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Antenna Ratings
Know anywhere I can find ratings to help decide which antenna?? I have a 23
foot center console. Generally range 10-15 miles offshore and plan to mount the antenna on my |
#2
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Antenna Ratings
On Mon, 29 Dec 2003 03:00:38 GMT, "sk" wrote:
Know anywhere I can find ratings to help decide which antenna?? I have a 23 foot center console. Generally range 10-15 miles offshore and plan to mount the antenna on my I don't recommend the very high gain fiberglass antennas for VHF on small boats that roll so much in swell. High gain antennas get their high gain from squeezing the vertical antenna's donut-shaped radiation pattern into one that looks like you stepped on the donut. As long as the antenna is vertical, this is fine and steps up the gain very nicely. But, as the antenna is TILTED in any direction, this flattened-out pattern also tilts, simultaneously, radiating all that power into the sea on one side and into the clouds on the other, putting the receiving station in an area of its radiation pattern that is more of a null, than it would be if you had used a simpler 1/2 wave vertical whip with the fat donut pattern. The effect on the signal at the receiver when the super antenna is rolling around is a signal that's fading in and out as the high gain pattern plays over the receiving station on its way to skyward or seaward as the boat rolls. The best small boat antenna is an end-fed, half wave as high up as you can mount it. I've used Metz antennas since they started making home-brew ham antennas in the 70's. Metz antennas have a no-nonsense LIFETIME warranty! Who could ask for more? http://www.metzcommunication.com/ I think the Manta 6 is the finest small boat antenna made. http://www.metzcommunication.com/manta6.htm It is a simple 1/2 wave, no ground plane, end-fed VHF antenna and will work anywhere you can put it, plastic, metal, ground or not. The connector on the bottom of it is the standard SO-239 "CB" connector. If you break the coil, they'll send you a new one. The whip is fine stainless steel and the only way you can break it is by FORCE. I have LOST a couple of them overboard over the years when the little clamp on top of the coil wasn't tight, though..... Shakespeare also makes an end-fed halfwave, but it sucks because the COAX IS PERMANENTLY MOUNTED IN IT so you can't FIX IT when it breaks! You also can't simply replace a lost whip with a piece of coat hangar wire, like you can on the Metz, out in the boondocks to get the radio working again...... Metz has lots of different crazy mounts for it..... |
#3
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Antenna Ratings
Larry: please define "very high gain"? 9db? 6db?
FWiW, I have have a 6db 8' Shake Galaxy on my 17' whaler. Works fine even when it's rock and roll. -- Chuck Tribolet http://www.almaden.ibm.com/cs/people/triblet Silicon Valley: STILL the best day job in the world. "Larry W4CSC" wrote in message ... On Mon, 29 Dec 2003 03:00:38 GMT, "sk" wrote: Know anywhere I can find ratings to help decide which antenna?? I have a 23 foot center console. Generally range 10-15 miles offshore and plan to mount the antenna on my I don't recommend the very high gain fiberglass antennas for VHF on small boats that roll so much in swell. High gain antennas get their high gain from squeezing the vertical antenna's donut-shaped radiation pattern into one that looks like you stepped on the donut. As long as the antenna is vertical, this is fine and steps up the gain very nicely. But, as the antenna is TILTED in any direction, this flattened-out pattern also tilts, simultaneously, radiating all that power into the sea on one side and into the clouds on the other, putting the receiving station in an area of its radiation pattern that is more of a null, than it would be if you had used a simpler 1/2 wave vertical whip with the fat donut pattern. The effect on the signal at the receiver when the super antenna is rolling around is a signal that's fading in and out as the high gain pattern plays over the receiving station on its way to skyward or seaward as the boat rolls. The best small boat antenna is an end-fed, half wave as high up as you can mount it. I've used Metz antennas since they started making home-brew ham antennas in the 