6m Lightweight Delta Loop for Sporadic E contacts

This week I put together a simple delta loop antenna for 6m. This is a simple, cheap, lightweight horizontally polarised somewhat-directional antenna. Fantastic for the sporadic E season which is going on right now – especially in a confined space where a beam isn’t practical.

It looks like this:

I made it out of 6m of hookup wire, some 75 ohm coax, 2m of plastic pipe, and a PL259 connector. All of which I found in the garage.

It is hung on a SOTABEAMS fibreglass pole, about 6m above ground level in my case. No guys, just strapped to the washing line with velcro.

Almost immediately after plugging it in, I decoded PV8DX in Brazil on FT8. That was exciting! Even if it was a fluke. Since then I have worked Italy on SSB, and Malta, Estonia, Spain, Italy, Sardinia on FT8. Right now I’m +8dB into Sardinia, so this thing gets out okay!

A quick test on a web SDR shows a 9dB difference between broadside and end-on, so it’s reasonably directional.

A word on matching. A delta loop has a nominal impedence of 100 ohms. To match to 50 ohm coax, this design uses a simple piece of 75 ohm coax as an impedence transformer. This works by taking advantage of the mismatch between 50 and 75 ohm coax. For 6m, a 99cm length is necessary. No other matching or choking seems to be required for this antenna, so it should be fairly low-loss.

The construction details:

  1. Cut 105cm of 75 ohm coax. Strip 3cm off each end, leaving 99cm intact.
  2. Cut 2x 3.1m lengths of wire. I used RS 356-713, obsolete, but any will do.
  3. Join one length of wire to the shield of the coax, join the other length of wire to the centre conductor of the coax.
  4. Drill a hole just wide enough for the wire at each end of the plastic pipe, through both walls. My two holes were 198.5cm apart from each other.
  5. Feed each piece of wire through one of the drilled holes. Bring together, strip and twist together the two pieces of wire at the midpoint of the pipe.
  6. Put a connector on the free end of the 99cm bit of coax.
  7. Drill a hole through the plastic pipe around 8-10mm, enough to push the second-to-top section of the SOTABEAMS mast a reasonable way through it.
  8. Connect your antenna to your coax, raise the mast, tune for SWR by chopping out short lengths of wire until the SWR is low at the desired centre frequency. I found perfect resonance (50 ohm resistive, 0 ohm reactive, 1.0:1 SWR) at 50.313MHz, the 6m FT8 frequency, with 598.5cm wire.  This bit is much easier with an antenna analyser but can be done with a rig set to lowest power and an SWR meter.
  9. Finalise and seal the connection between the two pieces of wire.

Finding the perfect length of wire for resonance will depend on the accuracy and quality of the 75 ohm section, and on the velocity factor of the antenna wire, which can vary. The maths predicts that resonance at 50.313MHz should be 5.963m of wire, so 5.985m (22mm difference) is damn close.

I quite fancy making 2m and 4m versions of this. The following table shows scaled measurements for 2m and 4m, based on this successful implementation for 6m.

Band   Centre freq   Wire length   Coax length   Boom length
6m      50.313       5985mm        990mm         1985mm
4m      70.200       4290mm        710mm         1423mm
2m     144.300       2087mm        345mm          692mm

Here’s where I’ve been heard over a few hours this evening:

and these are the stations we’ve heard this evening:

If you make one of these, let me know how you get on!

4 thoughts on “6m Lightweight Delta Loop for Sporadic E contacts”

  1. Hi Tom,

    I just put one of these together today, from a slightly different design found on the web – the only real difference is that the 75 Ohm stub is coiled around a 150 mm piece of 40 mm plastic waste pipe, so that it acts as an RF choke as well as a matching stub… it slides nicely over the SOTA pole I am using nicely and stops it swinging around.

    Interestingly the calculations I used give a slightly different length for the loop – I was given the formula L=306.3/F (MHz) – I wanted to centre my loop at 50.750 so that I can use for both FM and DX CW/SSB and this worked out at 6040 mm – the matching stub calculated out the same as yours, at 990 mm, assuming the velocity factor of the 75 Ohm coax to be 0.66 (It was an off-cut of some fairly nasty TV coax, so I’m really not sure!!).

    I cobbled it together temporarily using 2 mm bell wire and found the loop length to be about perfect, using my antenna analyser, although the SWR came in at 1 : 1.14 with about 2 Ohms reactive – I’m hoping that decent stranded antenna wire and some pruning will get me to the same level of perfection as yours.

    Pity 6m never opened today – hopefully testing “in live” tomorrow!!

    Kind Regards

    Nick

    1. Hi Nick!
      Interesting idea to coil up the matching stub like that. Might give that a go!
      The SWR you have achieved is as good as you need. Fingers crossed for an opening for you.
      One thought- in this configuration – bottom fed – the polarisation is horizontal. Thus you might find most FM traffic is 25+dB down on what it should be. As I remember, rotate the feed point around to one of the sides of the delta and it becomes vertical. Great for FM. Can’t remember where I saw this, and haven’t tried it, but let me know if you do!
      Cheers
      Tom

  2. Hi Tom,

    I’ve designed the loop so that I can move the feed-point at will from top left corner, through the bottom of the apex to top right, so I can change the polarisation. Since I am really only interested in DX portable, where the polarisation can be skewed a lot just through the E-layer reflections (and it is even more pronounced when we get F2 layer in the winter) I’m not too concerned.

    This is for me to use portable, when we are away with our caravan, or if I feel the need to take my car out to a high point, rather than using the ATAS-120-A, which has served me well in getting me to 208 DXCC entries in my log, but I came across the design quite by accident and it was a fun couple of hours yesterday knocking it together – just waiting for some decent 75 Ohm Co-ax and coated woven antenna wire from Fleabay – then I will re-make it “properly”!!

    Regards

    Nick – G7EQM / 3B8FG

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