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- Subject: [IRCA] Craig Barnes' 3.5 inch Baby FSL Rocks at Rockwork
- From: Mark Connelly <markwa1ion@xxxxxxx>
- Date: Thu, 23 Aug 2018 14:44:25 -0400
- Delivered-to: irca@xxxxxxxxxxxxxxxx
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Why would one want to make a broadband FSL when perfectly good reasonable-size broadband loops already exist?
https://www.wellbrook.uk.com/loopantennas/ALA1530LN
https://www.w6lvp.com/product/w6lvp-amplified-receive-only-magnetic-loop-antenna/
http://www.lz1aq.signacor.com/docs/wsml/wideband-active-sm-loop-antenna.htm
Or one can homebrew, as in:
http://www.qsl.net/wa1ion/loop/car_roof_loop.htm
As far as making the FSL compatible with 50 ohm receivers in a tuned mode, a buffer amplifier (input impedance greater than 100K) could be placed across the tuning capacitor. This would be powered by battery right at the amp and the 50 ohm output would go to a 1:1 transformer to isolate the amplifier card's ground from that of the coaxial cable going back to the receiver input. This is necessary to keep the antenna close to balanced, a condition desirable for good null depth and noise rejection.
If the input capacitance of a given buffer is too high, the upper extent of the tuning range might be reduced. If the effective shunt resistance is too low, Q (selectivity) may be reduced. For either or both of those cases, use of a link coil, closely coupled to the main coil, would be recommended to feed the amp.
High-impedance input buffer amplifiers are used in many applications such as active probes for oscilloscopes, spectrum analyzers, and medical electronics. There are many suitable circuits around. The Clifton Labs Z10130A, derived from a Dallas Lankford design, is one of the better ones. No longer manufactured, it can be built from the design shown at:
https://www.okdxf.eu/files/Z10130A%20Manual%2001.pdf
DX Engineering has another former Clifton Laboratories model, DXE-Z10000U-KIT. It might also be suitable.
https://www.dxengineering.com/search/product-line/dx-engineering-clifton-laboratories-z10000-buffer-amplifiers
This page might also help.
http://www.qsl.net/wa1ion/buffers/bufamps.htm
>From the '70s through the '90s, the LH0063CK was a workhorse in my designs but those are obsolete.
https://pdf1.alldatasheet.com/datasheet-pdf/view/72714/MAXIM/LH0063CK.html
A bit of Googling should produce numerous circuits using more modern parts.
One thing to remember is don't pick an amp that goes to > 1 GHz when 30 MHz or less may be your maximum frequency of interest. You don't want TV, FM, cellphone, etc. mixing products getting down into the MF and HF ranges. Even when you don't hear actual spurs, elevated noise floor could still be a side effect of an amplifier that is much more broadband than what you need.
Construction of such gear is not quite as easy as it used to be. Surface mount micro-miniature parts and lead-free solder are more difficult to use.
Mark Connelly, WA1ION
South Yarmouth, MA
<<
Hi Rick,
<<< Has anyone done any experiments with an inductive secondary coupling loop on an FSL or a high-impedance unity gain preamp with an unbalanced low impedance output across the loop winding? I.e. what would be needed to couple to a receiver or an SDR? >>>
Local buddy Guy Atkins (also living here in Puyallup) has performed multiple experiments attempting to match the compact gain performance of the FSL antenna with the Perseus-SDR receiver, and back in 2013 Guy tried almost every conceivable interface (low-noise Wellbrook preamps, sensing coils, direct connections, etc.) in order to somehow match the single-optimized-frequency performance of the FSL with the state-of-the-art Perseus-SDR receiver. Because of intense interest among local broadband DXers over $1K was used in these experiments, which unfortunately ran into the stark reality that a high-Q, single-optimized-frequency antenna with razor-sharp tuning peaks is not very compatible with a spectrum capture receiver requiring a broadband antenna to record all MW frequencies simultaneously. That's not to say that the FSL antenna couldn't be used on single frequencies to provide exceptional gain for the Perseus-SDR (or other 50 ohm antenna input receivers) once an impedance-m
atching system is devised, and I'm pretty sure that Guy experimented with those type of matching systems, and how to get good performance on narrow frequency ranges with a Perseus + FSL combination. But the essential fact remains that almost all Perseus-SDR DXers are oriented toward spectrum capture of the entire MW frequency range, and will not be satisfied with exceptional antenna performance on one single frequency.
73, Gary DeBock (in Puyallup, WA, USA)
>>
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