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Message-ID: <323C824311F2470F88D6EA386A1262FF@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
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Date: Sat, 17 Dec 2011 00:45:22 -0000
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Subject: LF: Re: Litz Wire
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Dear Terry, LF Group,

Re: is litz wire useful? It depends on the individual antenna. Consider 3 
examples:

1) Very large antenna in an open field environment (e.g. G3KEV). The antenna 
might have capacitance of 1nF or more, requiring only a few hundred uH of 
inductance for resonance, with a reactance of a few hundred ohms. The 
antenna resistance is probably several ohms. In order to keep the proportion 
of TX power dissipated in the coil to a reasonably small fraction of the 
total, the coil resistance must be below a few ohms. This will be achieved 
if the coil Q is < 100 or so. This is not a demanding requirement and  does 
not require special coil construction.

2) Small antenna in a back garden. Antenna capacitance is a few hundred pF, 
requiring several mH of inductance, with reactance of several kilohms. 
Antenna loss resistance is also high due to proximity to trees and 
buildings, probably 50ohms or more. So provided the coil resistance is below 
a few 10s of ohms, the coil will not greatly reduce overall efficiency, 
which will be achieved with a coil Q of a few hundred. This is fairly easy 
to acheve with the much larger coil neccessary than in case 1, without 
requiring special coil construction.. For instance, a coil of 1mm enamelled 
wire 150mm diameter should have a Q of well over 200. However, in order to 
achieve anywhere near 1W ERP with the very low antenna efficiency, High TX 
power of the order of 1kW or more will be needed. Using litz wire can reduce 
the resistance of the coil by something like a factor of 3, which will 
certainly help stop the coil overheating, even if it has little effect on 
overall antenna efficiency. The high reactance also means high voltage 
(>10kV not unusual), which requires a physically large coil to avoid 
breakdown.

3) Small antenna in an open field site. Now the coil reactance must be 
kilohms similar to the back-garden antenna, but the loss resistance of the 
antenna can be much lower; in my experience, less than 10 ohms. So you now 
require a large coil with a very high Q, of the order of 1000, so that the 
coil does not seriously reduce overall antenna efficiency. Litz wire is 
practically essential to achieve this, although even if a relatively low-Q 
coil does compromise the antenna losses, antenna efficiency will still be 
better than the back-garden antenna overall. Again, the voltage will be 
high, so a large coil will be required anyway.

> I have just seen an eBay ad for LITZ WIRE 10/44 250 ft at 15
> USD including shipping to the UK (the seller has more than 10 available). 
> Is
> this size any good for winding loading coils?

I guess this means 10 strands of 44AWG wire, which would be rather small for 
winding a coil that has to support a few amps of RF current.

Cheers, Jim Moritz
73 de M0BMU