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Antenna for Marine VHF
In article , rickman
wrote: On 4/25/2017 12:07 PM, Fred McKenzie wrote: In article , rickman wrote: On 4/24/2017 3:39 PM, Michael Black wrote: On Sun, 23 Apr 2017, highlandham wrote: On 23/04/17 04:03, Jeff Liebermann wrote: quote ===== When you're on the water line, antenna height does make a big difference. Prepare a roll of coax cable setup as an RF extension cable. Attach connectors and adapters so that they fit the radio and the antenna. If in trouble at sea, lash the antenna to the top of a pole or oar to gain altitude. A separate antenna, such as a common ground plane or coax sleeve antenna at the end of the coax cable would make a better antenna than a rubber ducky. unquote ===== A simple alternative to the J-Pole antenna would be a ground plane antenna made of welding rod material on an SO-239 socket with a 4 hole square flange with a vertical quarter wave radiator and 4 hinged radial elements,the latter under 45 degrees when in use . The SO-239 socket would be connected to a PL-259 connector and a suitable length of coax. But on a kayak? It won't do much good lying on the kayak, and unless you get it high enough (which has problems in itself), you'll poke your eye out. Why not go for something that doesn't need horizontal radials. Actually I gave this antenna some thought. If the radials were attached by some sort of hinge for example a heavy braid, they could fold up while still being electrically attached. The antenna could slide into a tube when not in use. It might be a bit complex to rig up, but this is the sort of thing I can give some thought to and maybe come up with something workable. Back to the J-Pole! You can make a light, flexible J-Pole out of 300 Ohm twinlead. Feed it with RG-174 cable. Use some kind of oar or pole to raise it when needed. I wouldn't know where to begin in sizing the components. I did a quick search for 300 ohm j-pole. The first result was: http://www.lowra.com/antenna/flexjpole/twinlead.pdf Fred |
Antenna for Marine VHF
In article , rickman wrote:
Yesterday I couldn't think of the term for the antenna style they use in the commercial marine VHF antennas, but I believe it is called "co-linear" or something like that. It is a bunch of coax sections connected inner to outer at specific lengths. I have never seen a Ham recommend using that type. But I guess Hams go more for permanent installations with ground planes of some type. The co-linear needs no ground plane I believe. "Colinear" or "collinear", depending on whom you read. They're basically a vertically-stacked array of individual radiating sections. http://www.rason.org/Projects/collant/collant.htm There are quite a few commercial ham antennas which use this approach (the "Stationmaster" probably being the best known). They're fairly popular for use on the 70 cm ham band, and some repeaters and base stations use them on 2 meters. This design is generally used when you want a substantial amount of directional gain, and are willing to pay the price (length) for it. I don't think this design would be a great choice for a kayak antenna, because the individual coax sections in the "stack" are a half-wavelength long (at the coax's velocity factor) and there are usually quarter-wave sections at the top and bottom. The shortest 2-meter collinear (one half-wave section and two quarter-wave) would be 2 meters in length - over six feet - and a marine VHF antenna wouldn't be much shorter. With a collinear of the type shown in the above link, you'd need to mast-mount it up some distance - the bottom quarter-wave tube is RF-hot, and if its bottom end is near water (or anything grounded) it would tend to de-tune the antenna. As others have noted, the OP really doesn't need a high-gain antenna. |
Antenna for Marine VHF
On 4/25/2017 3:01 PM, Fred McKenzie wrote:
In article , rickman wrote: On 4/25/2017 12:07 PM, Fred McKenzie wrote: In article , rickman wrote: On 4/24/2017 3:39 PM, Michael Black wrote: On Sun, 23 Apr 2017, highlandham wrote: On 23/04/17 04:03, Jeff Liebermann wrote: quote ===== When you're on the water line, antenna height does make a big difference. Prepare a roll of coax cable setup as an RF extension cable. Attach connectors and adapters so that they fit the radio and the antenna. If in trouble at sea, lash the antenna to the top of a pole or oar to gain altitude. A separate antenna, such as a common ground plane or coax sleeve antenna at the end of the coax cable would make a better antenna than a rubber ducky. unquote ===== A simple alternative to the J-Pole antenna would be a ground plane antenna made of welding rod material on an SO-239 socket with a 4 hole square flange with a vertical quarter wave radiator and 4 hinged radial elements,the latter under 45 degrees when in use . The SO-239 socket would be connected to a PL-259 connector and a suitable length of coax. But on a kayak? It won't do much good lying on the kayak, and unless you get it high enough (which has problems in itself), you'll poke your eye out. Why not go for something that doesn't need horizontal radials. Actually I gave this antenna some thought. If the radials were attached by some sort of hinge for example a heavy braid, they could fold up while still being electrically attached. The antenna could slide into a tube when not in use. It might be a bit complex to rig up, but this is the sort of thing I can give some thought to and maybe come up with something workable. Back to the J-Pole! You can make a light, flexible J-Pole out of 300 Ohm twinlead. Feed it with RG-174 cable. Use some kind of oar or pole to raise it when needed. I wouldn't know where to begin in sizing the components. I did a quick search for 300 ohm j-pole. The first result was: http://www.lowra.com/antenna/flexjpole/twinlead.pdf Thanks. -- Rick C |
Antenna for Marine VHF
On 4/25/2017 3:19 PM, Dave Platt wrote:
