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#1
posted to rec.boats.cruising
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Ham radio backstay antenna feed line
I'm replacing the antenna feed line between my antenna tuner and
insulated backstay. It's currently a coax cable but then I read that I should use insulated antenna wire and NOT use coax for the antenna feed line. This raises some questions though, isn't the whole reason to use coax because it's shielded which prevents the feed line from radiating and causing RF burns in the cockpit? The source I read it on is: http://www.geocities.com/bill_dietrich/Radio.html "from the output terminal of the tuner/coupler you want insulated VERY high voltage antenna wire (usually about 14 or 16 gauge) led to a point on your backstay just above the insulator ... You CANNOT use coax for this part of your antenna lead." Thanks, Todd -- http://sailsugata.com s/v Sugata Hans Christian 38 Mark II http://windandtides.com San Francisco Sailing Weather http://gearandboats.com Free San Francisco Boating Classifieds |
#2
posted to rec.boats.cruising
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Ham radio backstay antenna feed line
On 25 Mar 2007 10:06:08 -0700, "thuss" wrote:
"from the output terminal of the tuner/coupler you want insulated VERY high voltage antenna wire (usually about 14 or 16 gauge) led to a point on your backstay just above the insulator ... You CANNOT use coax for this part of your antenna lead." Lets just say that you should not use coax for the antennal lead since the lead becomes part of the antenna and you want it to radiate. It does need to be insulated however to prevent RF burns to those in the cockpit. There is special wire made for that purpose which you should be able to get at your local marine electronics dealer, but in a pinch you could take some large coax cable like RG-8 or RG-11, strip off the outer insulating jacket and coper braid, leaving only the insulated center conductor. |
#3
posted to rec.boats.cruising
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Ham radio backstay antenna feed line
Thanks for clarifying Wayne. I had a look and it appears as if West
Marine carries the correct cable called GTO 15 made by Ancor: http://www.westmarine.com/webapp/wcs...-1/10001/22784 I'll be curious to see how this effects my HAM radio performance since the original installation had coax as the feedline! Thanks, Todd -- http://sailsugata.com s/v Sugata Hans Christian 38 Mark II http://windandtides.com San Francisco Sailing Weather http://gearandboats.com Free San Francisco Boating Classifieds On Mar 25, 11:58 am, Wayne.B wrote: Lets just say that you should not use coax for the antennal lead since the lead becomes part of the antenna and you want it to radiate. It does need to be insulated however to prevent RF burns to those in the cockpit. There is special wire made for that purpose which you should be able to get at your local marine electronics dealer, but in a pinch you could take some large coax cable like RG-8 or RG-11, strip off the outer insulating jacket and coper braid, leaving only the insulated center conductor. |
#5
posted to rec.boats.cruising
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Ham radio backstay antenna feed line
"Larry" wrote in message ... wrote in news:1174870222.221326.262340 @n59g2000hsh.googlegroups.com: I'll be curious to see how this effects my HAM radio performance since the original installation had coax as the feedline! It won't help. Boats running low power (150W) and end-fed, untuned short wire antennas have SUCKY signals, no matter what you do....especially on the lower HF bands. A 55' backstay, between the insulators, is an antenna at 8.5 Mhz, if you have a good ground on the tuner (1/4 wavelength) and a great 1/2 wavelength antenna at 17 Mhz, where it doesn't even need a ground system to work against. At any other frequency band, especially below 8 Mhz, the antenna is way too short to fit the RF wave onto and the lower you go the worse it gets. Between 8.5 and 17 Mhz, the antenna is a complex impedance with a lot of inductive reactance. The wave doesn't fit well the further away from 8.5 and 17 Mhz you get, killing its radiating potential. Use 468/ft length = Mhz to determine what resonance is for your backstay. It'll work fair there on 150W. Far away from there in frequency, it'll suck, just like everyone else's. Hams use resonant antennas for a reason...(c; It might help, and shouldn't hurt. Larry, the reason ham use resonant antennas is because they can. All things equal, a resonant antenna will perform better, because there will be less loss in matching networks, ground systems, feedline loss, etc. HOWEVER, on a boat (on *my* boat, anyway), puttiing up, and keeping up, a resonant antenna for all the frequencies I regularly use is too much of a nuisance. I have an insulated backstay antenna, about 50 ft long, and a pretty good ground system. I have an Icom AT-130 tuner right below the backstay, with a short length of high-voltage (GTO-type) wire from the tuner to the backstay. There is a wide copper strap from the ground system to the tuner. The radio is an Icom 710-RT. This is a pretty standard installation, and it works very well. I'm sure it could work better, but it during last summer's race from San Francisco to Hawaii, and our trip back, we were participating in a marine-band net once or twice a day. VALIS (my boat) typically had