Message-ID: <2000120522213368199@zetnet.co.uk>
Date: Tue, 5 Dec 2000 22:21:33 GMT
To: rsgb_lf_group@blacksheep.org
From: "Peter Dodd" <g3ldo@zetnet.co.uk>
Subject: LF: Re: LF keep cool
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Mal, G3KEV said:

> With the advent of large LF amplifiers and proposed super large amplifiers
> capable of running up to 3kw output to the antenna, there is a requirement
> to keep the loading coil COOL.

With a transmitter running 900watts I can get 1watt erp even with my 
'bit of wet string' so there is no need to run more power. To get 
1watt erp I have to obtain 4.5amps antenna current with my antenna. 
The coil is wound with 1.5mm Litz wire (2.5mm including plastic 
insulation) with an RF resistance of approximately 5ohms and will 
dissipate around 100 watts during key-down periods. The coil area is 
cylinder 250mm long and 250mm in diameter.
In practice the coil temperature rise is not detectable to the touch 
(after a long period or QRSS but with the transmitter off!).
The coil inductance (and RF resistance) of a coil used with larger 
antennas is smaller so the heating effect will be even less.
If you have a very small antenna then the coil has to made to Tesla 
coil proportions as in the I5TGC coil, see Fig 1.10 of the Low 
Frequency Experimenters Handbook. In this case the coil is made large 
to prevent flashover but serves to dissipate heat as well.
My view is, that with the legal power, coil heating is not a problem.

I was quite surprised to find that the 3C8F core will handle 900watts 
and is only slightly warm after a prolonged period of QRSS.


> Use microbore copper tube to wind the coil. Interleave the turns with one of
> the poly varities ropes to keep turns evenly spaced on coil former.

Microbore copper tube would make a nice low loss coil but you would 
need over 250m of the stuff - a bit expensive. I wonder how much it 
would weigh?
It would have an RF resistance of less than 2ohms so water cooling 
would hardly be neccesary!


-- 
Regards, Peter, G3LDO

<g3ldo@zetnet.co.uk>