SPMcGreevy "OFSL" (open FSL) loop test-results and photos


Stephen P. McGreevy
 

It has been a few months since I reported on the results of what I now call my “OFSL” or “Open Ferrite Sleeve Loop” that I had constructed around a 16 oz. / 453g size Folgers “Classic Roast” plastic jar, in that the six 200mm (R40C1 type) are not adjacent to each other as I have seen of Gary’s designs, but are widely spaced around the jar (I had only ordered six of the rods for initial testing/experimentations).
The OSFL loop - previously I’d dubbed “the Ugly Duckling FSL”  - employed an external (and large/heavy) old Soviet 60-600 pF var. cap. that is designed for transmitting with widely spaced rotor/stator plates), and it was rather “kludgy” to rotate easily.
It occurred to me that a smaller open-air var. cap. could be mounted inside the jar’s red lid along with a small SPDT switch to switch in the off-center tap of the Litz winding and some fixed “padding” caps of about 330pF.  As such the loop is “dual-band” and goes down to about 340 kHz just incase I wish to use it in the upper NDB band and in the former Maritime segment (430-510 kHz).
The result(in the photos) is a very robust and quite water-resistant OFSL that is lightweight and marvelous for taking on Expeditions - even just into a knapsack as I walk out of my small town into a noise-free location.
In-yard comparisons with a considerably larger air-core “diamond” loop find that the OFSL has vastly higher-“Q” (sharper tuning) and it also has higher-gain than the larger stand-mounted diamond open loop due to its larger induction-field and the advantages of Litz wire.
I can’t wait to take this on a flight to Hawaii (maybe this November 2019) on a DXpedition tentatively planned in a similar fashion to Gary’s Kona (Big Island) Hawaii DXpeditions.  
Include a couple of photos for presentation here.  The OFSL makes AM stations with “barely audible audio” in the daytime to clear/clean copy provided there is no local noise nor QRN (such as 740 KCBS, 620 KIGS, 1070 KNX and many others).  Tests were made with one of my two trusty Sony ICF-SW6700GR portables.  I can imagine with a modified ultralight portable with switchable pass-band filters, this would be even more superb for coastal DXpeditions!

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