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Old 10-03-2013, 04:32   #1
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Are standoffs necesary for hf on backstay?

I have rarely seen them used here in aus, but when I google hf connection on backstay they are always mentioned.
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Old 10-03-2013, 05:23   #2
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The feed wire from the tuner to the backstay is an active part of the antenna. As such it it best kept as far as practical from other parallel conductors. Seems to be common to space it away a few centimeters. I use zip-ties threaded thru small flexible vinyl tube.

Maybe someone could report on the loss per foot for not spacing it away ... Or just do it and forget about it.
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Old 10-03-2013, 08:55   #3
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This almost certainly helps radiate the signal, but you wont find any definitive information about how much. There are so many factors governing successful HF communication that it can be extremely hard to quantify the influence of any single ground/transmission line/antenna variation. My approach is to incorporate as many best practices as possible, and as few compromise shortcuts as I can manage.

Chip
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Old 10-03-2013, 09:11   #4
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Re: Are standoffs necesary for hf on backstay?

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Originally Posted by SoonerSailor View Post
This almost certainly helps radiate the signal, but you wont find any definitive information about how much. There are so many factors governing successful HF communication that it can be extremely hard to quantify the influence of any single ground/transmission line/antenna variation. My approach is to incorporate as many best practices as possible, and as few compromise shortcuts as I can manage.

Chip
+1 on this. The amount of variables which would go into calculating the straight-line inductive losses by having a [non-grounded] conductor running parallel to the antenna would lend itself to resolving the problem through computational modeling. As the majority of cruisers don't have super-computing power on their boats to analyze their setup accurately, Chip's suggestion of mitigating as many losses as possible by following best/established practices is a good one.
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Old 10-03-2013, 09:27   #5
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Re: Are standoffs necesary for hf on backstay?

The loss is directional. If you attach directly to the backstay there will be an area where your signals are weak, about 120 degrees. The antenna will continue to receive and radiate well away from the stay.
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Old 10-03-2013, 09:36   #6
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Re: Are standoffs necesary for hf on backstay?

A lot depends on whether your backstay is grounded at the bottom.
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Old 10-03-2013, 10:19   #7
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Re: Are standoffs necesary for hf on backstay?

I drive the stay from the bottom. No insulator.
No problem, shocks etc. up to and including 20 meter band, although I haven't tried it on any higher freq.
I've even proved it to a few other HAM friends by holding the stay while they call "CQ".
They expected to see me writhing in pain, but I didn't feel anything at all.

They became believers after that.
The end fed antenna has maximum CURRENT at the feed end, but maximum VOLTAGE at the other end.
I'd certainly NOT want to grab the top.

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Old 10-03-2013, 10:43   #8
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Re: Are standoffs necesary for hf on backstay?

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I drive the stay from the bottom. No insulator.
No problem, shocks etc. up to and including 20 meter band, although I haven't tried it on any higher freq.
I've even proved it to a few other HAM friends by holding the stay while they call "CQ".
They expected to see me writhing in pain, but I didn't feel anything at all.
Steve
I'm sorry but this is not a smart trick. Standing on an insulated fiberglass deck may offer protection but it proves nothing. Under the right conditions you can badly burn deep tissues. I urge all readers of this thread not to try this stunt nor show it to others. We should not be showing people ways to injure themselves.

Dan
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Old 10-03-2013, 11:09   #9
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Re: Are standoffs necesary for hf on backstay?

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The end fed antenna has maximum CURRENT at the feed end, but maximum VOLTAGE at the other end.
I'd certainly NOT want to grab the top.

Steve
What you say is true for an antenna that is 1/4 wavelength at the driven frequency. If it is 1/2 wavelength, the voltage maximums will be at each end. I think maybe you were just lucky, assuming the radio was being driven at high power.

At other frequencies, the voltage max may be at different points along the antenna.

Chip
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Old 10-03-2013, 11:14   #10
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Re: Are standoffs necesary for hf on backstay?

And those conditions would be?

It's like a bull whip.
One end (the person holding and operating the whip) puts a lot of push into the handle, but only relatively slow speed, while the other end moves often at over the speed of sound. That's where the "crack" of the whip sound comes from.
Same theory. A tuned element has high current at relatively low voltage at the feed end, while it gradually transitions to high voltage at nearly zero current at the opposite end.

Yes, it was tuned before transmitting, and it was at full power of the Icom 735/AT120.

I did not say I was swimming around the boat and hanging on the backstay 3 feet above the water.

Steve
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Old 10-03-2013, 11:22   #11
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Re: Are standoffs necesary for hf on backstay?

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What you say is true for an antenna that is 1/4 wavelength at the driven frequency. If it is 1/2 wavelength, the voltage maximums will be at each end. I think maybe you were just lucky, assuming the radio was being driven at high power.

At other frequencies, the voltage max may be at different points along the antenna.

Chip
It's not the voltage that kills/harms.... it's the amperage....
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Old 10-03-2013, 11:32   #12
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Re: Are standoffs necesary for hf on backstay?

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It's not the voltage that kills/harms.... it's the amperage....
True, but you need VOLTAGE to get amperage through a resistance.
Ohm's Law.
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Old 10-03-2013, 11:44   #13
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Re: Are standoffs necesary for hf on backstay?

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And those conditions would be?
Not possible to list all the conditions for which this stunt would be safe or dangerous. A stunt is dangerous because the possibility for serious harm exists. Trying something that is dangerous and surviving without incident does not prove the thing tried was not dangerous. We must be careful when drawing conclusions from experiments that are flawed. Your experiment is flawed.

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It's like a bull whip.
Uh, no it's not like a bull whip. If you want to use that analogy the bull whip is fed energy at many frequencies from one end. The whip has a fixed length and therefore a small number of natural resonant frequencies. It filters out all the other frequencies and it pops at the natural resonant frequency. An HF wire antenna is only resonant at a relatively few frequencies like the bull whip. The transmitter is producing only a single frequency. The tuner makes up the difference which then makes the whole system (wire+tuner) resonant at whatever frequency the radio is tuned to.

Deadly and injurious voltages exist at the output of the tuner at any significant power. Just because your body was insulated and therefore did not conduct much current does not prove that the practice of bare handing the wire antenna is not dangerous. It is very dangerous. We need to end this discussion in an agreeable manner before someone gets seriously injured trying to prove or disprove your hypothesis. I have personally seen it proved more than once that burns will happen and that should be enough for anybody to never try this stunt.

I don't know what I can say to convince you but I hope other readers of this thread will heed my warning. Please, do not try what Senormechanico is suggesting. It is a really bad idea.
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Old 10-03-2013, 11:52   #14
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Re: Are standoffs necesary for hf on backstay?

I have worked professionally in electronics for most of my life, and am fully aware of what hi frequency and high voltage can do.

End of discussion.
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Old 10-03-2013, 13:31   #15
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Re: Are standoffs necesary for hf on backstay?

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I have worked professionally in electronics for most of my life, and am fully aware of what hi frequency and high voltage can do.

End of discussion.
To be blunt, you're wrong.

An end-fed random-length antenna will present a high impedance at some frequencies. For example, if the backstay happens to be a half-wave long between the top and the driving point, the drive-point impedance will be quite high. If the antenna is longer than a half-wave, there will be a high-voltage node some distance from the feedpoint, as well as at the top end.

This is why we need a wide-range antenna tuner: to provide an impedance match between the 50 Ohms of the transmitter output and the varying impedance (resistance and reactance) of the antenna feedpoint.

There can be many hundreds of Volts at these high impedance points -- enough to give you a serious R.F. burn.
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