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Old 28-10-2013, 15:30   #1
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Coax cable between SSB tuner and backstay antenna

I am renewing the connector wire between my ICOM AT 120 SSB tuner and the insulated backstay antenna on a sailing yacht. The current wire is a coax cable with just the core connected at each end.

Any ideas why a coax would have been used? Is it to prevent the connector wire acting as part of the antenna? Should I replace with coax?

Any help appreciated
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Old 28-10-2013, 15:53   #2
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Re: Coax cable between SSB tuner and backstay antenna

Coax was used because someone was lazy or ill-informed. GTO-15 is the standard for connecting the tuner to the backstay. You can get GTO-15 from any business that installs neon signs.
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Old 28-10-2013, 16:08   #3
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Re: Coax cable between SSB tuner and backstay antenna

If it has worked well over time then it would have been near enough to being a short between the centre & shield & would act like a single wire. Practical & provided a connection that is UV stable.

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Old 28-10-2013, 16:16   #4
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If it has worked well over time then it would have been near enough to being a short between the centre & shield & would act like a single wire. Practical & provided a connection that is UV stable.

Regards Bill
Coax with the shield left unconnected is a common firm of feed wire in Europe , GTo15 is just high voltage wire , any wire with voltage ratings in the 5 to 10kv , will do , even ordinary600v wire will do if there's no grounds nearby.

Dave
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Old 28-10-2013, 19:10   #5
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Re: Coax cable between SSB tuner and backstay antenna

I doubt this was coax. It's easy to mistake GTO 15 for coax, since they look very similar. Replace with GTO 15. Chuck
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Old 29-10-2013, 03:15   #6
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Re: Coax cable between SSB tuner and backstay antenna

It seems GTO-15 has a "Holy" status....

I used to use the ineer core and it's insulation of RG-213 coax - I guess this will have a good high voltage insulation! - but the inner core insulation is not UV stable and cracks.
Now I use marine garde RG-58, and I connect inner core and shiled together at both ends.

Easy, cheap. Never any issues.

By all means use standoffs if you have to lead this close to any metal (pulpit; lower part of backstay).

Jan

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Old 29-10-2013, 03:16   #7
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Re: Coax cable between SSB tuner and backstay antenna

mmmh....typing too fast....
Inner core - marine grade - shield - ....
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Old 29-10-2013, 04:16   #8
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I am renewing the connector wire between my ICOM AT 120 SSB tuner and the insulated backstay antenna on a sailing yacht. The current wire is a coax cable with just the core connected at each end.

Any ideas why a coax would have been used? Is it to prevent the connector wire acting as part of the antenna? Should I replace with coax?

Any help appreciated
Streetcar,

Coax with the outer conductor left open at both ends is ok to feed your back stay. The cover and outer conductor protect the inner insulation from UV. The coax is still part of the antenna. Eventually sunlight will damage the end and you may have to keep cutting off and reterminating at the back stay. Use a couple layers of shrink tubing and dielectric grease to seal the outer conductor from moisture at both ends.
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Old 29-10-2013, 15:42   #9
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Re: Coax cable between SSB tuner and backstay antenna

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Originally Posted by goboatingnow View Post
Coax with the shield left unconnected is a common firm of feed wire in Europe , GTo15 is just high voltage wire , any wire with voltage ratings in the 5 to 10kv , will do , even ordinary600v wire will do if there's no grounds nearby.
Not my experience in Europe. Also not a good idea. The radiating antenna starts at that output bolt of your antenna tuner. The shield on coax just blocks some of your radiating element. Not so good.

There isn't anything holy about GTO-15. It just works - high voltage single conductor wire. Grab some from anywhere that supports neon signs. Cheaper and better than any alternatives other than bell wire, and great insulation.
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Old 29-10-2013, 16:29   #10
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Re: Coax cable between SSB tuner and backstay antenna

Co-ax works but the insulation value of the casing is a bit low that's why GTO15 is recommended. Round wire tends to present a higher impedence at RF frequencies which is why you use flat tape for the earth. The shield on co-ax being braid has much more surface area than the centre wire so some poeple use that as the conductor.
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Old 29-10-2013, 17:42   #11
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Re: Coax cable between SSB tuner and backstay antenna

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[...] The shield on coax just blocks some of your radiating element. Not so good. [...]
As long as the shield is floating there should be no blockage of the signal. It is capacitively coupled to the center conductor, giving you, in essence, a fatter conductor.
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Old 30-10-2013, 03:43   #12
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Re: Coax cable between SSB tuner and backstay antenna

So what would there against using coax (appart from high voltage insulation properties) ...

...but having the outer shield and inner lead connected at both sides...

indeed inclusing the outer shield braid increases the copper and increases the diameter of the conductor.

Jan
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Old 30-10-2013, 09:04   #13
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Re: Coax cable between SSB tuner and backstay antenna

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Originally Posted by Goudurix View Post
So what would there against using coax (appart from high voltage insulation properties) ...

...but having the outer shield and inner lead connected at both sides...

indeed inclusing the outer shield braid increases the copper and increases the diameter of the conductor.

Jan
You said it: high voltage insulation properties. The insulation over the shield is pretty thin, since normally the shield is at a low voltage potential. The coax when connected in this way may be more difficult to protect against water intrusion and corrosion, but no doubt with enough heat-shrink, grease, tape, etc. you could do OK.
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Old 30-10-2013, 13:31   #14
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Re: Coax cable between SSB tuner and backstay antenna

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As long as the shield is floating there should be no blockage of the signal. It is capacitively coupled to the center conductor, giving you, in essence, a fatter conductor.
I'm sorry Paul, it just doesn't work that way. A floating shield won't be as good a shield as on grounded, but it isn't an effective radiator.
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Old 30-10-2013, 18:35   #15
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I'm sorry Paul, it just doesn't work that way. A floating shield won't be as good a shield as on grounded, but it isn't an effective radiator.
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.
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