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