More measurments on the Magnetic Loop Antenna

Band conditions yesterday on 20m were really quite bad. I had difficulty even maintaining PSK31 contacts due to QSB. I asked W3OQ across town to monitor my 5W transmission on 20m using the Magnetic loop. He was able to pick me up without issue. He was coming in at about a 54 with 100W from across town so this was further confirmation that the conditions were limiting performance evaluation. The good news is that the antenna did radiate!

So now on to the tuning range issues. I was expecting to be able to tune this antenna between 20m and 10m. As it stands now I can do 20m and 17m only. This implies that my capacitance at the low end is higher than expected. I have actually taken the antenna apart now to do some modifications to the mast and used the opportunity to re-measure the capacitance range. I have gotten some inconsistent results. The lowest measurements I get are 9pF to 108pF. The highest measurements I have gotten are 15pF to 155pF. Using AA5TB’s Magnetic Loop Calculation Spreadsheet I get the following table of expected values:

Frequency – MHz       Capacitance – pF
7                                  174
14                                    43
18                                    26
21                                    19
24                                    14
28                                    10

Using the Mini60 Antenna analyzer with the cap at either extreme gave me a result of 8MHz with a predicted capacitance of 133pF and on the other end 20MHz corresponding to 21pF. This indicates that the actual capacitance of the system is running a bit higher than my measurements. This sets up the following experiments next time I get the antenna reassembled:

  1. Tune antenna to each frequency and then measure the capacitance at that point to compare against predicated values
  2. Determine how much capacitance needs to be added to achieve tuning on 40m. This is fairly straight forward by placing various fixed capacitors in parallel with the existing variable cap. My thought is to add a switch to allow use on 7m

The big problem will be fixing the tuning issue on the higher bands. I am searching for another capacitor now just in case. This will pretty much all wait until I receive the 6:1 shaft gear so that I can improve the tuning sensitivity. Meanwhile I am going to work on the tripod mount and the mast to get rid of the cable ties.

UPDATE: Found the actual specs on the capacitor I am using an confirmed that the range is 20 – 120 pF. This would confirm the tuning range from about 8.4MHz to 20.7Mhz. This is a two gang Capacitor with a range of 10 to 70pF and the other with a range of 10 – 50pF. I have these wired in parallel giving me the 20-120pF range. I should be able to use just one gang and get the range back in line. Simple enough fix!

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