Saturday, December 05, 2015

Simple copper pipe loop for 40m

After several false starts, including complex schemes involving stepper motors and long rods, I've settled on a very simple design for a loop that nicely covers 40m.

The loop is about 1m in diameter, a cat is shown for scale.

Contrary to some other designs, I've put the coupling at the top. Two turns on a FT-114-43 toroid. (Since this picture I've anchored things with some cable ties and removed one turn to get a lower SWR).

The tuning is at the bottom. I needed an extra 180pF to get this on to 40m. A larger knob is better, a reduction drive would be even better. (Tuning is very sharp).

Here's a rough drawing of the circuit:

Sitting at the table in the back yard yields decent reception - not as good as the full size dipole - but not bad.

The 3m length of copper pipe was a standard cut length at the local hardware store.

To find where the loop was resonant and make sure it was working, I used a miniVNA pro  and the vna/j software. I'm running Ubuntu Linux and ran in to some problems getting vna/j running. The trick is to remove the open source Java implementation and install the version from Sun. Also there's the JNI libraries that are required the Arduino IDE that needed to be removed to get it running.

Here's the plot showing the SWR dip at 7.1Mhz to 1.4:1. The KX3 tuner nulls out the mis-match without problem.

I've transmitted up to 50W SSB without any bad things happening.


jim.brownza said...

Stunning idea Sir... as a new ham looking for a simple way to get out on 40m on my ic-720a on loan from om Leon ZR6LU, that looks good to me.

So a couple of questions if I way....

How did you bend it so nicely?
What are the specs of the variable cap?
Ditto the one across the ends?
I can't see where the wires go out from the tuner to the radio. Remember I'm new ;) so maybe another pic or sketch if you have a chance please?

73 Jim ZS6JMB

jim.brownza said...

Ah on looking at pic again... the variable cap is across the gap along with the normal cap? The connection to the radio is the two wires from the toroid?


Peter Marks said...

Thanks Jim,

* The pipe came as a 3m length already bent in a multi-turn loop so I just un bent it such that it made one loop - it's not perfect by any means.
* Not sure about the variable cap, it's one from the junk box, I suspect it's about 100pF
* The extra capacitor is 180pF. Before I added that it tuned from about 8MHz to 13Mhz - it didn't quite make it to 20m.
* Yes I have three loops through the toroid that simply connect to thin coax. As you might see the SWR is pretty high but my auto-tuner was able to match it. I'll go back and try fewer or more turns to get the impedance right. The idea of using a toriod rather than a gamma match came from Ross VK1UN.
* Yes the extra cap is in parallel with the tuning cap. Another good thing about this is that it makes the tuning cap less sensitive. These loops tune extremely sharply.


jim.brownza said...

Thanks Peter... have to love this ham community for the freely available advice.

My son ZS6HMB and I just qualified. He's a paramedic / rescue guy by trade so has an interest in the emergency comms side of the hobby. I have no idea where I'm going to end up; right now happy to be talking on 2m through a couple of local club repeaters. (My loc is KG42at).

And soon I hope to be out on HF...


My Fledgling blog:

Joel Caulkins said...


It's amazing, every time I start to build something, I find out you just built it:-)
I just built a 1.5 meter copper tubing loop using PVC pipe to support it, but haven't finished it yet. The weather turned bad here on the west coast of the USA. I was going to use a piece of RG-8X as the gamma match, but now I'm wondering if I should use the toroid instead. What kind of toroid did you use?