On Sun, 09 Nov 2003 14:56:14 GMT, Mike Coslo
wrote:

A precision, scientific-grade instrument is needed.

Now I am really perplexed! If I need an instrument such as you
describe, how will I ever get this thing to tune with my lowly IC-745?

Hi Mike,

Don't get hung up in the Cassandra cries of impossible. Your receiver
will tell you quite well where the loop is resonant. You will get
much more background noise as you tune the band across that frequency;
and if you cannot tune your receiver with such restraint to observe
this, prospects for transmission tuning are far bleaker. Shelling out
bucks for "scientific-grade" instrumentation does nothing to improve
the situation when your receiver has more than enough "precision."

There are zero-power tuners on the market that have adjunct white
noise generators to allow you to tune an antenna system using the
receiver employing this same method. But as that is for antenna
systems of broader Q's, their the additional noise is useful for
emphasizing the resonant peak. Your Loop comes with an advantage
(more Q) in that regard.

73's
Richard Clark, KB7QHC

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

Roger Adam wrote:

a bunch of good info snipped

Thanks for all that Roger.

The icing on the cake would be if we establish contact via the loops on 20
metres!!

That would be great, Roger! I'm in the middle of Pennsylvania, and I
take it you're in Great Britain. The path should be good in my
afternoon/your evening.

I'm taking Reg's suggestion and skipping the Farady loop, and going to
plain wire. My coax just keeps getting shorter... and shorter, so I'll
switch before I have to buy new!

- Mike KB3EIA -

Mike,

I sure you're right in thinking that the coupling system is the cause of
your problems.

As far as the ability to "null" out any unwanted sigs, I have fitted a Yaesu
rotator (which when bought was fully rusted up inside!) to the vertical
drainpipe support of the loop, but haven't yet tried it out. Thanks to TAM
WB2TT for help on the Yaesu.

Friends have said why don't I position the loop horizontal and do away with
the rotator. I may post a request asking for REG's advice on that!

I was working on the principal that with the long wire being horizontal,
then the loop should be vertical to give me a different polorisation.

However, after looking through the books and 'net, It appears that the
initial polorisation becomes changed within a short distance of being
launched and that there is little point in spending a lot of time on this
aspect.

As far as a possible future QSO is concerned, I'll be monitoring around
14.325MHz from 20.00hrs UTC most evenings.

That will most probably put you under pressure to get the loop up and
running!

By the way, my QTH is Surrey, 10 miles south of London.

Good luck,

Roger G7JAQ