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#1
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mountains reflect signal anyway?
The setting: city to the west, mountains to the east. For 2 meter
transmissions, should one feel bad that half the coverage area of ones vertical antenna is wasted on unpopulated mountains, or might the mountains reflect most of the signal anyway, especially if right east of us is an idealized vertical mountain wall? Thus no hurry to upgrade to a yagi? |
#2
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On Tue, 15 Feb 2005 02:39:50 +0800, Dan Jacobson
wrote: The setting: city to the west, mountains to the east. For 2 meter transmissions, should one feel bad that half the coverage area of ones vertical antenna is wasted on unpopulated mountains, or might the mountains reflect most of the signal anyway, especially if right east of us is an idealized vertical mountain wall? Thus no hurry to upgrade to a yagi? Hi Dan, That's how it works here in Seattle for some. We have a lot of hills that hide repeater antennas. We also have a lot of mountains that reflect them (even from the opposite angle). 73's Richard Clark, KB7QHC |
#3
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The setting: city to the west, mountains to the east. For 2 meter
transmissions, should one feel bad that half the coverage area of ones vertical antenna is wasted on unpopulated mountains, or might the mountains reflect most of the signal anyway, especially if right east of us is an idealized vertical mountain wall? Thus no hurry to upgrade to a yagi? My guess - the mountains reflect some of the signal, but in a rather unpredictable manner and with much of the energy being reflected upwards. The mountain reflections might help fill in a few gaps in your coverage pattern. However, the reflections are likely to _hurt_ coverage in some areas, due to multipath interference with the direct (non-reflected) signal. You might find that your signal suffers from a greater amount of picket-fencing due to multipath, than it would if the mountains were not present or if the antenna's pattern didn't include the mountains. The more vertical (and the more reflective) the mountain wall is, the stronger the reflections, and perhaps the more pronounced the picket-fencing. It might well be worth your while to experiment with a simple somewhat-directional antenna. A single reflector, located perhaps .1 to .2 wavelengths on the mountainside side (sorry :-) of your current vertical, cut to perhaps 5% longer than a half-wavelength, could be used to shape your antenna's pattern into something vaguely cardioid. This would send your transmit energy (and your receive sensitivity) where it will do you the most good. Play around with the antenna-to- reflector spacing and see what it does to your pattern. -- Dave Platt AE6EO Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
#4
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D It might well be worth your while to experiment with a simple
D somewhat-directional antenna. A single reflector, located perhaps D .1 to .2 wavelengths on the mountainside side (sorry :-) of your D current vertical, cut to perhaps 5% longer than a half-wavelength, D could be used to shape your antenna's pattern into something D vaguely cardioid. Yes! Say, can one just cut the reflector 5% longer than the antenna? The antenna is some complex double 5/8 wavelength job. If the reflector is shorter than the antenna, it becomes a director? What if the reflector is just a pole jabbed into the ground, thus grounded and longer downwards than the 5%? What if the pole is "very much longer than the antenna in both directions"? How about the thickness of the reflector? Only as thick as the antenna itself (but wait, the antenna has two thicknesses, top thin, bottom thick), or just grab any iron pipe (wait, metal type and shape important?) |
#5
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In article ,
Dan Jacobson wrote: D It might well be worth your while to experiment with a simple D somewhat-directional antenna. A single reflector, located perhaps D .1 to .2 wavelengths on the mountainside side (sorry :-) of your D current vertical, cut to perhaps 5% longer than a half-wavelength, D could be used to shape your antenna's pattern into something D vaguely cardioid. Yes! Say, can one just cut the reflector 5% longer than the antenna? The antenna is some complex double 5/8 wavelength job. If the reflector is shorter than the antenna, it becomes a director? What if the reflector is just a pole jabbed into the ground, thus grounded and longer downwards than the 5%? What if the pole is "very much longer than the antenna in both directions"? How about the thickness of the reflector? Only as thick as the antenna itself (but wait, the antenna has two thicknesses, top thin, bottom thick), or just grab any iron pipe (wait, metal type and shape important?) Well, I think you'd probably need to do a bunch of modelling in NEC2 or similar to get a really solid analysis, for any given antenna and reflector. The "5% longer" rule of thumb applies to antennas which use halfwave radiators - it works with Yagis, and Cebik has an analysis which indicates that it works with Yagi-like beams based on vertical J-poles. I suspect that the issue is rather more complicated for antennas which use 5/8-wave or stacked/collinear