Topband: Pixel Technologies Magnetic RX Loop
andrew.ikin at btopenworld.com
Mon May 9 02:06:07 PDT 2011
Steve WB6RSE wrote on the 8th May.
"If you've built your own Mobius receiving loop of tried the commercial product, a report of your experience would be of great interest to the Top Band list."
Back in June 2010, I decided to try out this loop architecture to see if there was any benefit compared to the use of a single turn low inductance balanced loop/ amplifier combination. My initial thoughts were; that the Near E-field noise immunity would be the same, but there may be a problem with HF gain.
My reasoning being:
1. With a Mobius shielded loop, the outer tube is 2 halves of one turn of the loop (The loop is effectively 2 turns). Hence, the E-Field is not screened from the outer. Therefore, I don't believe that this loop is shielded in the conventional sense.
2. If my reasoning is incorrect (the Loop is shielded), does it make any difference compared to a single turn un-screen balanced loop? The answer is no; with a single turn balanced loop, any induced E-Field cancels out due the 180 degree output phase difference, providing that the antenna aperture is small in terms of wavelength. The reduction in E-field rejection is also afforded by using a low input impedance/balanced amplifier.
3. Single and multi-turn loops deliver the same power output, providing that they have the same area/volume. Only the Current/Voltage and impedance relationship is different.
4. Therefore, the Mobius shielded loop provides 2 x the Voltage and 1/2 the Current and 4 x the impedance of single turn loop.
5. The capacitance of the Mobius is an order of magnitude higher than a 1 turn loop, this combined with much higher loop impedance may affect the HF response.
Notwithstanding the above. A Mobius shielded loop was constructed using 38inch diam. aluminium tube, 3/4inch OD with a split at the top of the loop, the inner wire was made from the outer braid of RG58c. This loop is effectively 2 turns with a loop inductance of 10uH (res. freq is 5.1MHz). This compares to 2.2uH for a 1 turn loop with above size of tube (Res. Freq. >50MHz).
Both loops were compared using a test set-up that provides an H-field from 50kHz to 30MHz.
The amplifier used on the Mobius loop was a high gain balanced loop amplifier.
The net result was that below 18MHz any gain difference was only due to differences in the loop to amplifier match.
Between 18MHz and 30MHz the gain of the Mobius drops off significantly by up to 12dB due to the very high loop capacitance.
Both loops had identical nulls.
As far as can see; there is no technical reason to use a Mobius loop for a broadband antenna.
Andrew Ikin G8LUG
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