Optimum Ladder Line lengths

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Optimum Ladder Line lengths

Post  N1LAF on Sun Aug 11, 2013 2:18 pm

The use of ladder line to feed half wave dipoles for multi-band use is preferred due to low transmission line losses due to high SWR's, much better than coax. The limitation is ladder line lengths, not cut in multiples of half wave lengths of the bands used.

In free air space, the wavelength (in feet), is 492 / Freq (in Mhz).

Since we do not live in free air space, the constant used is 468.

Wavelength = 468 / Freq(Mhz)

Half wave is simply half of this number.

Then, use the same formula for the other bands, such as 29 Mhz, 21.3 Mhz, 14.2 Mhz, 7.2 Mhz, 3.7 Mhz, and 1.9 Mhz - and figure the best ladder line legnth - not so easy. Then there are the other odd bands.
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Re: Optimum Ladder Line lengths

Post  N8YX on Sun Aug 11, 2013 5:37 pm

For a given half-wave antenna fed with one wavelength of 450-ohm line at frequency F, what does the impedance at the transmitter end of the circuit do as F is increased or decreased? What happens to the system impedance when F is halved or doubled?

Under what circumstances will the transmission line itself start to radiate?

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Re: Optimum Ladder Line lengths

Post  N1LAF on Sun Aug 11, 2013 11:07 pm

I was interrupted and have not gotten back to this. I have developed a program that determines best ladder line lengths for multi-band use with ladder line and a dipole.

Good reading on this topic:

http://www.arrl.org/files/file/Technology/tis/info/pdf/9706057.pdf

http://www.athensarc.org/ladder.asp
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Re: Optimum Ladder Line lengths

Post  N9XR on Thu Oct 17, 2013 3:46 pm

Thanks for the links, Paul.

This topic is massively misunderstood by many. I am always trying to learn more.

On Figure 1, there are two resistances shown on the cartoon. The loss resistance appears to refer to the transmission line losses. Another large loss is the ground losses in verticals. Where does that resistance fit in the schematic here? logic tells me that it should be in series as well, but in reality it seems like it may be in parallel decreasing the overall radiation resistance. In a way, these losses are radiation resistances as they are resistances at the lumped impedance antenna input. On EZNEC, it appears to increase the resistance when the vertical is lower to the ground AND the resistance is higher when the ground conductivity is higher. This does not seem to be intuitive.

One thing that is consistent with these QST articles is that they think that if you have a lossless transmission line that you will send 100% of your signal to the antenna. There is no way that I see this to be the case.



If we rework their schematic to show a lossless signal source and source resistance as above, we would see a signal reflected from the antenna heading to the transceiver. If the transmitter section is working right, the signal will see a 50 ohm resistor running to ground. This is why your rig doesn't like high SWR because the high SWR turns your radio into a dummy load for itself. Does the signal bounce back and forth over and over again? Yes it does indeed, but this signal is seeing your radio and a portion of it is being dissipated back into the radio.

They (antenna tuners) do not change the SWR on the line, or the loss it produces. What can we do to reduce the loss?
This is a hard concept, but true.

Pretty good article, all in all.
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Re: Optimum Ladder Line lengths

Post  N9XR on Thu Oct 17, 2013 4:12 pm

N8YX wrote:For a given half-wave antenna fed with one wavelength of 450-ohm line at frequency F, what does the impedance at the transmitter end of the circuit do as F is increased or decreased?
The resistance increases as the frequency increases and decreases when the frequency decreases. The reactance becomes more inductive as the frequency increases from resonance and becomes more capacitive as it decreases from resonance.

What happens to the system impedance when F is halved or doubled?
The actual resistance becomes very small either way. Also on my simulator, they are both capacitive.