radiators. The phase relationships between driven and parasitic elements in such antennas are probably different enough that this simple rule might not apply. I believe that you could probably get a fairly effective pattern-warping by using a non-tuned reflector which is significantly longer than your collinear radiator. Try cutting a piece of EMT or other conduit (or aluminum pole, or whatever) so that it's a few feet longer than your antenna, and then install it 6" to a foot on the undesired side of the radiator... have one end of it sticking up at least a foot above the top of the radiator and the other extending down below the bottom of the radiator. Some commercial multi-bay stacked dipole antennas use the mast or tower in just this fashion, to convert a circular omni pattern into a cardioid or off-center near-circular pattern. Varying the antenna-to- mast/tower spacing changes the shape of the pattern. -- Dave Platt AE6EO Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
#6
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Dan Jacobson wrote:
Yes! Say, can one just cut the reflector 5% longer than the antenna? The antenna is some complex double 5/8 wavelength job. If the reflector is shorter than the antenna, it becomes a director? Nope, the reflector needs to be 5% longer than 1/2WL. A 5/8WL antenna is NOT resonant. Passive elements must be close to and referenced to resonance. I had a vertical loop resonant on 17m. I installed a reflector 1.05 x (1/2WL of 20m). It became a beam on 20m. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
#7
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Dave Platt wrote:
I believe that you could probably get a fairly effective pattern-warping by using a non-tuned reflector which is significantly longer than your collinear radiator. Passive non-resonant elements have very little effect. That's what allows a lot of multi-band beams to work. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
#8
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On Wed, 16 Feb 2005 04:29:38 +0800, Dan Jacobson
wrote: D It might well be worth your while to experiment with a simple D somewhat-directional antenna. A single reflector, located perhaps D .1 to .2 wavelengths on the mountainside side (sorry :-) of your D current vertical, cut to perhaps 5% longer than a half-wavelength, D could be used to shape your antenna's pattern into something D vaguely cardioid. Yes! Say, can one just cut the reflector 5% longer than the antenna? The antenna is some complex double 5/8 wavelength job. If the reflector is shorter than the antenna, it becomes a director? What if the reflector is just a pole jabbed into the ground, thus grounded and longer downwards than the 5%? What if the pole is "very much longer than the antenna in both directions"? How about the thickness of the reflector? Only as thick as the antenna itself (but wait, the antenna has two thicknesses, top thin, bottom thick), or just grab any iron pipe (wait, metal type and shape important?) I am going to take a stab at this. I would think that the 5% greater than the 1/2 wave will be a better reflector than the 5% greater than 5/8 wave. I am thinking that the wavelength of the signal is what we are dealing with more than the size of the transmission element. I did a very non-scientific experiments with rubber duck antennas and beam elements many years ago. One of the tricks I used was to put a rubber duck between the reflector and driven elements of a small beam. The signal improved radically. I also tried it with three rubber duck antennas. The reflector duck had an extension to make it a little longer. The change in the signal strength was, as expected, a little improved, but not greatly. I also placed a 5/8 wave j-pole between the reflector and director. It produced a better signal than the rubber duck did, but not as much as I had expected. I didn't go any farther with the experiment. I didn't keep a log or use calibrated equipment or anything. Like I said, I was just playing around with it a few minutes. -- 73 for now Buck N4PGW |
#9
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"Cecil Moore" wrote
Dave Platt wrote: I believe that you could probably get a fairly effective pattern-warping by using a non-tuned reflector which is significantly longer than your collinear radiator. Passive non-resonant elements have very little effect. That's what allows a lot of multi-band beams to work. _____________ As a point of reference, the radiation patterns of sidemounted FM broadcast transmit arrays are affected by a non-resonant mounting structure (the tower) -- especially for vertical polarization. Measured patterns from the manufacturer's test ranges demonstrate this, as do the NEC-2 studies in several of the papers on http://rfry.org . RF |
#10
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Richard Fry wrote:
"Cecil Moore" wrote Passive non-resonant elements have very little effect. That's what allows a lot of multi-band beams to work. As a point of reference, the radiation patterns of sidemounted FM broadcast transmit arrays are affected by a non-resonant mounting structure (the tower) -- especially for vertical polarization. True, and please note that I was talking about *antenna elements* not support structures. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
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