Doublet idea

[dougdoug]

After doing some research regarding multi band antennas I am thinking about constructing a doublet. I plan to cut the wire to 132 feet based on the 80 meter band. This length is from a functional antenna I read about. I would feed the doublet with approximately 50 feet of Ladder line to my shack window. At the point of entrance through the window, to help eliminate RF in the shack, I plan to feed it through with two pieces of coax, using the center conductors as the feeds with the shields connected to each other. At the point it enters the window, I plan to connect a 1:1 current balun. From here I would run approximately 6 feet of coax from the balun to the antenna tuner. I would think I should be able to use this for several bands with a good tuner? Does anyone see any glaring problems with this idea?

 

[LtDoc]

Rather than the two pieces of coax in parallel, just put that balun outside the window and run coax to a tuner. Works out just as well and is simpler to do. As long as that run of coax is short and of good quality (meaning capable of handling higher power) it should do just dandy.
Since the impedances are going to be all over the place depending on the band of use, a 1:1 balun is the ratio of choice. Any sort of 'all band' antenna like that is always a compromise, so you have to expect some 'losses'. Hey, it works, not always well, but it works. Go for it.
- 'Doc

 

[KE5MC]

That will work. Mine is similar, but with different dimensions. Doublet length should be of a length not resonant on any band. I use 137 feet. Also the length of my 300 ohm feed-line is 32.3 feet. I have found that length more important than antenna length. I have made some internal cable connection changes which are giving me fits at the top of 80m. My tuner does not want to match the complex impedance as now seen with the cable changes. I need to go outside to make a length change of the feed-line. I expect only a foot longer or shorter, not sure which. Once I get 80m to play well it possible I will have shifted the problem to another band. It's always a compromise. Give it a try and see how it play.

The balun outside the window is a good idea. Balun Design makes some very nice ones. I use his 1114T, but its not currently one he makes, but there are others to pick from. Construction and material are 1st rate.

Keep us posted on how it works out.

Mike

 

[dougdoug]

So, since I have a 35 foot tower, would it be better to just run to the tower base, give or take, with ladder line, connect a balun and run from there (approximately 40 feet) to the antenna tuner? Or, would it be better to run the ladder line all the way to the shack window?

 

[KE5MC]

I would keep the coax as short as possible. So I would be running the ladder line to the window with the balun just outside the window, then coax into the tuner. On paper that is better than coax to the base of the tower, but you might be hard pressed to tell the difference.

At the tower use standoff supports to keep the ladder line away for the metal frame of the tower. Not sure how long, but your should be able to find the recommendation with online search. (8-10 inchs ??)

 

[WA0CBW]

I use basically the same setup, ladderline and double coax but no balun. I use about 10 feet of RG62 (92 ohm nominal) which provides about 180 to 450 ohm transition. I use two blitz bug lightning suppressors (one in each lead) at the junction of the coax and ladder line where it enters my house. My doublet is only 43 feet in each leg mounted as an inverted flying "v" (my trees just didn't grow in the right places). I use a balanced line tuner with an MFJ balanced line antenna line current meter. This setup allows me to tune from 80 meters to 10 meters using up to 600 watts. I found that using the balun I had difficulty tuning certain frequencies. This type of antenna was quite popular when coax first came into use. It allows me to bring the ladder line in through the basement across the plumbing and HVAC ducts. I have no RF in the shack. I had a pair of amplified computer speakers that talked back to me when I was using the balun. After removing the balun and using the balanced coax to a balanced tuner they don't make a sound. I would suggest using the highest ohm coax you can find. If you use 75 ohm TV coax try to find the kind without the foil shield. I tried some double RG6 Quad shield stuff but had more success with plain old 75 ohm shielded cable. It worked even better when I went to the 90 ohm coax and a "real" balanced line tuner. At our place in the country I set up the same thing but with a 132 foot antenna stretched between two trees at about 50 feet. It works great! I wish I had room for a 132 foot antenna at my home. The only disadvantage is that it takes a few moments to tune up. But that is what I like about ham radio, the "twisting" knobs and watching the meters! Try your setup without the balun and see how it works. If it doesn't you can always add the balun later.
BB.

 

[rfradioconsult]

I use a doublet with great success, my setup is a doublet about 88 ft. end to end, fed with about 37 ft. of 450 ladder line to a MFJ auto tuner on the roof, this terminates in a 4:1 Balun. A short 2ft piece of RG-8 connects to the auto tuner, 35ft of RG-8x runs to my shack, through a Diawa dual meter to allow me to confirm everything is performing as expected. After the SWR meter is a coax switch that I can select any of my 3 HF rigs. I have not found any HF frequency that I cannot tune including the ham bands and my numerous MARS frequencies.

This setup is very stable and most importantly keeps the stray RF out of the shack.