Tag Archives: Variable Capacitor

The Hardware Store 40m Magnetic Loop Antenna

A while back, I wrote up and submitted some info to QST antenna design competition on my 40m Magnetic Loop antenna. The contest is over so I am now publishing the write up on this site on the Projects page. The direct link to the file is here.

The write up details the design and construction of both the 40m Loop antenna and its High Voltage capacitor made from Copper tubing. Nearly the entire antenna can be made from parts gathered at your local building supply store. Hope some of you find this Useful!


Testing of the 40m Loop Continues

Here is a plot of my 24hr 1W WSPR signal using the 40m Magnetic Loop Antenna:


Next up tested PSK31 and JT65 with 20-30W of power. I made two solid contacts within 380 mi of the QTH which somewhat verifies the NVIS aspect I’d hoped for. I made some additional solid contacts on JT65 including to Guatemala and Cuba. The jury is still out on SSB though. I tried the Daytime Texas Traffic Net on 7.285 this morning and while I closed a contact with Net Control in Amarillo, Texas I was weak and I know there was some transmit problems on my side as the antenna was not tuned with precision. I’ll try again soon with the antenna tuned for 7.285 Mhz. The antenna held up well through the night and through a very windy day here at the QTH in South Texas.It swings around a bit so I may setup a “guy” wire to help position it. It has been largely oriented East-West. I should also note that it is about 2 S units quieter when compared to my main end-fed antenna.  So far so good!

UPDATE 1/11/2017: Made a 100W SSB contact to the Daytime Texas Traffic net. This time I tuned the antenna to a 1.42 SWR at the net frequency of 7.285Mhz. SWR behaved while transmitting and the net control copied my signal without issue. Net control was just east of Dallas in Kaufman, TX. 

40m Magnetic Loop Antenna with Modifications 

Here is the final version of my 40m magnetic loop antenna using my homebrew hi-voltage variable capacitor. As I mentioned yesterday, I fixed an issue with the variable capacitor feed screw. Today I set it up outside handing from a tree as before. The difference is that the capacitor is now at the 3 o’clock position to facilitate tuning without a ladder. This moves the feed point to the 9 o’clock position. No changes were made to the pvc supports as I merely change the position. I read a 1.31 SWR at 7.039 MHz with am fully engaged. I am running 1W WSPR on it now and will report results tomorrow. Here is what it looks like in operation:

Modification to Hi-voltage Capacitor

Made a modification to the High Voltage capacitor today. I added a washer to keep the captive nut on. Seems to work well.


I am ready for my next field test of the 40m loop. This time the cap will be at the 9 o’clock position with the toroid at the 3 o’clock position.

40m Loop Update

I ran 1W WSPR on the 40m Magnetic Loop Antenna for about 18 hours before impending inclement weather forced me to bring it down.Here is what is looks like when taken down:


Very compact. Longest PVC pipe is about 3.5′ long. Here are the WSPR results:


The antenna held up well over night. Only issue is the captive nut came loose. Epoxy does not seem to do the job so I’ll have to come up with a fix. Also thinking about how to bring the tuning adjustment down to a more reasonable height.

40m Magnetic Loop Antenna

Finally got around to finishing the 40m magnetic loop antenna. This antenna is designed to be easily deployed and used as an NVIS antenna with up to 100W. The heart of this antenna is the hi-voltage capacitor I designed and previously discussed. Before setting up the new antenna I made a few changes to the capacitor:

  1. Wrapped exposed copper with electrical tape
  2. Replaced the steel 5/16″threaded rod with a two food nylon rod. This was to minimize coupling effects while tuning
  3. Made a new center post with better hole alignment. This helps keep everything from binding.
  4. Epoxied the capture nut into the center post.

Here is the capacitor in use on the new antenna:


The 40m loop consists of 22 1/2′ of RG-213 coax. The length was selected to match the capacitance of my prototype capacitor. Here is the design points via AA5TB’s spreadsheet:


The rest of the build involved cutting pieces of 1/2″ PVC pipe to length to support the antenna. I hauled the whole thing ip a tree branch with the feed point just about 1″ off the ground. Initially the SWR was a bit high but it settled into about 1.4 when I removed two turns from the toroid. Here is the SWR plot through 50 feet of RG-8X feedline:


Here is a view of the antenna deployed on a tree branch.


