2 meter 5/4 wave antenna in 3/4 inch PVC tube.
By Mike Martell N1HFX
Many RASON members truly enjoyed last month’s collinear antenna. This month I decided to build a 2 meter 5/4 wave antenna. This antenna is unique in that it is enclosed entirely in 3/4″ PVC which makes the design a little more complicated. The primary problem is that PVC tubing has a significant velocity factor which causes RF to slow down.
This means that an antenna encased in PVC will normally need to have its physical length reduced by about 19%. To further complicate the design, a 5/4 wave antenna’s impedance has a highly inductive component which must be tuned out to get a good match. Fortunately, the design in Figure 1 solves all of these problems.
This antenna is made with the following components:
About 2 ft of outdoor type 300 ohm TV twin lead (Used for the matching system.)
About 5 ft of #18 stranded insulated wire (Used for radiating element.)
About 5 ft of RG58/U coax
One PL259 Connector
One PL259 female to female coupler
About 8 ft of 3/4″ PVC tubing. (normally sold in 10 foot lengths)
Two 3/4″ PVC end caps
About 8 ft of 1/4″ hardwood dowel (normally sold in 4 or 5 ft lengths)
About 25 small tie wraps
Miscellaneous PVC cement, solder, small piece of tubing, etc.
The twin lead was originally cut for 20 inches with 4 7/8 inches cut back on the braid or ground side. The #18 insulated wire was cut to exactly 57 3/4 inches. The overall length of the antenna assembly is 77 3/4 inches. This indicates a velocity factor of about .81 compared to a normal 5/4 wave 146 Mhz antenna. See the calculation below:
234 * 5 / 146 X .81 = 6.49 feet or about 77.88 inches
Now that we have all our parts, let’s begin assembly by cutting back the insulation of the coax and the TV twin lead. We will need to cut back the coax to expose the centre conductor as well as part of the braid. It is a good idea to lightly tin the braid with solder to prevent any strands from shorting out to the centre conductor. Solder the centre conductor to one end of the twin lead and solder the braid to the other end of the twin lead as in Figure 1. Notice the braid of the coax is soldered to the shorter part of the twin lead which is left open. This serves as our matching system which adds capacitance to our antenna to offset the inductive component of the antenna. Trim the twin lead to 20 inches and solder about 60 inches of #18 stranded wire to the twin lead as in Figure 1. The insulation should not be removed except as necessary for soldering.
Prepare the 1/4″ hardwood dowel by joining two 4 or 5-foot lengths together. The ends can be joined by crimping a 1-inch length of 5/16″ aluminium tubing or using good quality wood glue. Now attach the coax, twin lead and wire assembly to the 1/4″ dowel using tie wraps about every 3 inches. Pull the twin lead and wire to keep it as straight as possible. Before attaching the PL259 connector to the coax, drill a hole in one of the PVC end caps and slide it over the coax to prepare for permanent mounting in the PVC. Now attach the PL259 connector as well as any other connectors needed to check SW R. Cut back the open end of the twin lead to about 16 inches as in Figure 1.
Now we are ready for final tuning. Slide the antenna and dowel assembly inside the 3/4″ PVC first. All SW R readings must be taken with the antenna, and dowel assembly inside the PVC tubing or the antenna will appear electrically shorter than necessary. Check SW R on both the top and bottom edges of the band. If the SW R is higher at 147.995 Mhz than at 144.005 Mhz then the antenna is too long and should be shortened. Cut off no more than a 1/4″ at a time of the #18 wire. Also, trim the open end of the twin lead by no more than 1/8″ at a time to further lower SW R. Remember the twin lead is simply a matching system which changes impedance and has no real ef f ect on the electrical length of the antenna.
The final lengths of the #18 wire and twin lead should very closely resemble those listed in Figure 1. The prototype antenna achieved SW R readings of less than 1.2 to 1 across the entire 2 meter band. Remember to keep the antenna away from metal objects when checking SW R. After the antenna is properly tuned, trim the antenna dowel assembly to about 7 feet. Leave a few inches of coax attached to the bottom of the dowel so that the mast will be away from the twin lead portion of the antenna when mounted. Trim the PVC tubing to about 7′ 2″ and cement the top end cap. Double check SW R before cementing the bottom end cap. After SW R has been doubled checked, slide the antenna, and dowel assembly into the PVC and cement the bottom end cap. If desired, styrofoam spacers may be used to get a very snug fit. Waterproof the bottom end cap where the coax leaves the antenna. When completed, the antenna should resemble Figure 2.
When mounting the antenna, use a PL259 female to female coupler. Do not use RG58/U for the entire feed line because it is too lossy. Use good quality RG8/U or similar coax for the feedline. Of course, do not forget to waterproof the female to female coupler. Mount to any mast using standard TV antenna clamps at the bottom of the antenna and keep it high and away from other metal objects for best performance and lowest SWR.
Although not measured, this antenna should give at least 6 dB gain if mounted high enough. Remember, the small diameter of the radiating element has no effect on the radiation resistance. The only real benefit of using a large diameter radiating element is durability and slightly improved bandwidth. This antenna should give many years of reliable performance for a fraction of the cost of a commercial antenna.
DE N1HFX