Message-ID: <4E5414EA.7070707@o2.co.uk>
Date: Tue, 23 Aug 2011 22:00:26 +0100
From: Peter Dodd <g3ldo@o2.co.uk>
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Subject: Re: LF: 136kHz ferrite rod antenna test
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In the early 1960s some aircraft in the RAF were fitted with Marconi ADF 
sets. The loop antenna was a slab of ferrite, which if my memory serves 
me right, was some 200mm x 120mm and about 30 or 40mm thick. The 
windings were just a few turns of litz wire.
As the equipment had to cover the whole of the NDB beacon band I guess 
it was a broad band technology.

In the early days of 73kHz I followed the example of Mike, G3XDV, in 
constructing a loop antenna and FET preamp from a defunct transistor set 
to make up a mobile receiving station. These were used to check out the 
relative field strengths of our primitive early transmitters. My 
transmitter was a audio signal generator, a 10w solid state vehicle 
sound system amplifier and a big coil

Peter, G3LDO.
>
> Having said quite a lot about ferrite rods on the reflector recently, 
> I thought I better try one as a quick "feasibility study"!
>
> I have a large bar of ferrite I had assembled from surplus "U" cores 
> some years ago as part of a portable SAQ receiving system (it's a long 
> story...). The"rod" is about 470mm long, with rectangular cross 
> section about 30mm x 35mm. This is rather larger than the rods used by 
> DK7FC and DF6NM, but the ferrite is more lossy - probably 3C8 or some 
> similar "power" grade. 30 turns of litz wire gave L about 360uH and Q 
> of 150 (so using litz probably wasn't justified, but I had that length 
> to hand). This resonates with about 3700pF at 137kHz, giving an 
> equivalent parallel resistance of about 45kohm and a bandwidth a bit 
> under 1kHz.
>
> The preamp is a compound JFET/bipolar follower using a J310 biased to 
> 6mA and a 2N3053 biased to 50mA (probably similar performance to a 
> "mini-whip" preamp at 137k). The high Z input is connected directly 
> across the tuned antenna winding. With the high QRN on the band this 
> evening, it is hard to get a good view of the noise floor, so I used a 
> "dummy antenna" consisting of a pot-core inductor wound with the same 
> inductance and Q, substituted for the real rod antenna to simulate a 
> zero signal and external noise condition. The noise output from the 
> resonated dummy antenna was 10dB or more above the RX noise floor 
> (SDR-IQ). The preamp with input shorted gave a noise level below the 
> RX noise floor. So this preamp arrangement gives sensitivity limited 
> by the thermal noise of the antenna, and adequate gain for the 
> "reasonably good sensitivity" SDR-IQ (also for my RA1792).
>
> I made a rough estimate of the overall sensitivity of the complete 
> system by measuring the level of DCF39, which is a fairly stable 
> 800uV/m here during daylight. On an arbitrary scale on the SDR-IQ 
> spectrogram, DCF39 was -19dB, while the noise floor at around 137.7kHz 
> with dummy antenna was -109dB with 0.75Hz FFT noise bandwidth. This 
> works out to an antenna noise floor of about 0.03uV/m per sqrt(Hz), 
> several dB below quiet 136kHz band conditions. So sensitivity of a 
> receive system with this antenna should be limited only by the band 
> noise; it should also be possible to reduce the amount of ferrite 
> somewhat, especially if lower loss ferrite antenna rods are used. I 
> await lower QRN levels for a "live" test !
>
> Cheers, Jim Moritz
> 73 de M0BMU
>
>