Sunday, April 14, 2019

The 50 ohm rectangular loop

Many of us have seen diagrams of the square quad loop. It's an effective antenna. You can feed it at the bottom centre for horizontal polarisation or in the middle of either side for vertical polarisation.

Something that makes its construction slightly harder is its 100-120 ohm feedpoint impedance. Unlike a half wavelength dipole (especially the inverted-vee variety) it can't be fed directly with 50 ohm coaxial cable.  A 4:1 balun is not quite right either. Instead you need to make a transmission line transformer (using 1/4 electrical wavelength of 75ohm coax) to reduce ~100 ohm to ~50 ohm.  Or you could feed it with open wire line and use a balanced antenna coupler. 

Another issue with a square loop is height. You don't want the bottom wire to be on the ground. Raising it a little helps but you still want it to be above head height to avoid crashing into it. So, assuming you make the square loop from wire slung between two poles, the masts need to be at least 1/3 wavelength tall.  

That's a little beyond the 7 or 8 metre fishing poles many portable operators use. Especially if you don't use the top metre or so due to its fragility.  However, especially if you're portable you can spare some extra ground space.

One solution is the rectangular loop. It's still a wavelength around but its no longer square. That reduces its feedpoint impedance to 50 ohm with certain dimensions. And if you make it 1/3 wavelength wide and 1/6 wavelength high then you can erect it with shorter masts.  The shorter side also make it easier to feed in the middle, which is desirable if you want vertical polarisation.  

Here are a couple of videos I've done. The first was with a loop 20m in perimeter. The second was a loop with a 30m perimeter. The latter, if fed with open wire line and a balanced antenna coupler, can work efficiently on popular bands including 7, 10, 14 and 18 MHz. 

Some rectangular loop ideas from others:

Rectangular loops are so simple to build. Try one - I think you'll like it. 

PS: I have written five books on amateur radio topics. They are available in electronic and paperback form (most countries). Ebooks are under $US 5 each. Find our more here or follow VK3YE Radio Books on Facebook.

1 comment:

  1. Does rectangular loop have any gain over a square one?


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