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Author Topic: Switched to ladder line....  (Read 2056 times)
AA4PB
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« Reply #15 on: July 30, 2008, 10:45:44 AM »

I've used many wires over the years. 1/2 wave dipoles, fan type, trap dipoles, and yes, long dipoles fed with ladder line
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Yes, his doublet fed with ladder line will perform about the same on 20M as a 20M 1/2 wave dipole fed with coax. The issue is that he initially had his doublet fed with coax. Nobody is saying that the doublet will outperform a dipole on its resonant frequency. A doublet will however perform well on all bands when fed with ladder line and it WILL NOT when fed with coax.

You've got to compare apples to apples. I wouldn't want new hams to get the idea that you can make ANY antenna perform well simply by placing a tuner at the transmitter. I would like new hams to understand that a 130 foot doublet fed with properly installed ladder line and a tuner makes a pretty good general purpose all band antenna while a 130 foot doublet fed with coax does not. They should also understand that there are a variety of other ways to make an efficient all band antenna, some fed with coax.
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K0OD
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« Reply #16 on: July 30, 2008, 10:53:49 AM »

It would take some doing to figure everything involved. Need to get an idea of the SWR. I was assuming a fairly short run of decent coax.

Try VK1OD's loss calculator:
http://www.vk1od.net/tl/tllc.php

Loss looks to be just a few dB which would hardly turn Italian big guns into "peeps" from the US east coast. Again, noise would drop too.
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N1LO
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« Reply #17 on: July 30, 2008, 11:43:27 AM »

Congratulations. You did the right thing and are now reaping the benefits. I saw that same article years ago, which inspired me and local Ham friends to use parallel line for multiband doublets.

Using a coax fed dipole on bands where it is not resonant incurs large losses in the feedline.

Good job!

Mark
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K5END
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« Reply #18 on: July 30, 2008, 12:14:06 PM »

Just curious, but were all other things equal? Same height? Same orientation? ...etc?

Other changes are unknown, such as the possibility of eliminating faulty or suspect connectors, and such.

I don't have an opinion either way on this topic, except that it isn't easy to form an accurate conclusion based on this comparision.

Whatever the case, it sounds like it's working well and you're happy with the result!

That is the important thing.

Good work and 73!
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AA4PB
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« Reply #19 on: July 30, 2008, 01:09:07 PM »

Don't forget, in addition to the loss in the coax, if the tuner is working at its limits in order to make the match then it could have considerable loss as well.
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N0CGF
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« Reply #20 on: July 30, 2008, 02:12:08 PM »

I experienced the same as KC2RVD back in the 80's, I switched between Ladder Line and Coax as the feed line, with the use of a Antenna Tunner on both.  I kept the Ladder Line because of the improvement.      It is just the loss of Coax vs Ladder Line and high SWR...  73's  N0CGF
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W5DXP
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« Reply #21 on: July 30, 2008, 02:26:17 PM »

The feedpoint impedance of a 130 foot dipole used on 20m is "1738+j1912 ohms" according to EZNEC. The SWR on 50 ohm coax is 77:1. The SWR on 450 ohm ladder-line is 8.7:1. Assuming 100 watts available, according to the feedline calculator at http://www.vk1od.net/tl/tllc.php:

The total loss for 75 feet of RG-8x is 9.4 dB or 88.5% of the available power.

The total loss for 75 feet of 450 ohm ladder-line is 0.314 dB or 7% of the available power.

With 75 feet of RG-8x, he would radiate 11.5 watts on 20m. With 75 feet of 450 ohm ladder-line, he is radiating 93 watts on 20m. I suspect almost any of us could detect the 9 dB difference between 11.5 watts and 93 watts.
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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.
N1LO
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« Reply #22 on: July 30, 2008, 03:17:27 PM »

nicely done, Cecil. Thanks for bringing us some numbers for discussion.

