HF Reception on a MW Ferrite Loop Antenna: Anyone Know about this Oddity?

[RadioDiscountCabbage]

I was tuning Radio New Zealand on 25 meters at around 07.30 zulu, when I noticed that despite good propagation conditions and greyline positioning, the signal was fading a bit and weaker than usual on my capable Icom r75. Band conditions were fine. Dumbfounded, I checked all of the functions of my receiver and realized that my antenna selector was set to my Grundig/Eton AN-200 passive ferrite loop built for MW reception. It was also picking up WWV on 10 MHz, and a south American station also on 25 meters. How was this thing, which is admittedly great for MW DX but not at all an HF antenna, picking up these signals?
 
To rule out some sort of interplay between the Ferrite loop and the bare copper wire of my homebrew antenna, I pushed both it and my ground outside my shack. No change. Moving the ferrite loop to a window on the other side of the room at a rough heading towards the transmitter site improved reception to about S7. Fading was present but not extensively so until the greyline had moved west of the transmitter site.  It continued to pick up RNZ on 31 meters until 09.30 zulu, when fading became so obtrusive that I could no longer copy the signal.  

Since then, I have pulled in signals on all of the HF bands below 25 meters during local nighttime hours. Strong reception above S5 has exclusively taken place when the greyline effect is present at either my location, the tx site, or both. Marginal reception of stations (primarily in the Americas, for example, CHU at 3.330 MHz) is possible without the greyline effect, but this is very unreliable and prone to fade out quickly.

This has led me to hypothesize that low- impedance antennas intended for MW reception such as ferrite loops can receive a subset of HF signals when conditions in the ionosphere are extremely favourable to the propagation of longer HF wavelengths. The greyline effect, combined with the ease of signal propagation at and below the 25 meter band, appears to bolster (if it does not entirely enable) shortwave reception on these antennas intended for reception of medium wavelengths. Exactly how is another question. But it is certainly a phenomenon worth further exploration.

Does anyone have experiences that might shed some light on this?

Apologies for the long post. Actually part of a draft on a working paper that started on this, since I can find nothing about it anywhere else.

Video of this happening is linked below.

https://www.youtube.com/watch?v=Snu_gV_RIAY

[Pigmeat]

I've got the Tecsun version of the same loop. It does the same thing. All loops will tune to frequencies that are multiples of the ones they were designed for, but you lose the directional properties and efficiency as you go up the bands.

You ought to try to build one, it's dead easy. Check out JFarley's loops in the blog section and build your own. He tells you all you need to know in a clear and concise manner.

BTW, you could plug a 3 foot hunk coax into that ICOM and hear something. Those radios are sensitive.

[ChrisSmolinski]

I'll echo what Pigmeat said, jFarley's loop is indeed extremely good, especially considering the size.

[BoomboxDX]

It stands to reason you're going to hear something, even with the MW loop -- after all, the antenna probably has somewhere around 100 ft of wire inside it, even though it's looped up into a relatively small space, and tuned for MW.

[Andy]

I'd agree with that - I dare say the loop is performing in the same manner as a length of wire stuck in the back of the receiver would. Inefficient, but still picking something up.

[Zoidberg]

Several years ago a fellow who specialized in high quality small loops for MW DXing said he could modify one for me to work pretty well on the most popular US shortwave funny bands, approx. 6800-7000 kHz.  It wouldn't be directional for reception, but the loop could be oriented to minimize local RFI/EMI from buzzing streetlights, plasma TVs, etc.

His suggestions prompted me to try my own homebrewed loop, rigged up from old TV coax.

"BTW, you could plug a 3 foot hunk coax into that ICOM and hear something. Those radios are sensitive."

Ditto the Palstar.  The fugly loop I used to have wasn't amplified. it wasn't even a good design.  But it was quiet enough to make SWL tolerable in a high RFI area.  And it was just directional enough to null out the worse local noise.  And pretty directional on MW too.

I finally ditched the fugloop recently after the old TV coax cable cracked.  Good while it lasted and cost nothing - I snagged the coax after the local cable company ripped it out of an old apartment and tossed it almost 10 years ago.  It was from the apartment building's first installation in the 1970s.

This thread reminds me I've been meaning to try a small MW loop again for SWL on the funny bands.  The main challenge remains heavy local RFI.  No point in a more sensitive antenna - those only pick up noise more efficiently.  But a small quiet loop would make it tolerable to leave the radio on while waiting for the stronger signals to punch through.