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Solar-Terrestrial Data

Single element Delta Loop

Building a simple delta loop is very easy and it doesn't require science fiction. With two fishing rods and some other material you can find in your local hardware store, you can build an efficient single element delta loop. It is convenient especially in portable operations but I built and used one for about two years at my home location without problems. Ok, let's start.

First of all, we have to calculate the size of the loop. Wavelenght in meters at a given frequency is obtained by the formula 300/F (MHz). Cut  the same size of electrical wire used in domestic installation (section from 1,5 to 2,5 mm2 ). This will be the radiator. The loop is essentially a triangle, so divide the wavelenght by three to have the size of one side. Now you have to buy two fishing rods in your local fishing shop, one meter longer than the size of the loop side (the last element of the fishing rod is not used, too thin).

PAY ATTENTION!

FISHING RODS MUST BE IN FIBERGLASS. DO NOT USE CARBON FIBER!

To build the antenna support, I used the legs of modular shelves you can find in your local hardware store.

Usually  they are one meter long and have an "L" profile. Make a cut in the center of one side of the "L" and fold it to  form an angle of about 60-70° (see picture below).

Close the triangle with another profile and the antenna support is finished. To fix the support to the mast, you ca use any available method. The above picture gives an idea.

Now we have to fix the fishing rods to the antenna support. Remember to remove the last element of the fishing rod becouse It is too thin and fragile. I used metal clamps to secure the rods to the support but any other method can be used.

With the fishing rods still closed, insert the electrical wire (the radiator) into them (inside the smallest element) and make a node at the base to prevent slips. Now extend the elements and put in traction the radiator.

The antenna is phisically ready but we have to do the last thing: the feed line and the matching system. The impedance of a single loop is about 120-140 Ohm so it's not possible to feed the loop with the standard 50 Ohm coax line. To match these different impedances, we will use a coaxial stub made with 1/4 wavelenght 75 Ohm coax line (RG59). To calculate the lenght of the stub we use the following formula:

Stub lenght in meters = ((300/F) /4)*0.66

where

300 is the light speed;

F is the frequency in MHz;

0.66 is the velocity factor of the RG59 coax cable.

If you intend to use satellite tv cable, the velocity factor is variable from 0.85 to 0.88. Refer to their own data sheet. The next image show how to connect the feed line to the antenna and to the transceiver. If you wind the stub on a diameter of about 10cm, you will have an rf choke usefull to prevent rf feedback.

Enjoy with your new delta loop antenna.


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