40 Meter 2 Element Delta Loop
"Wire Beam Construction"
By Chuck Stigberg K4GE
How would you like to build a full sized wire beam that is competitive with 2 or 3 element yagis? How would you like to have instantaneous switching - no rotor necessary? Best of all, how would you like it to cost under $200.00?
I've always wanted something better than a dipole on 40 meters. I had tried phased verticals, phased groundplanes, and I even had a 4 Square Vertical array up at one time. The 4 Square worked pretty darned good, but it didn't seem to have the gain of a yagi on the high angle signals.
I had tried full sized loops including changing the feedpoint at various methods to enhance the vertical or high angle, feeding on the side, bottom, top, etc. However, something always seemed to be missing.
The Idea . . .
Well . . .I had read an article in the ARRL Antenna book several times by Dave Pietrowski, K1WA . The article was about 5 Slopers suspended from a single tower. He would transmit on one sloper and the others acted as parasitic reflectors. He had an "ungrounded" coax switch mounted inside of his tower. He achieved forward gain and about 20 db front to back ratio with this system.
The way it worked was this . . .each element/sloping dipole had a 3/8 wavelength feedline. The feedline length by virtue of being 3/8 wavelength and "open circuited" (ungrounded) at the coax switch added inductance making the element appear 5% longer. In other words - you transmit on one sloper and the others looked like reflectors.
It occurred to me that maybe this would work with a pair of loops also. I'm blessed to have a lot of tall pine trees in my yard. I got out my slingshot and managed to put up another Delta Loop the exact same size 20 feet from the other one. I fed each element with a 3/8 wavelength feedline. You have to take into account the "velocity factor" of the coax when figuring out the proper length. I used foam RG-8x with a velocity factor of .81. The coax switch I used is the Ameritron RCS-8V. It has 5 antenna positions you can select from. Best of all? The RCS-8V comes from the factory in the "ungrounded" configuration. You can go into it and solder it to ground the unused positions - but you don't want to or this antenna won't work..
Feed both loops direct with the RG-8x 3/8 wavelength feedlines. There is no need for a 1/4 matching transformer. I have an SWR of about 1.2:1 at resonance. The loops are fairly broadbanded and the SWR never gets above 2.4 to 1 across the band. Mine are both cut for about 7.150 Mhz, but they ended up resonating around 7.225 Mhz.
My Delta Loops look like "upside down" triangles and I feed them at the bottom point. This would predominately favor horizontal polarization, but it still works wonderful for the long haul stuff.
All I can say is "wow." I still can't believe the signal reports that I receive. Although I don't have a field strength meter - on the air reports indicate this antenna has about +5 db forward gain or better over a dipole and the front to back ratio is in the range of 10-15 db at resonance. Sometimes the f/b ratio is 25-30 db, but f/b is determined on the angle of signal arrival and varies throughout the day when the band changes. I don't see any f/b on signals arriving from the side, but with a reference dipole it is obvious there is a reduction of both transmitted and received signals from the sides.
While QRO I typically receive +20 db over S-9 from VK/ZL and the same from Europeans. My Deltas favor NE x SW . . .so when I'm pointed SW I get a lot of rejection of the European broadcast while increasing the signals in that direction. Signals often literally "pop out of the noise" when I switch directions with the array. Also - when competing with folks who have yagis I often beat them. You also have to consider that this array's average height into the reports. My Deltas are 60' at the top and about 20 feet at the bottom (feedpoint). That is an average of only about 40 feet. Rarely do I have to call twice, and in the pileups I don't have any problem getting through.
1. LOOP - the formula to calculate the length of the full wavelength loop is:
2. FEEDLINES - I calculated my feedlines unscientifically, and figured it out this way:
a. If 234 = 1/4 wave then 4 x 234 = 936
b. L = 936/f
c. L/8= 1/8 wavelength
d. L x .81 (velocity factor) x 3 = 3/8 wavelength feedline
Now - yes I know . . .there are several ways you might go about doing this and come out with different numbers. There is test equipment that would certainly help you to be precise - especially when it comes to making both the feedlines the same exact length electrically. However - in the end - I found my feedlines seemed to obtain the best f/b ratio after trimming them somewhat. Mine ended up being less than the length calculated. Perhaps this was due to the spacing, imperfect symmetry, proximity to ground, or something. Experimentation may benefit you more and yield better gain or front to back.. You will also see changes in f/b and your SWR when you change the spacing. Experiment!
I've never had a better antenna for 40 meters than this one. There are lots of possibilities for you to utilize the same method. You might consider simply using a pair of dipoles? A pair of slopers? Feeding the Deltas for vertical polarization? Hanging them like righteous triangles rather than inverted? If your shack is right underneath the Deltas - you could run the feedlines inside the shack and use a cheap ungrounded 2 position coax switch rather than purchase the Ameritron. Radio Shack sells these for a few bucks.
I really enjoy the signal reports I receive and "hearing" the difference and "seeing" the difference on the S-Meter using "instantaneous switching."
Good luck & 73's!