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Mike,
to celebrate the change in my license conditions which allowed me to access HF freqs this year (no code experience), I decided to put up a long wire transmitting antenna (30metres long). This was connected via an Icom ATU connected to the Icom IC 725 rig. I also decided to have a go at building a Magloop as an alternative, to experience the trial and tribulations of getting it to work at minimum cost. I could also gain experience as to the ability for the loop to "null" out interference. Having no experience of operating on the HF bands, I was not sure what range of frequencies this antenna should be designed for. Initially, using freeware software, I could see the efficiency rapidly changing for the worse, as the operating frequency went lower, for a given size of loop. Having decided to choose a square loop of 1.33mtrs sides, using 22mm copper pipe for practical reasons, I set about making a piston capacitor. This consisted of approx 2x300mm of insulated 15mm pipe, fitted within 22mm outer tubes, being part of the main loop at the top. I had hoped to cover the 20metre to 40 metre bands. It actually did this, but the piston caps were either fully in or fully out and tuning at these extremes was very, very touchy. I discarded the piston idea and bought a single vacuum variable cap (5-100pf), a 12v geared motor, insulated coupling spindle and built a pulse width modulator kit from Vellerman to drive the motor. It was at this time that I decided that the way forward for me was to make this a monoband antenna (20M) with a reasonably high efficiency (The 40metre band seemed so busy at the time). I used some external 3 core copper cable (pyro) for the gamma match and using a MFJ 259 analyzer via 3mtrs of feed coax set about setting it up. A very obvious VSWR dip was found at an indicated resistance of 50 ohm, around 14 MHz. The analyzer could track the changing frequency when volts were applied to the motor. The antenna was tested in open space, but at ground level + 1 metre. This system is now installed at the top of the garden and via its connecting cables can be tuned from the shack. In operation, I usually find a reasonable station around 14.250 on the long wire, connect the MFJ and set it to the chosen freq, connect the magloop feed to the analyzer and adjust the PWM up and down to find best VSWR. Then switch the magloop to the rig and wait for the opportunity for a "break". It's fair to say that the received signal appears a few "s points" down compared to the long wire. The predicted spec of the antenna is as follows:- Circumference=5.33mtrs Conductor dia=22mm Band =20m (around 14 MHz) Bandwidth=44.1kHz Cap value 15.4pF Cap voltage=3.6kV Efficiency=87.3% Inductance=-4.325uH Inductive reactance=390ohms Loop area=5.8mtrs Loop dia=1.3mtrs Loop Q value=325.2Qres Radiation resistance=0.524 ohms Resistance loss=0.076 ohms So from my experience, you should see a tuning dip when using the MFJ Analyzer. I'm getting around 1.2:1 at around 50 ohms at the chosen freq. But I will say that when coming away from that freq, the VSWR rockets skywards very suddenly. As REG points out, the Q is extremely high and the bandwidth very narrow In the early days, if I attempted to transmit without getting the magloops' vswr down to a reasonable level, the Icom IC725 would go into protection mode and then it was impossible to determine in which direction the variable cap should be adjusted. I have never tried using the receiver to tune the loop. Well that was a long "over" from me, maybe some of it may help. The icing on the cake would be if we establish contact via the loops on 20 metres!! regards, Roger G7JAQ |
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