2m / 440 Copper Pipe Super J-Pole
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.
Building 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.
Feeding It
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.
Tuning It
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.
Results
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.
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 