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Author Topic: Shielding a high voltage line  (Read 5967 times)
KE5PPH
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« on: May 28, 2012, 12:46:35 PM »

Am I totally off base??   I have to mount my SGC inside the vehicle body. I'm thinking of  bolting it directly to the tailgate ( Jeep LJ). to help eliminate the possibility of stray RF inside the body tub, should I run the output line through some flexable conduit that is grounded?  I'd ground it to tuner base and tail gate itself as well.  I'm having to go this route, because Screwdrivers DO NOT hold up to serious rock crawling.   Any KISS Ideas will be greatly appreciated.
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AA4PB
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« Reply #1 on: May 28, 2012, 04:37:00 PM »

With an SGC (or similar) coupler, the antenna begins at the connection on the tuner. There really isn't a feed line since everything from the tuner to the tip of the antenna radiates RF. More important than shielding is adequate insulation and separation from metal vehicle parts. There can be some seriously high voltages present in such a system, even with 100W. If you are intending to run the "feed line" through the tailgate then you need to use a proper high voltage feed-thru insulator. In addition, the antenna mount needs to have a good insulator. The typical ball mount is not adequate. GeoTool makes some stake pocket mounts that have a good delrin insulator about 6-inches long that could be adapted to other mounts. You might also find some good heavy duty military mounts at one of the electronic surplus stores that would be suitable for off-road type usage.

Remember too that the feed impedance is often very high and any shunt capacitance between the antenna connection and the vehicle body will result in loss. You don't want to lay feed line against the body or use coax cable with a grounded shield.
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KE5PPH
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« Reply #2 on: May 28, 2012, 07:29:53 PM »

I was looking at a Breedlove mount and HF Insulator. I'm just trying to make sure every thing that comes out of the SGC, goes out to the antenna. Jeeps are pretty bad when it comes to electrical noise. Do you think that Heliax would work? since "coax" is not an option?
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K0BG
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« Reply #3 on: May 29, 2012, 02:58:57 PM »

Go to my web site, and look under Auto Couplers. It expands on what Bob, AA4PB, is saying.

Besides the need to be careful of the HVRF, you need to remember that the leads need to be short! That is inches, not feet!

You'd actually be better off RFI wise, to mount the coupler on the outside of the tail gate. If you do this, you need to use one of the waterproof ones, or mount the coupler in a weatherproof housing. The SG230 or SG235 are sealed well enough that they don't need and covering. The SG237 is questionable in this application.
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WX7G
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« Reply #4 on: May 29, 2012, 04:25:35 PM »

PPH, your plan will work. The electrical performance must be compromised to leave the SCG tuner inside the vehicle and the question is how much of a compromise is it?

An 8' whip at 4 MHz presents an input capacitance of 20 pF or a capacitive reactance of 2000 ohms. The tuner produces a voltage high enough to cause the desired antenna current though the whip. For 2.5 amps of antenna current - about what 100 watts will produce given losses of 15 ohms - 5000 volts is present at the base of the antenna.

Placing a feedline, or in this case specifically a coaxial transmission line of some sort, adds shunt capacitance thereby increasing the RF current out of the antenna tuner. This increases antenna tuner losses. But if you can minimize the capacitance of the coax you can minimize further heating of the antenna tuner. That means keep the coax short. One foot of RG-11 coax will add 20 pF and will double the antenna tuner output current. And yes RG-11 will take the voltage and current.
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AA4PB
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« Reply #5 on: May 29, 2012, 06:02:08 PM »

A 102-inch whip is already a compromise on the lower bands. Why compromise the system more by adding tuner loss caused by the capacitance of the coax? If the antenna is only a foot away then just use a single, well insulated, conductor for the "feed line". Keep it at least a few inches away from any metal and keep it centered in the tailgate hole with an insulator.

I'm guessing that you may be thinking of snaking the feed line around the edge of the tailgate, up tight against metal. I wouldn't recommend that.

A much better solution is probably to mount the tuner on the outside of the vehicle within a few inches of the antenna base. Then just run the RG58 coax (which is operating at 50 Ohms) from the tuner to the radio inside the vehicle.

I have an SGC tuner mounted on a metal bracket inside the bed of a PU truck. The antenna is on a stake pocket mount using a GeoTool insulator. The total length of the feed line is less than 6-inches and suspended 3-4 inches away from any metal body parts.

