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
<[email protected]>
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