When the 73 kHz band was made available in the UK, I asked Arnold Lynch 
(formerly of Post Office Research Department, Dollis Hill) about measuring high 
Q values. I believe the maximum he mentioned was 5000.

John F5VLF

On 19 Dec 2011, at 10:36CET, James Moritz wrote:

> 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 
> 
>