Delta Loop Matching

As I've often written, my proper DX career on radio began one, wet Christmas week morning, when I decided my long wire really wasn't good enough any more.

I recycled the LW into a 15m delta loop, and never looked back.

I have three delta loops now, having moved away from the usual desire at the beginning of operating that one antenna must do the whole damned lot!

Now, if you connect-up a delta, using twin wire and a 4:1 balun, then a delta will match-up on several bands.  However, its radiation pattern gets progressively higher as you move up the bands, and it really isn't much of a performer above its cut band.  This, of course, does not equate to not being able to make QSOs.  Far from it.

The problem with the 4:1 arrangement on a monoband delta, though, is that the impedance doesn't work out properly.  You can spend many a happy hour looking at various opinions about what a delta loop's natural impedance is, and a figure of anything between 100 and 200 Ohms is often bandied around.

Now, for a very long time, I had to rely on software models and implied impedance by simply 'feeling' how the antenna was matching at the ATU.  Although the SWR can be brought down from something like 2:1 to a perfect match very readily, it's clear that the matching is a bit odd, in that there is little change in SWR from one end of the band to the other.  In other words, it's got impedance issues!

Just remember, though, as we move on in this post, that the delta loops in their present configuration of twin-feed wire and 4:1 current or voltage baluns, have worked entirely satisfactorily for several years, have never generated any RFI or other problems, and have brought in a couple of hundred DX entities.  They have only needed a very slight tweak by my simple external ATU to bring a perfect match, and only then for higher power use.

So, if you have a delta in this configuration and don't want to change that which is already working fine: don't!

But, for those seeking a bit more natural antenna performance, and newly armed with my trusty SARK-110 analyser, I set to work to examine just what is going on with the delta system I have.

I looked at the less-used 17m delta first, to avoid hassles with my mainstay, 20m delta for the time being.  If the 17m worked out OK, I could then apply the insights gained to the 20m loop.

Here's how the SARK saw the 17m delta when connected up to the 4:1 voltage balun:

17m delta loop attached to a 4:1 voltage balun.

That SWR is fairly high, at about 2.5-ish across the whole 17m band.  There is no sign of a resonant dip, and my intuition that the impedance was off is dramatically shown to be correct!  At the lower end of 17m the impedance is about 25-ish Ohms, with a fair bit of imaginary component.

So this impedance result shows the delta loop doesn't have an impedance of 200 Ohms, but nearer 100 Ohms - the 4:1 is reducing 100 Ohms to 25 Ohms, resulting in the ca. 2:1 SWR.

Modelling using MMANA-GAL yields an impedance for the delta of about 130-160 Ohms, and this appears to be reasonably correct.

Now, having switched out the 4:1 and switched in a 2:1 balun, which ought to cut somewhere around 100 Ohms to something near 50 Ohms. This is how the SARK reports:

17m delta on a 2:1 voltage balun.

This now looks more like the expected curve to resonance!  The impedance is floating around the 50 Ohms mark, and the SWR curve dips beautifully to a perfect match - albeit in need of some minor shortening on the basis of this curve.

Once shortened both curves should look almost perfect.  That will have to wait for a sunnier, drier and less windy day!

Just remember that a 4:1 balun remains your best option if you want to multiband your delta loop, because it will generally allow you to match-up the antenna across more bands.

Update 1.

I tried the 2:1 balun on my 20m loop.  This didn't work!  The SWR climbed to 3:1 or so. The answer to this problem is addressed in a new post, which you can find here.