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600 Ohm Ladder Line vs Coax

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  #9  
Old 07-19-2010, 03:03 AM
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This is just a bunch of random things about using ladder line. No particular order to it, just as it comes to mind. It's also not any kind of definitive argument pro/con, every installation is going to be different in some ways, the 'trick' is to figure out how your installation behaves and the use that behavior to your advantage. Sometimes using ladder line isn't the 'best' way of filling your requirements because of how/where you can put an antenna.

First, ladder line, of any impedance, isn't a cure-all. It is less susceptible to damage than coax cable is to standing waves. That's because SWR can produce some really high voltages/currents. While the feed line may not be the limiting factor, other parts of the antenna system can certainly be. One of those parts of the antenna system is the tuner typically used. The physically larger tuners usually have larger components (variable capacitors) which can withstand those higher voltages/currents better than smaller tuners and their components. Wider spaced coils in the inductor(s) mean higher ratings too. So, if it's a 'smallish' tuner, don't expect a lot of power handling at all.
[Tuners are typically rated using Pep rather than AVG power levels. Figure half of the advertised ratings as what you can usually expect it to 'do'. Manual tuners can typically handle larger variations than the auto-tuners. The automatic tuners are only the 'best' when judging that 'best' by convenience.]
Weather does affect ladder line! The characteristic impedance (any size ladder line) will change when it get's wet, covered with snow/dust/dirt/etc, so expect things to change occasionally. Keeping notes on the tuner settings is a handy guide to keeping things tuned. 'Windowed' line is more susceptible to those changes than 'open-wire' type lines, there's just more area to get covered with stuff. The 'open-wire' type parallel feed lines are more prone to 'swarming' on you. Less structural surfaces that tend to keep things from snarling so much. Any ladder/open-wire line will 'swarm to some extent, so just be more careful with it. It's a factor, but not a huge one (until you have to untangle all that @#$%.
Unlike coax feed line, ladder line can't be laid on stuff. Keeping it 3 - 5 times it's width away from metal is a good idea. That's usually something like 4 - 6 inches for typical 1" ladder line, closer to two feet for 600 ohm stuff (4 - 5" wide?). As long as it's kept away from stuff ladder line isn't affected a lot. If/when you run it through something to get it inside the shack it will have 'lumps' of impedance changes. Those 'lumps' are typically not a biggy, a tuner should be able to 'smooth' them out with not problem if they are kept to a minimum.
Using a balun ('remote balun'?) to change to coax to get the stuff to the tuner/radio is the 'weak' point when doing that. The coax used has to be the 'best' you can find, there will certainly be some compromises with that. Keeping that coax as short as possible is a very good idea. If used on multiple bands, the impedance transformation ratio of a balun is -only- a guess, what will be required will change from band to band. It will never stay the same for all bands.
Be prepared to do a lot of soldered connections! I'm not aware of any connectors that are available and that work well with ladder lines. Not really a big deal, but be aware of it.
Anything that has a 'flat' surface is going to wiggle in the wind. One trick to using longer runs of ladder line is to twist it, make it into a spiral looking thingy. That lessens the amount of movement from wind. (Ever wonder why those 'tape' advertising thingys are usually in a spiral? Now you know.)
Can you make 'bends' with ladder line, make it bend around things? Yes. But keep those bends small, never a sharp bend to change it's direction. Hang it with nylon/synthetic/non-conductive cord, or held up off of a wooden 'T', or something.
There are more 'trick', look at all the installations you can find, see how they do things, then adapt those 'things' to your situation when you can.

Having been part of MARS for a long time (was, not at present), I found that retuning an antenna to the MARS frequency then 'making do' on the ham bands was my best way of going about it. Just a matter of priority, and that's up to you. There's more work to it than you see at first glance, but it is very rewarding.
Have fun!
- 'Doc

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  #10  
Old 07-19-2010, 07:19 AM
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Here is my antenna setup Robb. It is configured as an inverted VEE type with each leg about 65 feet or so long. As you can see it is a simple hook-up from feedline to antenna. The balun/choke is homemade and consists of "a bunch of turns" of #12 house wire on a plastic pipe form. I used green and white wires just to keep it easier to ID the ends. It was originally configured as a 4:1 but the impedance was much to low on 80 and 160m ( something like 2 or 3 ohms at the shack) so I rewired it for 1:1 and things are much happier. Basically the 450 ohm ladder line connects to one end of the choke across the green and white wires and the coax connects across the other end. The choke is in my basement directly beneath my shack with about 10 feet of Belden 8214 up to the shack. I use either a Ten-Tec 228 manual tuner or a Kenwood AT-230 manual tuner depending with radio I am using.







