I did not think that they were expensive $5.25 for a coil with a Q of over
500, and beautiful construction.
g3kev
----- Original Message -----
From: "James Moritz" <[email protected]>
To: <[email protected]>
Sent: Saturday, October 01, 2011 9:47 AM
Subject: Re: LF: Ferrite RX antennas
> 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&produ
ct_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
>
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