In article , rickman wrote: Yesterday I couldn't think of the term for the antenna style they use in the commercial marine VHF antennas, but I believe it is called "co-linear" or something like that. It is a bunch of coax sections connected inner to outer at specific lengths. I have never seen a Ham recommend using that type. But I guess Hams go more for permanent installations with ground planes of some type. The co-linear needs no ground plane I believe. "Colinear" or "collinear", depending on whom you read. They're basically a vertically-stacked array of individual radiating sections. http://www.rason.org/Projects/collant/collant.htm There are quite a few commercial ham antennas which use this approach (the "Stationmaster" probably being the best known). They're fairly popular for use on the 70 cm ham band, and some repeaters and base stations use them on 2 meters. This design is generally used when you want a substantial amount of directional gain, and are willing to pay the price (length) for it. I don't think this design would be a great choice for a kayak antenna, because the individual coax sections in the "stack" are a half-wavelength long (at the coax's velocity factor) and there are usually quarter-wave sections at the top and bottom. The shortest 2-meter collinear (one half-wave section and two quarter-wave) would be 2 meters in length - over six feet - and a marine VHF antenna wouldn't be much shorter. With a collinear of the type shown in the above link, you'd need to mast-mount it up some distance - the bottom quarter-wave tube is RF-hot, and if its bottom end is near water (or anything grounded) it would tend to de-tune the antenna. As others have noted, the OP really doesn't need a high-gain antenna. I think the real problem is this antenna for 2 meter operation is 20 feet long! For marine VHF it can't be used on shore, so hanging it from a tree would not work. When you say using a single half wave section wouldn't be much different from a marine VHF antenna, what type of antenna would a marine VHF antenna be? I thought they used a colinear design. -- Rick C |
Antenna for Marine VHF
rickman wrote:
I don't think this design would be a great choice for a kayak antenna, because the individual coax sections in the "stack" are a half-wavelength long (at the coax's velocity factor) and there are usually quarter-wave sections at the top and bottom. The shortest 2-meter collinear (one half-wave section and two quarter-wave) would be 2 meters in length - over six feet - and a marine VHF antenna wouldn't be much shorter. With a collinear of the type shown in the above link, you'd need to mast-mount it up some distance - the bottom quarter-wave tube is RF-hot, and if its bottom end is near water (or anything grounded) it would tend to de-tune the antenna. As others have noted, the OP really doesn't need a high-gain antenna. I think the real problem is this antenna for 2 meter operation is 20 feet long! For marine VHF it can't be used on shore, so hanging it from a tree would not work. When you say using a single half wave section wouldn't be much different from a marine VHF antenna, what type of antenna would a marine VHF antenna be? I thought they used a colinear design. I wonder, why is the "2 meter band" not called the "6 1/2 feet band" in the USA? This alternating between meters and feet is getting a bit funny. |
Antenna for Marine VHF
On 4/25/2017 4:24 PM, Rob wrote:
rickman wrote: I don't think this design would be a great choice for a kayak antenna, because the individual coax sections in the "stack" are a half-wavelength long (at the coax's velocity factor) and there are usually quarter-wave sections at the top and bottom. The shortest 2-meter collinear (one half-wave section and two quarter-wave) would be 2 meters in length - over six feet - and a marine VHF antenna wouldn't be much shorter. With a collinear of the type shown in the above link, you'd need to mast-mount it up some distance - the bottom quarter-wave tube is RF-hot, and if its bottom end is near water (or anything grounded) it would tend to de-tune the antenna. As others have noted, the OP really doesn't need a high-gain antenna. I think the real problem is this antenna for 2 meter operation is 20 feet long! For marine VHF it can't be used on shore, so hanging it from a tree would not work. When you say using a single half wave section wouldn't be much different from a marine VHF antenna, what type of antenna would a marine VHF antenna be? I thought they used a colinear design. I wonder, why is the "2 meter band" not called the "6 1/2 feet band" in the USA? This alternating between meters and feet is getting a bit funny. Why is the 70 cm band not the 700 mm band or the 0.07 meter band? Not sure what issue you have with feet other than it not being familiar perhaps. As much as I use metric, feet and inches are still ingrained in my soul. When I look at a flag pole I don't think, geeze, that's 10 meters high! I think 30 feet. It's that simple. -- Rick C |
Antenna for Marine VHF
In article , rickman wrote:
I think the real problem is this antenna for 2 meter operation is 20 feet long! Yup. Great for side-mounting on a tower, so-so for top-mounting on a tower or mast (it'll sway in the breeze and this can wear out the solder junctions), not so great for vehicle mounting, and downright impractical for a hand-held :-) For marine VHF it can't be used on shore, so hanging it from a tree would not work. When you say using a single half wave section wouldn't be much different from a marine VHF antenna, what type of antenna would a marine VHF antenna be? I thought they used a colinear design. For "shortened" antennas, what you'll often find is that they're "loaded" antennas. You can significantly shorten a radiating element, and still have resonance, by adding an inductor of some sort - the shortened element has capacitive reactance, and the inductor's reactance (equal magnitude, opposite sign) cancels it out. One way to do this is to wind the element itself into a spiral... this adds inductance and lets you use a longer piece of wire (i.e. something close to a quarter-wave). Most "rubber duck" antennas are of this sort - they often have an inductor in the base (right by the radio), and the whip consists of a wire wound spiral-fashion around some sort of insulating core. If you buy a hand-held marine-VHF radio, that's very likely what you'll get. An example: http://www.gandermountain.com/modper...ntenna&i=91538 Note they call it "Heliflex" - probably shorthand for "helically wound flexible". It's only 1' long. http://www.gandermountain.com/modper...ntenna&i=91513 This one is 6', or almost 2 meters. It might contain some sort of center-fed dipole (fed up through the center of the lower element), or it might be sort of the equivalent of a J-pole (half-wave or 5/8-wave radiator with a matching stub or coil at the bottom). |
