QRP magnetic loop antenna for 40 meters

I know I will have to start doing a bit more with circuits on this blog of mine. Lately I have been focusing more on antennas. This post is no exception. I have been sitting on this magnetic loop antenna for some time. It has been near complete in my basement since early winter. The reason I have not completed it is that final tuning and testing needed to take place outside. Winter makes it cold and miserable out and there is far too much metal in my basement for it to couple to and throw everything off.

This weekend was great, yesterday was sunny and around 80F. I did try taking this thing outside once before but I made the loop too large to fit through the stair well. The original circumference of the main element was about 18 feet with original testing and experimenting done in the basement during the winter. I had to cut this down and make it closer to 16 feet.

I used this site to calculate the antenna characteristics: http://www.66pacific.com/calculators/small-transmitting-loop-antenna-calculator.aspx

Magnetic loop leaning on tree - AC9LF

I used a 20 foot roll of 1/4" copper tube from the local Lowe's store which cost about 10 bucks. The antenna isn't very efficient per the calculation from the above website at 5 watts:

Note that these numbers are a bit off from what I measured:

Antenna efficiency: 23% (-6.4 dB below 100%)
Antenna bandwidth: 15.1 kHz
Tuning Capacitance: 157 pF

Capacitor voltage: 580 volts RMS
Resonant circulating current: 4.11 A
Radiation resistance: 0.034 ohms
Loss Resistance: 0.114 ohms
Inductance: 3.12 microhenrys
Inductive Reactance: 141 ohms
Quality Factor (Q): 477
Distributed capacity: 13 pF

I measured the main loop with my LC meter which turned out to be about 4.6uH, The variable capacitor I used is of the butterfly type. This allows lower loss because there is no mechanical wiper connection. It also allows higher voltages across the capacitor. The trade off is that the capacitance is lower and butterfly capacitors go from fully meshed to fully unmeshed with a 90 degree change in rotation where as standard variable capacitors are 180 degrees. For my purpose this turned out to be perfect as I was only interested in covering the 40 meter band which is 300khz wide. Measuring the capacitor it is roughly 5pF to 20pF. 

Close up of capacitor and connections -AC9LF

To resonate the main loop at 7Mhz with 4.6uH of inductance I needed roughly 113pF of capacitance. With only the small capacitance of the variable butterfly capacitor I needed to add some capacitance in parallel. I chose to go with RG58 a/u as my extra capacitance. This coax uses solid polyethylene insulation which increases its maximum voltage rating over foam insulation. According to a datasheet the max voltage is about 1,400V RMS. This is well above the 580V RMS calculated at 5 watts. Capacitance per foot is around 30pF so I cut a length of slightly over 4 feet to bring me a bit above the 100pF needed for resonance. 

I trimmed the coax away slowly which shifted the frequency up to 7Mhz with the plates of the capacitor fully meshed. I used the receiver of the VX-7R to follow the frequency with the highest noise level while pruning the coax. The frequency with the highest noise level is the frequency of resonance. This was confirmed with my Dip Meter. Once satisfied I switched to my BITX 40 to perform final transmitter SWR testing.

Here is the data sheet for Belden RG-58 a/u: http://www.belden.com/techdatas/english/8259.pdf

I used something a bit different but should be pretty similar in characteristics.

Coax used as parallel capacitor - AC9LF

This combination gave me a calculated tuning range of a bit more then 300Khz which as stated before fits the whole 40 meter band. It also allows for easier tuning as having a wider tuning range makes it harder to hit the resonant "sweet spot" at about 50 ohms for the transceiver to be happy. I used a hard plastic straw on the shaft of the capacitor to decrease coupling between me and the antenna when tuning it.

The frame is made out of remnants from a wood trellis that I scrapped. The inner driven loop is made out of the same RG58 a/u coax cable with one end of the shield connected to the center conductor. The center conductor on the opposite end of the driven loop is connected to the center conductor of RG-174 which has a SMA connector on it. The shield of the RG-174 goes to the Shield/Center conductor side. Connecting the outside shield to the inner conductor on the driven loop helps to reduce electric field noise pick up. The circumference of the driven loop is 1/5 of the outer loop. 

Using the setup below I was able to get SWR close to 1:1 on any frequency of interest on the 40 meter band. Even though fully meshed to fully unmeshed of the capacitor only covers a little more then 300Khz hitting the "sweet spot" was still a little tricky. I plan on putting a stepper motor on this in the future for remote tuning. This will also allow micro stepping for more accurate and easier tuning of the capacitor.

QRP setup -AC9LF

Even though the XYL says it isn't, this really is a small antenna seeing as a dipole on 40 meters would be around 60 feet long.

I still have yet to make a 40 meter contact with the BITX40. My operating skills must not be up to par for QRP.