Thursday, February 26, 2009

Building a 40-M 1/4-wave Vertical for KC8UHE

Today KC8UHE and I undertook to build and install a 1/4-wave, 40-Meter vertical at his QTH. Tim had a 21-foot telephone pole already installed (left-over from a prior antenna project) so we decided to use that as the support for the antenna. We had previously ordered 6 telescoping pieces of aluminum tubing from DX Engineering. The smallest was 1/2" in diameter and the largest was 1-1/8 inches. We planned to slip the pieces into each successively larger piece to build up a 33-foot long radiator. We also purchased two Resin Support Block Clamps from DX Engineering (see one HERE) and planned to screw these to the telephone pole to support the vertical.

Last night I cut off a short piece of perforated metal strap, used a chassis punch and drill to make the mounting holes for a chassis-mount SO-239 connector, and bent it into an L-shape to attach to the telephone pole. A short jumper from the center pin of that connector would connect to the vertical radiator.

Tim drilled some pilot holes and then I lifted up the 33-foot piece of tubing and he secured the Upper Mounting Clamp to the vertical at about 20-feet above ground. This meant Tim was on one ladder near the top of the pole and I was on a shorter one to the side where I could hold the bottom of the vertical about 10-feet above ground. Then Tim moved down and installed the second Mounting Clamp. At this point, the vertical was UP!

We then screwed the mounting bracket with SO-239 connector to the telephone pole and connected the center pin to the vertical tubing. Three elevated radials were then cut at 33-feet and a spade lug soldered to the end that would attach to the SO-239 bracket. We put a couple of lag bolts into the pole to take the strain of the radial wires off the spade lug ends and just wrapped the wire around the lag bolt in a simple over-hand knot. Below you can see the Lower Mounting Bracket, the coax connector & bracket and the radials attached to the lag screws. Click on the photo for a larger view.

The final step was to wind the end of the RG-213 feedline into an RF Choke to decouple the feedline from the antenna. We purchased a piece of PVC that was 4-1/2 inches in diameter and wound about 23-feet of feedline in 18 turns on the form and secured it with Ty-Raps. See THIS PAGE for similar chokes. Below is a photo of our RF Choke (click on the photo to see a larger image):

Checking the antenna with the MFJ-259 we found the antenna to be a bit too short. So, Tim climbed back up the ladder and loosened the clamps to let the pieces of tubing telescope back down for three sections. He then lengthened the amount of tubing that was above each section so that we raised the height of the antenna by about a foot. This brought the resonant point of the antenna down to 7.19 MHz. At that point the antenna had an SWR of 1.1:1 and exhibited a 50 ohm impedance. Down at 7.025 MHz, the SWR had climbed to 1.5:1 so that was still acceptable. We may lengthen the antenna some more after Tim has a chance to use it a bit but for now, it seems to be A-OK. And, total cost of this antenna (less the feedline) was under $100.

Below is a photo of the completed antenna. Click on the photo for a larger image. You can see more photos of our project on THIS PAGE.

At 4:45 p.m. we checked the radio and had excellant copy on 9K48NLD in KUWAIT on 40-M SSB!! This was more than 1-1/2 hours BEFORE our sunset! I just can't wait to see who Tim works with this Monster! (Now I need to get the splinters out of my hands!)

1 comment:

ON8EI said...

Lovely write up, gives ideas. I am trying to fit a 40m 4sq in the garden. 73's ON8EI TNX