Antenna for Marine VHF
In article ,
Rob wrote: I wonder, why is the "2 meter band" not called the "6 1/2 feet band" in the USA? This alternating between meters and feet is getting a bit funny. Fewer syllables? :-) The ham bands have always been described in terms of meters, I believe. Since amateur radio use is defined to a significant extent by international treaty, and since the U.S. is very definitely in the minority in terms of measurement units, it's just easier for all of the hams to use the same (metric) terminology. Also, wavelength and frequency convert back and forth a bit more conveniently in metric, at least in terms of rough numbers (divide 300 by one, to get the other). Hence, "2-meter" is around 150 MHz. |
Antenna for Marine VHF
On 4/25/2017 5:43 PM, Dave Platt wrote:
In article , rickman wrote: I think the real problem is this antenna for 2 meter operation is 20 feet long! Yup. Great for side-mounting on a tower, so-so for top-mounting on a tower or mast (it'll sway in the breeze and this can wear out the solder junctions), not so great for vehicle mounting, and downright impractical for a hand-held :-) For marine VHF it can't be used on shore, so hanging it from a tree would not work. When you say using a single half wave section wouldn't be much different from a marine VHF antenna, what type of antenna would a marine VHF antenna be? I thought they used a colinear design. For "shortened" antennas, what you'll often find is that they're "loaded" antennas. You can significantly shorten a radiating element, and still have resonance, by adding an inductor of some sort - the shortened element has capacitive reactance, and the inductor's reactance (equal magnitude, opposite sign) cancels it out. One way to do this is to wind the element itself into a spiral... this adds inductance and lets you use a longer piece of wire (i.e. something close to a quarter-wave). Most "rubber duck" antennas are of this sort - they often have an inductor in the base (right by the radio), and the whip consists of a wire wound spiral-fashion around some sort of insulating core. If you buy a hand-held marine-VHF radio, that's very likely what you'll get. An example: http://www.gandermountain.com/modper...ntenna&i=91538 Note they call it "Heliflex" - probably shorthand for "helically wound flexible". It's only 1' long. http://www.gandermountain.com/modper...ntenna&i=91513 This one is 6', or almost 2 meters. It might contain some sort of center-fed dipole (fed up through the center of the lower element), or it might be sort of the equivalent of a J-pole (half-wave or 5/8-wave radiator with a matching stub or coil at the bottom). Interesting. Any idea what the specs mean? Gain 3dBi "Marine Gain" 6dB I know what dBi is, but what is Marine Gain? Is there some reference antenna they use such as the rubber ducky? -- Rick C |
Antenna for Marine VHF
In article , rickman wrote:
Interesting. Any idea what the specs mean? Gain 3dBi "Marine Gain" 6dB I know what dBi is, but what is Marine Gain? Is there some reference antenna they use such as the rubber ducky? That's probably taking into account an assumed 3 dB of additional gain, due to the fact that the antenna is located a short distance above the water surface. Salt water is an excellent reflector of RF energy. At certain angles and distances, the primary wave from the antenna and the reflected wave from the water surface will be in-phase with one another, and will reinforce, doubling the strength of the received signal. In other places the two signals will largely or entirely cancel out. There Ain't No Such Thing As A Free Lunch. |
Antenna for Marine VHF
On 4/25/2017 6:11 PM, Dave Platt wrote:
In article , rickman wrote: Interesting. Any idea what the specs mean? Gain 3dBi "Marine Gain" 6dB I know what dBi is, but what is Marine Gain? Is there some reference antenna they use such as the rubber ducky? That's probably taking into account an assumed 3 dB of additional gain, due to the fact that the antenna is located a short distance above the water surface. Salt water is an excellent reflector of RF energy. At certain angles and distances, the primary wave from the antenna and the reflected wave from the water surface will be in-phase with one another, and will reinforce, doubling the strength of the received signal. In other places the two signals will largely or entirely cancel out. There Ain't No Such Thing As A Free Lunch. I'd be very surprised at that. My understanding is that the water gets in the way by obstructing the fresnel zone. But then as I read more about the fresnel zone I see this is really about reflections rather than the main wave propagation. So whether reflections off the water are good depends on the geometry of the antennas and water surface, no? Then there is the difference between salt and fresh water. -- Rick C |
Antenna for Marine VHF
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Antenna for Marine VHF
On Tue, 25 Apr 2017 15:39:57 -0400, rickman wrote:
