Mounting insulators for SSB with a gimballed radar

[Alls Well]

Just purchased an Icom 802 with tuner and want to mount insulators on my backstay. I have a gimballed radar (Simrad 4g) mounted about 10' up on the backstay and also a backstay adjuster(Sailtec)at the lower end of the backstay.
Does anyone have experience with this? I'm wondering if there are any issues with the antenna and radar? Also, is it alright to mount the lower insulator below the gimballed unit, rather than above?
Any advise would be appreciated.

[roverhi]

You might get by without installing the lower insulator. If the chainplate is not bonded or otherwise attached to a ground, the lower insulator isn't needed for the backstay antenna to work. Of course you need to warn people not to hang onto the backstay while you are transmitting. Not sure what effect the radar gimble mount might have. If it grounds the backstay, could suck up the RF from the radio. Install the upper insulator a foot or more down from the mast head and see how the radio works. Mounting the insulator below the radar mount would be the same as not having the insulator if the radar mount is going to be a problem. FWIW, the antenna lead is hot from the tuner. Insulation on the antenna lead keeps it from grounding out but still emits RF energy.

[robert sailor]

You can mount the lower insulator as low as you like, it can be just above deck level if you like. No issues with the radar, I had a similar set up on my second last boat.

[btrayfors]

With the backstay adjuster inline, the "feed belowdecks" solution might not be viable.

Another possibility, which avoids the need for expensive insulators altogether, is to use an "alternate backstay". I have used, installed, and written about these for many years.

Basically, you use a length of insulated s/s lifeline, strung from near the top of the mast with a spare halyard to the pushpit on either side (whichever side works best for you, and allows the tuner to be installed underdeck nearby). This solution will work providing that it can be done so that the mainsail roach clears the alternate antenna OK.

I've used this solution for over 20 years, and while my newish mainsail with full battens and a sizeable roach doesn't quite clear, it works just fine because I'm not tacking 20 times a day every day which could cause wear at the roach.

Bill

[Paul Elliott]

Originally Posted by btrayfors

With the backstay adjuster inline, the "feed belowdecks" solution might not be viable.

I've got a hydraulic backstay adjuster between the deck and the insulator, and my SSB antenna feedwire runs through a deck gland next to the chainplate. It works as long as you have enough slack in the feedwire to accommodate the maximum backstay extension. This means that with the adjuster cranked down tight there is quite a bit of slack in the feedwire. I have my insulated standoffs arranged to keep the slack out of the way (usually).

Sorry, I have no experience with the radar mount question. I would rather see the insulator above the radar, since this will reduce the chance of the SSB interfering with the radar signals / controls, and might make the tuner's job easier, but perhaps this isn't a real problem...

[Alls Well]

Originally Posted by robert sailor

You can mount the lower insulator as low as you like, it can be just above deck level if you like. No issues with the radar, I had a similar set up on my second last boat.

Robert sailor, did you have a gimballed radar? Did you have any issues with the operation of the radar while transmitting on your SSB? Did you have to be careful not touching the backstay while transmitting SSB?
Thanks


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[W3GAC]

The bottom insulator should go above the (gimbaled) radar if it's mounted on the backstay. The radar ground/ wiring with shunt your RF receive/ xmit signals to boat ground. Generally speaking you like to have about 31-33 feet of insulated backstay. With many antenna tuners more (or less) than this length is not better. If your antenna tuner output is coax... then just run it with tie-wraps up the lower (grounded) part if the backstay, over the lower insular and connect the center wire of the coax to the upper (insulated) part of the backstay. The shield of the coax should be connected to the lower (grounded) side of the insulator. If the output of your tuner is a single wire... it should be spaced 3-4 inches away from the lower (grounded) part of the backstay/ around the radar gimbal and attached to the backstay above the lower insulator.
If you system uses the single (unshielded) method, be aware your crew can get a nasty RF burn if they grip any part of that wire during a transmission... right through the wires insulation. Coax fed backstays don't have that issue because they have a grounded shield that keeps all the RF energy inside the coax cable along the route until the point where the center connector is connected to the insulated backstay. (Don't touch the insulated backstay either during transmission!)

[btrayfors]

Originally Posted by W3GAC

.......If you system uses the single (unshielded) method, be aware your crew can get a nasty RF burn if they grip any part of that wire during a transmission... right through the wires insulation. Coax fed backstays don't have that issue because they have a grounded shield that keeps all the RF energy inside the coax cable along the route until the point where the center connector is connected to the insulated backstay. (Don't touch the insulated backstay either during transmission!)

The backstay should be fed with GTO-15 transmission line. This is a single-conductor heavily insulated cable....actually neon sign cable. The "15" means 15,000 volts insulation. You certainly will not receive a burn thru this cable, nor do you have to worry about it passing close to a grounded part of the system.

The issue of RF burns has taken a life of its own. I've never (in over 50 years of hamming while MM) heard of anyone injured by a RF burn from the rigging. And, because high voltage points and high current points are different, and are distributed along the antenna, you'd have to grab the backstay in just the right place to feel it at all.

Again, I'd urge you to consider an alternate backstay which avoids all the costs and problem(s) with a traditional backstay with an adjuster and a gimbled radar antenna.

Bill
WA6CCA

[Jim Cate]

Again, I'd urge you to consider an alternate backstay which avoids all the costs and problem(s) with a traditional backstay with an adjuster and a gimbled radar antenna.

Very good advice! By far the best solution IMO.

Jim

[Alls Well]

WG3AC and btrayfors, thanks for your reply.
I will be using the gto15 wire from my icom 140 tuner to my insulator. Is it necessary to stand-off the wire if it is totally insulated?


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[Paul Elliott]

Originally Posted by Alls Well

WG3AC and btrayfors, thanks for your reply.
I will be using the gto15 wire from my icom 140 tuner to my insulator. Is it necessary to stand-off the wire if it is totally insulated?

Standoffs are a good idea, especially if the GTO runs alongside the backstay for more than a foot or so. Otherwise, the tuner *may* not be able tune at some frequencies. Often it's not a problem, but with standoffs it's less likely to be an issue.