I have been running WSPR on 40m now for about an hour and transmitting 1W. Here are the contacts thus far:


This seems to be working well so far. My only complaint is that I have to stand on a ladder to tune it. I’ll need to work on that. I’ll run some additional tests now that its built.

Magnetic Loop Update

I rearranged some PVC fittings to my Magnetic Loop antenna to accommodate the new High Voltage capacitor. Here is what it looks like:


Remaining improvements are to cover the exposed copper pipe with heat shrink tubing and replace the metal threaded rod with a nylon rod, add a tuning extender and install some band limit nuts to aid in single band tuning. I operated this indoors today but had no luck on 5W QRP. Had to use the amp to make any contacts at 45W.

I also verified the SWR 2:1 points and they measured 48kHz bandwidth which is a definite improvement over the other tuning capacitor which was more in the range of 70kHz. I also verified that the AA5TB spreadsheet was giving the proper tuning capacitor values. To do this I would tune the antenna to a specific frequency, say 14.200 Mhz. The AA5TB spreadsheet indicates a tuning capacitor of 42.42pF minus 8.2pF of loop capacitance for 34.22pF. I removed the capacitor from the loop and measured a capacitance of 38.5pF.

This confims that the spreadsheet gets me the right values for the capacitor which I will now use to finalize the 40m loop design. A quick trip to Home Depot this afternoon yielded the remaining PVC fitting I need for this version.


Hi-Voltage Capacitor Tweaks

Testing the Hi-voltage capacitor on the small magnetic loop antenna did uncover the need for a few improvements. First up is the tuning rod. The metal threaded rod does seem to couple with my body when I was tuning the loop. I plan to replace it with a nylon threaded rod of the same size (5/16″-18). Tuning the capacitor on the side with the exposed copper outer pipe is a bit risky. I grazed it a few times while trying to tune it while the tuning signal was being transmitted. No burns, but I definitely want to reduce the risk of that.I plan to either add some heat shrink tubing or just wrap some more PE sheet over the exposed sections. Having the capacitor up high does make adjustment a bit awkward. I may have to add an extension rod to facilitate that. I still plan to configure the 40m loop with the captor high and the coupling match low. For the small loop, I can adjust the vertical PVC posts again to accommodate the capacitor in the low position.I’ll replace the PVC cap and eye bolt with another reducer and coupling. Having the capacitor low should also help the balance of this loop on the tripod (not a problem with the larger loop as I plan to hang it).

Testing the Hi-Voltage Capacitor on the Magnetic Loop Antenna

I did some preliminary testing of the Hi-voltage capacitor I completed last week and found some interesting things.Rather than commit the 40m loop design I decided to try and adapt the existing loop antenna I have previously described. To make this work in a configuration similar to what I have in mind with the 40m loop, I installed the capacitor at the top and used a toroid instead of the shielded coax loop for the coupling at the bottom of the loop. To do this is needed to replace the vertical PVC supports to accommodate the configuration. The reducer I used for the 1/2″ pipe on the 40m loop was replaced with a 3/4″ version. This configuration worked well. The capacitor is in the range of being able to tune this loop on 20m. Previous experiments indicated a good match with 5 turns on the FT140-43 toroid. It turned out that 6 turns would give me a perfect match 1:1 SWR at resonance. Here is the SWR plot on 20m.


The  bandwidth at the 2:1 points measured at 45kHz. This compares favorably to the original capacitor which was indicating a bandwidth of around 68kHz. This indicates that the Ohmic losses are indeed lower and therefore efficiency should be higher as well.

I did not have enough time this weekend to run all the measurements I had planned but I did operate the antenna indoors at 45W with the FT-817ND and amplifier.The Califonia QSO party was underway and I made numerous contacts in California without difficulty with this loop. The capacitor handled the power without any issues at all!


Hi-Voltage Variable Capacitor – Finished!

I wrapped the PE film around the center copper pipe lined everything else up and basically finished it! The feed mechanism works great and is not sensitive to me touching the tuning shaft. I should be able to finish the 40m loop now as the RG-213 that forms the loop will just connect to each end of the capacitor directly. I already have a toroid ready to work the matching so I will just have to size and cut the spreaders.The capacitor will be at the top of the loop this time with the feed point at the bottom. Been a great afternoon of homebrewing and realizing an idea that I had been working on for some time.