Summary - A delta loop antenna is described which is cheap, simple to make as well as being very quick and straight forward to put up for portable work. With a balanced ATU it will work well on 20, 15 and 10m.
I've been really very surpised how well this antenna works even when, at my limited QTH I can only have it a few feet off the ground. This simple design for a versatile HF loop comes from the pages of the Radio Communication Handbook. Its a classic full sized Delta loop for 20m (i.e. about 20m in circumference) which can be loaded up via a balanced ATU. Best results are for 20, 15 and 10m band but (with an ATU) its worth exploring other bands with this simple antenna.
The loop is composed of a aluminium top section of length 6-8m (ca. 18-24') with wire sides 6m (ca. 20' each) in length. I used 12m (40') of 300 ohm slotted ribbon feeder with the last 6m (20') cut down the middle and each part used as the wire side arms of the loop (going up to the sides of the top section).
I used a section of Tufnell to support the top section. This not only has excellent electrical insulation properties but is a extremely strong building material. Varnished wood would do just as well.
Almost any metal sections can be used for the top section of the antenna. Ideally concentric Al tubing could be used but can be expensive and well fitting concentric tubing does not seem to be readily available in the UK. I used 2m lengths of right angle Al strip (as used to finish off the edges of kitchen surfaces etc). 3 strips were fixed together to make up a 5 1/2 m (roughly 18') top section. Butterfly nuts and bolts means the whole thing comes apart easy in the field (that is when you want it to!).
Bolt and butterfly nuts can also be used to attach the wires to the ends of the top section but for speed of assembly I replaced them by banana plugs/sockets. The side arms taper down seamlessly to the feedpoint (the start of the 300 ohm feeder proper). I used a short 2' section of 1 1/2" plastic pipe with which to hold the feed point securely and away from the mast.
I used an 8m Al mast with the top section of the loop fixed at the top of the mast. Alternatively you can use guy ropes from the ends of the top section and attach to trees or other high objects. I fibre mast might be better but I have not yet tried one. The feed section attaches about 5m below and in my limited space is feed only about 2-3m above ground level. The feeder then goes to a balanced ATU, SWR meter and then to the radio.
RESULTS
The loop loads up on all bands from 80m to 10m (with an ATU of course) On first trying the loop on 20m with about 40 Watts I immediately got a call from a Russian near Moscow, 59 both ways. I have worked all round Europe using 5W from an FT817. The summer sporadic-E on 10m has been a joy with this antenna. Best DX so far has been Indonesia. I have even used the loop on 2m with a suitable homebrew atu (its roughly 10 wavelength around the loop) it seems to work suprisingly well - although as expected the radiation pattern is 'strange'. I am having a lot of fun experimenting to see what this loop will do on the HF bands!
A MKI version
Note: I built an early version of the loop consisted of two telescopic side sections (made from two scrap cb antennas) with a wire top-section (the opposite of the version described here). This had the advantage that the loop was mounted at the base so much greater heights could easily be achieved with a simple mast. Although this worked well the whole thing was very heavy to manipulate; the wind loading at an exposed location was considerable and I was never able to get the mast up to the full height. The current version is very much lighter, simpler and also cheaper.
Other comments
The right angle Al is not a great material for the antenna (I used it simply because I had a few length in the garage so the antenna cost me next to nothing to make). I found that I needed to double up the central section for strength. You also need to be careful while raising the antenna so that the Al sections do not bend due to the leverage on the end sections by the wires.
References
1) Radio Communication Handbook, Sixth Edition, 1994, Section 12.71
