Re: Connecting the Tecsun PL-360 to a large air-core loop

jim_kr1s <jkearman@...>

--- In ultralightdx@..., "sdwillingham" wrote:

> For air loops with high inductance, directly matching the Si4734's tuning range, it is difficult to make a 1:1 balun transformer. It takes a lot of windings on both the primary and secondary, leading to more parasitic capacitance and series resistance. Also, a larger core must be used.

To put this in perspective: For an untuned balun, we like the reactance of a winding to be 4 times the source or load reactance at the lowest frequency of interest. Reactance is proportional to frequency, so we'll calculate at 530 kHz. Let's go with an inductance toward the low end of the range to minimize coil size. We'll use 240 uH.

At 530 kHz a 240-uH inductor has a reactance of 800 ohms. So the winding reactance should be 3200 ohms at 530 kHz. As reactance is directly proportional to inductance as well as frequency, the inductance should be 960 uH. Both windings will have the same inductance. It takes about 113 turns on a 1.14-inch toroid to get 960 uH, and it would take mighty thin wire. As Scott mentions, that raises the resistance and losses.

As Scott mentioned last night, a 6-inch air-core loop has about the same signal-gathering ability as a 7.5-inch ferrite rod antenna. Your relative RSSI readings on your 24-inch air-core loop vs the 7.5-inch ferrite rod sound about right. A 6-inch, low-inductance loop is easy to tote around. As you can locate it a few feet from the radio, so you can turn the antenna and not have to turn the radio. I find that to be a great convenience, even with the larger, 23-inch Hoop Loop. Cores and wire are available from (With a quick transformer change the loop will work on LW, though I'd use something larger than 6 inches there. I don't think you could squeeze a LW transformer onto a half-inch toroid, as you need >2000 uH.) Quilting and embroidery hoops are available in a variety of shapes and sizes. Use your favorite search engine or check out a local Michael's, Jo-Ann Fabrics, or similar store.

Jim, KR1S 

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