70's. Metz antennas have a no-nonsense LIFETIME warranty! Who could ask for more? http://www.metzcommunication.com/ I think the Manta 6 is the finest small boat antenna made. http://www.metzcommunication.com/manta6.htm It is a simple 1/2 wave, no ground plane, end-fed VHF antenna and will work anywhere you can put it, plastic, metal, ground or not. The connector on the bottom of it is the standard SO-239 "CB" connector. If you break the coil, they'll send you a new one. The whip is fine stainless steel and the only way you can break it is by FORCE. I have LOST a couple of them overboard over the years when the little clamp on top of the coil wasn't tight, though..... Shakespeare also makes an end-fed halfwave, but it sucks because the COAX IS PERMANENTLY MOUNTED IN IT so you can't FIX IT when it breaks! You also can't simply replace a lost whip with a piece of coat hangar wire, like you can on the Metz, out in the boondocks to get the radio working again...... Metz has lots of different crazy mounts for it..... |
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Antenna Ratings
Chuck Tribolet wrote:
Larry: please define "very high gain"? 9db? 6db? FWiW, I have have a 6db 8' Shake Galaxy on my 17' whaler. Works fine even when it's rock and roll. I have an 8' "Digital" brand antenna hooked up to my VHF on my 25' Parker pilothouse. Works fine in the rock and roll of the Atlantic Ocean. Visit the boat at: http://photos.yahoo.com/hakrause -- Email sent to is never read. |
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Antenna Ratings
On Sun, 28 Dec 2003 21:49:17 -0800, "Chuck Tribolet"
wrote: Larry: please define "very high gain"? 9db? 6db? The higher the "gain" of the antenna, the flatter and less "thick" its radiation pattern. The 9db antenna has a much flatter radiation pattern than the 6db. FWiW, I have have a 6db 8' Shake Galaxy on my 17' whaler. Works fine even when it's rock and roll. "In range" you'll really not see the difference on FM, which signal strength has less effect on until it's near the fringe. I just don't like the big, long fiberglass whips on small boats. Neither does USCG. They got fed up with them breaking off and all use the Metz, now. Metz brags about it on their webpage. If jumping waves with the SeaRayder jetboat can't break it, your boat won't, either. Mine is even mounted way up in the bow and has survived even submarining the bow at speed.....drowning us all in the process...(c; A fiberglass tube filled with hookup wire antenna elements would never make it....hee hee. |
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On Wed, 31 Dec 2003 11:30:07 GMT, Jack Burton
wrote: On Tue, 30 Dec 2003 04:01:15 GMT, (Larry W4CSC) wrote: On Sun, 28 Dec 2003 21:49:17 -0800, "Chuck Tribolet" wrote: Larry: please define "very high gain"? 9db? 6db? The higher the "gain" of the antenna, the flatter and less "thick" its radiation pattern. The 9db antenna has a much flatter radiation pattern than the 6db. So, how does the horizon work into the gain equation? Picture the vertical antenna standing vertically. It's a high-gain fiberglass type with many phased elements imbedded into it. Its radiation pattern is perpendicular to the antenna in a flattened donut shape. When you step on a donut, its outer edge moves outward as its thickness is squeezed. That's exactly what's happening to get more radiation to the horizon where the receiving station is located. The phased array squeezes the fat round donut of the 1/2 wave, which bulges out in the middle making a bigger signal perpendicular to the antenna. This pattern moves with the antenna, always perpendicular to it. Now, tilt the antenna towards the receiving station. (Boat pitched?) The flattened donut is now pointing into the water on the side towards the receiving station. The signal level at the receiving station drops, drastically, because this pattern is so flattened. On an FM receiver, you hear no difference in signal "loudness" as signal varies, UNTIL that signal drops below the receiver's noise floor, usually a few hundredths of a microvolt, at which point the receiver "hiss" of its FM detector gets louder and louder. As the signal increases, again, the FM receiver "quiets", the hissing drops. This is the only time the effect of the high gain antenna's flattened pattern will cause communications problems. If the antenna's received