section consists of a piece of 3/4 inch copper pipe. The base is soldered into a brass fitting with 3/4 inch threads to mate with a nylon male to male fitting. At the top, solder on a 3/4 inch to 1/2 inch copper reducer, and a short piece of 1/2 inch copper pipe topped off with a T fitting. The total height of this assembly is 46.8 cm, measured to the middle of the side of the T. The phasing section joins the T at a 90 degree angle. Insert a 10 cm piece into the T, leaving the top of the T empty for now. Add a 90 degree fitting, then another 10 cm pipe, followed by another 90 and the 20cm piece. Keep adding to this until you reach a point 11cm above the main vertical. To keep everything aligned and stable, I used a 9 cm piece of 1/2 inch PVC pipe joining the two T fittings. Carefully align the phasing section, keeping the sides parallel with a steady incline. Once everything is aligned, solder it together. Then add the top 130.7 cm copper pipe for the top mast.
For the ground plane section, you need 2 galvanized flanges with 3/4 inch thread. Take 4 copper pipe caps for 1/2 inch pipe, put them in a vise, and drill a 1/4 inch hole in the centre of each. Insert a 1/4 x 1 inch carriage bolt inside the cap. Put both flanges together, and stick each of the the bolts into the flange holes with the cap on the bottom. Tighten nuts on the top. Once this is assembled tightly together, take 4 1/2 inch copper pipes cut about 47.3 cm long, and insert a 45 degree street fitting on one end of each. Put the other end into the caps, and after aligning the pipes 90 degrees apart, solder into place. This gives a total length of 51.3 cm for each ground plane radial.
The top section then gets threaded into the upper flange, and you are ready to attach the feed point. Various methods can be used. Probably the best would be to solder the coax centre conductor to the inside of the brass fitting, and attach the braid to the flange base. Then the coax can go inside the supporting mast, which is a length of 3/4 inch galvanized pipe threaded into the bottom flange. A one-turn coil of 3 inch diameter can be added on the outside between the flange and mast to provide DC grounding and fine-tune the match (feed point made little difference during tests). Make sure everything is watertight – it’s possible to get condensation dripping onto the coax.
Check your SWR. Using these instructions, it should be very close, making fine tweaking unnecessary. When installed away from metal objects, the SWR will be centred around the middle of the band, with the entire band under 1.5:1. In reality, it was a bit higher than predicted, but less than 2:1 across most of the band. Beware if adjusting the height of the top section to lower the match. The modeling software showed that if resonance was too low, it had to be lengthened to bring the resonance higher. If adjustments are needed, use a hose clamp to clip a #14 copper wire on the top to find out the proper height, raising or lowering the wire. Just be careful since at this point, the tuning seems counter-intuitive!
As a final touch, double check all soldering, clean the copper pipe of any excess flux, and shine it up with steel wool. Add a coat or two of shellac to preserve the colour of the copper and prevent oxidation.
The antenna was very tricky to tune up. The dimensions calculated for the lower section were too long with the 3/4 copper pipe. Several inches had to be cut off. Then, using the analyzer, the top section was added and trimmed for the lowest SWR. A coax fitting was added at the base to make threading the antenna on the mast easier. The loading coil is about 3/4 turn of #12 copper wire going from ground to the center mast. The lowest feed SWR was found right where the coil was attached to the mast. A larger coil might provide more tuning range.
Overall, measurements were very close to the modeled values. The lower mast had to be shortened substantially from the dimensions given. SWR bandwidth of 2.0:1 or less is from 143.1 to 151.9.