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Re: LF: Re: LITZ WIRE

To: [email protected]
Subject: Re: LF: Re: LITZ WIRE
From: "James Moritz" <[email protected]>
Date: Tue, 12 Feb 2002 17:09:19 +0000
In-reply-to: <000a01c1b3cf$7f5d2ee0$6a01a8c0@cianciolo>
References: <[email protected]>
Reply-to: [email protected]
Sender: <[email protected]>
Dear Paul, LF Group,

I have yet to try a transmitting loop on LF, but from experience with loading coils and various other LF inductors, you can expect that replacing normal wire with Litz of similar size gives a reduction in RF resistance due to the wire itself by a factor of the order of 3. Obviously this would reduce the losses in an antenna, but by how much depends on what proportion of the loss occurred in the wire, and how much was due to other causes. If the total loss resistance of the antenna was dominated by external factors such as losses in the ground, using litz would make little difference. This will depend on the particular antenna.

...To what do you attribute the changes in tuning using the higher Q litz wire?
I mean when the wind blows?. Is it movement in the antenna wire itself? Or
is it an effect of the air/moisture,temp. change passing over the coil?

An antenna with a higher Q will have a narrower bandwidth - or equivalently, an impedance that varies more rapidly with changing inductance or capacitance - therefore the effect of any slight change in inductance or capacitance caused by movement in the wind or water droplets, etc, will have a greater effect on the antenna impedance for a high Q antenna than for a low Q one. The tuning can be made less critical by reducing the Q; however, for electrically small antennas, the efficiency will be reduced in exact proportion to the Q reduction. So halving the Q will give you twice the "SWR bandwidth", but only half the radiated power, for a given transmitter power and antenna dimensions. I believe some low-power MF beacons actually deliberately introduce resistive loading to achieve this, but in these cases efficiency is not a major issue; if you want the maximum possible efficiency from a given antenna, you will have to live with high Q.

My question all stem from the root question of  is it possible to build a
small multiturn loop, say 3 meters on a side,
that would radiate reasonable well if it were elevated from the ground, and
out in the open with no trees.

Also if great care was taken in the design to eliminate losses.  Thus the
litz wire questions etc.

What gives the best results can only be found by experiment in each individual case - however experience shows that the prime factor in maximising LF antenna efficiency is to maximise the critical dimensions of the antenna; effective height in case of a vertical, effective area in the case of a loop. Other improvements like reducing losses in wire or environment can be significant, but are unlikely to be as effective as making the antenna as big as possible in the first place. An inevitable consequence of reducing the size of an antenna is that the Q required to achieve the same efficiency as a larger antenna is increased - so even if it can be achieved, it will lead to practical operating difficulties. Making a small, efficient, wideband antenna is the holy grail of antenna specialists, but none have done it - although a very good small antenna can sometimes give better performance than a very poor big antenna.

Cheers, Jim Moritz
73 de M0BMU



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