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Author Topic: 135+-' dipole fed with 450 ladderline  (Read 287 times)
KD8Z
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Posts: 169




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« on: November 10, 2007, 09:08:12 PM »

  I need how to ideas on the transition from 50 ohm coax at the transceiver, through the wall to 450 ohm ladder line just outside the shack?

Also I need to know the latest thoughts on a balun at the dipole feed point?

Will be using full power at times.
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AD5VM
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« Reply #1 on: November 10, 2007, 11:09:01 PM »

The best (most efficient) way in my opinion is to bring the ladder line into the shack to a fully balanced antenna tuner like the Palstar BT1500A. As long as the currents on each side of the ladder line remain the same, the RF fields around the feedline will cancel each other out and there will be no RF in the shack. MFJ makes a meter so you can see the current levels on either side of the line. As long as the ladderline is kept away from the ground and metal objects the currents will remain equal. Otherwise, you'll need a 4:1 balun to go from coax to ladderline, Palstar and DX engineering both make good ones. You don't need a balun at the antenna feedpoint. Bal-un stands for BALanced to UNbalanced. Your ladder line is balanced and your doublet is a balanced antenna therefore you don't need anything except a piece of plastic with a few holes drilled for a strain relief.
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W5DXP
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« Reply #2 on: November 10, 2007, 11:13:51 PM »

Here is just such an antenna: http://www.w5dxp.com/notuner.htm

You need a balun at the BALanced feedline (ladder-line) to UNbalanced feedline (coax) junction. You do NOT need a balun at the antenna feedpoint since the ladder-line is balanced and the dipole is balanced.
--
73, Cecil, w5dxp.com
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
W5DXP
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« Reply #3 on: November 10, 2007, 11:16:40 PM »

> AD5VM wrote: ... you'll need a 4:1 balun to go from coax to ladderline, ...

Why do you think a 4:1 would be superior to a 1:1 current balun?
--
73, Cecil, w5dxp.com
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
KV9U
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Posts: 166




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« Reply #4 on: November 11, 2007, 09:00:23 AM »

If you have a high resistance in the feedline, a 4:1 balun might help in some cases, but baluns really are not designed to handle highly reactive loads and many of these kinds of antennas have a great deal of reactance which can be a major component of the SWR.

Personally, I have never really liked open wire antennas due to RFI issues in the shack and immediate area of the antenna. Also, you will have many lobes as you go up in frequency and while this may help in some directions it will be a problem in others. I have done many comparisons of ground mounted verticals (with radials) and horizontal wire antennas at about 35 to 45 feet, and except for HF NVIS operation with the horizontal, tend to find better performance with the vertical on 40 meters and higher.

But if I could only put up one wire antenna and it could not have multiple dipoles from a common feedpoint or a trapped dipole, with either fed with coax, I would opt for the open wire line antenna and switch to coax using a 1:1 choke balun, just outside the shack. This also gives me a practical way to handle lightning protection.

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WW5AA
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« Reply #5 on: November 12, 2007, 06:52:28 AM »

My 177' doublet is 76' of 450 Ohm ladder line to a Coax (75 Ohm) choke (5' plus 5' to the antenna switch). No RFI issues and tunes 160-6 meters with an auto tuner.

73, de Lindy
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WA3SKN
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Posts: 5419




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« Reply #6 on: November 13, 2007, 05:25:43 AM »

Since you plan full power, plan on getting a "full power tuner".  Most already have a balun built in.  The 450 ohm line will handle full power OK, just connect it to the dipole and go!
Have you considered a loop?

-Mike.
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KD8Z
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Posts: 169




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« Reply #7 on: November 13, 2007, 09:01:56 AM »

Tried a couple of loops, they don't seem to perform well at this location.  I have several full power tuners with baluns and a couple of amps to boot. Also 5 hf rigs of various levels of difficulty and several wire antennas already in place.  I was mostly interested in the transition method of connecting the coax to the ladder line, thanks to those who answered my questions.
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W4VR
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« Reply #8 on: November 14, 2007, 11:21:55 AM »

What I've done in the past is use open wire line all the way into the rear of the antenna tuner in the radio shack.  My tuner was next to a window, so I made a horizontal slot using a small drill bit at the bottom of the double hung window so I could thread the open wire line...when you move simply fill up the slot with caulk and paint it.  If you can't run the open wire line inside because the shack is too far from the window, then use coax to a remote balun..but it's a good idea to keep that piece of coax as short as possible (15 feet or less).
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