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Old 30-10-2013, 19:18   #16
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Re: Coax cable between SSB tuner and backstay antenna

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Paul is right. Unconnected shield is invisible to the antenna. It causes no attenuation unless it is more than a few meters long. Even then it does not cause much attenuation. The trick is to leave both ends of the outer conductor open. It's not a shield. It's as if it doesn't exist.
It isn't about attenuation. It's a loss in radiation. Efficiency is reduced. If your stuck without options you're better off letting the inner wire float (or pull it out) and using the shield to connect the tuner antenna connection to the backstay or other antenna.

GTO-15 is cheaper and easier than coax so why not use it? It's just heavily insulated 14 gauge twisted wire.
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Old 30-10-2013, 20:22   #17
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It isn't about attenuation. It's a loss in radiation. Efficiency is reduced. If your stuck without options you're better off letting the inner wire float (or pull it out) and using the shield to connect the tuner antenna connection to the backstay or other antenna.

GTO-15 is cheaper and easier than coax so why not use it? It's just heavily insulated 14 gauge twisted wire.
Attenuation is another word for loss in radiation. They mean the same thing. Efficiency is not reduced. Using coax is just fine so long as the outer is not connected to anything. It is every bit as good as GTO-15. In some ways it's better. If coax is what you have then don't hesitate to use it.
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Old 30-10-2013, 21:18   #18
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Re: Coax cable between SSB tuner and backstay antenna

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Originally Posted by transmitterdan View Post
Attenuation is another word for loss in radiation. They mean the same thing. Efficiency is not reduced. Using coax is just fine so long as the outer is not connected to anything. It is every bit as good as GTO-15. In some ways it's better. If coax is what you have then don't hesitate to use it.
Attenuation is a reduction in power along the transmission line. That's one of the best reasons for paying a little extra for good quality coax. Attenuation is a function of material and circuit.

Radiation efficiency is a different factor. A dummy load is the perfect example - a perfect match but no to little (nothing is perfect, not even dummy loads) radiation.
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Old 30-10-2013, 21:42   #19
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Re: Coax cable between SSB tuner and backstay antenna

Re. the effect of a floating shield on a radiating element, I'm going to have to ponder this some more. Intuitively I think the difference, if any, would be unmeasurable for a run of a few feet, but at this point I'm not really sure. The distance between the center conductor and the shield are a tiny fraction of a wavelength, the only current in the shield is induced by the current through the center conductor, or the current from radiation of the capacitively-coupled voltage on the center conductor. There is also shield skin-effect to consider.

But ultimately, if the radiation efficiency is reduced, where does the energy go? As I see it, it has no where to go but via radiation, so there is no loss in efficiency.

Or does the floating shield act as a core (similar to a ferrite choke with a single-turn through it), thus increasing the inductive reactance of the series circuit? If so, it can't be very lossy.

But I could be wrong. Somebody please convince me, one way or the other.
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Old 30-10-2013, 22:06   #20
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Re. the effect of a floating shield on a radiating element, I'm going to have to ponder this some more. Intuitively I think the difference, if any, would be unmeasurable for a run of a few feet, but at this point I'm not really sure. The distance between the center conductor and the shield are a tiny fraction of a wavelength, the only current in the shield is induced by the current through the center conductor, or the current from radiation of the capacitively-coupled voltage on the center conductor. There is also shield skin-effect to consider. But ultimately, if the radiation efficiency is reduced, where does the energy go? As I see it, it has no where to go but via radiation, so there is no loss in efficiency. Or does the floating shield act as a core (similar to a ferrite choke with a single-turn through it), thus increasing the inductive reactance of the series circuit? If so, it can't be very lossy. But I could be wrong. Somebody please convince me, one way or the other.
The energy is converted into heat, i.e. The cable warms up.