one of the best signals out there, on 6 and 8 MHz. Yesterday, I had re-installed the radio after some maintenance, and my first radio-check contact was on 20 Meters with a ham in the Carribean -- I was in San Francisco. My point is that it works well enough. Yeah, it isn't as good on the lower frequencies as a full-length antenna would be. A different installation may be better, but so would a tri-band yagi at the top of my mast. I'm happy to stick with what I have. As has been mentioned, using coax from the tuner to the end of the backstay has two disadvantages: It adds shunt capacitance, and some loss, which can makes the tuner's job more difficult. Also, it may arc through from the center conductor to the shield, during operation at certain frequencies where the voltage can rise to very large values. (Larry, and anyone else interested -- I will be asking a question about Raymarine vs Furuno nav systems, and would appreciate some advice. I will start a new thread over in rec.boats.electronics so I don't hijack this one.) - Paul (wb6cxc) - S/V VALIS -- PSC44 #16 -- Sausalito, California - www.sailvalis.com |
#6
posted to rec.boats.cruising
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Ham radio backstay antenna feed line
Paul wrote:
SNIP As has been mentioned, using coax from the tuner to the end of the backstay has two disadvantages: It adds shunt capacitance, and some loss, which can makes the tuner's job more difficult. Also, it may arc through from the center conductor to the shield, during operation at certain frequencies where the voltage can rise to very large values. SNIP - Paul (wb6cxc) - S/V VALIS -- PSC44 #16 -- Sausalito, California - www.sailvalis.com Hello Paul, Part of the problem in analyzing a run of coax between the tuner and the bottom of the backstay is agreeing on what the alternative is. If the alternative is having the tuner right at the base of the backstay, then that will usually give the best results. (But see below) But if the tuner is, say eight (or more) feet away from the bottom of the backstay and the run is more or less along the waterline, then that's not going to be a very attractive alternative. On frequencies where that length constitutes a current node, (especially at higher frequencies where the antenna might be a short, automotive-type whip) that length is an important part of the radiating system. But its radiation is essentially into the water where it does no good. A variation on that is where the connection between the tuner and the "ground" is several feet long: a not unusual arrangement. In that case, the antenna actually begins at the ground system and the ground wire running to the tuner is a full, radiating part of the antenna! So the rules are not just to avoid coax between the tuner and the backstay, but to install the tuner as close to the backstay as possible and to install the ground system as close as possible to the tuner. Fortunately, these rules are tempered with the knowledge that many installations work acceptably despite their departures from the ideal. ;-) I would second Bruce's recommendations, and also point out that on the Chesapeake and its tributaries as well as on the Great Lakes, you will need to provide lots of area for your RF ground because of the water's lack of salinity. 73, Chuck NT3G ----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
#7
posted to rec.boats.cruising
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Ham radio backstay antenna feed line
"thuss" wrote in news:1174842368.471514.316370
@n59g2000hsh.googlegroups.com: I'm replacing the antenna feed line between my antenna tuner and insulated backstay. It's currently a coax cable but then I read that I should use insulated antenna wire and NOT use coax for the antenna feed line. This raises some questions though, isn't the whole reason to use coax because it's shielded which prevents the feed line from radiating and causing RF burns in the cockpit? The initial installation was just WRONG. Coax cable left open on the backstay end is simply a Faraday Shield the also makes an output capacitor because of the proximity of the ground (shield grounded) and the center conductor hot with RF. What SHOULD have been installed was a high voltage cable on standoff insulators or just hanging out AWAY FROM ALL METAL OBJECTS...NOT TYWRAPPED TO THE BOTTOM METAL PARTS OF THE BACKSTAY, PLEASE.... How long is the run from the tuner's antenna output insulator to the top of the backstay's bottom insulator, the feedpoint of the antenna? The tuner should be located as close to that point as is practical. Lionheart's is less than 2' and I use a stainless wire to it. Every piece of metal sticking up above the deck is a passive part of the backstay antenna system and can have high voltage points, of a sort, on them if they are of a resonant length at the frequency you are operating on. How high depend on their proximity and parallelness to the radiating element. This is RF, not AC power. At some frequency, all metal rods/shrouds/masts/anything that conducts becomes resonant because of its length. Ride by any AM radio station, whos tower is the radiating element at all of them, and look at the insulators installed in every guy wire around the tower to make sure each section between those insulators is NOT a resonant length of wire (too short) which reduces secondary radiation and absorption to insignificant values. Now to safety. 