HF Magnetic Loop for Chasing SOTA – Part 3

The Australia Day public holiday long weekend was an ideal time to spend an hour or two updating the HF Magnetic Loop antenna between many domestic duties.  I have changed the small inner loop from wire to copper pipe and purchased a 12 volt 2 rpm DC motor to drive the tuning capacitor.  I didn’t get time to fit the motor this weekend.  😦

The arrangement for the small inner loop has been temporary for some time.  5 months ago I made a 200 mm copper pipe loop which was to be mounted permanently as the driven element.  Earlier I had made a 200 mm loop out of 7 strand copper wire and temporarily fitted it to the center support with cable ties.  The aim was to test the frequency range of the Magnet Loop which BTW is 6.8 Mhz to 25 Mhz (40m to 12m) then replace the wire with a copper pipe loop.   I’m confident to say like some amateur radio projects the temporary wire loop arrangement had the potential to be the permanent solution as I struggled to find time to finish it off.   🙂   Check out the post from 12 Oct 13 for pictures of the wire loop.

This morning I was blessed with one or two hours (very early!) to mess about in the man-cave, the copper pipe inner loop is now fitted, boy that took a long time to fit.  See picture below.  The top of the loop passes through the center support while the ends of the loop are connected to the SO239 UHF panel socket via short lengths of 3 mm solid core copper wire.  The top of the driven loop is in-line with the plane of the main loop.  The bottom of the driven loop is slightly forward of the main loop. Cutting the plastic pipe to recess the loop would create a weak joint and eventually break the plastic pipe.

200 mm inner loop connected to a SO2359 panel socket.

200 mm inner loop connected to a SO239 panel socket.

1 metre Magnetic Loop

1 metre HF Magnetic Loop antenna completed.  Not a perfect circle, please forgive me.

Tuning capacitor is a Johnson 13 to 200 pf.

Tuning capacitor is a Johnson dual gang 13 to 200 pf per section. The shaft diameter is 1/4 inch.

The two variable gangs are isolated from each other and connected across the loop in series by the rotor.  The rotor has no physical connection to the loop. Whilst this helps to reduce losses it effectively halves the total capacitance to 7 – 100 pf.   To enable the capacitor to tune the loop at 7 MHz, I have fitted a 63 pf Door Knob capacitor.  The Door Knob capacitor is connected ‘in-circuit’ via a large alligator clip (improvisation).  When not in use the alligator clip is a part of the braid.

The Johnson variable capacitor is temporarily secured to the center support by cable ties.  What would we do without cable ties?  The capacitor rotor can rotate 360 degrees, making it ideal for a DC motor drive.

80 pf Door Knob capacitor connected in parallel to tune 40m

63 pf Door Knob capacitor connected in parallel to tune 40m

Circuit diagram:  dual gang capacitor connected across the main loop in series.

Dual gang capacitor in series

Dual gang capacitor connected in series

Top down view

Top down view.  RG213 braid to connect the loop to the capacitor sections

DC Motor to drive the variable capacitor

The 12 volt drive unit will be fitted in the coming weeks.  🙂

12 volt DC motor complete with 2 rpm output reduction gear box

12 volt DC motor complete with 2 rpm reduction gear box.  I have added a 6:1 ball drive to reduce the rpm.  This should smooth out tuning on 40m

5:1 reduction ball drive

1/4 inch 6:1 reduction ball drive salvage from the junk box

Chasing SOTA with the magnetic loop antenna – 27 Jan 14

Having finished the loop at 8 am this morning (not the DC drive unit) I check SOTAWatch for activity.   Activator alerts on SOTAWatch were Greg VK2FGJW, Ian VK1DI and Rod VK2TWR with Peter VK3PF.   I parked the Mag Loop on the deck, XYL was okay with that, set up the Yaesu FT-817 low power (QRP) transceiver and successfully chased the following SOTA activators:

21:57 UTC, Greg VK2FGJW on Mt Coree VK1/AC-023, 21.125 Mhz, report 5-7, 5-9

22:07 UTC, Marshall VK3MRG on Hyde Hill VK3/VC-008, 7.090 MHz report 5-1, 5-2

22:14 UTC, Greg VK2FGJW still on Mt Coree this time on 7.100 MHz report 5-8, 5-9

23:50 UTC, Rod VK2TWR/3 on Mt Delegate VK3/VG-034, 7.090 MHz report 5-7, 5-3

00:07 UTC, Peter VK3PF also on Mt Delegate VK3/VG-034 with Rod, 7.090 MHz report 5-7, 5-3

I did hear Ian VK1DI at 00:10 UTC operating from Pheasant Hill VK1/AC-021 near the southern tip of the ACT/NSW border, on 7.095 MHz working S2S with Greg VK2FGJW on Mt Coree, then without warning Ian’s signal faded very quickly.  Being a very patient person, I listened to VK3 stations working Ian until 00:44 UTC when Ian’s signal lifted to 5-8.  I had a quick exchange with Ian before fading (QSB) away a second time.

At the time of writing the mobile phone issued another RRT SOTA alert.  A quick dash outside to the deck.

04:42 UTC, Peter VK3PF, on Dinah Divide Range VK3/VG-139, 7.100 MHz report 5-7, 5-8

Here is a picture of the gear set up on the deck.   A very productive public holiday in the man-cave and on the deck!

Chasing SOTA from the deck operating a FT-817 into a HF Magnetic Loop antenna

Chasing SOTA from the deck operating a FT-817 into a HF Magnetic Loop antenna.  Antenna is aligned due south to Pheasant Hill and Mt Delegate

Related posts:

HF Magnetic Loop antenna – Part 1

HF Magnetic Loop antenna – Part 2

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2 thoughts on “HF Magnetic Loop for Chasing SOTA – Part 3

  1. Pingback: HF Magnetic Loop for Chasing SOTA – Part 1 | VK1NAM Summits on the Air (SOTA)

  2. Pingback: HF Magnetic Loop for Chasing SOTA – Part 2 | VK1NAM Summits on the Air (SOTA)

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