This is constructed from the stiffest wire I could find and still cut. The brown string is just for physical support, to try to keep the geometry stable. The junk in the middle corner is a matching element. The coax soldered onto the middle of the matching element is a 1/4 wl RG62 (93 ohms). (ideal would be 86 = sqrt(150*50)) I used a plastic piece from a floppy disk as a support for the matching element, which is shorted near the inside pair of holes.
This is not a perfect match -- it could be better, but it was hard enough to solder it as it is, let alone retune it and resolder the coax again. (Close enough!!) Improvements would probably require recutting and replacing the RG62, and moving the attachment point and short a bit more in.
I've played with this antenna enough to know it works fairly well. It has sharp nulls in the plane of the loop, as theory predicts. I made this so Mattk can use his mobile radio in his office; a dipole did not work well surrounded by filing cabinets. Hopefully, this can be hung close to the ceiling, well clear of surrounding metal objects. I've found that loops typically do better in small spaces than dipoles. I was able to hit the repeater from my house on 5w. I expect it should be able to hit it from his office on 1w or less. It should receive even better than it transmits.
Vital statistics, as measured:
|dimensions:||~20" per side, ~29" tall, 0.128" dia wire, 0.22" dia insulation|
|label on wire:||"E23919 (UL) S 8 CU TYPE TW 600V OIL RESISTANT I RW/LE"|
|untuned impedance:||about 150 ohms (+-50)|
|impedance with rg62 and match:||43+22j = ~48ohm magnitude --> 1.6 SWR, ~0.07 db loss|
|with 13ft of RG58:||38+0j --> 1.3 SWR, ~1.1db loss total|
|2.0 swr bandwidth:||143mhz - 165mhz = 22mhz|
I think most loops are square, because people use floppy wire, and then use (two) spreaders to make it a loop. Since I'm using heavy wire for mechanical stability anyway, I figured, why not make the loop round?
The round loop has a very slightly higher gain (less than 0.3db higher), but the impedance is way different. Lots of experimentation has led me to the picture shown. Note the dimple in the top to hang it. The matching section is RG62.
This antenna unfortunately is extremely sensitive to the length of RG58 connected to it, and even sensitive to the way the RG62 is attached. There is probably a significant amount of common mode conduction in the matching section; however, this seems to still work quite well.