Thursday, December 13, 2007

SAP-A-LOOP

from WA0SAP --

Here is the design of the "SAP-A-LOOP." I think it is close to what Brother Dave did for Fred, only his is hung upright in a vertical diamond shape. Basically the same thing.

Using the standard formula for a loop: 1005 divided by the frequency, in my case 7.125, that comes out to be about 141 feet of wire for a full wavelength. Using three conductor Romex wire still in the jacket, I form the loop in a rectangle pattern 44 by 27 feet, stuffed along the eaves of the house in the attic. Solder all three wires together in hopes to make it broadbanded, and it does.

The solid 12 gauge wire in the Romex forms very easy by using a ten foot piece of 1x2 with a "V" shape cut on one end to push the wire to the eaves of the house. So you don't have to go swimming in all the insulation to make a perfect rectangle. It is fed broadside in the middle of the 44 foot section on one side.

The only thing that concerned me was to having enough distance between the broadsides of the antenna. It would act like an inverted vee with the wires to close to each other, thereby canceling out your signal. So at 27 feet spacing I felt good about that, thinking that the minimum should be 24 feet.

Technically, a quarterwave piece of 75 ohm coax should be used at the feed point and then 50 ohm coax after that.

I was busting through pile-ups on twenty meters this afternoon, knowing that the other guys were running lots of power, beams and towers. I just had to let them all know about my attic antenna just beat up your beam, HA, HA. The other good thing about the "SAP-A-LOOP" is it has about 2 to 3 dB of gain, not bad for an attic antenna. Plus you don't have to worry about the ice, snow, and lightning.

2 comments:

Anonymous said...

Sap,
I have a home brew attic antenna. Drop me a note im good in the book and ill describe it to ya.

Warren
AB0HX

wa5mlf said...

For a detailed description, with photos, of an outdoor loop antenna, visit http://www.donkeith.com/skywire.htm