My mainantenna for general use on 20m is my homebrew delta loop. I was introduced to this antenna by M0DOL who I saw using one at the KDARS site over a year ago. After seeing the results he was having I decided that I would give it a go and see if I could get similar results.
The Mk1. loop was my first foray into serious antenna building. I had thrown up dipoles and long wires in the past but never a specific design. I decided to make this as low a cost as I could but still keep it symettrical. I had a lot of Lexan in my stock of 'useful bits to get in the wifes way' and some cable ties but had nothing that would do for the wire supports, so a quick look at ebay got bme some 8m long fibreglass fishing poles for £16 gbp each. I had some wire laying around and after doing the maths for the length I cut a bit of wire 21m long and fed it up the inside of one pole and down again through the other. I cut a bit of 6mm thick lexan into a sort of delta shape and drilled holes for the cable ties to support the spreaders. I also drilled holes for the support brackets to fit a standard 2 inch U Clamp.
I then faced a dilemma. Do I feed the antenna with a balun or do I feed it via a quarter wave stub to acheive resonance? I decided on using the 75 ohm stub method and after soldering it to the wire loop it all worked well. I then decided to mount it on my wall mounted mast at 30 feet above ground and the reports were great! I was convinced that this was the antenna for me.!
Disaster stuck a few days later. We had a bit of a week of high winds and during that time the sections of the fishing rods telescoped back into each other, making the antenna useless! Undeterred, I decided to try again..Enter Mk2.
Delta loop Mk2 is really not much different. I decided to change the way the supports for the spreaders was done, instead of cable ties I went to smaller U Clamps. I also decided to change the feed method. Although the stub method worked well on 20m, it didn't lend itself well to multiband use. I replaced the stub with a homebrew 4:1 balun, with the bits salvaged from an old Radioworks balun that had melted due to RF heating (It had actually caught fire whilst on the mast!). Strictly speaking, the balun does not provide a good match to 50 ohms as the impedance of the loop is around 120 ohms at the feed point but the mismatch at the radio end can be dealt with by my atu. The last modification was to put insulation tape over each section to stop it telescoping back down!!
With the antenna in this form I worked a fair ammount of DX, even though I decided to put it on my floor mounted mast, a 20 foot long pole (6.5m) half way down my garden rather than on the wall mast at 30 feet. I figured that it was easier to repair on the garden mast should it need it.....
Another day of heavy wind and it did need a repair.... The fibreglass bottom section actually collapsed under the flexing stress, leaving one spreader laying ofer my neighbours fence with the other being pulled over by the weight of the fishing pole and wire.....
A phone call to M0DOL who manufactures deltas and other antennas provided the solution. I purchased two aluminium spreaders which came complete and all I had t do was fit them in place of my fibreglass poles. I did have to drill a pair of holes in each at the bottom to attach the balun but it only took half an hour to have the damaged one down, repaired and back in the air.
Since then, no problems at all (famous last words) and even more DX in the log. It also works a bit better on the higher bands due to the larger diameter of conductors and will tune well with my ATU from 20m (where the vswr is 1:1.5) right up to 6m with no obvious problems. The balun hasn't caught fire, even at legal limit of 400w, and the wind hasn't claimed another victory. Yet.
The Photo to the left shows the feedpoint of Mk2. This is just the homebrew balun along with a simple RF Choke 'Ugly Balun' to isolate any RF from the feedline