I think the real problem is this antenna for 2 meter operation is 20 feet long! Yep. The problem with the alternating coax cable antenna design is that only every other 1/2 wave section radiates. The result of half the radiation is half the gain. Or, as you've noticed, the antenna is twice as long as it might be with phasing elements between 1/2 wave sections. For marine VHF it can't be used on shore, so hanging it from a tree would not work. I've shoved them into a fiberglass radome filled with urethane foam. Works nicely hanging from a tower. I guess it might also work hanging from a tree. When you say using a single half wave section wouldn't be much different from a marine VHF antenna, what type of antenna would a marine VHF antenna be? I thought they used a colinear design. Lots of ways to build a marine VHF antenna. The key is the length. They come in 3ft, 6ft, 9ft, and 12ft lengths. The shortest are for top of the mast, where low gain is needed to compensate for pitching and rolling. The 12ft is for deck mounting, where the effects of pitch and roll are somewhat less. Inside the radome is usually a coaxial sleeve dipole with various numbers of 1/4 wave phasing sections. You start with a coaxial antenna something like this: http://www.hamuniverse.com/w4bwsverticalbazooka.html That's good for about 2dBi gain if the manufacturer bothers to use a brass sleeve instead of folding the braid over the outside of the coax cable. For more gain and lower radiation pattern, put another 1/4 wavelength sleeve 1/4 wavelength below the first sleeve and connected to the coax shield at the top: http://www.w8ji.com/Image1/Skirt_feed.jpg I think that page was lifted from an early version of Henry Jasik "Antenna Engineering Handbook". The sleeve does not need to be cylindrical such as in the Isopole type antenna: https://www.google.com/search?q=isopole+antenann&tbm=isch. You can keep adding them until you run out of space. However, there's a catch. As the gain goes up and the vertical radiation pattern goes down, the usable transmit bandwidth also goes down. Coaxial antennas are better than most, but you still have to be careful not to add so many elements that the VSWR is too high at the transmit band edges. Also, some junk out the http://www.thehulltruth.com/marine-electronics-forum/343939-here-s-what-shakespeare-5206-vhf-antenna.html I've seen worse. I won't mention the manufacturer, but there's one that's nothing more than a length of copper tape stuck to the inside of a fiberglass radome, with a very narrow band matching system in the base. Some are just a 5/8 wave antenna with a matching transformer in the base. Incidentally, marine antennas are very much like land mobile commercial antennas. However, there's one difference in the coax cable. Marine antennas use various schemes to prevent water from wicking up the braid by capillary action, such as silicone grease filler in the braid or having the outer jacket reflowed into the braid. The idea is to not have any air gaps inside the cable that might fill with water. However, I doubt the kayak will be in the water long enough for this to be a problem, but if your friend has money, it wouldn't hurt to spend a few dollars extra for marine grade coaxial cable. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Antenna for Marine VHF
On 25 Apr 2017 20:24:44 GMT, Rob wrote:
I wonder, why is the "2 meter band" not called the "6 1/2 feet band" in the USA? This alternating between meters and feet is getting a bit funny. Chuckle. Yeah, that's a problem. I sometimes irritate the local hams by referring to various HF bands by the equivalent feet, yards, or cubits. When asked which is the right way, my usual answer is when test equipment manufacturers start labeling their equipment in units of feet or meters, I might consider using those designations. Until then, since most test equipment is labeled by the frequency, I'll use Hz, KHz, MHz, GHz, etc. Incidentally, if you need more confusion, there are the various microwave frequency bands designed by letters, such as DBS broadcasting is on K and Ku bands in the US. Then, there are bands that are designated by their service type or spectrum auction block numbers: http://wireless.fcc.gov/auctions/default.htm?job=bandplans -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Antenna for Marine VHF
On Tue, 25 Apr 2017, rickman wrote:
On 4/25/2017 3:19 PM, Dave Platt wrote: In article , rickman wrote: Yesterday I couldn't think of the term for the antenna style they use in the commercial marine VHF antennas, but I believe it is called "co-linear" or something like that. It is a bunch of coax sections connected inner to outer at specific lengths. I have never seen a Ham recommend using that type. But I guess Hams go more for permanent installations with ground planes of some type. The co-linear needs no ground plane I believe. "Colinear" or "collinear", depending on whom you read. They're basically a vertically-stacked array of individual radiating sections. http://www.rason.org/Projects/collant/collant.htm There are quite a few commercial ham antennas which use this approach (the "Stationmaster" probably being the best known). They're fairly popular for use on the 70 cm ham band, and some repeaters and base stations use them on 2 meters. This design is generally used when you want a substantial amount of directional gain, and are willing to pay the price (length) for it. I don't think this design would be a great choice for a kayak antenna, because the individual coax sections in the "stack" are a half-wavelength long (at the coax's velocity factor) and there are usually quarter-wave sections at the top and bottom. The shortest 2-meter collinear (one half-wave section and two quarter-wave) would be 2 meters in length - over six feet - and a marine VHF antenna wouldn't be much shorter. With a collinear of the type shown in the above link, you'd need to mast-mount it up some distance - the bottom quarter-wave tube is RF-hot, and if its bottom end is near water (or anything grounded) it would tend to de-tune the antenna. As others have noted, the OP really doesn't need a high-gain antenna. I think the real problem is this antenna for 2 meter operation is 20 feet long! For marine VHF it can't be used on shore, so hanging it from a tree would not work. When you say using a single half wave section wouldn't be much different from a marine VHF antenna, what type of antenna would a marine VHF antenna be? I thought they used a colinear design. Someone pointed out the classic groundplane antenna, with radials. But, that's just a variant on the basic dipole, where there are two elements of the same length. But the form of the groundplane puts the radiation more where you want it. For mobile, the whip antennas are often just a variant on the groundplane, except the body of the car acts instead of the radials. There are longer antennas, but still single pieces, that provide some gain, but more important, don't need a ground plane, which of course is hard to find in a kayak that isn't made of metal. So those are longer whips, with some matching in place. The collinear is like stacked dipoles, providing more gain, but needing more height, and of course the matching stubs stick out the side. They are fine on a tower, not so useful on a kayak. Michael |
