Loopy Heights?

A nice, if rather glacially-paced video popped-up on my YouTube feed last night. It's an unboxing-through-to-testing sequence that does a nice, if rather superficial job of demonstrating the potential of a magnetic loop antenna.

It's fairly small and multibanded, yes. But I would expect most neighbours to commence immediate warfare, should they wake up to this, one morning. Image: Lutz-electronics.

 

Wading through comments reveals the inevitable: "imagine what it [the loop] could do if it were at the same height as the [comparator] dipole antenna!"

I've made and used magnetic loops for over a decade now. I'd never buy one, because they are extremely expensive - the Baby Loop is currently priced at a staggering £1,489 in the UK - and their proprietary tuning circuits mean they are more difficult to repair than if you made your own. Companies also sometimes go out of business, making repairs even more difficult, or impossible. I don't like relying on others for my continued peace of mind!

So, does raising a loop to the standard height of 10m improve a magnetic loop's performance? I should first point out I've used MMANA-GAL for modelling, with real, good ground constants and an understanding that the absolute gain numbers it produces are typically substantially lower than a real magloop. This is due to the use of disputed equations for the efficiency of a magloop - a topic that has raged-on for decades and that is well outside the scope of this post.

Well, firstly, you can't just sit a large mass of metal on top of a casual mast - you would need at least a lightweight lattice tower or extremely well-guyed scaffolding-type mast to even contemplate it in windy countries like the UK. If you have a tower, chances are you won't choose to put a magloop on top of it; you can put a directional antenna up, instead. 

And I'm not dealing with the special case of being in a high-rise building, because there are other factors to allow for there, such as being indoors, using the loop horizontally out of a window, and being surrounded by steel reinforcement.

Assuming a base height of 2m, at 14MHz, erected as a vertical antenna as most magloops are, you get a vertically-dominant 'loaf of bread' pattern, with good gain at all elevations. Not so good to eliminate NVIS signals if DX is your thing, but it won't prevent DX-hunting - you might just  get some interference from loud Italian stations on occasion:

Veritcal pattern of a magloop at 2m base hight.

The horizontally-polarised radiation occurs at 90 degrees to the plane of the antenna and is a little weaker than the vertical radiation at peak, with a certain loss of gain at very low elevations, compared to the vertically polarised stuff:

Horizontal pattern of a magloop at 2m base height.

Let's now raise the antenna to a base height of 10m, hoping there is no winter storm on the way! The vertical pattern now looks decidedly different, turning from a loaf of bread to a bowler hat:

Vertical pattern of a magloop at 10m base height.

The peak radiation strength has now increased by 4.6dBi, up from a peak for the 2m-mounted loop of 0dBi. This is reasonably good - if you are interested in NVIS and can somehow get your loop up that high, because the peak is at the overhead point.

At 10 degrees above the horizon - useful for DX though still a little high for the best outcomes - the 10m-high loop gives around -0.8dBi, whereas the 2m-high loop gives -1.2dBi; an extremely marginal and likely undetectable difference, given the massive difference in complexity of achieving the required additional height. 

At 5 degrees elevation, the difference is about 1dB in favour of the higher loop - again an inconsequential amount (an output power of 50W, for example, would increase only to 62W effective, with that 1dB extra gain).

But, look at that deep waist in the pattern for the 10m-high antenna. This is likely to impair performance considerably, and at the reasonably low angles important for medium-to-longer DX paths, relative to the 2m loop, which doesn't have this feature. At the peak of the 'pulled in' waist, at around 30 degrees elevation, the gain for the 10m antenna is -6.4dBi (50W out now becomes just 11.5W!). For the 2m-high antenna, it's around -1.3dB (50W reduces to a more tolerable 37W) at the same, roughly 30 degree elevation. But I stress again, these figures should be taken as relative, rather than absolute, due to the limitations of the modelling equations used, as previously noted.

You will, incidentally, see this 'waistline effect' with all loop antennas, including vertical delta loops, which also tend to frequently attract the wrong belief that great height is necessary.

The peak strength for the horizontal radiation is the same for both heights at around -2.4dBi. 

So, whilst putting a loop up at 10m height can yield very slightly better numbers, the detail of the pattern and the sheer mechanical, cost and planning difficulties of getting an antenna to that height means that there is no practical benefit to be achieved and that, for DX, the lower antenna is likely to outperform the higher one.

You can read about some of my many real-world, WSPR-based and thus human wishful thinking-free magloop tests here and here.