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KE5PPH
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« Reply #6 on: May 29, 2012, 06:30:30 PM »

The reason for trying it inside the body, was there's no place on the outside of the tailgate. the brackets for the tire carrier don't leave enough room. Inside however I can go directly thru the gate and only have about 4" fron coupler to Antenna conection. And I have the 237 model. I'm now thinking What ever guage wire, nested inside a couple concentric pieces of radiator type hose. I'll be punching a hole straight thru the tailgate itself.
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K0BG
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« Reply #7 on: May 30, 2012, 06:40:04 AM »

The thing you don't want to do is underestimate what high RF voltage can do! If you're going to drill a hole anyway, then use a proper high-voltage standoff (thru-hole insulator). Nebraska Surplus sells several different ones.
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WX7G
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« Reply #8 on: May 30, 2012, 08:46:54 PM »

You have the wire from the tuner to the antenna, now you need a return path. With the scheme you mention the return path is along the metal of the tail gate, through the hinges and so on back to the tuner. The way to do this is with a coaxial transmission line and the best you can do in terms of low capacitance is 75 ohm coax. An alternative is to run a wire to the antenna then run a return wire from the body at the base of the antenna to the case of the tuner.

The peak voltage at 4 MHz and 100 watts will be about 7 kV. The dielectric strength of air is 70 kV/inch but with field enhancement due to sharp points you may experience corona discharge with 1/4" spacing. I would keep the antenna wire spaced a half inch or more from metal objects. For HV creepage and clearance with dry insulators 5 volts/mil is often used and this works out to 350 mils or 0.35 inches.
« Last Edit: May 30, 2012, 08:53:33 PM by WX7G » Logged
M6GOM
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« Reply #9 on: May 31, 2012, 03:57:26 AM »

Do you think that Heliax would work? since "coax" is not an option?

Heliax IS co-ax...

 Roll Eyes
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M6GOM
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« Reply #10 on: May 31, 2012, 03:59:04 AM »

The thing you don't want to do is underestimate what high RF voltage can do! If you're going to drill a hole anyway, then use a proper high-voltage standoff (thru-hole insulator). Nebraska Surplus sells several different ones.

TBH if he's going to drill holes, I'd just get a screwdriver antenna.
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AA4PB
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« Reply #11 on: May 31, 2012, 05:49:06 AM »

WX7G: I disagree with using the coax shield as the "ground return path" for the tuner. The tuner needs to be bonded to the tail gate and the tail gate needs to have bonding straps to the vehicle body (which is why I would prefer mounting to a non-movable component rather than the tail gate). Any type of coax between the tuner and the antenna in this application is not a good thing in my opinion.

OP: Have you considered other mounting locations for the antenna like on the side of the vehicle where you can mount the tuner on the inside, close to the mount?
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KE5PPH
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« Reply #12 on: May 31, 2012, 10:55:50 AM »

GOM...IT FAILED to stand the test. Keep up!

It's a JEEP, look at one in the parking lot some time. The only flat spot inside is tailgate,or the floor if you never haul anything. Bonding the Coupler mounting, really isn't a problem. keeping the High voltages contained and getting to the Antenna properly is the challenge. The feed thru insulators I have found so far, won't work. they are for less than 1.5" thick material or built for a battle ship hull.  Is there a better SGC model number the Ya'll think might survive mounted on the outside of the body tub? Weather would probably be the most likely cause of damage, but there's no practicle way to shield it.   The rason I thought about Heliax was that it might provide the needed insulation, if I just used the center conductor, and not the braided shield.
« Last Edit: May 31, 2012, 11:24:41 AM by KE5PPH » Logged
AA4PB
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« Reply #13 on: May 31, 2012, 12:58:39 PM »

I have an SGC-237 mounted in the weather on the pick up and haven't had any problems. I did add a light grey plastic shield over it to keep it out of the direct sunlight and cool it down. Repainting the SGC cover light grey may have accomplished the same thing.

I wouldn't mount it under the vehicle where it would get sprayed with water from the road but vertically on the tail gate it should be okay.

I used the center conductor from RG213 as my connecting lead. I put heat shrink tubing over it for added protection from the weather.
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KCJ9091
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« Reply #14 on: May 31, 2012, 02:14:54 PM »

I mounted my FC-40 in the rear most compartment on the service bed.  I could have mounted outside on the rear of the box right under the antenna and had an inch or two of wire from the tuner to the antenna as long as the tuner would have stayed in place and not have been stolen.
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