----- Original Message ----- From: "SM Ling" To: Sent: Thursday, February 20, 2003 7:17 PM Subject: Re: [EE]: Looking for good ref for antenna grounding? > > > I'm flattered to be included in the list of "RF experts" ;-) (Of > > course, assuming I'm the Sean you are talking about!) > > You should be :-) > > > However, from what I do know, I like Michael's suggestions: either make > your > > radials more robust or use an antenna type that doesn't require them. > > The antenna sits at the end of a narrow (2m wide) concrete platform, the > platform is on a 45 degree-slope rock (sea-bank). Using pipe is an > interesting idea, might have to test it out first. Think about it, pipe > might suffer a greater force from the water-movement than wire as its > surface area is larger. They would have to bolted even more tightly. > Perhaps the problem is the wire are bolted too lossely. I am no structure > engineer but have to second-guess their work now. Is there any way to get the radials that are in the water to be under the sand? I've never been on a small island, but I suspect that the sand moves around to much due to the tides. Maybe you could put the end of each radial in a bucket of concrete to help keep it from moving around so easily? > > AFAIK, theres really not much else that he could do. ;-) > > > What kind of soil do you have on this island? It might also be > > possible to just place the radials in the ground, but probably not if it > is sandy. > > > > AIUI, the radials don't actually have to be in contact with earth ground > > to still function well. He could spiral the radials around so that they > > are not so far into the sea and this should work nearly as well, as the > > electrical length of the radials would still be the same. He may have > > to tweak the matching to the antenna though, since nearly anything that > > he does will have some effect on the feedpoint impedance and possibly > > the radiation pattern. I'm a bit rusty on my antenna theory, as I'm > > sure you can see. > > There is an Antenna tuning unit (ATU) then do the matching. We found out > that if the ground (if we are right) is no good, the transmitter tripped > faster with an ATU than without. Well...that sure sounds odd. I could understand the ATU getting hot, but not the transmitter if the ATU is able to tune out the reactance. Does the ATU self adjust? If so, maybe it's just not able to tune to a suitable SWR so it's hunting causing it to make things worse for a while. Perhaps bad enough that the SWR is much higher with the ATU than without. > Could you tell more about "spiraling" the radials? AIUI, the length of the radials is what's most important, not so much that they extend outwards perfectly. My experience with HF in amateur radio shows that you can usually get away with curling them or bending them without much negative effect. It's not ideal, but it does work. But from your description of the site, there is not much dry area around the antenna to place the radials on. Maybe the copper pipe idea would be a better solution. > Thanks for respnding. As usual, please shoot freely. Please understand that I am not what anyone might call an absolute "expert" on this. I've read allot of material, and even make my own antennas, but please don't consider my opinions to be anything more than that. Antenna theory is just that, theory. IMO, it is probably the one EE subject that is as much opinion and anecdotal evidence as it is fact. Because there are so many factors, it is closer to voodoo than it is to science. ;-) If I happened to be in your position, I (me personally) would be asking someone for advice. ;-) There is no perfect antenna besides an isotropic point, and it doesn't even have any gain. ;-D michael -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu