Bi-Square Loop - Fun All The Way on 10m

Yes folks, it's autumn in full swing, as is the 10m band!

For the past week or so, conditions on 10m have been outstanding.  Of particular interest to me is the excellent signal coming across from the Big Apple from KQ2H, a repeater that was once located on one of the ill-fated World Trade Centre buildings.

The bi-square in action.  An 8m-fishing pole supports the top.  Remember that this is not actually a loop - it's open at the top!

And by 'excellent', I am not exaggerating; the signal for hours on end swings to full-scale deflection on the signal strength meter - that's 59+60dB on my rig!

Of course, that's just the repeater's signal; the signal you get into it depends on the usual parameters.

But conditions are not always this good on 10m, and it can be expected that, come the end of this sunspot cycle's peak, the band will be mostly dead.  That makes big investment in a 10m antenna a poor investment choice.

A bi-square, correctly depicted - unlike many other internet resources - as two wires, open at the top.  It is NOT a closed loop!

Enter the 10m bi-square loop wire antenna.  I first came across this truly amazing antenna in 'The International Antenna Collection' book, which compares it against a yagi, highlighting some interesting and often conveniently overlooked points.

Remarkably, this most basic of wire antennas - it is just a loop, open at the top (so not a loop at all!) with both sides pulled out into a diamond shape, each 'leg' being a full wavelength long.  It's fed with twin at the bottom; I use 300Ohm ribbon, to an ATU which makes matching very easy (but do look out for strange effects with twin - keep it well away from other conductive materials.

I once asked a modelling question about the bi-square, and for the record, I reproduce below (in orange) the kindly WB6BYU came up with the following analysis, and interesting points.  I don't disagree with him about full wave loops, but I can say the bi-square has performed admirably from here, often making me the subject of mini-pile ups on KQ2H - all with just 100W, which gets boosted, as a minimum, to about 380W in effective radiated power.  At optimal configuration, this can be as high as 560W EIRP. 

Another useful thing about this antenna is that, if you open it at the bottom and connect one wire to one pole of a 4:1 balun, and a 1m-long counterpoise to the other pole (the other leg remains free), you get a very useful 2-element vertical beam for 20m (see Les Moxon's 'HF Antennas for All Locations').

'OK, I ran some models in EZNEC.  Starting with a 15m Bi-Square loop with the top at 11m.
and the bottom about 1m above ground.  I rounded the dimensions to make it easier to
vary the model.  Then I progressively squashed the loop, keeping the bottom 1m off the
ground, and recorded the gain and vertical angle of maximum radiation:

11m   8.6dBi @ 28 degrees
9m    7.8dBi @ 34 degrees
7m    6.9dBi @ 45 degrees

The vertical angle of radiation increases significantly.  Height is important for low angle
radiation.  In the process the feedpoint impedance dropped from about 3000 ohms at
resonance to 1400 ohms.

Now let's see what happens if we raise the bottom of the antenna rather than
lowering the top (while leaving the top at 11m.)

1m   8.6dBi @ 28 degrees
3m   9.2dBi @ 28 degrees
5m   9.6dBi @ 25 degrees

Clearly the higher average height above ground improves antenna performance, even
though the loop is more squashed.

Now, for comparison, let's look at just the top wires, so a sloping "Two Half Waves
In Phase."

6.7dBi @ 23 degrees

If we use a standard inverted vee at the same height (11m) then we get:

6.8dBi @ 19 degrees

For comparison, let's go back to the original Bi-Square and compare the radiation at
these vertical angles:

8.3dBi @ 23 degrees
7.7dBi @ 19 degrees

And we can check a full wave loop with a top height of 11m at the same angles:

7.6dBi @ 28 degrees
8.1dBi @ 22 degrees (maximum)
7.9dBi @ 19 degrees

So while the Bi-Square loop has a higher peak gain, the standard full wave loop has equal
or better gain at lower elevation angles.

Let's see what happens when we lower it:

9m  7.1dBi @ 28 degrees
7m  5.7dBi @ 39 degrees

(This is without squashing the full wave loop, which would raise the average radiation height.)

While the Bi-Square may be fun to experiment with (and should give a good match to 50 ohms
using the common window twinlead that is around 400 ohms impedance as long as the antenna
is square), I'm not sure there is a lot of benefit to using one at low height compared to a full
wave loop, or possibly even a dipole or inverted vee with the same top height, especially if
your primary objective is DX.'