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Old 07-19-2010, 09:20 AM
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The reason for the recommendation of the Alpha Delta DX-CC (or some homebrewed variation of it) was that he mentioned that he has space limitations and getting up a full sized doublet for 75 meters was going to be a challenge. You could feed something like that with either coax or ladder line and it will work fine. Because that antenna is really a fan dipole, you have elements cut for 10, 20, 40, & 75/80 meters. When tuned properly, you won't experience very much loss due to high SWR on any of those bands when using coax. Most of the MARS frequencies that you will use won't be that far out of those bands, so you should still be fine with coax. If you use ladder line, you will have a lot more flexibility with tuning other bands like 60 meters, and others that can be used with MARS. To me, it really comes down to your installation requirements and whether or not you can make the run of ladder line within all of the restrictions that 'Doc gave you. If you can do it, use the ladder line. If the installation gets to be a headache and you don't want to deal with feedline issues, use coax.

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Old 07-19-2010, 04:17 PM
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Thanks all for the info. It has given me some directions to study more about what is happening with the dynamics of wire-fed dipoles. Still need to understand more.

I will probably go the route that Mole suggested - using coax on the Alpha Delta DX-CC fan dipole. I wanted to steer away from coax, as running it some 80 ft at an angle to my radio shack might over-stress the connector on the end and tear it off. I'm trying to get as much right as I can - before I get the antenna, the fiberglass 35 ft mast, 1:1 current balun, and the coax purchased and set up.

Any more thoughts and help would surely be appreciated.
I intend to set the antenna up as an inverted "V" diagonally above a 1-story ranch-type home - 35 ft from the feedpoint to the surface of the 10 ft high roof. Or 45 from the apex to the surface of the ground - same thing - right? (NW to SE orientation) The dimensions of the roof are 60 ft x 25 ft...
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Old 07-19-2010, 04:53 PM
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Do you have any reasons why you cannot use the ladder line? With a fiberglass mast, you can even just tape the stuff to it, then run it at an angle down to where you're going to run it into the house...

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Old 07-19-2010, 05:20 PM
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Quote:
Originally Posted by Robb View Post
Thanks all for the info. It has given me some directions to study more about what is happening with the dynamics of wire-fed dipoles. Still need to understand more.

I will probably go the route that Mole suggested - using coax on the Alpha Delta DX-CC fan dipole. I wanted to steer away from coax, as running it some 80 ft at an angle to my radio shack might over-stress the connector on the end and tear it off. I'm trying to get as much right as I can - before I get the antenna, the fiberglass 35 ft mast, 1:1 current balun, and the coax purchased and set up.

Any more thoughts and help would surely be appreciated.
I intend to set the antenna up as an inverted "V" diagonally above a 1-story ranch-type home - 35 ft from the feedpoint to the surface of the 10 ft high roof. Or 45 from the apex to the surface of the ground - same thing - right? (NW to SE orientation) The dimensions of the roof are 60 ft x 25 ft...

Put a loop in the cable and anchor the loop thus taking the stress off the connector.
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Old 07-19-2010, 05:51 PM
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Or, if using coax, just bury the stuff. No strain, except for the burying part, then it's just your back. Having it in dirt should even help with some RFI/CMCs.
- 'Doc

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Old 07-19-2010, 05:59 PM
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Quote:
Originally Posted by moleculo View Post
Do you have any reasons why you cannot use the ladder line? With a fiberglass mast, you can even just tape the stuff to it, then run it at an angle down to where you're going to run it into the house...
My shack is in my garage; not in the house. The house is oriented lengthwise east to west - and the garage is just west of the house - and the line needs to go to the far west side of that garage - at the base of that palm tree.

18734 Loree Ave - Google Maps
To the left of the picture is a palm tree. My shack is under there - just to the right of it. One can barely make out the IMAX on the garage roof. To the east of it - is the house - where I am planning to put up the fan dipole mounted from the NW corner to the SE corner of that roof. The distance between the center of the roof of the house to the west side of the garage is approximately 60 ft. Plus, the slope from the antenna apex is going to add more length.

The length of this dipole is 82 ft; so it should work well on that roof as an inverted "V" configuration.

How would you set it up?
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