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Author Topic: Coaxial Inverted L for 75mtrs  (Read 2516 times)
KE2TR
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Posts: 129




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« on: November 08, 2012, 11:14:53 AM »

I have seen a few posts on the net about these antennas, looks like half of a bazooka antenna which might have a lower noise level than a standard L plus have the advantage of a 30 degree take off angle for dx'ing and some hi angle for local rag chew. Most of what I have looked at on the net has been for 160, has anyone tried this antenna for 75 and maybe has anyone has phased a pair of these. My problem is most of the pine tree's that are still standing top out at 40' and I have tried some linear loaded verticals here but Sandy took them out. I would be using them with n elevated radial system as well, I wonder if anyone has used these antenna's on any other band's besides 160.
TNX
Jim
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WB6BYU
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« Reply #1 on: November 08, 2012, 12:13:12 PM »

The ONLY difference between a standard inverted-L and a coaxial one would be
that the coaxial one has somewhat higher losses, which will widen the SWR
bandwidth slightly.  You may also see a change in reactance at the feedpoint
if the stub length isn't resonant.

And, of course, the coax version will weigh more.
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W4OP
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« Reply #2 on: November 08, 2012, 12:17:15 PM »

There is so much wrong information on the internet with regard to the double bazooka, it is difficult to know where to start.
There is no theoretical reason why that design would be any quieter than a regular dipole. Second, the stubs acting as conjugate matches to the antenna impedance and thus yielding wider bandwidth is also false. The wider BW is a result of fatter conductors and loss in the stubs. Here is an excellent paper on the double bazooka:
http://vk1od.net/antenna/DoubleBazooka/index.htm

Walt Maxwell and Frank Witt have done some nice mathematical treatments illustrating the fact that the stubs cannot perform as bazooka manufacturers would have you think.
Finally, there is no reason that a double bazooka's design is such that it  does not benefit from a balun or isolation choke.

Save yourself  some time, money and antenna weight and put up classic 1/4 lambda monopoles.

73,

Dale W4OP
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WX7G
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« Reply #3 on: November 08, 2012, 12:30:31 PM »

The horizontal portion of a 50/50 (half vertical and half horizontal) inverted-L radiates about 10% of the total energy. It adds a (little) bit of fill within a couple hundred miles.

If you want wider bandwidth a two wire inverted-L can be built to act like a larger conductor.
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KE2TR
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Posts: 129




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« Reply #4 on: November 09, 2012, 11:24:32 AM »

Well the coaxial l would have loss over a regular wire inverted L but if I connect both the braid and center conductor together it should just act like a larger conductor which would give slightly wider bandwidth. I have a bunch of RG59U left over from a house wiring job and don't have any use for it so that's why the coaxial L but using it tied together should act like a larger conductor.
Jim
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W5LZ
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« Reply #5 on: November 09, 2012, 12:15:30 PM »

That 'extra' diameter will make some difference, sure.  But, at the frequency you're talking about I very much doubt if it would be much -if- it's noticeable at all.  Using coax instead of wire also means that the support for that antenna will have to be stronger, it'll just weigh more.  Happen to have a lot of some particular size/kind of conductor?  Use it, but don't expect there to be any significant benefit.
 - Paul
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W4OP
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« Reply #6 on: November 09, 2012, 05:54:09 PM »

Spot on Paul,
I see  a difference of 25KHz wider on 80M when going from #14 gauge to 1/4" conductor size in EZNEC

Dale W4OP
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KE2TR
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Posts: 129




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« Reply #7 on: December 12, 2012, 07:47:32 AM »

Well for those who care I tried the coaxial inverted L for 75mtrs, it was easy to convert it back to a basic L cause I used pigtails at the end. The results on local qrn was better using it as the coaxial L over the basic L, about 6-10db lower qrn and the difference in transmit was none as tested with a local ham but either way the antenna works. I could only see a 25khz improvement in swr so the broadband idea was not a great deal but what it did show is before the swr starts to rise it was flatter at resonance. The antenna is 6-10db better than the 43ft vertical I have at my qth and does a better than expected job on both local and DX. For only 30ft vertical it really surprised me and I am trying to see if I can build another then phase the pair. I matched the antenna at the base with a simple hairpin match like what was used on a F12 40mtr yagi I used to have back over 10 years ago, this also may help bleed off rain static.
Moral of the story sometimes it pays to just go out and build it over modeling it to death, modeling help you to a point but I feel some ham's have lost the field testing approach cause some antennas just don't model all that well.
Jim
KE2TR
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N4OGW
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Posts: 283




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« Reply #8 on: December 12, 2012, 10:12:39 AM »

Well for those who care I tried the coaxial inverted L for 75mtrs, it was easy to convert it back to a basic L cause I used pigtails at the end. The results on local qrn was better using it as the coaxial L over the basic L, about 6-10db lower qrn and the difference in transmit was none as tested with a local ham but either way the antenna works. I could only see a 25khz improvement in swr so the broadband idea was not a great deal but what it did show is before the swr starts to rise it was flatter at resonance. The antenna is 6-10db better than the 43ft vertical I have at my qth and does a better than expected job on both local and DX. For only 30ft vertical it really surprised me and I am trying to see if I can build another then phase the pair. I matched the antenna at the base with a simple hairpin match like what was used on a F12 40mtr yagi I used to have back over 10 years ago, this also may help bleed off rain static.
Moral of the story sometimes it pays to just go out and build it over modeling it to death, modeling help you to a point but I feel some ham's have lost the field testing approach cause some antennas just don't model all that well.
Jim
KE2TR

How did you conclude it had "lower qrn"? Did you have both antennas up at the same time?

Tor
N4OGW
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W0BTU
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« Reply #9 on: December 12, 2012, 08:53:14 PM »

How did you conclude it had "lower qrn"? Did you have both antennas up at the same time?

My thoughts exactly. That's the only way to compare two antennas: being able to instantly switch between them.

And the spacing between the two must be great enough that they do not interact.
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