THE ALL BAND HF DOUBLET
This antenna project will get you up and running with an all band Hf antenna using one of the oldest and least expensive ham antennas around.....the all band doublet. If you've got some TV twinlead or ladder line laying around and an antenna tuner, some wire, insulators and a bit of time then read on.......Project also includes a novel way of getting the rf to and from the shack using coax rather than ladder line.
ALL BAND DOUBLET DIAGRAM
Shown using 450 ohm Ladder line
Shown with standard TV type 300 ohm ribbon line
Details and construction
The all band doublet antenna is nothing more than a 1/2 wave dipole cut for your lowest operating frequency and fed with twinlead, ladder line, open wire, etc to a tuner that will accept a balanced line connection.
IT IS NOT FED WITH COAX!
It can be designed for use from 160 thru 10 meters very easily using the standard 1/2 wave dipole formula:
468/freqmhz = total length (ft) end to end
The exact length is not critical!
If you don't have room for the 160 or 80 meter version...then design it for 40 meters and up! Just remember, don't operate it on a lower frequency than it was designed for...tuner damage may result! You can always tie the two ends together at the tuner and use it as a random wire antenna with the tuner and it may tune lower bands than it was designed for! Always check swr at low power before attempting.
It can be installed in the horizontal fashion or inverted V style. Get it up as high as possible and have fun!
Remember when working with twinlead (Flat TV feed type) don't use over about 100 watts of power to be safe. For higher power, use the heavier, ladder, open or window type.Construction:
After you have determined the total length of the horizontal section of the antenna, lay that amount of your antenna wire out and cut it in exactly in half. This will give you two identicallengths for each half of the antenna. It is suggested that you use #14 or #12 gauge wire. You can use smaller size wire but it will tend to break easier with longer antennas due to weight of ice, snow, birds, wind loads, etc.
The Center insulator/strain relief:
Attach a center insulator between the two lengths of antenna wire. This center insulator can also provide strain relief for the twinlead, ladder line, antenna wire etc. Leave enough bare wire from each half of antenna wire exposed for soldering to the feedline and for strain relief at the center insulator. See example drawing below:
Using the drawing above as one example, the center insulator can be made from any non-conductive material such as sealed wood or Plexiglass, standard commercial insulators, etc. Use your imagination and ham engineering. It should be of a size that will allow the antenna wires to be attached to it from each half of the antenna with strain relief for each wire including the feedline. Your feedline also needs strain relief. It can be provided by using nylon ties going thru the center insulator (drill holes) and tightened on the other side so as to press the feed line against the center insulator. In the drawing above, they are the heavy black lines going across the twinlead. If you use TV type twinlead, this will be a must. TV twinlead is very fragile and can break easily from too much strain. The weakest point on the twinlead is where the conductors come out of it on the ends. The wires are very small inside and break easily. Each half of the antenna can go thru holes drilled into the center insulator....use at least two holes on each side of the center insulator as in drawing...make certain there are no sharp angles on the edges of the holes to cut the wire. Thread each side of the antenna wire into the first hole near the side of the insulator and out the back....then back thru the other hole leaving enough wire to work with in soldering to the feedline. This type of arrangement provides some strain relief for the antenna wires using the mechanical pressure of the wire against the center insulator. It is important that there are no sharp edges where the wire enters or exits the holes. Use whatever method that works best for you.
The two bare wires from each half of the antenna are attached (soldered), one at a time to each side of the 2 conductor twinlead, ladder line, etc. (Meaning one side of antenna to one conductor of feedline and the other side of the antenna to the other conductor of the feedline.) Do not connect all together in the center!
You should end up with 2 continious conductors side by side with one continious conductor from the very end of one half of the antenna to the very end of your feedline at the tuner and the same thing with the other half of the antenna. Do yourself a big favor and do not get in a hurry and just twist the wires together at their junctions! They will soon corrode at the twist and create more problems for you than the time saved by not soldering them together! Believe me, it will take much more time in the long run to do it poorly than to do it properly with solder. You should provide some sort of weatherproof sealer to the solder joints after you are done soldering...and as a last resort...tape well and then tape again. If you "cut corners", sometimes a "temporary" installation tends to become permanent when forgotton about......then later it will remind you when it does not work!
Attach end insulators to both ends of the antenna. Attach the insulators to their supports with UV resistant rope, cord, etc and make sure you have enough to extend to the outside support tie off points. As a further note for those that are not experienced with wire antenna building, there are many ways to build center and end insulators. Do a search on Google.com using their "images" section for more ideas.
Now assuming that you have plenty of feedline to run from the final operating position up in the air for the antenna after raising it.......get help if needed....tie off the end supports.....run the feedline away from the antenna preferably at a 90 degree angle and keep the feedline several inches from any metal conductor such as rain gutters, down spouts, metal house siding, metal windows, etc.With very long antennas, the weight of the wire and feedline, center insulator etc, causes some sizes of wire to sag in the center. If this is the case with yours, some support in the center may be needed by attaching another support rope to the center insulator.
(Another option for the center insulator/strain relief would be to take the feedline and wrap it OVER a "dog bone" type (round), insulator and then back down parallel with and touching the feedline making sure you have a couple of inches left over for attachment of the bare wires from the feedline to each half of the antenna. Then use nylon ties to secure it tightly against the main feedline.
By wraping the feedline over the insulator and securing it to the feedline below the insulator, you will be adding a strain relief to help prevent the weight of the feedline from tearing apart the connections.)After your antenna is up and secure....attach the feedline to your tuner's balanced output connectors....and you're done!
Use your tuner as per mfg's instructions...have fun.Added notes of information:
There are many methods of "hanging" an antenna like this one and various center supports can be used, like towers, metal pushup poles (masts), etc. The use of a small cross arm made from heavy PVC or other insulated material extending out a couple of feet or more from the tower or metalic pole will help to prevent the feedline or radiating parts of the antenna from touching any metal and shorting out. This also helps to prevent the feedline from rubbing against anything in the wind and eventually coming apart at that point.
When bringing the feedline down from the antenna to the radio, always keep it away from sharp corners that can cut it due to rubbing in the wind.If you have a shingled roof, try to keep the feed line away from any edges that (from rubbing) will eventually cut into the feed line.
Remember to keep the feedline away from any metalic object by several inches.
Below is a very handy way of getting your feed line into your radio room by going thru a window, metal wall, etc.LADDER LINE OR TWINLEAD SHACK ENTRY OPTIONS
Note: in the options in the drawing, the 4:1 balun is NOT used if your tuner already has a balanced output. Only use it if your tuner does NOT have balanced line connections. In option #2, the balun would go between the feed line and the tuner if the tuner does not have a balanced output.