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Old October 26th 03, 11:57 PM
John
 
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Default array of magnetic loops?

Hi there,

I want to make a receive loop for HF transatlantic flight monitering,
Gander, Ascension and Shanwick around 5.5Mhz to 6.5-ish megs. That bits
easy. But I have a lot of local QRN, dimmer switch/thermostat noise or
something, that obliterates anything below 7Mhz. Using a single small loop,
I can null out much of the noise, but what Id like to do is use a second
loop, orientated 90degrees to my receive loop, firing straight into the
noise, but phase shifted by 180degrees to cancel the noise out. Is this
possible? Alternatively, but similarly, Is it possible to use two magnetic
loops, somehow phase linked like a mini array, to have a system with greater
gain and deeper nulls?

Thoughts please,

John G0WPA



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Old October 27th 03, 10:38 AM
Reg Edwards
 
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To use two or more very small loops in an array to produce a particular
radiating/receiving pattern, the physical spacings between them cannot be
less than the order 1/4 or 1/2 free-space wavelengths.

You may just as well use the well-known arrays of 1/2-wave dipoles.

This is because the radiation pattern of one small loop is identical to that
of one small dipole except that is twice rotated in space by 90 degrees.

You can do no better than use a receiving magloop in conjunction with an
elevated, well-distanced noise-cancelling length of wire, the two being
combined at the receiver, the wire via a variable-phase and
variable-amplitude network. If the wire picks up a smaller noise signal than
the main antenna it will be necessary to include an amplifier in the
phase-shifter.

Incidentally, the noise-cancelling wire + phase-shifter technique behaves
identically to rotating the wire direction in space, the null in the
combined receiving pattern also being rotated.

Of course, if the noise or interfering source is in the same direction as
the wanted signal you will get nowhere.
----
Reg, G4FGQ


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Old October 27th 03, 05:57 PM
John
 
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Thanks Reg,

I think you are right. I need a variable phase and variable amplitude
system. The loop does a good job nulling out the noise, and "phasing out"
any residual noise wrt a sense wire should just about kill it off. Why dont
wellbrook or MFJ incorporate noise cancellers into their RX magnetic loops?
It would make sense.

Thanks Reg,

73s John.


"Reg Edwards" wrote in message
...
To use two or more very small loops in an array to produce a particular
radiating/receiving pattern, the physical spacings between them cannot be
less than the order 1/4 or 1/2 free-space wavelengths.

You may just as well use the well-known arrays of 1/2-wave dipoles.

This is because the radiation pattern of one small loop is identical to

that
of one small dipole except that is twice rotated in space by 90 degrees.

You can do no better than use a receiving magloop in conjunction with an
elevated, well-distanced noise-cancelling length of wire, the two being
combined at the receiver, the wire via a variable-phase and
variable-amplitude network. If the wire picks up a smaller noise signal

than
the main antenna it will be necessary to include an amplifier in the
phase-shifter.

Incidentally, the noise-cancelling wire + phase-shifter technique behaves
identically to rotating the wire direction in space, the null in the
combined receiving pattern also being rotated.

Of course, if the noise or interfering source is in the same direction as
the wanted signal you will get nowhere.
----
Reg, G4FGQ




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Old October 27th 03, 09:08 PM
Richard Harrison
 
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Reg Edwards wrote:
"You may just as well use the well-known arrays of 1/2-wave dipoles."

In most instances Reg is right and this is probably no exception.

There is an April 1986 artivcle in "CQ" magazine by Richard Fenwick,
K5RR on using two small rectangular loops, 85 in. on a side, and spaced
30 to 60 feet apart for 160-mtr. reception.

The loops are tuned to 1812 KHz using 620 pF capacitors. High voltage
capacitors are used for transmitting. K5RR says that "ten watts into the
loop will produce about 450 volts RMS across the capacitor."

This may be the place to use Reg Edward`s observation about the voltage
across reactances in a series resonant circuit; the ratio of reactive
voltage to applied voltage is equal to Q.

Page 2-27 of the 1987 ARRL Handbook agrees with Reg: "The voltage across
either the inductor or capacitor is QE, where E is the voltage applied
to the series circuit."

My ARRL L/C/F Calculator says 12 microhenries resonates with 620 pF, the
capacitance used to resonate the loops at 1812 KHz. The reactance is
only about 137 milliohms.

The author, K5RR writes: "The loops nave a very narrow impedance
bandwidth - only about 13 kHz for a 2:1 VSWR.

The author shows a unidirectional pattern for his two-loop array and it
was intended to replace a 400-ft. Beverage antenna in a distance of less
than 60 feet. A delay line of 180-degrees minus the physical degrees of
spacing beteen the loops is switched into the feed of one or the other
of the loops to reverse the sense of the array. Equal currents are fed
to the loops using a "Wilkinson power divider".

This two-loop array is an inefficient antenna meant only to compete with
a Beverage. It is fine for reception but better used with a more
efficient antenna for transmission.

Best regards, Richard Harrison, KB5WZI

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Old October 28th 03, 12:11 AM
Reg Edwards
 
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John asked -

Why dont Wellbrook or MFJ incorporate noise cancellers into their RX

magnetic loops?
It would make sense.

====================================

Because it wouldn't make sense.

The noise-collecting wire MUST be spaced an appreciable distance from the
loop. Otherwise the noise picked up by both antennas would either be in
phase, or would appear to be coming from the same direction.

More economical to put the noise cancelling arrangements in a separate box
as may be purchased commercially.

If there is only ONE dominent noise source the magloop does a very good job
all by itself. Its pair of 180-degrees apart nulls are extremely sharp
whereas its wanted signal pick-up lobes are very broad. The solitary magloop
fails only when the noise and wanted signal come from very nearly the same
directions. The extra noise cancelling wire would be seldom needed.

If there are TWO noise sources, to simultaneously cancel BOTH of them out is
well nigh impossible. It would require a well-distanced (1/2-wavelength)
noise-pick-up wire to distort the overall radiation pattern such that the
two magloop-nulls are no longer at 180 degrees to each other.

For perfection you would need TWO well-spaced noise-cancelling wires, each
with its own phase and amplitude controls, all of which would interact with
each other and with the rotation of the magloop.

By the time you had twiddled the phasing and amplitude knobs to your
satisfaction you would be half way into the next sun-spot cycle.

Back to the drawing board ? ;o)
----
Reg, G4FGQ




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Old October 28th 03, 09:01 PM
Tarmo Tammaru
 
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"John" wrote in message
...
...............................
Why dont
wellbrook or MFJ incorporate noise cancellers into their RX magnetic

loops?
It would make sense.

Thanks Reg,

73s John.

That's an extra cost option! Both MFJ and Timewave sell noise cancellers
that let you vary the amplitude and phase of the second (noise) antenna.
Priced at around U$200.

Tam/WB2TT




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