For those who connect core and shield together at both ends: congrats, you just made a shorted coil with two taps. Makes for all kinds of nice stuff that happens, starting with good reflections and impedance madness
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Old 30-10-2013, 22:31   #21
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Re: Coax cable between SSB tuner and backstay antenna

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The energy is converted into heat, i.e. The cable warms up.

For those who connect core and shield together at both ends: congrats, you just made a shorted coil with two taps. Makes for all kinds of nice stuff that happens, starting with good reflections and impedance madness
No, that can't be right. How is that different than using a solid 1/2" diameter copper wire for a conductor? No excess loss there. Or a thick bundle of Litz wire? Again, no excess loss. Please describe the path this heating loss current is taking.

Besides, a shorted single-turn coil connected in this manner (?) has very low impedance and low loss. Sort of like a wire.
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Old 30-10-2013, 22:33   #22
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Re: Coax cable between SSB tuner and backstay antenna

Note to the OP: Only you can stop this madness! Use GTO-15 and we will all stop arguing!
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Old 30-10-2013, 22:58   #23
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Re: Coax cable between SSB tuner and backstay antenna

I used electric fence wire, cheap and cheerful, and bought some ignition HT wire (solid wire, not carbon core) to fit at a later stage. 8 yrs later it's still stored under a bunk!

John
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Old 31-10-2013, 04:25   #24
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Quote:
Originally Posted by Paul Elliott View Post
Re. the effect of a floating shield on a radiating element, I'm going to have to ponder this some more. Intuitively I think the difference, if any, would be unmeasurable for a run of a few feet, but at this point I'm not really sure. The distance between the center conductor and the shield are a tiny fraction of a wavelength, the only current in the shield is induced by the current through the center conductor, or the current from radiation of the capacitively-coupled voltage on the center conductor. There is also shield skin-effect to consider.

But ultimately, if the radiation efficiency is reduced, where does the energy go? As I see it, it has no where to go but via radiation, so there is no loss in efficiency.

Or does the floating shield act as a core (similar to a ferrite choke with a single-turn through it), thus increasing the inductive reactance of the series circuit? If so, it can't be very lossy.

But I could be wrong. Somebody please convince me, one way or the other.
The effect of a short (few meter) floating shield is nil. There will be zero current in the shield (where can it go?) and thus no power loss. The magnetic field around the coax is not affected at all. There will be a slight difference in the velocity along the coax as compared to insulated wire but this is just going to change the electrical length of the antenna a few tenths of a degree so nothing to worry about there. I have no idea why people think this configuration causes any problem whatsoever. If the shield were connected to the tuner or the boat then it would matter but floating it is invisible to RF.
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Old 31-10-2013, 04:27   #25
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The energy is converted into heat, i.e. The cable warms up.

For those who connect core and shield together at both ends: congrats, you just made a shorted coil with two taps. Makes for all kinds of nice stuff that happens, starting with good reflections and impedance madness
I fail to see how how shorting center and outer together looks like anything other than a fat piece of wire. It certainly is not a shorted coil.
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Old 31-10-2013, 04:41   #26
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Re: Coax cable between SSB tuner and backstay antenna

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The effect of a short (few meter) floating shield is nil. There will be zero current in the shield (where can it go?) and thus no power loss. The magnetic field around the coax is not affected at all. There will be a slight difference in the velocity along the coax as compared to insulated wire but this is just going to change the electrical length of the antenna a few tenths of a degree so nothing to worry about there. I have no idea why people think this configuration causes any problem whatsoever. If the shield were connected to the tuner or the boat then it would matter but floating it is invisible to RF.
You are going to have to provide some technical references for this. My primary ones readily at hand (Maxwell's Reflections and the ARRL Antenna Book) focus on what works and not on why other things don't.

By the logic presented ferrites won't work.

By the logic presented running an antenna up the inside of an aluminum mast would work.