150 watts on a marine HF isn't much RF power, relatively speaking. Stop by Lionheart and I will let you run 150 watts whistling into its SSB transmitter while I'm wrapped around the backstay antenna, unharmed. Tower climbers never turn off AM radio stations to climb up and change the lights, or even those insulators while hanging upside down from the guy wires they are working on...which scares me to death. You aren't burned from an high voltage wire AS LONG AS YOU ARE AT THE WIRE'S POTENTIAL. Where you get into trouble, which is really minimal and not really dangerous at 150 watts HF, is when you get ACROSS high voltages of different potentials/phase...where current, which is what burns/shocks, flows through your body. Don't touch the backstay and boom or mizzen or that seawater-grounded metal helm wheel when the transmitter is on. The wire coming from the tuner to the backstay, by the way, is part of the antenna, which starts at the high voltage insulator on the tuner. Larry -- Message for Comcrap Internet Customers: http://tinyurl.com/3ayl9c Unlimited Service my ass.....(d^ |
#8
posted to rec.boats.cruising
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Ham radio backstay antenna feed line
In article .com,
"thuss" wrote: I'm replacing the antenna feed line between my antenna tuner and insulated backstay. It's currently a coax cable but then I read that I should use insulated antenna wire and NOT use coax for the antenna feed line. This raises some questions though, isn't the whole reason to use coax because it's shielded which prevents the feed line from radiating and causing RF burns in the cockpit? The source I read it on is: http://www.geocities.com/bill_dietrich/Radio.html "from the output terminal of the tuner/coupler you want insulated VERY high voltage antenna wire (usually about 14 or 16 gauge) led to a point on your backstay just above the insulator ... You CANNOT use coax for this part of your antenna lead." Thanks, Todd -- http://sailsugata.com s/v Sugata Hans Christian 38 Mark II http://windandtides.com San Francisco Sailing Weather http://gearandboats.com Free San Francisco Boating Classifieds There have been MANY different Ideas on what type of wire to use for the connection between an AntennaTuner and an EndFeed MF/HF Wire Antenna. The Accepted Standard used in Commercial Marine Installations has always been, GTO-15, which is a Copper Conductor, Insulated to 15Kv, and shiethed with a very thick Black Vynel Outer Jacket. Some folks have used coax cable by only using the Shield Braid as the conductor, but this doesn't have the same 15Kv Insulation qualities. Using Coax in a Standard Configuration, with the Inner conductor as the Antenna Connection and a Grounded Shield, or Braid, is a BIG NO-NO, as the shield would add Significant Shunt Capacitance, that the tuner must deal with, as well as shunting a significant portion of the RF to Ground. The same can be said for Tye-Wrapping ANY portion of the Antenna to ANY Grounded Surface, or even ungrounded surface of a conducting material. When designing a Effective MF/HF Marine Antenna System, one needs to Drag Out the Special RF Specticals, and view the vessel thru these, which will only let the user see those things that have significance in the MF/HF Spectrum. So lets look, and see what we can see. First we see the SeaWater. It looks like a Flat Plate of Copper with the vessels hull sticking out of it, if the hull is made of a Conducting Material, OR, a Hole in the Copper sheet, where the hull displaces the SeaWater. Then we see the Engine, Gearbox, Wiring, and all the Grounded conducting material inside the hull, including any Grounded Stays. On closer inspection we see the Antenna Tuner connected to the RF Ground System. Hopefully, this is done with a LOW Impedance connection at MF/HF Frequencies. Now if the Hull/House Material is conductive, then we need a RF Feedthru Insulator, of a size to make as SMALL of Capacative Coupling as possible to the Antenna Connection between the Antenna Tuner and the bottom of the EndFeed Wire. Now look at the External side of the RF Feedthru Insulator, and see that the Antenna Wire stays away from ANY of the Grounded things that we looked at earlier. The farther the better, but anything over 6-10 inches should be sufficent. No reason to become ANAL Retentive on this point. If the Hull/House Material is non-conductive, then all we really need to do is see that the Antenna Wire stays away from ANY of the Grounded things that we looked at earlier. The farther the better, but anything over 6-10 inches should be sufficent. No reason to become ANAL Retentive on this point. Now concerning RF Grounding of MF/HF Marine Antenna Systems, the better (Lower Impedance) the RF Ground, the BETTER the System will WORK. More is Better, less is worse. Conductive Hull Material is the BEST, when a non-conductive material is the case, then ANYTHING that adds to the Surface AREA of the RF Grounded, is better, and the CLOSER these things are to the SeaWater, the BETTER. You are building a Capacative Coupling to the SeaWater, and you make a BIGGER Capacitor (Lower Impedance) by having MORE surface AREA, and less space between the two plates of the Capacitor. :end of MF/HF Marine Antenna Lecture 101 Bruce in alaska I use to give this lecture a lot, but MF/HF Marine Radio is a very Mature Technology, and soon to be a DEAD Technology....... -- add a 2 before @ |
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