Antenna for Marine VHF
On 4/25/2017 7:23 PM, Jeff Liebermann wrote:
On Tue, 25 Apr 2017 15:39:57 -0400, rickman wrote: I think the real problem is this antenna for 2 meter operation is 20 feet long! Yep. The problem with the alternating coax cable antenna design is that only every other 1/2 wave section radiates. The result of half the radiation is half the gain. Or, as you've noticed, the antenna is twice as long as it might be with phasing elements between 1/2 wave sections. For marine VHF it can't be used on shore, so hanging it from a tree would not work. I've shoved them into a fiberglass radome filled with urethane foam. Works nicely hanging from a tower. I guess it might also work hanging from a tree. My point is it is illegal to use *marine* VHF when on shore. When you say using a single half wave section wouldn't be much different from a marine VHF antenna, what type of antenna would a marine VHF antenna be? I thought they used a colinear design. Lots of ways to build a marine VHF antenna. The key is the length. They come in 3ft, 6ft, 9ft, and 12ft lengths. The shortest are for top of the mast, where low gain is needed to compensate for pitching and rolling. The 12ft is for deck mounting, where the effects of pitch and roll are somewhat less. I'm not necessarily looking to build an antenna. Someone had injected the construction project into the conversation that my friend asked me about. I did some digging on the interweb and realized I should ask here. Mostly what I've been told is the same as what I found myself although the flexible J-pole is new. I was hoping there might be some more insight into the issues. Personally I would recommend a commercial antenna unless they want something that can be rolled up which some of the suggestions could be. Inside the radome is usually a coaxial sleeve dipole with various numbers of 1/4 wave phasing sections. You start with a coaxial antenna something like this: http://www.hamuniverse.com/w4bwsverticalbazooka.html That's good for about 2dBi gain if the manufacturer bothers to use a brass sleeve instead of folding the braid over the outside of the coax cable. For more gain and lower radiation pattern, put another 1/4 wavelength sleeve 1/4 wavelength below the first sleeve and connected to the coax shield at the top: http://www.w8ji.com/Image1/Skirt_feed.jpg I think that page was lifted from an early version of Henry Jasik "Antenna Engineering Handbook". That's interesting. I wish I were building one. The sleeve does not need to be cylindrical such as in the Isopole type antenna: https://www.google.com/search?q=isopole+antenann&tbm=isch. You can keep adding them until you run out of space. However, there's a catch. As the gain goes up and the vertical radiation pattern goes down, the usable transmit bandwidth also goes down. Coaxial antennas are better than most, but you still have to be careful not to add so many elements that the VSWR is too high at the transmit band edges. Also, some junk out the http://www.thehulltruth.com/marine-electronics-forum/343939-here-s-what-shakespeare-5206-vhf-antenna.html I've seen worse. I won't mention the manufacturer, but there's one that's nothing more than a length of copper tape stuck to the inside of a fiberglass radome, with a very narrow band matching system in the base. Some are just a 5/8 wave antenna with a matching transformer in the base. Incidentally, marine antennas are very much like land mobile commercial antennas. However, there's one difference in the coax cable. Marine antennas use various schemes to prevent water from wicking up the braid by capillary action, such as silicone grease filler in the braid or having the outer jacket reflowed into the braid. The idea is to not have any air gaps inside the cable that might fill with water. However, I doubt the kayak will be in the water long enough for this to be a problem, but if your friend has money, it wouldn't hurt to spend a few dollars extra for marine grade coaxial cable. Yeah, water is amazingly corrosive, even fresh water. -- Rick C |
Antenna for Marine VHF
On 4/25/2017 7:36 PM, Michael Black wrote:
On Tue, 25 Apr 2017, rickman wrote: On 4/25/2017 3:19 PM, Dave Platt wrote: In article , rickman wrote: Yesterday I couldn't think of the term for the antenna style they use in the commercial marine VHF antennas, but I believe it is called "co-linear" or something like that. It is a bunch of coax sections connected inner to outer at specific lengths. I have never seen a Ham recommend using that type. But I guess Hams go more for permanent installations with ground planes of some type. The co-linear needs no ground plane I believe. "Colinear" or "collinear", depending on whom you read. They're basically a vertically-stacked array of individual radiating sections. http://www.rason.org/Projects/collant/collant.htm There are quite a few commercial ham antennas which use this approach (the "Stationmaster" probably being the best known). They're fairly popular for use on the 70 cm ham band, and some repeaters and base stations use them on 2 meters. This design is generally used when you want a substantial amount of directional gain, and are willing to pay the price (length) for