80M (3.7MHz) Loop Antenna
8/9/14, UPDATE: I took and passed the general class today. Now the learning (leaning) curve begins……
One area of vacuum tube technology that I have not (yet) explored is (amateur) radio. I have a technician license and have been putting off doing anything more because I found the antenna (and ham jargon) issue daunting. One thing is that I want the feedpoint at the house, not in the middle of the yard! This makes a classic dipole less attractive given my house and yard. A friend (ham mentors are called Elmers) suggested a loop and sent me an excellent article describing the setup and performance of a loop antenna. Because it is a loop, the feedpoint can be anywhere so I have mine where I want it! The loop length is made equal to the wavelength of the lowest frequency of interest, in this case 3.7MHz giving 272ft. The larger the area enclosed by the loop the better, so the ideal shape is a circle. Mine is a pentagon because that gives me the best compromise given the wire length and the trees I have. The feed is RG8X 50Ω coax. I now need to learn how to use it!
I have a Heathkit SB-102 transceiver that I have repaired and checked out into a dummy load. It has a solid state Variable Frequency Oscillator (VFO) that is supposed to be significantly more stable than a tubed VFO. Using a MFJ Deluxe Versa Tuner II with the Heathkit, I obtained Standing Wave Ratio (SWR) figures of 80M, 1 to 1.1; 40M 1.2 to 1.3, 20M, 1 to 1.1 and 10M 2.5 at best. I need to upgrade my license so that I can use voice (SSB) on bands other than 10M! Some people use a 2:1 balun with a loop and I do wonder if this would yield better performance at 10M? I am a beginner….
By the way, the J in MFJ may stand for junk. The vanes of the variable condensers in my unit were rubbing due to massive slop in the spindle bushings that allowed the vane assemblies to flop around. I bought the unit on ebay however, it shows no signs of use much less abuse, so I think this issue is due to rubbish quality parts. I was able to slip plastic strips into the spindle bushings and remedy the problem. Even so, I would not use it with solid state transmitter finals.
The wire is 14ga stranded and insulated house wire. I placed loops of rope around the trees / branches that looked to be good locations for suspension. The loops are fitted with pulleys. Each suspension point on the wire is fed through an insulator tied to another rope (1/4in Daycron) that passes through the pulley down to ground level. In this way, the suspension points can be dropped down to ground level when the wire snaps (it will for sure). The length combined with the elasticity of the suspension ropes provides some accommodation for tree sway however, the mid-point suspension has a dead weight load consisting of two house bricks. (The dead weight tension method is used for the suspension of railway power catenaries.) At the lowest point, the chimney it is 24′ high and at the highest, around 40′. I hope this arrangement will accommodate most wind activity! The feed point is tied to the chimney of my house (which seems a better scheme than tying it to my neighbours house). It consists of a small plastic Radio Shack project box fitted with an eye bolt for the anchor. The ends of the antenna come in, then out and back in again through small holes providing sufficient grip to prevent them pulling out. The feed cable comes in through the bottom of the box and a tie wrap inside prevents it pulling out. All the entry points are sealed with silicon goop.
Oh, I had great fun playing with a potato gun to launch 3 of the suspension points……….