I assume we can agree that the antenna begins at the tuner stud. I hope we can agree that the best place for a tuner is dry, as high above the water as practical, and with as short a run as possible below deck. On boats radiating length is pretty short for a long-wire and any degradation is suboptimal. Running a portion of the radiating element through a metal tube is sub-optimal. Skin-effect is real but second order at HF.

There is a reason GTO-15 is the solution of choice for connecting a tuner to the backstay. There is credible, peer-reviewed treatment to support a direct connection of a single conductor. Insulation is simply to avoid any personal hazard from RF burns, themselves unlikely at the power levels on most boats.
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Old 31-10-2013, 04:52   #27
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No, that can't be right. How is that different than using a solid 1/2" diameter copper wire for a conductor? No excess loss there. Or a thick bundle of Litz wire? Again, no excess loss. Please describe the path this heating loss current is taking. Besides, a shorted single-turn coil connected in this manner (?) has very low impedance and low loss. Sort of like a wire.
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I fail to see how how shorting center and outer together looks like anything other than a fat piece of wire. It certainly is not a shorted coil.
The difference between a solid 1/2" conductor and this is the dielectricum which insulates for RF and thus creates two parallel conductors, each with different impedance. Connecting braid and core at each end and expecting same as a single conductor is a DC view, not RF. The heating loss statement was separate and for the case where the outer braid is not connected. The loss caused by the shield, small as it is, appears as heat. Losses almost always appears as heat.
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Old 31-10-2013, 05:07   #28
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You are going to have to provide some technical references for this. My primary ones readily at hand (Maxwell's Reflections and the ARRL Antenna Book) focus on what works and not on why other things don't.

By the logic presented ferrites won't work.

By the logic presented running an antenna up the inside of an aluminum mast would work.

I assume we can agree that the antenna begins at the tuner stud. I hope we can agree that the best place for a tuner is dry, as high above the water as practical, and with as short a run as possible below deck. On boats radiating length is pretty short for a long-wire and any degradation is suboptimal. Running a portion of the radiating element through a metal tube is sub-optimal. Skin-effect is real but second order at HF.

There is a reason GTO-15 is the solution of choice for connecting a tuner to the backstay. There is credible, peer-reviewed treatment to support a direct connection of a single conductor. Insulation is simply to avoid any personal hazard from RF burns, themselves unlikely at the power levels on most boats.
Let's use logic:

Current flows in the center conductor.

The outer conductor is not connected to anything.

So long as the outer conductor is short relative to a wavelength there will be no current in the outer conductor.

Ohm's law says if there is no current there can be no heat and thus no loss.

People use GTO because it is cheap, not because it is the only thing that will work. If coax is what you have then it's ok to use it. If GTO is what you have use it. No need to start a war between these two equally good solutions.

Nothing said above have would affect the working of ferrites.

This is not the same as putting the antenna inside the mast for several reasons. The mast is grounded to the water for starters. Next, it is longer than a wavelength. So a mast is not similar to a short piece of open outer on a coax.

To the OP, I apologize that this thread has gone off the rails. I'm sure all this back and forth isn't helping you much so I won't repeat myself a fourth time. GTO is ok and so is a short piece of coax with the outer left open at both ends.
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Old 31-10-2013, 05:30   #29
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Re: Coax cable between SSB tuner and backstay antenna

This is getting hard.

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Current flows in the center conductor.

The outer conductor is not connected to anything.
Not relevant. From that perspective antennas shouldn't work as they are unterminated. RF doesn't work the way that DC does.

Look at all the directive antenna designs with unconnected directors and reflectors. Current flows in those elements. That's why they work.

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So long as the outer conductor is short relative to a wavelength there will be no current in the outer conductor.
Simply not true. The floating shield will be a component in an LC circuit. First, second, or third order effect is dependent on a lot of variables. Lost energy will show up as heat. Heck, the whole concept of radiation resistance is an artifice to describe what happens.
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Old 31-10-2013, 05:34   #30
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Auspicious,

If you want to continue this discussion PM me.

Dan
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