Mark
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K0OD
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« Reply #23 on: July 30, 2008, 06:11:35 PM »

"The SWR on 50 ohm coax is 77:1"

Just modeled it in EZNEC and came up with a similar outrageous SWR for 14 mhz which surprised me. That would be quite lossy with a long run of RG8X. Again, shorter and better coax would help. 5-9 dB of loss shouldn't affect your ability to hear. Would be like turning down the gain a bit.

Modeling that antenna sure didn't reveal much merit in it. It does exhibit low SWR on some frequencies but all, except 3.6 mhz are between ham bands. No doubt the pattern is odd above 80, too.

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VK1OD
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« Reply #24 on: July 30, 2008, 07:15:17 PM »


For a more complete model, balun and ATU losses need to be considered.

In the case of a coax feedline centre feeding a full wave dipole, a feedpoint balun will improve feedline losses because of the loss in the balun.

The article at http://www.vk1od.net/balun/W2DU/index.htm looks at a coax centre fed 80m half wave dipole on 80 and 40 with W2DU style balun and ATU, estimating the system efficiency.

Owen
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N3OX
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« Reply #25 on: July 30, 2008, 07:28:45 PM »

"Again, shorter and better coax would help. 5-9 dB of loss shouldn't affect your ability to hear. Would be like turning down the gain a bit. "

9dB loss on receive assumes that the receiver is matched to the antenna by the tuner.  This will be true if the receiver is 50 ohms but what if it's not (not all of 'em are).

I would generally agree that 10dB loss would not keep you from hearing stuff on 20 because many signals (including most people's background noise) will stay well above the receiver's internal noise threshold.

But, you crank that up to 15dB loss or 20dB loss and then  take a liberal definition of "not hearing anything," I can really imagine the change "making the band come alive"

And, of course, radiating 90W or 95W makes it a lot easier to make contacts !

There are other  considerations here as well.  The *exact* impedance of the antenna and the *exact* length of coax will affect both the loss in the coax and the impedance that the tuner has to match (and therefore affect how much tuner loss there is).  And, all these loss calculations have assumed shiny new coax with brand new matched line loss.

I think it would be hard to get more than 20dB loss on receive in most configurations of coax and wire, but you might be able to get up there.

Now, stick Europe in a null of the antenna.  We don't know the exact orientation but we do know it's long enough to lobe out a bit.  The ZLs and VKs are also in a null then.  

Sure, you still hear stuff with 20dB loss, but what about a combined 35dB in the interesting directions?

Given the terrible antenna advice, pseudoscience, and awful products that pass for antennas out there, I think we should not underestimate how bad some installations can be.

In most random multiband installations, a casual switch from coax to ladderline will be somewhere between neutral and a massive improvement.  There are occasional situations in which it might not be worth doing, but in most of them, as the original poster found out, it can be a big deal.

73,
Dan

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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
KC2RVD
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« Reply #26 on: July 31, 2008, 04:56:56 AM »

Wow what a discussion I started....

Who knows maybe my coax feeder was bad/had water in it... or maybe the moons of Jupiter were not in the correct alignment... all I know is that the antenna is performing nicely now....

Sure it is no high performance beam, but it works for now...
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K1BXI
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« Reply #27 on: July 31, 2008, 05:22:34 AM »

Nice explanation Dan.

John.....K1BXI
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AA4PB
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« Reply #28 on: July 31, 2008, 06:00:26 AM »

K.W.Ham says 10dB loss in coax due to SWR is no big deal, no need for ladder line in his station. On the other hand he uses expensive "N" connectors on the coax because he is worried about the 0.1dB loss in a UHF connector. Go figure :-)

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WW5AA
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« Reply #29 on: July 31, 2008, 07:12:51 AM »

....and most will continue using coax because it's easy! A 3 dbi loss on my RX antennas would be a big set back working 160/80 DX. I guess that a 9 db loss for rag chew and net guys is OK...?

73 de Lindy
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