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  #11  
Old 01-04-2011, 02:32 PM
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I don't think balanced feedline would be necessary and for VHF open 450 ohm ladder would be bad, more so if the air was moist. Using 300 ohm TV feeder would be an option, though. In particular to avoid tons of loss at >144MHz the conductors would have to be close, others it's no longer balanced since they don't couple and just act like independent wires.

Probable that I'm missing something, what are you trying to solve using the balanced feed here Brian? If you feed an unbal 50 ohms into a 9:1 balun into 450 ohm ladder, which you connect to a high impedance, you still get a huge reflection and essentially make the ladder line the radiator. So yes, it's keeping you from blowing the radio finals but doesn't solve the feedpoint inefficiency.

I 100% agree that solving a full wavelength dipole problem is not worth the time and doing a matching stub or changing to a 5/8 radiator makes a lot more sense. :-)

BTW, teachable moment... If you look at the construction of this antenna, you'll see the author wrapped the coax several times at the end. He's making a 1:1 current balun to solve the balanced-to-coax issue. In this case being a correct Hertzian dipole the feedpoint impedance will be ~72 ohms, so you don't need the impedance transformation that Brian and I are discussing.
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Last edited by DaveInDenver; 01-04-2011 at 03:14 PM.
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  #12  
Old 01-04-2011, 03:04 PM
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'Cuz even with a 9:1 balun you are still looking at an impedance miss match of around 3:1. Your transmitter would tolerate it but coax would become very lossy.

Last edited by Seldom Seen; 01-04-2011 at 03:34 PM.
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Old 01-04-2011, 03:10 PM
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Quote:
Originally Posted by Seldom Seen View Post
'Cuz even with a 9:1 balun you are still looking at an impedance miss match of around 3:1. Your transmitter would tolerate it but coax would become vary lossy.
That was my more or less my guess. I think there really isn't a practical solution to this for high band VHF because the ladder line becomes lossy at those frequencies. Darned if you do, darned if you don't. My thinking is that a wound transformer would be less lossy than any cable, but then a 9:1 balun is a basically a great big lumped heater that leaks a little RF, forgetting 20:1. Really 4:1 is about the limit of practicality. On the up side the antenna would be resonant at least, if almost impossible to feed. So the antenna itself would be efficient at radiating whatever makes it down the pipe.
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  #14  
Old 01-04-2011, 06:22 PM
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Wow, I am glad to see all this discussion!! Basically, I have never built an antenna from scratch, which I can only assume other new Ham operators have not. I was interested in a design which is portable, breaks down and sets up easily, does not take up much room and works well. I was thinking if it can break down and be put inside of a larger piece of PVC tubing with end caps, that would be tough, durable and easy to pack. If we can get enough interest I am happy to volunteer my house for assembly, I have all the tools necessary to put the antennas together and the MFJ-269 for tuning.. I am in the South West area Simms & Belleview.. Kick this around some more and lets see if we can come up with a design that everyone likes and would like to try, then we can put together a list of materials and work on a date and time.. Just a note, I for one get just a bit intimated with the level of expertise and knowledge you guys have and want to start grabbing some books to look up some of the things you are saying I think it is GREAT and I hope others can take advantage of this generous knowledge and willingness to help..
So: THANKS!!!
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Old 01-04-2011, 06:30 PM
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Default I might have some materials to offer

I need tO remove an old UHF/VHF tv antenna from my house. Would those materials be useful tO donate to the cause?
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Old 01-04-2011, 06:44 PM
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If it is a weekend I would be tempted in coming up from the Springs. Since you're offering your house I would understand if you want to keep the group small. I'll keep watching this post. Great stuff, not that I get it all.
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  #17  
Old 01-07-2011, 09:54 AM
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Quote:
Originally Posted by CardinalFJ60 View Post
I need tO remove an old UHF/VHF tv antenna from my house. Would those materials be useful tO donate to the cause?
I do not know if we could use any of these materials or not but thanks for the offer..

Quote:
Originally Posted by CO Hunter View Post
If it is a weekend I would be tempted in coming up from the Springs. Since you're offering your house I would understand if you want to keep the group small. I'll keep watching this post. Great stuff, not that I get it all.
I would like to do this on a weekend, I think there would me more interest and more time available. Let's see what the level of interest is and we will go from there..
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