it. I don't think this design would be a great choice for a kayak antenna, because the individual coax sections in the "stack" are a half-wavelength long (at the coax's velocity factor) and there are usually quarter-wave sections at the top and bottom. The shortest 2-meter collinear (one half-wave section and two quarter-wave) would be 2 meters in length - over six feet - and a marine VHF antenna wouldn't be much shorter. With a collinear of the type shown in the above link, you'd need to mast-mount it up some distance - the bottom quarter-wave tube is RF-hot, and if its bottom end is near water (or anything grounded) it would tend to de-tune the antenna. As others have noted, the OP really doesn't need a high-gain antenna. I think the real problem is this antenna for 2 meter operation is 20 feet long! For marine VHF it can't be used on shore, so hanging it from a tree would not work. When you say using a single half wave section wouldn't be much different from a marine VHF antenna, what type of antenna would a marine VHF antenna be? I thought they used a colinear design. Someone pointed out the classic groundplane antenna, with radials. But, that's just a variant on the basic dipole, where there are two elements of the same length. But the form of the groundplane puts the radiation more where you want it. Yes, that has some potential for a portable antenna by making the radials hinged and letting it all fold up and slide into a tube. I expect I'm not the first to think of this. I know there are Ham events where the bring equipment into the field and have contests. For mobile, the whip antennas are often just a variant on the groundplane, except the body of the car acts instead of the radials. There are longer antennas, but still single pieces, that provide some gain, but more important, don't need a ground plane, which of course is hard to find in a kayak that isn't made of metal. So those are longer whips, with some matching in place. The collinear is like stacked dipoles, providing more gain, but needing more height, and of course the matching stubs stick out the side. They are fine on a tower, not so useful on a kayak. Out the side? I haven't see that yet. -- Rick C |
Antenna for Marine VHF
In article ,
Jeff Liebermann wrote: On Tue, 25 Apr 2017 15:39:57 -0400, rickman wrote: I think the real problem is this antenna for 2 meter operation is 20 feet long! Yep. The problem with the alternating coax cable antenna design is that only every other 1/2 wave section radiates. The result of half the radiation is half the gain. Or, as you've noticed, the antenna is twice as long as it might be with phasing elements between 1/2 wave sections. Jeff, Do you have a sim or model of that situation, showing that there aren't significant RF currents on every other half-wave section? Somehow I can't make sense of how that would happen. My mental model of the alternating-sections design has been that all of the sections do radiate... the alternating hookup forces them to radiate in phase with one another, rather than out of phase (and thus tending to squint the pattern badly upwards and downwards). Now, I have heard that the alternating-coax collinear doesn't have as much gain as an array of separate dipoles hooked up with a phasing harness... but I've always read that as being explained by the fact that the upper sections are carrying smaller RF currents than the lower because some power has been radiated away before the signal reaches the upper part of the antenna. So, I'd appreciate enlightenment here! |
Antenna for Marine VHF
On Tue, 25 Apr 2017 17:24:37 -0700, (Dave
Platt) wrote: In article , Jeff Liebermann wrote: On Tue, 25 Apr 2017 15:39:57 -0400, rickman wrote: I think the real problem is this antenna for 2 meter operation is 20 feet long! Yep. The problem with the alternating coax cable antenna design is that only every other 1/2 wave section radiates. The result of half the radiation is half the gain. Or, as you've noticed, the antenna is twice as long as it might be with phasing elements between 1/2 wave sections. Do you have a sim or model of that situation, showing that there aren't significant RF currents on every other half-wave section? Somehow I can't make sense of how that would happen. No, I don't. I'll see what I can find but I don't recall ever seeing such a model. My mental model of the alternating-sections design has been that all of the sections do radiate... the alternating hookup forces them to radiate in phase with one another, rather than out of phase (and thus tending to squint the pattern badly upwards and downwards). Now, I have heard that the alternating-coax collinear doesn't have as much gain as an array of separate dipoles hooked up with a phasing harness... but I've always read that as being explained by the fact that the upper sections are carrying smaller RF currents than the lower because some power has been radiated away before the signal reaches the upper part of the antenna. On every other 1/2 wave element, the wire that carries the signal is inside a shielded and grounded conductor. I don't think it's going to radiate. So, I'd appreciate enlightenment here! Well, I'll see what I can find and do. If necessary, I'll throw together a model. Modeling coax cables with NEC2 might be difficult or impossible, but I'll see if I can fake it: "The Dipole and the Coax" https://www.antennex.com/w4rnl/col0606/amod100.html "Neither software core (NEC and MININEC) is capable of physically modeling conventional coaxial cables. The transmission line function within NEC creates lossless non-radiating mathematical models of lines and hence cannot capture common mode radiation. Therefore, the method used to show common mode radiation is to place a third leg into the dipole." and so on... This is probably more than I want to attempt without some study time. "Collinear antenna structure" https://www.google.com/patents/US6771227 "Collinear antenna of the alternating coaxial type" https://www.google.com/patents/US20040125038 -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Antenna for Marine VHF
