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Re: LF: Loop (was Re: IGBT in 136 KHz TX?)

To: <[email protected]>
Subject: Re: LF: Loop (was Re: IGBT in 136 KHz TX?)
From: "James Moritz" <[email protected]>
Date: Wed, 22 Jul 2009 22:33:27 +0100
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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" <[email protected]>
To: <[email protected]>
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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