The 2m/440 Super J-Pole
This antenna is also known as the “copper cactus” (because it kind of looks like one of course!). It is an outstanding performer, especially when you get it up in the air. A friend of mine (KC0UYK) convinced me I should build this antenna and I’m certainly glad he talked me into it.
I built mine from 3/4″ copper pipe. This provides you with a greater bandwidth than 1/2″ pipe and the SWR stays low throughout the 2m Amateur band. I’ve built two of them to date, and they both work wonderfully with very little tinkering. Even if you’ve never used a soldering torch to weld copper before it’s a good project to learn on. (I had never done any copper pipe work before my first cactus antenna).
If I was going to do it again, I’d use a stronger type of copper pipe simply because it tends to flex in the wind just a bit. I chose the cheaper of the materials and in hind site I regret it a bit.
I’m not going to go over all the details of how to build one since there are links below to help you with that. I’m only going to discuss the lessons I learned while constructing and using it.
The instructions I followed were from this site:
Here is a nice calculator as well showing the proper lengths for a given frequency. I strongly recommend using this to do the design:
The only modifications I needed to make were to increase the space between the two vertical components by 1/4″ to account for the increase in the size of the pipe (those directions are for 1/2 inch pipe).
Finally, if you’re one of those modern “I’d rather watch an instructional video” kind of folk, here’s a youtube video of someone building a smaller J-pole (non-super) for just the 70cm band.
It’s recommended you use a short coil of 4 turns of coax as close to the input of the antenna as you can. The super-J’s tend to be affected by near-by metal structures and this is supposed to help alleviate that. I’ve done this in the past and I think it did help, but the way I have my current one mounted I don’t have a coil at the moment and it still works wonderfully, but there isn’t a huge amount of metal near it either.
Getting It In The Air
I had mine in the attic for a while, and it did just fine. But when I finally got it up on the roof connected with high quality co-ax boy does it reach out better.
I mounted mine to the chimney of my roof using a chimney mount I picked up from Radio Shack. The mount is somewhat of a pain to put up and I don’t think it’s a high quality ratchet system on the mount, but it seems solid and doesn’t move. I added extra metal braces to the corners of the chimney to keep the metal from eating into the stucko (I don’t have a brick chimney which would be better). The metal corner brackets are actually just held there by the mount strips itself.
To tune it with a SWR meter, take a reading and if it’s off move the two clamps (ie, the feed-point) simultaneously up or down the copper piping until you hit near a 1:1 SWR. Now, if you’ve already put it on the roof you’ll either have to run up and down a lot or take a radio (with power) up to the roof with you.
With this mounted on my roof I’ve had conversations at 5 watts quite far away. For those familiar with the Davis area, I’ve talked to people mobile from my house in Davis simplex at 50 watts out to nearly Hazel on the far side of Sacramento. My wife (KI6UTP) and I held a conversation between each other from Davis to beyond Sunrise without straining too much to hear each other except when I dropped down into a dip on the highway. That’s a total flat-land distance of 25 miles. We also talked as I was roaming around the mountains up near the Echo Summit (7500 feet or so), which was a distance of 90 miles. But the elevation helps a bit there. WB6ISO (I think) in Pine Crest (elevation 2000 feet or so) said I was 40 over S9 at 5 Watts.
One interesting aspect is that a friend of mine, who lives about a mile from me, said my lower antenna in the attic actually gave him better performance. I suspect that the downward angle of the antenna propagation isn’t as good as it is closer to the horizontal plane. This makes some sense since it performs really well out to the horizon. Since he could hear me anyway, only a mile away, I’m hardly worried about it since I think it’s more important to have the main propagation closer to the horizontal to get a better reach.
[Update Jan 2011: Check out the results of measuring the antenna along with adding a balun and a lightning ground strap.]
Still To Do
Simulate it. I generally like to run antennas through simulators before building them, but I haven’t done so with this one. It just works