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location: fandipole

Fan dipole

Kludge's antenna project

This antenna is made from 14 gauge stranded copper-clad. The stranded cable is less brittle and more flexible and the copperclad is much stronger and cheaper than regular copper. I used whatever the Wireman was selling at the last hamfest.


There are actually four individual dipoles here, each cut for a different band, all held parallel to one another, and all sharing a common feed point.

There are six spreader bars, each made of 24-inch schedule 30 PVC pipe with four holes drilled in them with a hand drill. In this illustration they are the original white color although I later painted them chrome green in order for them to blend into the scenery a little bit better.


The two ends of the fan dipole are attached to two trees in the yard about 90 feet apart, with a pulley at each end up in the tree so that the antenna can be raised and lowered for maintenance or alteration. The feedline is in the center of the yard and is supported only by the antenna itself. My wife says the feedline going into the ground is ugly (which it sort of is) and I am looking for a solution to get it out of view. The feedline is RG-154B because I got a lot of it cheaply at a hamfest but you can use whatever low-loss cable you want. Loss is not too critical since the antenna is very close to 50 ohms on most bands.


This is a close-up of one of the ends. There is a steel aircraft cable around the tree, with some old garden hose around it to keep it from damaging the tree itself. A U-bolt holds the steel cable into a loop, and a small pulley from the hardware store is at the end. 1/4" nylon rope goes through the pulley and a loop is made at the end of it which goes through the spreader at the end of the antenna. The loop is not attached to the spreader so that with a little judicious tugging you can adjust the angle of the spreader when it's tensioned.


The end of each of the ropes is secured to a cleat. It looks flimsy but it holds indefinitely, but is easy to remove when you want to check something.


The balun is homemade; it is just a 2" PVC plumbing cap with a hole drilled in the bottom and an SO-239 screwed into it. About a foot of 12 ga. magnet wire scavenged from an old motor is soldered to the center socket and the sleeve of the connector, and they are wrapped together four turns around a large hamfest toroid that is probably material 43, then the leads pulled out of holes drilled in each side of the cap. Eyehooks are attached to each side and the inside of the cap is filled with silicone potting material. You could use roofing tar if you didn't mind a little more weight.

In this picture you can also see some extra thin nylon cord used to try and keep the two inner spreaders in position. This didn't work at all but I didn't bother removing it.


A sideways picture; lifting the antenna into place.


Another sideways picture of the whole operation. You can see that the top dipole is cut for 20m (17 feet on each end), the one below it cut for 17m (13 feet on each end), the third one from the top is the long 60m one (44 feet on each side), and the bottom one is the 40m one (33 feet on each side). I laid them out in this way to keep the 20 and 40 meter ones separated and reduce interactions between them. That seems to have worked, but now I have issues with the 60 and 20 meter dipoles interacting with one another. If it's not one thing, it's another.

I cut all of these slightly long (to the measurements above) and then trimmed them into resonance in the middle of the phone portions of each band using a grid dip meter. If you aren't concerned with having it perfectly tuned, you can just use it as is. There is an insulator at the end of each one of them (except the 60m section), and thin nylon cord is used to continue them so that all four antennas have the same physical length for the purposes of tying them off at the ends.

If I had to do it again I might consider dropping 20 meters altogether. This antenna works beautifully on 15M because the 40M section has a secondary resonance there. My suspicion is that the 60m section alone would perform well on 20M.


A close-up of one of the insulators in the rain, copperclad on one side and nylon on the other.


One end about to be raised. You can see here that the 60M copper is tied off at the end, and the other three dipoles have the thin nylon cord tied off at the end.


A sideways picture of raising it.


One end silhouetted against the sky! You can see that there is a knot tied in the rope going from the cleat to the pulley in such a location that the antenna cannot be dropped down below about three feet from the ground. I like doing this because it prevents the rope from inadvertently being pulled all the way through the pulley and someone having to climb the tree and reattach it.


Closeup of the balun. I actually did use a jeweler's torch to solder the leads from the balun to the bundle of copperclad on either end later on. This is totally unnecessary; just wrapping and clamping it is fine, but soldering makes me feel better.


Another shot of the end attachment. This was originally set up at a fairly low height; the pulley was moved another 20 feet up the tree after I made sure everything was secure and worked well.


This picture actually is from a different antenna and probably ought to be on another page but is a good illustration of a simple dipole attachment.

Many thanks to Brian Case who originally set this up, and to my wife Chakaal who has been very tolerant of strange things in the yard.

fandipole (last edited 2012-09-20 16:16:40 by sauron6)