Insulated backstay question for SSB experts

As I said earlier, the wire feeding the antenna doesn't actually get hot, nor does the antenna itself. It is the RF energy that will produce heat in tissue, by exactly the same mechanism as a microwave oven works. That is, the RF energy hitting the tissue in your hand will be absorbed by the tissue giving rise to localised heating inside the tissue. This would make you think the wire is 'hot' when it fact it's your hand that's hot. Although 150 watts of RF energy should not be enough to cause permanent tissue damage, it is advisable not to be holding the antenna or antenna feed during transmission.

I doubt that 150 watts would be enough to ignite a cigarette but it might happen. It would happen by exactly the same mechanism: localised heating from absorbed RF energy.
 
prv said:
He's describing a method of mounting the feeder cable on standoffs a couple of inches long, rather than strapping it tight along the non-aerial lower part of the stay. A similar method I've seen is to use short lengths of plastic pipe, with a long cable tie round the wires at each end and through the pipe in the middle:

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If you simply strap the feeder to the lower part of the stay, you are connecting the two together. RF is not DC, it is not particularly impressed by the few mm of rubber between the two conductors.

Pete
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Yep, that's the way to do it, but I haven't found anything to make the standoffs from that doesn't fall apart in a season due to UV degradation.
 
Yep, that's the way to do it,
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Ah, I see, I hadnt imagined such a large gap between the base of the backstay and the connector. I was told to avoid this at all cost, and this is also made clear in the Icom tuner installation manual. The gap is far too large. The insulator should be on the base of the stay so the feeder wire has no more travel than through the deck and over the tang and isolator onto the stay. Well that is how mine is arranged. I will try and manged a photo at the weekend.

I am also not sure about the tape - it is a sure source of corrosion under the tape; the suggestion put to me was to avoid tape and just bond the wire to the stay with a copper ferrule.

Whether all these refinements make a huge difference I dont know - I have nothing scientific to support the argument but I can say the installation works really well for me - I get almost no interference and remarkable transference. I also used a KISS-SSB grounding aerial with which I have been very pleased and greatly simplified the installation.
 
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jerrytug said:
Perish the thought, but some bits of wood? ;)

Thanks for all your learned replies to my question anyway, Jerry
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Why not tape over the plastic spacers with PVC tape so that the PVC degrades and not the spacer? I put PVC tape over the self amalgamating tape at the base of my VHF aerial and then cover it in scotchcote for exactly that reason.
 
ip485 said:
Ah, I see, I hadnt imagined such a large gap between the base of the backstay and the connector. I was told to avoid this at all cost
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I haven't heard that specific admonition - just proves how much of this stuff is black magic and old wives' tales :)

The advice I've seen is to have the lower insulator above casual hand height, to reduce the shock risk. Obviously this involves a reasonable length of feeder cable parallel to the wire below the insulator, hence the importance of the standoffs to avoid losing power into that wire.

Maybe your instructions want a minimum length below the insulator because they're assuming people will strap the feeder tightly against it? So they're solving the same coupling problem in a different way.

Pete
 
I use a setup just about the same as the picture already posted above.
My insulator is very close to the lower end of the backstay, perhaps 1m of stay below the insulator. The insulator is just above the pushpit rail. I use standoffs just like those pictured, using plastic tube and cable ties to secure. As I demast each year they don't need to last longer, but I think they would. I only replace the cable ties.
For connection of feeder to stay (above insulator), I use a cable clamp. Feeder total length perhaps 1200mm. Tuner inside hull at stern. Feeder through a gland.
Top insulator is about 500mm down from masthead.

Antenna does not get hot in terms of thermally hot, but Radio Frequency "Hot" - ie high voltage. If you hang on to a HV cable, its not hot, but you get burnt. That's the same with VHF too. RF burns can be quite nasty, but a bit unpredictable. The voltage isn't the same all along the length of the antenna. There are parts that you could (theoretically) touch, and other parts that are the highest voltage and give worst burn. So you might hear someone say that it didn't burn. That's chance; if they slid their hand up or down probably would have suffered the burn.

I would not insulate the other backstay (if I had two), but would recognise that this would give the antenna some directional properties. Not easily calculated, and would change for different frequencies, but likely to have higher signal output away from the non-fed stay. So your best signal might be heard by receivers 90 degrees off your port side (assuming the port side stay is the antenna, vice versa if not).
 
Can I ask what the correct cable to use to go from tuner, through gland to backstay above insulator?

Could we not use UV resistant tubing for the antenna cable and the stand-offs?

TudorSailor
 
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tudorsailor said:
Can I ask what the correct cable to use to go from tuner, through gland to backstay above insulator?

Could we not use UV resistant tubing for the antenna cable and the stand-offs?

TudorSailor
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The cable used between the Tuner and the connection to the backstay will form part of the antenna, and will be "live" - that is will be transmitting. Its not critical what cable to use, but get plenty of cross section of copper. So a heavier copper wire of some type. One option is to use coaxial cable but bind the braid and inner together at both ends. (That what I have, and mainly as it was available and fits the gland nicely).

Don't forget that a tuner has two connections. The antenna one and another. Sometimes called earth, this goes to one of several options. 1. Wires inside the hull sometimes called counterpoises. 2. Bronze sintered plate immersed in the sea. 3. Some other connection to "ground" (sea). For eaxmple a steel or aluminium hulled boat it might just be a connection to the hull.
 
MM5AHO said:
Don't forget that a tuner has two connections. The antenna one and another. Sometimes called earth, this goes to one of several options. 1. Wires inside the hull sometimes called counterpoises. 2. Bronze sintered plate immersed in the sea. 3. Some other connection to "ground" (sea). For example a steel or aluminium hulled boat it might just be a connection to the hull.
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Presumably the "earth" is DC isolated?

Mike.
 
tudorsailor said:
Can I ask what the correct cable to use to go from tuner, through gland to backstay above insulator?
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They sell suitable cable at yachtcom.com and all the other stuff. I got mine from a neon sign installer. It needs high voltage insulation specs.

I was strongly advised against the use of coax.
 
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I mentioned my installation earlier - and you can see here the insulator is very close to the deck - I originally used stand offs but there was no difference without so I have reverted to cable ties. The tuner is almost directly beneath the backstay so the "red" cable is very short.

Being centre cockpit I guess it does have the advantage of being well away from the crew.
 
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