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RogerTango.com - Build a multiband doublet
Build a multiband doublet
Posted on Sunday, 2/18/2007

Just got my Kenwood TS-120S up, with the help of Extra Class Phil/W4VPI. I have a 40m dipole setup, with a 949E tuner... But, as expected... All Ill be able to work is 40m. Sigh, Id love to work more than one band. So, I do some extensive research. I find many plans for multiband antennas. Everything from fan dipoles, to trap dipoles to the renouned G5RV. I decide Im going to make a G5RV, so I get the materials I need and proceed in building one! I built it to specs for an 80m full size version... only to find out that silly me has not the yard to setup such an antenna. I already finished building it, but the legs are *way* too long... Oh well, I go ahead and position them anyway, its not anywhere near right, but I give it a whirl. Cant tune squat. So much for that. The next 2 days I search more. And in one article I read, a ham posted basicly ""just put up as much wire as you'r yard can handle and feed it with balanced line"". Well, that sounded too simple, and a big claim for less than $30 worth of supplies... when there a lots of antennas claiming to do multi band for $100 and up!

Well, what the heck! I already have the legs and the twinlead, all it would be is a simple matter to modify the current faux G5RV to a balanced fed doublet! The pictures will outline the process... Keep in mind Ive deleted some of the pictures that were not necessary to this write up... as I took them during the G5RV build. You will see the pieces I used for the pigtail, which was no longer used with the resulting antenna. #12 solid copper wire with black plastic shield and PVC couplings from Home Depot, 300ohm twinlead from Radio Shack. Total invested: About $30.




Time to start drilling holes, in the following pictures pay close attention to where I drilled.




I measured 1/2 inch back to drill 1/4 inch holes. The straight couplings are insulators for the legs, the "T" is for the apex.




One of two leg insulators finished.




Both the leg insulators and the "T" drilled... there is only one hole on the base of the "T", two on each end, and one on the top for the pull string.




Top view of the "T".




Back view of the "T".




Here is about how much you want the split end of the twinlead to extend from both ends. Use a pencil and mark about where you want to make a hole to secure it to the "T".




I used the un-tipped end of a princes iron to melt a hole where I marked for the mounting of the twinlead to the "T".




The resulting hole.




With a single tie-wrap, the twinlead is secured to the "T" with the hole we made.




The core of the "T" is built... now for the legs, and a few finishing touches.




A double knot in some nylon rope prevents the pull line from coming out the top of the "T" which is now the apex of the antenna.




This is how the antenna apex will be suspended.




A loop is tied and the end is melted to prevent fraying. The apex will be pulled up with another rope, of your choice.




About 4 inches of insulation is stripped in segments from the copper wire, this will go through the apex and end insulators, both ends stripped the same.




The first leg is attached to the "T".




Twist on the exposed twinlead wire and soldier. Almost done.. This setup will allow for flex to be used either in an inverted-v (my choice) or as a flat top.




The end insulators are twisted and soldiered the same. Were ready to run the twinlead into the house, put up the apex and test the system out!




I did "encase" the open ends of the "T" in Scotch Super-33 electrical tape (anything less is junk). This will offer some protection from the elements, at least for a year in my estimate.




The antenna is up! Lets play the "Find the dipole" game! Seriously, look... you can find it! I ran it up with black polyester flat rope, and also used the same to secure the legs. The rest of any white PVC that was exposed was fully taped up. What I ended up with is what I was hoping for, a dipole that you cannot detect in the yard. A covert antenna.




The apex is only up about 20 feet, though after testing I was able to tune from 80 to 10 meters without a problem. 80 and 10 I can get down to as little as 1.5:1, 40, 20 and 15 I can tune almost flat.




Can you see the leg, and the rope that secures it?




How about now?




The leg end insulator and rope. Covert.




From this angle, about all you notice is some twinlead coming out of the tree, if you were looking. Most would likely not even notice, Im sure.




So for review, about $30 in supplies and about an hour of messing around (about half an hour if your serious about the project and not taking pictures), you will have a 66 foot (33 feet each leg) dipole antenna. Mine is feed by about 75 feet of twinlead, though you could use the entire 100 foot roll, or more if you need to splice for length without problems. The twinlead is connected to the back of a tuner at the "balanced line" post.






UPDATE: 14 MAR 07

I had the wrong measurments prior to this update, the legs are 33 feet each, giving a 66 foot tip to tip doublet.



UPDATE: 14 MAR 08

I have come to find, although it can tune up 80m with a Z-100, the efficiency is lacking so I do not suggest anything other than 40m - 10m including all WARC bands. I did run 40m, 30m and 20m CW and PSK recently with 100w CW and 25-50w PSK and have had VERY good reports.















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