Re: [IRCA] ALA 100 and its beam pattern

[Nick Hall-Patch <nhp@xxxxxxxx>]

At 07:45 1/28/2009, Steve wrote:

>Lots of folks on ndblist use various size loops with ALA100 preamps for LF reception. They have found, at least on LF, that the lobe is fairly narrow and you can't just put up an E/W loop for example and expect to hear 90 degrees on each end. 

This is a valid point, Steve, though I'm not sure how narrow is "fairly narrow"

Using the standard equation for voltage output from a loop antenna, I derived the following:

0       0.0 
10       -0.1 
20      -0.5 
30      -1.2 
40      -2.3 
50      -3.8 
60      -6.0 
70      -9.3 
80      -15.2 
90      null (maybe -35 dB on a real world loop)

The first column is degrees off the bearing of the loop, and the second column is the number of dB down from the signal strength found from a station in line with the loop (so a similar strength station at 60 degrees off the bearing of the loop would be down 6 dB.)

Doesn't sound like much, given that an S-unit is usually thought to represent 6 dB.   However, in a weak signal situation, my impression is that 2 or 3 dB can make the difference between having readable DX and not having readable DX, so perhaps we should say that the lobe on your loop antenna should be thought of as being +/- 50 degrees from the bearing of the antenna.    

So, Patrick is perhaps right to be concerned about an east-west loop antenna's response to TA's  which are from the zero to 50 degree range for those in the Pacific Northwest.    Zero degrees of course won't cut it at all on an east-west loop.   Still, I wonder how many degrees north of east the loop could actually be mounted?   Ideally, it would be a 30 degree shift to the north, but even 15 or 20 would help quite a lot.  A borrowed ALA100 will allow for a reasonably painless experiment.

As to whether theory lines up with reality....I tested this using KAPS-660 on my tuned box loop, and it was within 0.5 dB of theory.  I'm doing some other ALA100 tests just now, so it was convenient to repeat the test using an ALA100 and a copper pipe loop such as Craig Healy described, 1.3 meters on a side, a bit small for an ALA100, but presumably relevant for a larger well-made loop.  I don't have a rotator, so my null position may not have been exact, but even so, theory was vindicated to within 1.5 dB (and at least a 30 dB null to boot), on both 660 and 240 kHz.

Hope this helps.

best wishes,

Nick





*****************************
Nick Hall-Patch
Victoria, BC
Canada 

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