signal drops from 800 uV to 40 uV, you won't notice it. 40 uV will keep the receiver hiss quiet. But, if we are talking about adding this pattern tilting effect to the boat riding down into the trough of those 30' rollers in a "worst case scenario" where it really counts, THEN we are talking about disrupted comms. The best antenna for this situation is NOT the 9dB fiberglass beast mounted on the side of the helm console.....but the 1/2 wave Metz stainless whip mounted as high up as you can get it.....with its fat pattern less effected by tilting that's still sticking up above the waves on top of the sailboat mast....like a beacon from the lighthouse. FWiW, I have have a 6db 8' Shake Galaxy on my 17' whaler. Works fine even when it's rock and roll. "In range" you'll really not see the difference on FM, which signal strength has less effect on until it's near the fringe. I just don't like the big, long fiberglass whips on small boats. Neither does USCG. They got fed up with them breaking off and all use the Metz, now. Metz brags about it on their webpage. Interesting - all the USCG small boats around these parts use the Shakespeare 396-1 which is a center-fed 1/2 wave. I use one and it's been great antenna. That's the best antenna mounted as high as you can get it. It's end-fed by a transformer in that cylinder base. It requires no groundplane. I just don't like the way I can't replace the broken cable permanently mounted to it up inside.....or the screwed-on whip because I can't slip a 34" piece of coat hanger into the end when the whip gets busted off by that little pitchpole we did sideways to that Perfect Strom roller in the night. The Metz whip is mounted in a gripping ferrule. Coat hanger wire slips right in where the broken whip comes out. |
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Antenna Ratings
On Thu, 01 Jan 2004 18:06:55 GMT, Shortwave Sportfishing
wrote: How can an antenna work without a ground plane? At the frequencies we're discussing, the ground effect in FM is about the same as it is in AM if I understood your discussion points (deleted from this post) correctly. Modulation has no effect on antenna physics. The object is to fit at least 1/2 wavelength onto a conductor. Where it is fed is of no consequence to radiation, but does effect the feedpoint impedance. As to the "ground plane"..... In an HF antenna, we are always dealing with an antenna where the practical length is far shorter than the ideal length. At 7 Mhz, a 1/2 wave antenna is about 65' long. Any 1/2 wave dipole antenna is complete and there is no ground plane. A ground plane is only required if your antenna design includes an image antenna. Case in point: Look at 90% of the AM broadcast antennas in your area. Some AM stations actually DO have 1/2 wavelength antennas, but a 1000 Khz this is 468' long and is expensive to erect and keep erected, so they only do it when they have no other choice. A 1/2 wavelength AM tower requires no extensive ground radial system, either, so it would be located in a dense city where you cannot lay out long radials to bury. Most AM transmit antennas are near 1/4 wavelength in length (a 108" CB stainless whip on Bubba's pickup is a 1/4 wavelength antenna on 27 Mhz). To get this 1/4 wavelength to "tune" (resonate), we have to bend the other half wavelength and lay it out sideways in an L pattern. However, erecting just an L 1/2 wave antenna fed at the corner of the L creates a radiation pattern in the direction of the horizontal of the L. To counter this effect, more horizontal elements are laid out around the base of the 1/4 wave vertical part to make the pattern omnidirectional, like the 1/2 wave resonant antenna. The end result is like a CB "ground plane" antenna for 27 Mhz. A 1/4 wave vertical over a set of "ground plane" radials, which don't have to be buried to work. On VHF marine at 160 Mhz, a whole half-wave antenna needs only be 34" long, so there is no need to resort to shortened whips working against "ground plane". The Metz antenna is a 34" stainless whip with an autotransformer on the end of it that raises the feedpoint from 52 ohms, the cable impedance to the transmitter, up to several hundred ohms, which excites the already-resonant element. No ground of any kind is required, or desired. Like you say, to each his own. |
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