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Author Topic: Loop antenna math help  (Read 3914 times)

Posts: 53

« Reply #15 on: December 09, 2005, 07:11:55 PM »

You can also build a half square antenna. I've made mine with telescoping tubing mounted on 3 foot tripods. To change bands I have different length wires cut for each band 20 through 10 meters. I adjust the length of the tubing and put on the appropriate length top wire for the band I want to operate on. Being on the tripods I just tilt it over to change length. I also can rotate it around to cover different directions.
You can feed it at the top corner for a decent match to coax or as I have done feed it at the bottom of one end. I have a T match tuner built on one of the tripods that I tune for each band. Not the most convenient but they work better than my trap vertical which has plenty of radials in most circumstances. They are vertically polarized and have a low radiation angle even at ground level.

Posts: 1

« Reply #16 on: December 21, 2005, 05:47:30 PM »

RE: Loop antenna math help  Reply  
by KC0RDG on December 7, 2005  Mail this to a friend!  
Goes to show you want I know. I was going to do something like this picture. I have a dormer and I was going to put up a pole and use the house as a brace and then attach the wire to the pole, the house and then at ground level.

Got any other good ideas for a low noise DX antenna that you can install on a city lot? Power lines run on the other side of my house both to the house and the a light on the street. I have 1 tree in my backyard, it's maybe 40 feet tall. I have no backyard to put down radials.  

You sound as if you want to make a square loop (It's difficult to make perfectly round loops because of all the supports needed, etc.)  If that is the case, setup four 40 foot push-up poles in an area approximately 140 feet square.  Inside that square, you can consider making the antenna length like you stated as an estimate 1005/frequency.  Remember that you must consider the frequencies you want the loop resonant on to begin with (160 meters?).  Then, you can find the harmonics easy enough in the ARRL Antenna Book.  I think I have seen that for several different frequencies but I don't have it in front of me.  

Now, when you put your loop up, leave about two-four feet of wire extra at each insulator.  You will use this to tune.  If you wrap the extra wire around the terminated wire before the insulator, this should shorten the wire length and change the resonant frequency.  You'll be wrappging the line instead of cutrting it off so you can go back a turn or two if needed.

Make sure to figure the length for the lowest frequency you will operate (For instance 1.800)  This will make the loop a little larger, but antennas work best at resonant frequency and higher.  

If you are not expecting to be used as a vertical, then why would you use the radials?  If you are going to do this, I suggest that you leave the feedline ladder line as vertical as possible and at LEAST 40 feet high.  Solder the ends of the ladder line together and feed that to the center wire of your coax or balun.  Then, punch a ground rod in the groun below the vertical horizontal line and feed the coax shield to it.  Next, place as many quarter wave radials as possible beneath this type of antenna.  120 would be the figure for commercial broadcast stations.  

What you are doing here is using the loop as a large capacitance hat.  

A friend four miles from me has a full 1/4 wave vertical with 90 1/4 wave radials and it sounds awesome.  Still, it isn't perfectly efficient.

You are going to have to get that loop as far off the ground as possible and if not a 1/4 wave high, at least 40 feet.  It will not be as efficient as you probably like, but 160 requires a lot of space and height.  

The other thing you can do as look into using an antenna shortener such as sold by Spiro Manufacturing.  You can find them online.  Please don't ask me to recommend that because I haven't tried it and I don't know the efficiency, etc.  One thing you might want to know too, is how many watts can you burn off on that antenna.

Another loop that will be a resonant a little below 1.800 MHZ is a tri-loop.  Create a Triangle of 186 feet on each side of the triangle.  It works great and has great gain on 40 meters.

Well, just my two cents worth.  I know you'll hear a myriad of different information.  

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