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Old 23-10-2020, 19:49   #1
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Shielding a HF antenna wire.

One for the HF crew.

Iíve found a very nice place to put my ATU which would locate it less than a meter from the base of the HF whip and about the same distance from the ground plane.

But...

If I do it this way the feed wire from the ATU to the antenna will cross (at exactly 90 degrees) a small bundle of critical wires that lead to the stern of the boat.

These include the Raymarine radar cable, a VHF antenna cable (backup VHF antenna, not the primary which is on the mast head), the TV antenna feed (not really all that important since I donít watch tv) plus a handful of DC wires for the rear solar, the wind gen and the stern light.

What is the feeling about shielding those wires from HF, and how close to them can I afford to go?

Would I try to shield the HF antenna feed itself, or the bundle of cables? If so, how long should the shielding be?

Over to the crew...
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Old 24-10-2020, 01:48   #2
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Re: Shielding a HF antenna wire.

In theory there is no electromagnetic coupling between two conductors crossing at 90 degrees. I expect you will see much more coupling from the HF antenna radiation than the antenna feed line. If interference occurs you would need to shield the non-HF cables.
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Old 24-10-2020, 02:02   #3
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Re: Shielding a HF antenna wire.

What is the air gap between the proposed antenna feed wire and the other bundle (measured where they cross)?

If it isn't say an inch or two, try to make it greater than say 50mm.

I don't think it necessary to shield the bundle but if you do decide to do it, then shield maybe 300mm either side of the X point. Don't shield the HF feed wire.
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Old 24-10-2020, 04:12   #4
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Re: Shielding a HF antenna wire.

Thanks guys, sounds reassuring.

I think Iíll be able to get more than 50 mm of clearance. If not, Iíll shield the bundle and not the HF feed as advised.
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Old 24-10-2020, 08:57   #5
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Re: Shielding a HF antenna wire.

Most hf whips are fed by a coaxial cable. If this is the case you are already shielded in the main. Use the best quality coax you can find because the standing wave will be high at the frequencies at which the whip is not resonant.
If the line from atu to whip is to be a plain wire then I suggest using stranded automotive ignition wire inside a rubber fuel line hose. This is a practical protection against electrocuting the crew. Keep that wire as far as possible from all other conductors. Any conductor closer than the intended ground plane will become part of that groundplane including all devices at both ends. So yes shield them with metal tubing or a wire sleeve to ground. All of these expedients will add to lightning protection as well. KC3LZR
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Old 24-10-2020, 08:58   #6
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Re: Shielding a HF antenna wire.

What will you use to shield the bundle?
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Old 24-10-2020, 09:05   #7
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Re: Shielding a HF antenna wire.

Quote:
Originally Posted by GILow View Post
One for the HF crew.

Iíve found a very nice place to put my ATU which would locate it less than a meter from the base of the HF whip and about the same distance from the ground plane.
[...]
Would I try to shield the HF antenna feed itself, or the bundle of cables? If so, how long should the shielding be?

Everything from the insulator atop the tuner onward *IS* the antenna, and can't/shouldn't be shielded.



As others replied, space other wires away as much as possible, shielding them would not hurt, but realize you are dealing with a part of the antenna.
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Old 24-10-2020, 09:29   #8
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Re: Shielding a HF antenna wire.

Electrically shielding the bundle: check this for a source or info on one type of product to look for: .
If it were me, I'd add a braided ground wire from that shielded bundle to the boat's ground. And then mechanically protect the whole mess with a split sided plastic accordion shield. Broader discussions of the 'RF in the shack' problem are an important topic in the Amateur Radio Relay League's antenna forums.
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Old 24-10-2020, 11:07   #9
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Re: Shielding a HF antenna wire.

I'd not worry too much about this until you test for any problems. The radar and VHF receivers are quite sensitive but to very different frequencies, the main issue is not exceeding a level that would actually damage them. But those both have their own shielding in the wires you are looking at.

DC wires collecting some RF can feed it back into any items on the boat - we had trouble with CO detector going off when I transmit on certain frequencies.

I think the foil wrapping is a great idea and not too difficult. If you can wire it to an RF ground like rudder post or thru-hull (not just a battery DC return wire), do that.

If you experience problems after that, then you could add ferrites around the whole bundle or individual wires. They can be bought in various sizes that can clip on over wires. They are lossy to RF frequencies.


Off-topic, but someone mentioned RF protection for crew (and captain!). Your feedwire to the whip should be about #10 stranded tinned copper, plastic insulated. And you can put any sort of insulator over the whip itself without affecting it's operation. Maybe use the same plastic stuff available for going over SS rigging wire.
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Old 24-10-2020, 13:51   #10
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Re: Shielding a HF antenna wire.

So far I agree with everyone who said not to shield the GTO15. And I agree with the comment that if the RF is 90į to the other cables you shouldn't have too much interference. I also agree that if there is interference it is probably coming from the actual antenna. It was mentioned a 50mm (2 inches or so) is good, but if you can get 75mm (3 inches or so) it should be a little better.

Now remember that anything that is metal near the base of the antenna (the GTO15 connected to the tuner is the base of the antenna as it is also radiating RF energy) could detune the antenna. The antenna tuner will compensate for that but there is a slight chance the compensation will be off some, thus changing the impedance at the base of the antenna.

I am sure you know this but repeating it just for taking up space here LOL. The purpose of the tuner is two fold. First, it tricks the radio into thinking it is working into a 50Ω load. That makes the radio VERY HAPPY. Then it tricks itself into thinking the antenna is a beautiful 50Ω impedance, which we know it is not. So the tuner uses the magic of inductors and capacitors, combined with the impedance of the wire connected to the tuner, to find a very nice match for a 50Ω impedance, or as close as possible. So placing something metallic might change the second half of the tuners job to find a good match for that wire connected to tuner. But this is probably an over statement and you should be fine.

I would recommend going with distance from the other cables before using anything metallic as a shield. But, your call as you are the one who is looking at the available space.

JMHO and good luck!

If you are curious about the impedance of just the wire connected to the tuner, you can use a Vector Network Analyzer (like the NanoVNA) to check it. Nice piece of test equipment, inexpensive, and small so it doesn't take up space in your vessel. It will also measure SWR upto 900MHz.
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Old 24-10-2020, 14:59   #11
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Re: Shielding a HF antenna wire.

GTO 15 is not a coax cable and is not shielded like a coax cable but is a high voltage rated wire, Ancor GTO 15 is rated at 15,000 volts. It is insulated but not shielded (no copper or aluminum foil) I can’t say what a whip antenna wire is supplied with but if it meets the same specs it is not a coax nor is it shielded. I agree that crossing at right angle and trying to get a bit of space is a good idea and I would try it and see about results.
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