[600MRG] Ferrite antennas

[Marshall Cross]

Couple of thoughts: 1) The virtue of ferrite loaded loops is that you can obtain the same (or better) sensitivity and directivity as a large air core loop using a much smaller structure. 2) At 500 kHz (+/-), performance is usually externally (manmade and atmospheric) noise limited so, unless the receive site is in a quiet manmade noise area and operation is during an atmospheric-noise quiet season/time of day, the sensitivity (sometimes called effective height) of the loop isn't important until its thermal noise combined with that of its LNA (if used) and the receiver, begins to equal external noise. Bottom line, the ferrite serves as a flux gathering mechanism (good) but at the same time increases the loops's inductance (bad), the latter being so since reactance leads to stored energy which lowers the performance of electrically small loops, and for that matter, all electrically small antennas. Lowering the inductance can be accomplished using alternative winding strategies and core configurations.

In my opinion, one of the better tutorials on air and ferrite core electrically small loops is in R.C. Hansen's seminal book: "Electrically Small, Superdirective and Superconducting Antennas," sections 1.3.3 through 1.3.3.4, Wiley Interscience, 2006. This book is also very entertaining to read in that it exposes many of what he calls "nut-house antennas," many of which are foolish, and as he points out on page 85: "Foolishness if pursued, can lead to denial and that sometimes leads to fraud," taken from "Voodo Science" by Robert Park, Oxford Univ. Press,2000.  

  73, Marshall, W1HK   

-----Original Message-----
From: 600MRG [mailto:600mrg-bounces at w7ekb.com] On Behalf Of Warren K2ORS
Sent: Tuesday, February 16, 2016 9:06 AM
To: J Mcvey; david vanhorn; 600mrg at w7ekb.com
Subject: Re: [600MRG] Ferrite antennas

There are limits to the effective area of a ferrite loop which can be overcome with a large air-core loop. Ferrite increases the effective capture area of the antenna, but there are asymptotic limits. There are a number of papers on the subject with both theoretical and experimental results that back this up. If you want to do real DX you will need something better than a ferrite loop.

Ribbon cable is a poor choice for a multi-turn loop as the close proximity of the turns results in a high capacitance between turns and a limits the tuning range and Q. Ribbon cable is also made up of thin diameter wire which has a relatively high resistance (recall the skin effect at RF increases the effective resistance compared to DC), this further degrades the Q.

Almost anything will work to a degree, it's often the case that the ham with one antenna and nothing else to compare it to thinks that he has the best antenna in the world :-)


73 Warren K2ORS




On 2/15/2016 9:56 PM, J Mcvey wrote:
> Does it have to be ferrite based?One of the more innovative things I saw, was loop antenna made from some old (40 -50?? conductor) flat ribbon computer cable.He offset the end to end alignment by one conductor and soldered each wire end to end which resulted in a multi turn loop for VLF and MF . He said he used it with very good results. It was a diamond shape and about 3 or 4 foot  across if I can remember dimensions by just looking...
> Without lossy media, I think it would perform better , no?
>
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>      On Monday, February 15, 2016 12:11 PM, david vanhorn <kc6ete at gmail.com> wrote:
>
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>   I am looking for a good reference on ferrite rod antennas.  
> Something like the ARRL antenna book but focused on ferrite antennas.
> _______________________________________________
> 600MRG mailing list
> 6

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