Dear LF Group,
Since I have used several designs of loop antennas almost exclusively for
136k and 500k reception, here are some thoughts:-
For small VLF - MF RX loops, the available signal power from the loop
depends mostly on the area of the loop and the unloaded Q of the loop (the
shape, and in multi-turn loops the distributed capacitance have some smaller
effect too). You get maximum power delivered to the receiver when the loaded
Q with the receiver connected is 1/2 the unloaded Q. This happens when the
impedance connected to the loop has a resistive component equal to the loss
resistance, and a capacitive reactance equal to the inductive reactance of
the loop, i.e. resonance and conjugate match. To a first approximation, the
number of turns does not affect the signal power level, only the design of
the impedance matching. With very small area loops you want to try to
achieve conjugate matching in order to maximise sensitivity. But with
relatively large area loops (of the order of 1 square metre or more), and a
suitable low-noise receiver or preamp, etc. you can get enough signal so
that band noise is the limiting factor even with quite a large mismatch. In
particular, mismatching to reduce the Q increases the bandwidth, which is
often very useful. At LF/MF, it is usually possible to make preamps whose
noise level is either below the band noise, or below the thermal noise level
generated by the loss resistance of the loop, so the loop itself is normally
the limiting factor. With several square metres area, you can get ample
signal level without bothering to tune to resonance, and have really
wideband loop antennas. The EMF induced in a given loop is proportional to
frequency, so the output decreases at low frequencies, but since the band
noise level increases at low frequencies, this isn't usually a problem
How many turns the loop has is largely a matter of convenience for the
tuning and impedance-matching point of view. The traditional high-Q loop has
many turns so that it can be tuned to resonance with available variable
capacitors; also the resulting high parallel resistance gives a low noise
figure when connected directly to a valve input stage, although this is not
so much of an advantage these days. There are several disadvantages , such
as being easily de-tuned, susceptible to capacitive noise pick-up, requires
remote tuning, mechanically complicated.
Small single-turn tuned loops have the drawback of requiring a large tuning
capacitor. For instance, my 136k, 1m^2 "bandpass loop" design has a 0.4uF
tuning capacitance. But since a fairly narrow band is to be covered, the
tuning is fixed, and the loaded Q is quite low this isn't really a problem,
and the loop is a simple square of tubing that is not significantly de-tuned
even if laid on the ground.
As has been pointed out, you can add a 1:n step-up transformer to a
single-turn loop, and the combination essentially behaves as a n turn loop,
with loop EMF increased by a factor n and resistance/inductance n^2 times
that of the loop element itself, so this is a flexible way of getting the
impedance/inductance level you want. The transformer will add some loss
resistance, but usually this can be made small enough not to make much
difference. Also, the transformer winding inductance will be effectively
shunted in parallel with the transformed loop inductance, and a small
leakage inductance appear in series. In Alberto's case, with a single-turn
winding through the core, the problem is likely to be getting sufficient
primary inductance. You would want to choose a core where a single turn has
a relatively large inductance compared to the inductance of the loop, i.e.
with AL >> loop inductance. Small single turn loops have inductance of the
order of a few uH - this requires quite a large core in high permeability
material, or several stacked smaller cores. Or you could incorporate the
shunt inductance into the circuit design. It would be harder to optimise the
transformer design if you are restricted to a single turn primary, which
might cause problems if you were trying to make a high Q design.
Some people have made quite large, high Q tuned loops - the main
justification for this would seem to be to get a large signal output so that
a receiver with poor sensitivity can be used directly without a preamp. The
high loop Q helps to prevent the RX being overloaded by out-of-band signals.
The same result could be achieved with a small/wideband loop with a suitable
preamp/preselector.
A large area wire loop can be connected directly to a low-impedance receiver
input with reasonable results as a wideband antenna- or a considerable
improvement in matching can be obtained using a wideband transformer as
PA0RDT and DJ1ZB have done. The loop inductance can be incorporated as part
of a filter/matching network, as I have done in the "bandpass loop" designs.
Low-pass designs are also quite feasible - I am currently using 4 x 5m
single-turn wire loops incorporating low-pass filters matched to 50ohms,
with cut off frequencies around 550kHz to reduce the local broadcast
signals. These give plenty of signal over the 10 - 550kHz range without
re-tuning.
Small loops have the advantage that they are easier to re-position away from
noise sources. At LF , mains wiring seems to be a potent source of H-field
noise, so it is often essential to experiment to find the best location. The
height above ground, or the presence of trees and buildings has little
effect on the signal level. Whether the loop is big or small, narrow- or
wideband, it has the same figure-of-eight pattern, so the nulls can be used
to reject local or distant QRM. Unless your real estate is big enough for LF
phased arrays, the loop is the only feasible LF directional antenna. At
M0BMU, 500kHz reception would currently be almost impossible without loops
due to wideband QRM radiated by the broadcast stations.
Cheers, Jim Moritz
73 de M0BMU
----- Original Message -----
From: "Alberto di Bene" <dibene@usa.net>
To: <rsgb_lf_group@blacksheep.org>
Sent: Wednesday, July 22, 2009 6:04 PM
Subject: Re: LF: Loop (was Re: IGBT in 136 KHz TX?)
I would like especially to comment on using a one
winding loop combined with a step-up transformer:I am interested in
knowing opinions about using the loop itself as
a primary of the transformer, as in this picture (heavily compressed
to stay into the size limits). Here it is a three-turn loop, but it could
be a single turn one, as discussed.
73 Alberto I2PHD
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