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Old 10-03-2013, 13:38   #16
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Re: Are standoffs necesary for hf on backstay?

Back to the original question, without the stand-off insulators the closely-coupled feed wire and grounded backstay will create a low-impedance section, probably mostly capacitive at the frequencies and dimensions we are dealing with. There probably won't be much loss in this section, but it may be difficult for your tuner to deal with. Antenna tuners *do* have loss, and for some conditions it can be higher than you would like.

So, it will probably work without the standoffs, and it will probably work well at most frequencies. There may be some frequencies where your tuner will not be able to provide a match, or where the loss in the tuner is high.
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Old 10-03-2013, 15:36   #17
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Re: Are standoffs necesary for hf on backstay?

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Old 11-03-2013, 00:51   #18
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Quote:
Originally Posted by Paul Elliott View Post
Back to the original question, without the stand-off insulators the closely-coupled feed wire and grounded backstay will create a low-impedance section, probably mostly capacitive at the frequencies and dimensions we are dealing with. There probably won't be much loss in this section, but it may be difficult for your tuner to deal with. Antenna tuners *do* have loss, and for some conditions it can be higher than you would like.

So, it will probably work without the standoffs, and it will probably work well at most frequencies. There may be some frequencies where your tuner will not be able to provide a match, or where the loss in the tuner is high.
Thanks Paul.
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Old 12-03-2013, 23:01   #19
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Re: Are standoffs necesary for hf on backstay?

Just to take sides, Paul Elliot has described the situation perfectly. There are antenna's of all types, some are voltage fed high impedance at the driven end, some are current fed low voltage. In the boats case the tuner makes adjustment to accommodate the varying frequency with the fixed length antenna.

Just to add a bit more data: If you drive the backstay directly, or wrap the antenna wire directly around it, closely, you are inducing currents in the back stay. The stay is connected at the mast head through swivel connection. The swivel action can look like a varying resistance, or disconnect momentarily in a sea way as the mast swings. When this happens the antenna tuning changes dramatically. Your listeners will likely experience rapid fading of the signal. Your tuner will rapidly attempt a re tune and then be wrong when the connection changes again. On top of that issue your are energizing the mast and are now surrounding yourself in a RF field. More importantly, you are bathing other gear, like your autopilot, in more RF then it may prefer for reasonable operation.

All that said, it works, YMMV. I have used standalone masts, direct connections, wire spaced from the B.S., and hitech kevlar backstays with wire tucked under the core. You can get them all to work. Note what your tuner instructions say regarding length. They normally suggest a length to avoid 1/2 wave resonance at the base of the antenna so that you don't see very high voltages at the tuners output, which could ruin the tuner.
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Old 13-03-2013, 07:57   #20
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Re: Are standoffs necesary for hf on backstay?

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Originally Posted by brianb00 View Post
The swivel action can look like a varying resistance, or disconnect momentarily in a sea way as the mast swings. When this happens the antenna tuning changes dramatically. Your listeners will likely experience rapid fading of the signal. Your tuner will rapidly attempt a re tune and then be wrong when the connection changes again.
That is not the case with Icom tuners. Once tuned, they will not retune while you are transmitting. In fact, they are not even monitoring swr while you are in communication. Your signal may still change due to your changing antenna parameters and your radio possibly cutting back power due to high swr, not because the tuner is attempting a retune. SGC and SEA tuners on the other hand will automatically retune when swr exceeds 4:1 while you are communicating.

Eric
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Old 13-03-2013, 09:52   #21
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Re: Are standoffs necesary for hf on backstay?

I have a standard rigging insulator near the top, and the autopilot is not in the least affected, nor is anything else in the boat.
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Old 13-03-2013, 10:28   #22
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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.

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.

Steve
Now you've got me wondering--are you talking about the dragonfly tri? When I look at the dragonfly rigs, I think I see twin stays going out to the amas, both split on the bottom end. Is one of those what you are driving? When you say no insulator, do you mean the bottom of the stay(s) is just attached to fiberglass which acts as an insulator?

If your electrical panel bulbs don't light up when you transmit at full power, yours would be the first boat I've been on that doesn't.

I think the "30 thousand volts" at the tuner is just lawyer language if you are only using 100 watt transmitters. I used regular electrical cable as a feedline and never had any problems.
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Old 13-03-2013, 11:47   #23
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Re: Are standoffs necesary for hf on backstay?

Don,
Both my previous boat, a Lord Nelson 35 and my Dragonfly have been set up the same way, an insulator near the top and the stay driven at the bottom with the tuner mounted only about a foot away from the chainplate.
In the LN35, it drove the backstay chainplate and in the Dragonfly, the port cap shround.

On the LN35, the lighting outputs did get modulated a bit due to the current draw of the transmitter, and on certain bands, our stereo speakers would bark a bit even though the stereo was off.
I fixed that by installing shielded speaker wires.
Also, the fridge would start its cycle when the transmitter was keyed.

With the same antenna configuration, the dragonfly has no issues whatever.
I suspect the lithium batteries are the reason for no discernable voltage fluctuation which can cause all kinds of interaction between systems.

See the thread about LiFePo4 batteries for house banks.
I'm a believer in these cells.

The first thing I did after the radio installation on the Dragonfly was to see if any other boat system acted up.
All of them worked normally.
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Old 13-03-2013, 14:11   #24
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Re: Are standoffs necesary for hf on backstay?

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Originally Posted by donradcliffe View Post
I think the "30 thousand volts" at the tuner is just lawyer language if you are only using 100 watt transmitters. I used regular electrical cable as a feedline and never had any problems.
I think you're right. The impedance numbers I've seen (read about, not personally measured or calculated) for an end-fed half-wave antenna range from (roughly) 1800 to 5000 Ohms. Assuming a transmitter power of 150W, and a 5000 Ohm impedance, the voltage will be the square root of (150 * 5000), or 866 Volts. This is the likely worst-case high-voltage we will see on our backstay antenna. If we run the numbers for 100W and 1800 Ohms, we get 424 Volts.

It seems likely that the voltage will be well under 1000V, but certainly 400V of RF can still cause serious injury. Obviously the half-wave case is the worst for high voltage, and other lengths (frequencies) can see much lower voltages.
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