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Re: LF: Ferrite RX antennas

from [James Moritz] [Permanent Link][Original]
To: <[email protected]>
Subject: Re: LF: Ferrite RX antennas
From: "James Moritz" <[email protected]>
Date: Sat, 1 Oct 2011 10:47:16 +0100
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References: <68BE37BD69E54DEE89CEBA8E30E9B94E@PcMinto> <[email protected]> <[email protected]> <0A4ED65EC3B244A3BD0DAD6ACFF43793@JimPC> <[email protected]> <3746D901368F4C139E414F9110153EEE@JimPC> <[email protected]> <1B55B625C3CD4D529EC361D9C5D79D96@JimPC> <[email protected]> <[email protected]> <1FF2FAD9854F4890A338A9F862D93FE9@JimPC> <[email protected]> <[email protected]> <001f01cc5cf9$4b734780$0401a8c0@xphd97xgq27nyf> <[email protected]> <003301cc5d07$d4e2a840$0401a8c0@xphd97xgq2! 7nyf> <[email protected]! delberg.de> <002501cc5d33$a7f37f90$0401a8c0@xphd97xgq27nyf> <[email protected]> <000e01cc7f59$8fcc9cb0$0401a8c0@xphd97xgq27nyf> <20108458101B4C2F82BCA69885FB890E@JimPC> <[email protected]> <000901cc800e$67610210$0401a8c0@xph d97xgq27nyf>
Reply-to: [email protected]
Sender: [email protected] 
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

Attachment: Helium_coil.jpg
Description: JPEG image

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