Re: LF: Ferrite RX antennas

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

Dear Mal, LF Group,

G3KEV wrote:
> NEW
> 500 series  HI-Q COILS
> Q over 400, mobile antenna individual coils, one for each band 160 - 10
> metres
> ( These coils are air wound and sealed in a transparent gas tube.
> DAVIS ELECTRONICS,  BURBANK, CALIFORNIA

A Q of over 400 for a large coil is not unusual or suprising.

Re the "helium coils" I guess you mean something like the advert in the 
attachment, which came from the web site of  K0BG 
(http://www.k0bg.com/history.html). In such a coil, most of the losses are 
due to RF conductor resistance, perhaps significant Q reduction is also 
caused by dielectric losses in the solid insulating components, eddy current 
losses in conducting parts, etc. The losses in the air around the coil would 
be minute in comparison. So filling the casing with helium would make no 
detectable difference - whether the helium would stay there for any length 
of time, or if they actually bothered to put it in initially, is debatable. 
So the helium bit is just a marketing ploy.
There are a lot of similar scams around, where something expensive is 
justified by claims of exotic-sounding materials and techniques, and results 
that are in some way remarkable. The hi-fi world seems especially prone to 
this - the Tara Labs website is always good for a laugh, see 
http://taralabs.com/shop?page=shop.product_details&flypage=flypage.tpl&product_id=118&category_id=17 
if you want to buy a 1m long audio cable for nearly 16,000 dollars!
Coming back to loop receiving antennas, the Q of the winding is only one 
factor; the "effective area" is another factor which determines the amount 
of signal power available. This depends mostly on the actual area of the 
coil in the case of an air-cored loop, and on the length, geometry and 
permeability of a ferrite rod, as was discussed some weeks ago on this 
reflector. You can certainly make an air-cored loop with a similar Q to a 
ferrite rod, but it will be more bulky and less portable than an equivalent 
ferrite rod that achieves adequate noise performance, which was the original 
reason for discussing ferrite rod antennas.
Cheers, Jim Moritz
73 de M0BMU

Helium_coil.jpg
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