Under what circumstances will the transmission line itself start to radiate?
If we look at an antenna as a radiator and a transmission line feeding that antenna, ideally the waves flow into the feedpoint from a non standing wave feedline. HOWEVER the waves on the antenna are standing waves as the voltage and current swings stay in the same place. So I am venturing to guess that the transmission line begins to radiate when there are standing waves. There are coaxial antennas as well and they will radiate quite well.
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Re: Optimum Ladder Line lengths

Post  N1LAF on Thu Oct 17, 2013 7:51 pm

If the ladder line is balanced, it doesn't radiate RF. If the ladder line is placed near metal objects, it may have some inbalance introduced into it
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Re: Optimum Ladder Line lengths

Post  N1LAF on Thu Oct 17, 2013 7:55 pm

From the Athens Amateur Radio Club page: http://www.athensarc.org/ladder.asp

"A resonant length of ladder-line, just like the shield of coax, will pick up RF from the antenna and conduct it into the shack. The only difference is that the shield of the coax is grounded, and the ladder-line is not, so it acts in common-mode to bring in and radiate induced RF. A non-resonant length of feed-line will present a high impedance to common-mode currents. And, as with any feed-line, it’s best to run it perpendicular to the antenna as far as you can so the EMF from the dipole will cancel itself instead of inducing current in the feed-line."
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Re: Optimum Ladder Line lengths

Post  JuanLargo on Sun Dec 01, 2013 12:23 am

N1LAF wrote:The use of ladder line to feed half wave dipoles for multi-band use is preferred due to low transmission line losses due to high SWR's, much better than coax.  The limitation is ladder line lengths, not cut in multiples of half wave lengths of the bands used.  

In free air space, the wavelength (in feet), is 492 / Freq (in Mhz).  

Since we do not live in free air space, the constant used is 468.

Wavelength = 468 / Freq(Mhz)

Half wave is simply half of this number.

Then, use the same formula for the other bands, such as 29 Mhz, 21.3 Mhz, 14.2 Mhz, 7.2 Mhz, 3.7 Mhz, and 1.9 Mhz - and figure the best ladder line legnth - not so easy.  Then there are the other odd bands.
Right idea except that 468 divided by the frequency in MHZ is the formula for approximately a half wavelength.  I you halve the number you get approximately one quarter wavelength.

I have a program done in Excel that will calculate the appropriate lengths to avoid 1/2 wave mulltiples on all bands between 160 and 6 meters.  It also works for end fed antennas, loops, and dipoles.  For multi band flat top antennas it is preferable to avoid quarter wavelengths to keep the feed point impedance manageable over as many bands as possible.

If anyone would like a copy just send me a PM with your email address and I will forward it to you right away.
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Re: Optimum Ladder Line lengths

Post  N2IZE on Sun Dec 01, 2013 2:40 pm

Thanks. interesting facts and useful to me since I use ladder line almost exclusively on HF. Can't remember when the last time I ever used coax on an HF installation.

Also, by "ladder line" are we talking about real 600 ohm ladder line, usually bare copper line with the wide spacers between them. Or the 450 ohm plastic covered "ladder line" ? I usually use the plastic covered 450 ohm line because it's hard for me to get a clean clear run from the antenna into the house. The line has to feed through branches and leaves and come in via metal storm window frames, etc. Bare 600 ohm ladder line would be too difficult to manipulate here. The plastic covered 450 ohm line makes life a lot easier and is a nice compromise. I have a friend who fed his 1690 meter flatop dipole with cheap 300 ohm TV twin lead and got excellent results. from what I hear it can handle a fairly high amount of power too. But I wouldn't want to push its limits too far. Again the 450 ohm line is a nice compromise for me.

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Re: Optimum Ladder Line lengths

Post  N9XR on Mon Dec 02, 2013 7:07 pm

N2IZE wrote:Thanks. interesting facts and useful to me since I use ladder line almost exclusively on HF. Can't remember when the last time I ever used coax on an HF installation.

