First, determine the lowest desired frequency of operation. Typically this is either the 160 meter, 80 meter, or 40 meter band. Once the lowest operating frequency is decided, use the standard 1/2 wavelength dipole formula to determine the overall length:
It should be noted, however, that length is not critical here. So long as the overall length is > 0.4λ the antenna will work well.
In addition it is important to keep the ladderline feed to odd multiples of 1/8λ. Doing so will prevent an easier match for your antenna tuner. This may be calculated by:
Where the velocity factor may be obtained from the specs of your chosen feedline. 0.88 is a typical value for 300Ω ladder line.
Suppose we wish to construct a doublet and our lowest frequency we wish to operate is 3.5 MHz.
The overall length of the antenna is:
And rounding the length up to 135′ may be more convenient for measuring purposes.
The feedline should be an odd multiple of 1/8λ at the lowest frequency. So if our feedline has a velocity factor 0.88, the 1/8λ is therefore:
So if the required feedline distance is 75′, we should use 92.7′ of feedline since the 75′ distance from antenna feedpoint to the tuner is not an odd multiple of 30.9′. The excess ladderline may be “zig-zagged” or run in a non-direct route in order to use up the extra length. But under no circumstances should the ladderline be coiled or left to lie on the ground. Also, it is important to keep the feedline at least 6″ or more from nearby metallic objects.
So in this example, a doublet antenna that is operable on every band from 80 meters – 10 meters with a feed point located 75′ away from the radio, we need 135′ of wire and 92′-8″ of ladderline.
But before building the antenna, we must have a way to connect the feedline to the wire and hang the wire between a convenient set of trees.
Center and End Insulators
Electrically connecting the antenna wire to the feedline is simply a matter of soldering the wires to each side of the feedline. However, we must have a way to remove strain from the soldered connection or the antenna will fail mechanically in a very short period of time. You can either consruct a simple center insulator from plexiglass or PVC or purchase a specially made center insulator just for this purpose. Many choose to use the “Ladder-Loc” by WA1FFL which is available at many dealers.
In addition to the center insulator, means of connecting a rope to the ends of the antenna for support is required. Again, this device may be constructed out of PVC or a purpose specific part may be purchased. These items are also available from nearly every dealer. DX Engineering also offers a kit that includes both the center and end insulators.
Once the antenna is built, we will need some way to feed it into our radio room and into the antenna tuner. While antenna tuners exist that can accommodate ladder line, most do not. Plus, feeding ladderline into a house is a more difficult than using coax. Thus a balun (short for BALanced UNbalenced) is useful. Simply connect the ladderline to one side of the balun and coax to the other side of the balun and connect to your tuner. For this antenna, a 1:1 current balun is recommnded in order to use coax as a means of ingress to your ham shack.
Note that you will want to use the highest quality coax possible. LMR-400 or RG-213 at minimum. Smaller diameters such as RG-8X, RG-58 and similar will simply have too much loss owing to the very high SWR present. The advantage of the ladder fed doublet is that ladderline has extremely low loss even in the presence of very high SWR. The same cannot be said of coaxial cable. Like the insulators mentioned above, baluns and coax are available at nearly every dealer.
Despite the rather verbose explanation above, a ladderline doublet is really quite simple. Using our example above, constructing a single antenna usable on every amateur HF frequency from 80 meters and higher consists of only:
- 135′ of antenna wire
- 92′-8″ of ladderline (or whatever amount you need keeping in mind the 1/8λ caveat
- Center Insulator and two end insulators
- Support rope
Where can you get all this stuff? Dealers abound and while CARC doesn’t endorse any specific dealer, here are a few links to get you started: