At Last! Perfecting the Delta Loop

One of the reasons I started this blog was as a personal diary of developments with antennas and other equipment.


For several years now, my bread-and-butter antenna has been the trusty 20m delta loop.  For all that time, it's been an apex-up, coax corner fed unit, using a 4:1 balun at the feedpoint.  At the start, the antenna was much too long - about 23metres - but it worked most of the world anyway, coupled to an ATU.

The revised, almost-perfect delta loop for 20m in a Frank Hurley-inspired night-time image.

Later, I shortened the loop to a lower SWR, and fed it with higher quality RG213 coax, again to a 4:1 balun.  Again, the delta carried on adding DX to the list, reaching 116 entities in a pretty short period.

So, the first take-home message for anyone starting out with a delta loop is: a corner fed delta is easier to put up than one with a feed a quarter wave down from the apex.  It yields low angle radiation but doesn't really need to go up more than 8m at the top. It will also certainly let you work the world with decent signals.

And on that note of being 'only' 8m at the top (or about 2m off the ground), and to answer many, mostly US-based criticisms of this, raising a delta loop too high is a bad, not a good thing.  This is because as the loop rises, it starts sprouting high angle lobes.  High angle lobes allow high angle signals to come in, which negates one of the main benfits of a delta: low noise.  At 20m, these lobes are well-developed when the bottom wire is above about 5m, regardless of ground type.  So keep that bottom wire within 2 to 4m of the ground (it also becomes increasingly complex to rig a delta above this height.)

The only drawback with a corner-fed delta is that its pattern in elevation is very broad because cancellation of the horizontal radiation is not complete. Consequently, it has much the same gain from near the horizon to overhead.  That means closer-in stations coming in at steeper angles can cause excessive QRM.

It is to rid the high angle stuff that necessitates the use of a feed point a quarter wave down from the apex, or 0.08 wave up from the bottom corner if you prefer that.

I decided I would try, at long last, the latter feed arrangement, and used a very lightweight dipole centre attached to 300Ohm twin.  A broken fishing pole holds up the twin at a 90ish degree angle from the antenna, keeping it reasonably balanced, and is led off into a 4:1 current balun outside the shack.  A very short section of RG213 completes the journey to the rig.

So, having braved the cold, cut the delta down to 21.4m, and soldered once more in stiff winds (a propane/butane torch is extremely useful for this, but aim its jet downwind to avoid accidents!),  I put the rig into CW mode and pressed the mic.

Had it worked?

Yes, the delta's now working at an SWR 1:1.2 in the digital and CW sections, rising to 1:1.5 at the upper band edge of 20m.  If I had an analyser to confirm my reasoning, I would say I can afford to trim another 10cm or so off the loop.  But for now, the loop has finally reached the kind of place where it is pretty much perfected with low SWR and very low angle radiation.

So, that is the culmination of years of experience of a delta loop.  The final wire length for resonance is indeed in keeping with online calculators.  Books are not always so accurate...