Also, by "ladder line" are we talking about real 600 ohm ladder line, usually bare copper line with the wide spacers between them. Or the 450 ohm plastic covered "ladder line" ? I usually use the plastic covered 450 ohm line because it's hard for me to get a clean clear run from the antenna into the house. The line has to feed through branches and leaves and come in via metal storm window frames, etc. Bare 600 ohm ladder line would be too difficult to manipulate here. The plastic covered 450 ohm line makes life a lot easier and is a nice compromise.  I have a friend who fed his 1690 meter flatop dipole with cheap 300 ohm TV twin lead and got excellent results. from what I hear it can handle a fairly high amount of power too. But I wouldn't want to push its limits too far. Again the 450 ohm line is a nice compromise for me.
The higher impedance, the lower the loss. Just use what you can. Even 300 ohm twinlead is better than or similar to the losses in RG8. I have used 300 ohm TV twinlead for antennas before. I'm not proud.
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Re: Optimum Ladder Line lengths

Post  JuanLargo on Tue Dec 03, 2013 3:22 pm

I agree, 300 ohm TV twinlead is better than most people think and has very low loss at HF. If it is properly matched it can handle up to about 350 watts but I have never done that. It will handle 100 watts easy even with a high standing wave ratio.

I have 500 feet os 300 ohm ladder line in my junk box and plan to use it to feed a loop one of these days. It is smaller than 450 ohm line and can handle a KW. Because it is smaller it is less visable and people don't get freaked out over what they think is TV lead in even if it isn't, they don't know that.

My very first antenna was made from 300 ohm twinlead. It was a folded dipole made from twinlead and fed with twinlead. Worked like a champ on 40 and 15 meters but my Adventurer would load into almost anything and I'm sure that helped a lot for a dumb kid who didn't know much at the time.Wink Smile 
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Re: Optimum Ladder Line lengths

Post  N9XR on Tue Dec 03, 2013 5:33 pm

I only run 100W here as well so I don't worry that TV twinlead won't handle 2kW. But I am sure the voltage limits would be pretty low since it has such a high impedance and so close to each other.

Folded dipoles are great. I have used them before as well. You just have to match them and all is well.

My philosophy is to make the feed into the transmission line equal to the characteristic impedance of the line. If it is 450ohm line, then feed it a 450 ohm (9:1) matching. Then match once again on the other side. A loop is probably pretty high resistance anyway.
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Re: Optimum Ladder Line lengths

Post  N2IZE on Wed Dec 04, 2013 3:36 pm

An 80 meter Inverted Vee doublet fed with 450 plastic covered balanced line and matched via a link coupled tuner (an old Johnson 350 watt matchbox) worked great for me for decades. It gave me an excellent signal with 100-200 watts on AM/CW/SSB. deed point about 65-70 feet in a tree I got awesome signal reports from the low end of 80 right on up through 10. I ran 350 watts carrier 100% modulation (120% positive peaks via asymetrical limiting) and the old Matchbox didn't even get warm. I have been told that the Johnson Matchbox is very conservatively rated and will handle well above the manufacturers original ratings without melting down or flashing over. I have friends who also had incredible results with 300 ohm twin lead although I doubt I'd want to run much over 100 watts + 100% modulation into it.

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Re: Optimum Ladder Line lengths

Post  N9XR on Wed Dec 04, 2013 5:03 pm

I would not run a 350W AM carrier into 300 ohm TV twinlead. It might be okay, but I would not chance it either. Smart move.
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Re: Optimum Ladder Line lengths

Post  N2IZE on Wed Dec 04, 2013 11:18 pm

N9XR wrote:I would not run a 350W AM carrier into 300 ohm TV twinlead.  It might be okay, but I would not chance it either.  Smart move.
Especially the el-cheapo 300 ohm twinlead they were selling in the late 70's. It was like one thin strand of conductor with a thin coating of plastic. 350 Watts of RF would probably melt that stuff. The old 300 ohm twinlead from the 50's and 60's used heavier gauge multi-strand wire with a heavy rubber or plastic insulation. That stuff might have been able to handle 350 watts of RF. But the newer stuff forget about it. 100 watts max and even that was probably pushing it.

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