Delta Loop Balun

These past days, I've been trying to get to the bottom of just what value balun my 20m delta loop needs for the best match possible.

My bread-and-butter antenna: a vertical 20m loop, used almost exclusively as a monobander.

This has been a long-running investigation, lasting years with on and off experiments.  But now, with the SARK-110 analyser, life is a lot more enlightened, and progress much quicker.

Like most people, I've been using a 4:1 balun with both my 20m and 17m vertical delta loops (fed with 300 Ohm twin) for many years.  Both can be used at relatively low SWR without antenna matching unit (AMU), but in general, a small amount of tweaking by the AMU makes things tidier.

In other words, these antennas were not in any way 'way off' or being forced to work outside of their natural limits.  WSPR tests showed the efficiency of the antennas was very good, often outlasting other EU stations across the Atlantic on 20m by up to an hour.

The SARK-110 analyser is a real revelation when applied to your antenna systems.

I decided to try a 2:1 (Balun Designs model), which I happened to have lying around from the days when I was too scared to build my own, on the 17m delta.  This brought it to a reasonably sharp and very good match at or near the band.  As a result, I've now kept the 2:1 in place, using a minor amount of adjustment from the AMU, which is principally necessary due to the proximity of telephone and domestic voltage electricity lines across a far corner of our garden.

Moving onto the 20m delta, the main focus of this post, things were quite different.  This was also fed with 300 Ohm twin and a 4:1 current balun for many years, bringing very good performance without issue, and is located much further away from environmental metal.

This is how the SARK-110 saw my 20m delta, about 15m of 300 Ohm twin and a homebrew 4:1 current balun:

My 20m delta at the end of 300 Ohm twin and a 4:1 current balun.

You can clearly see that the SARK proves there was not an awful lot wrong with this arrangement, the impedance and SWR being within acceptable limits.  This is, after all, an antenna that has worked the world many times over for five years or more.

But that impedance interested me.  It was too low for my liking, and surely, the SWR would improve if the impedance could be brought closer to 50 Ohms.

In particular, when you consider that a high imaginary component to the impedance means that a fair amount of energy is being contained in the antenna's near field, rather than radiated into the far field, then this is something worth addressing.

So, the next question to answer was: what does the antenna and 300 Ohm wire look like when connected to the SARK without a balun?  This is the question that you simply can't answer without an analyser, and which makes them essential to a rational approach to understanding antennas:

Here's the result:
 

 20m delta, 300 Ohm twin, no balun.

So, the impedance is up at about 245 Ohms, which results in a SWR of about 5.4.  Now I could see that to get a better match, I needed to divide 245 by some value to get 50 Ohms.  This directs what kind of balun value we need and, obviously, the answer is division by 6 -  a 6:1.

After a lot of online searching, which included searching in Italian, rather than English - yielding different search results - I eventually found a comprehensible schematic for a 6:1 voltage balun.  This transforms 245 down to about 40 Ohms, which is not bad.  Here is how it looks:

Fairly easy to construct 6:1 balun.  The right most lead to the output is a tap on the fifth-but-last turn of one of the trifiliar turns. You scratch the insulation off the top of the wire, and solder carefully, making sure the joint is not dry or weak. Tinning each wire before joining is often the best way.

With wind and rain forecast (again!), I quickly set off outside to disconnect the 4:1 and make some new eyelets to fit the chunky new 6mm outlets of my 6:1.  Somehow, it always ends-up being dark and windy when I work on antennas!

This is the system in place:

Here is how the SARK now sees the new arrangement, fully connected-up, and at the end of a couple of weeks of dry weather, where the ground was beginning to turn from moist to dry:

A 6:1 balun brings near-perfection at an impedance of nearly 50 Ohms real and very little imaginary, meaning pretty much all of the supplied power is radiated.  The SWR is pleasantly low across the 20m band.

And the following day, during a spell of rain, here is how the same antenna system changes:

6:1 attached to the delta, in the rain.

Rain, as one might expect, brings a slight shift in system characteristics.  In this case, the system works rather nicely, in that the best match frequency oscillates within the 20m band, according to the weather.

But the overall result is that the impedance of the system has been brought under very good control, lying around the 50 Ohm mark.

So I now seem to have achieved the best possible match for my 20m delta, where there is little further prospect - or indeed any need - for improvements.  What I do need to do is build a current, rather than a voltage 6:1, now the simpler voltage unit has proven the concept.

UPDATE:

Having now received a fine 6:1 current balun from Geoff Brown, 'G-Whip', the system continues to function as expected, although Geoff's balun yields a slightly higher resonant point than with my voltage 6:1, so I'm sticking with my own unit for now.  After adding about 50cm of wire to the base of the delta, which was known to be slightly short, the antenna is now singing right in the middle of, and shows a very good SWR across, the whole 20m band.  Here's how things now stand:

You can't get better than this with a wire antenna across a pretty wide band!

Other than explaining the slight difference between the current and voltage baluns, this really does now signify the end of my years-long work to reach perfection with my delta loop.  If only I'd bought an antenna analyser all those years ago!

Remember that your environment is probably very different from mine, and you may of course be using coax, not twin.  Your twin may also be a different length, and terminate at a different impedance.  So a 6:1 may need to be replaced with a 4:1, or even a 2:1.  If you have a go at making some baluns, which are not difficult if you can find a schematic online, then you can try different values without spending much money.

Remember, also, that with twin wire feed, you can multiband the antenna quite readily and, unlike with coax, with extremely low losses, even at high SWR.  My efforts above are simply because I use the delta for its fundamental frequency of 14MHz and the first harmonic of 28MHz, where the match is equally good.