LF: Re: TX system at DK7FC, schematic

["James Moritz" <james.moritz@btopenworld.com>]

Dear Mal, LF Group,

> The highest Q coils I have seen are self supporting encased in a helium
> container and the Q specified was only in a few hundred. What sort of coil
> construction yields 4000 and above ?
> I have yet to encounter such a specimen
Errm.... The sort of specimen shown in the picture you sent to everyone 
yesterday, i.e., a coil a few metres in diameter wound using very large litz 
wire.
The highest Q LF coil I have made so far had 109 turns of 729-strand litz 
wire wound on a plastic former about 400mm diameter. L was about 4mH and Q 
measured at 1100. Stefan's loading coil uses similar construction and has 
similar Q. A coil of similar size and inductance wound with PVC insulated 
copper wire had a Q of only about 300.
For a given inductance and type of construction, Q increases with size, but 
only slowly. The size required to reach a given Q seems to reduce fairly 
slowly with increasing frequency. This is part of the reason for VLF loading 
coils being so large. The "helium coils" were discussed on this reflector 
some time ago - they were loading coils for HF mobile whips. A Q of a few 
hundred for a fairly large coil  is not remarkable; obviously, the size of 
coil is limited for a practical mobile whip. The helium was just a sales 
gimmick that would make no significant difference to Q.
Cheers, Jim Moritz
73 de M0BMU