From: Glen Baker (gmb@sr.hp.com) Subject: Re: A FAQing question about shovels Newsgroups: rec.skiing.backcountry Date: 1997/11/21 Richard Ray (richard_ray@xbd.com) wrote: > There was a lot or question last year about possible attenuation of signal > from avalanche beacons if a metal shovel blade were in the wrong place, > but there is _no_ empirical evidence that that occurs, and considerable > experimental evidence that it makes no practical difference. Ah, I was waiting for this to come up! Time to re-post the results of my experiment (so that I feel justified in having gone through the effort in the first place! :-) Original posting date was 22 Oct 1996. OK, as promised I brougt in an Ortovox F1 in order to get a quick idea as to how much a metal shovel would attenuate the signal. Everything that follows is the result of a very quick, very messy, very non-scientific look at performance. Use this data at your own risk. Some restrictions apply. Offer void where prohibited. You get the idea. The quick answer is: "A metal shovel made no appreciable difference, I'm sticking with my metal blade". Now for the hard data: All tests used an HP 8562A spectrum analyzer. Center Frequency set to 457 kHz. Zero span. Video triggered. Resolution BW auto coupled (100 kHz). The receiving antenna was carefully designed and fabricated by wrapping "several" turns of some magnetic coil wire around a plastic syringe that looked like it might be a nice size. All tests were performed in the anechoically pure environment of my lab cubicle with distance measurements facilitated by the 1 foot square tiles on the floor. The "shovel" was a 1'4" x 1'4" piece of sheet metal that I bent to approximate the shape of a shovel blade until it looked "pretty good to me". ------------------------------------------------------------------------- Test 1: get a real quick idea of the effect. The transmitter and receiver were placed on the floor 4 feet apart oriented such that the signal was maximized on the analyzer. The shovel was placed directly on top of the tranmitter and the resulting attenuation was measured. Shovel on transmitter attenuation: -2.44 dB (at 4'). Then, sez I, "that's measureable, but how much does it represent?". So I moved the transmitter 1 foot. Transmitter with no shovel, 5', attenuation = -5.2 dB. So placing the shovel directly on top of the transmitter had less effect than moving one foot away. ------------------------------------------------------------------------- Test 2: Repeat test 1 with a slightly more realistic orientation of the transmitter/receiver. The transmitter was left on the floor and the receiver was elevated exactly 1.5' (calibrated eyeball measurement) to provide some angular displacement. Horizontal Distance Shovel Position Attenuation 4' none 0 4' on top -5.9dB 5' none -4.8dB 6' none -8.2dB 4' 6" above -0.3dB Note that (a) again the effect is similar to a one foot displacement and (b) if the "shovel" is moved into a slightly more realistic position 6 inches away from the transmitter the effect is essentially unmeasurable -------------------------------------------------------------------------- Test 3: more goofing around in order to try to find the absolute worst orientation of the shovel. After playing around with various orientations of the shovel and transmitter the worst one I found was as follows: +-------+ | S | <- Shovel | | +-+ | +-+ | |R| | |T| | +-+ | | | | +--+-+--+ | | Receiver Transmitter The transmitter was on it's side with the antenna oriented into the page. The shovel was oriented 90 deg to the tranmitter. The antenna was a coil that is oriented into the page. In this configuration, with the shovel directly adjacent to the transmitter, the resulting attenuation was 4.5 dB. Note that (a) this is again similar to a one foot distance change and (b) in the real world, achieving this orientation would be somwhat painful since the shovel blade would typically need to be protruding through the chest. --------------------------------------------------------------------------- Test 4: Sanity check on attenuation vs distance. Just for grins I measured the attenuation vs distance for two different orientations of the transmitter (again, using my calibrated floor tiles). Signal Strength (dBm) Distance(ft) 0 degrees 45 degrees 1.0 -40.43 -43.7 1.5 -50.67 -54.57 2.0 -56.53 -59.6 2.5 -63.5 -67.03 3.0 -67.1 -69.0 3.5 -70.7 -73.1 4.0 -74.1 -76.0 4.5 -77.2 -80.0 5.0 -80.7 -82.2 5.5 -82.5 -85.0 6.0 -84.0 -86.5 6.5 -87.5 -91 7.0 -88.3 -93 If you plot this out and squint at it properly it appears that the attenuation is relatively linear in dB which is what I would expect. Admittedly if you squint at it differently you may decide it's got some curveture to it as well but hey, it's not exactly carefully gathered data! Anyway, it's dang linear in the 2.5 - 5 foot range and using that as an indicative region the attenuation appears to be approximately 5.7dB/foot. The 45 degree data tracks the 0 degree data very well as expected. -------------------------------------------------------------------------- Conclusions: 1) It's not always a good idea to draw sweeping conclusions from hastilly gathered data. 2) Conclusion 1 notwithstanding, the effect of the shovel in the near field was negligable. For the realistic case of a slight separation between the shovel and the transmitter the effect of the shovel was so slight as to be well within the uncertainty of the measuring techniques used. 3) The maximum effect observed in all of these tests was equivalent to an approximate change in distance of 1 foot. 4) I'm keeping my metal shovel, thanks very much! ..glen