On 4/25/2017 10:12 PM, Jeff Liebermann wrote:
On Tue, 25 Apr 2017 17:24:37 -0700, (Dave Platt) wrote: In article , Jeff Liebermann wrote: On Tue, 25 Apr 2017 15:39:57 -0400, rickman wrote: I think the real problem is this antenna for 2 meter operation is 20 feet long! Yep. The problem with the alternating coax cable antenna design is that only every other 1/2 wave section radiates. The result of half the radiation is half the gain. Or, as you've noticed, the antenna is twice as long as it might be with phasing elements between 1/2 wave sections. Do you have a sim or model of that situation, showing that there aren't significant RF currents on every other half-wave section? Somehow I can't make sense of how that would happen. No, I don't. I'll see what I can find but I don't recall ever seeing such a model. My mental model of the alternating-sections design has been that all of the sections do radiate... the alternating hookup forces them to radiate in phase with one another, rather than out of phase (and thus tending to squint the pattern badly upwards and downwards). Now, I have heard that the alternating-coax collinear doesn't have as much gain as an array of separate dipoles hooked up with a phasing harness... but I've always read that as being explained by the fact that the upper sections are carrying smaller RF currents than the lower because some power has been radiated away before the signal reaches the upper part of the antenna. On every other 1/2 wave element, the wire that carries the signal is inside a shielded and grounded conductor. I don't think it's going to radiate. Huh, what? Doesn't the shield carry the signal as well? Every "other" section is exactly the same as the non-other sections. Which ones radiate and which other-ones don't? Maybe I've got the wrong image in my mind. I thought each section was coax but they connect wire to shield both ways at each junction. http://www.rason.org/Projects/collant/collant.htm So, I'd appreciate enlightenment here! Well, I'll see what I can find and do. If necessary, I'll throw together a model. Modeling coax cables with NEC2 might be difficult or impossible, but I'll see if I can fake it: "The Dipole and the Coax" https://www.antennex.com/w4rnl/col0606/amod100.html "Neither software core (NEC and MININEC) is capable of physically modeling conventional coaxial cables. The transmission line function within NEC creates lossless non-radiating mathematical models of lines and hence cannot capture common mode radiation. Therefore, the method used to show common mode radiation is to place a third leg into the dipole." and so on... This is probably more than I want to attempt without some study time. "Collinear antenna structure" https://www.google.com/patents/US6771227 "Collinear antenna of the alternating coaxial type" https://www.google.com/patents/US20040125038 -- Rick C |
Antenna for Marine VHF
On Tue, 25 Apr 2017 17:24:37 -0700, (Dave
Platt) wrote: My mental model of the alternating-sections design has been that all of the sections do radiate... the alternating hookup forces them to radiate in phase with one another, rather than out of phase (and thus tending to squint the pattern badly upwards and downwards). Sheesh. I'm an idiot. I just noticed that I already have a model of an alternating coax collinear on my own web pile: http://802.11junk.com/jeffl/antennas/CoaxVert/index.html It's not my design. I vaguely recall asking some questions about this antenna, and someone sent it to me. Notice in the Geometry that every other 1/2 wave elements do the radiating. The comments are useful: CM Coaxial Vertical Antenna, converted with 4nec2 on 28-Nov-08 22:18 CM This "Franklin" array model was created by Linley Gumm, CM K7HFD. Coaxial cable is modeled as a combination of CM transmission line model, to represent the inside of the CM coax, and a wire to represent the outside. The technique is CM described in the EZNEC manual. See "Coaxial Cable, CM Modeling" in the index. Notice that if you replace the non-radiating 1/2 wave coax delay line sections with a 1/4 wave wire stubs (or end shorted ladder line), the antenna is now about half its previous length, with no loss in gain, and probably little change in pattern. Also: https://ukradioscanning.com/viewtopic.php?f=5&t=3028 Note that the drawing shows that all elements radiation, but the nearby comments say "(on outer conductor for radiation)" which means that only those segments that have exposed outer shield conductors connected to the feed coax center conductor, do the radiating. That's every other 1/2 wave segment. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Antenna for Marine VHF
On Tue, 25 Apr 2017 22:38:56 -0700, Jeff Liebermann
wrote: Sheesh. I'm an idiot. I just noticed that I already have a model of an alternating coax collinear on my own web pile: http://802.11junk.com/jeffl/antennas/CoaxVert/index.html I fixed a few things, made some better images, and renamed the folder: http://www.11junk.com/jeffl/antennas/CoaxVertical/index.html Looks like the 1/4 wave section at the top is missing. I'll fix it tomorrow. It's late, I'm tired, etc... -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
Antenna for Marine VHF
rickman wrote:
On 4/25/2017 4:24 PM, Rob wrote: rickman wrote: I don't think this design would be a great choice for a kayak antenna, because the individual coax sections in the "stack" are a half-wavelength long (at the coax's velocity factor) and there are usually quarter-wave sections at the top and bottom. The shortest 2-meter collinear (one half-wave section and two quarter-wave) would be 2 meters in length - over six feet - and a marine VHF antenna wouldn't be much shorter. With a collinear of the type shown in the above link, you'd need to mast-mount it up some distance - the bottom quarter-wave tube is RF-hot, and if its bottom end is near water (or anything grounded) it would tend to de-tune the antenna. As others have noted, the OP really doesn't need a high-gain antenna. I think the real problem is this antenna for 2 meter operation is 20 feet long! For marine VHF it can't be used on shore, so hanging it from a tree would not work. When you say using a single half wave section wouldn't be much different from a marine VHF antenna, what type of antenna would a marine VHF antenna be? I thought they used a colinear design. I wonder, why is the "2 meter band" not called the "6 1/2 feet band" in the USA? This alternating between meters and feet is getting a bit funny. Why is the 70 cm band not the 700 mm band or the 0.07 meter band? Not sure what issue you have with feet other than it not being familiar perhaps. As much as I use metric, feet and inches are still ingrained in my soul. When I look at a flag pole I don't think, geeze, that's 10 meters high! I think 30 feet. It's that simple. Maybe yes, but it should be obvious that when you are discussing antennas for the "2 meter band" their dimensions will be nice multiples of those same 2 meters. When I think about a "halfwave dipole for 2 meters" I think "1 meter", not "3 1/4 feet" or "39 inches". So when you want to discuss antenna sizes I think it would be more convenient to use the same units all around. For me, that of course is meters. Scaling like "milli" or "centi" is natural in the metric system, I don't have to think about that. (contrary to converting feet to inches or meters to feet) |
Antenna for Marine VHF
On 4/26/2017 3:55 AM, Rob wrote:
rickman wrote: On 4/25/2017 4:24 PM, Rob wrote: rickman wrote: I don't think this design would be a great choice for a kayak antenna, because the individual coax sections in the "stack" are a half-wavelength long (at the coax's velocity factor) and there are usually quarter-wave sections at the top and bottom. The shortest 2-meter collinear (one half-wave section and two quarter-wave) would be 2 meters in length - over six feet - and a marine VHF antenna wouldn't be much shorter. With a collinear of the type shown in the above link, you'd need to mast-mount it up some distance - the bottom quarter-wave tube is RF-hot, and if its bottom end is near water (or anything grounded) it would tend to de-tune the antenna. As others have noted, the OP really doesn't need a high-gain antenna. I think the real problem is this antenna for 2 meter operation is 20 feet long! For marine VHF it can't be used on shore, so hanging it from a tree would not work. When you say using a single half wave section wouldn't be much different from a marine VHF antenna, what type of antenna would a marine VHF antenna be? I thought they used a colinear design. I wonder, why is the "2 meter band" not called the "6 1/2 feet band" in the USA? This alternating between meters and feet is getting a bit funny. Why is the 70 cm band not the 700 mm band or the 0.07 meter band? Not sure what issue you have with feet other than it not being familiar perhaps. As much as I use metric, feet and inches are still ingrained in my soul. When I look at a flag pole I don't think, geeze, that's 10 meters high! I think 30 feet. It's that simple. Maybe yes, but it should be obvious that when you are discussing antennas for the "2 meter band" their dimensions will be nice multiples of those same 2 meters. When I think about a "halfwave dipole for 2 meters" I think "1 meter", not "3 1/4 feet" or "39 inches". So when you want to discuss antenna sizes I think it would be more convenient to use the same units all around. For me, that of course is meters. Scaling like "milli" or "centi" is natural in the metric system, I don't have to think about that. (contrary to converting feet to inches or meters to feet) So what should I have said rather than 20 feet? -- Rick C |
Antenna for Marine VHF
In article ,
Jeff Liebermann wrote: On Tue, 25 Apr 2017 22:38:56 -0700, Jeff Liebermann wrote: Sheesh. I'm an idiot. I just noticed that I already have a model of an alternating coax collinear on my own web pile: http://802.11junk.com/jeffl/antennas/CoaxVert/index.html I fixed a few things, made some better images, and renamed the folder: http://www.11junk.com/jeffl/antennas/CoaxVertical/index.html Looks like the 1/4 wave section at the top is missing. I'll fix it tomorrow. It's late, I'm tired, etc... Thanks! I'll pull down this model, read through it so I understand it, and run a few sims myself. |
Antenna for Marine VHF
On Wed, 26 Apr 2017 11:23:23 -0700, (Dave
Platt) wrote: In article , Jeff Liebermann wrote: On Tue, 25 Apr 2017 22:38:56 -0700, Jeff Liebermann wrote: Sheesh. I'm an idiot. I just noticed that I already have a model of an alternating coax collinear on my own web pile: http://802.11junk.com/jeffl/antennas/CoaxVert/index.html I fixed a few things, made some better images, and renamed the folder: http://www.11junk.com/jeffl/antennas/CoaxVertical/index.html Looks like the 1/4 wave section at the top is missing. I'll fix it tomorrow. It's late, I'm tired, etc... Thanks! I'll pull down this model, read through it so I understand it, and run a few sims myself. Great. Maybe you can explain it to me. Here's a rec.radio.amateur.antenna 24 article thread on the topic from Nov 2008: https://groups.google.com/forum/#!msg/rec.radio.amateur.antenna/DREJnRznluQ/58Z0gIimqdwJ This is where I totally blew it when I incorrectly declared that most of the RF comes out of lowest element, and very little out the top. https://groups.google.com/d/msg/rec.radio.amateur.antenna/DREJnRznluQ/bZyCgwa0JvwJ That was like saying that in a series string of identical light bulbs, the lowest lights would be brighter. Argh. Corrections and comments by Roy Lewallen (W7EL) including the original model of the alternating coax vertical antenna: https://groups.google.com/d/msg/rec.radio.amateur.antenna/DREJnRznluQ/LOwnb-eZmjMJ I converted it to 4NEC2 format. See: http://eznec.com/misc/rraa/ for original model in EZNEC format. This model does NOT run in the Demo version of EZNEC 5.0 because it contains 80 segments and the demo program only allows 20. https://www.eznec.com Good luck. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
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