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Antenna Ratings
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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