Magnetic Loop Antenna
July 28, 2015
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Powered through the construction of the magnetic loop antenna for HF last night after a last run to Home Depot for supplies. I had purchased three different capacitors from Ebay for this. One was unsuitable due to voltage handling. One was great for voltage handling but a bit bulky and stiff to turn. The one I decided to go with has a capacitance range of about 10pF to 110pF when both halves are ganged together. Voltage handling should work well with QRP and it is not too large. I found a 4x4x2 plastic junction box that housed the capacitor and the connections nicely.
I had spent some time thinking about how to motorize the cap from the start but I started having some issues with that and decided to try manual tuning to start out with as I can always add on the motor drive later on. Wiring up the cap in a clean way is by far the most difficult part of this project. I am using a 9 ft piece of RG-213 for the main loop so I added some SO-239 panel connectors to the box. This is handy as I can always replace the loop with different lengths of coax and try on optimize operation on other bands. Here are some photos of the capacitor build progression:
The coupling loop was made from an RG-8X patch cable. I cut off one end of the connector and formed a loop 1/5 the diameter of the main loop. The loop was wired as an unshielded type loop. The center counductor on the feedline end is soldered to the loop braid. On the other end the loop braids are soldered together. Once soldered I used electrical tape to cover the joints and give them some strength.
Next up was putting the whole thing together as I was anxious to test it out. The main frame is built from four pieces of 2 foot 3/4″ PVC pipe and a PVC cross. I had these on hand and the 3/4″ PVC fits on my camera tripod so I went with it. I may try and change this to 1/2″ PVC for the final version. I laid out the parts on the floor and used cable ties to hold the coax loop and the coupling loop to the frame. The box was a problem so I temporarily used electrical tape to tie it the PVC pipe. I’ll secure it long term with a clamp (back to Home Depot…), Here is the finished antenna ready for testing:
Once all this was done it was about 11pm local time here at the QTH. I set it up in my Shack and connected it to my FT-817ND. I found a QSO and tuned the antenna for maximum signal strength on the S meter. Receive on this antenna is really pretty good. I was picking up signals that I could not hear well at all on the end fed wire and FT-450D. I picked up EA3AKP in Spain with a solid 59. Tuning up to transmit was a bit frustrating and I was not successful in getting the SWR down. The Q is so high on this antenna that without gear reduction it is very hard to fine tune the capacitor so the antenna resonates. Overall happy that it is “somewhat” finished and that I can start experimenting with it. Later today I will try again and use the Mini60 to try and tune it to resonance.