Re: Topband: Best wire antenna for roof top location
No. You'd be missing the essential point which has to do with the "shape" of the tool and whether it was remotely suited to the problem. The best estimate of NEC applied to this problem, even with a cage for the building, will likely be buried in inescapable inaccuracies. The cage's good point is that at least it is not based on a doomed misconception of how the NEC code works. IF the building is all metal construction, the cage may return some halfway decent description of RF behavior. If it's concrete over rebar posts with indeterminate connections to the facing, the outcome is probably unknowable in advance of antenna installation. And it's important to understand that the model is NOT useful in this case, rendering all-issue (e.g. noise, interference) anecdotal advice from those who have attempted the same the better counsel. 73, Guy K2AV On Tuesday, August 11, 2015, Jim Brown wrote: > Ah, perfection. Sadly, EZNEC does not provide that set of options. If I > could build that model, I would. But I can't. I will take this first > approximation, understanding its limitations. Brown's 99th law -- never > let perfect be the enemy of good. We don't know the nature of that roof, > so modeling several possibilities and using the result to TRY things seems > like a pretty good approach to me. > > 73, Jim K9YC > > On Tue,8/11/2015 12:42 AM, Guy Olinger K2AV wrote: > >> In order to estimate your hypothesis you need to create a building shaped >> cage of dielectric wrapped, resistively loaded, interconnected wires. It is >> necessary to bookkeep in the model that the roof, as conductor, is >> connected to a vertical conducting face on four sides and then to ground. >> >> These faces electrically are at least in the quarter to half wavelength >> range vertically and can radically effect the appearance of the roof as >> conductor. These faces can have significant radiation. These faces can be >> the majority radiator with the dipole serving mainly as a matching device >> to the building as majority radiator. >> > > _ > Topband Reflector Archives - http://www.contesting.com/_topband > -- Sent via Gmail Mobile on my iPhone _ Topband Reflector Archives - http://www.contesting.com/_topband
Topband: Laurel & Hardy
Frank, Interesting that in the 1930s you folk in the colonies called the thing an “aerial” and not an “antenna”! 73 Tom G3OLB _ Topband Reflector Archives - http://www.contesting.com/_topband
Re: Topband: Best wire antenna for roof top location
Ah, perfection. Sadly, EZNEC does not provide that set of options. If I could build that model, I would. But I can't. I will take this first approximation, understanding its limitations. Brown's 99th law -- never let perfect be the enemy of good. We don't know the nature of that roof, so modeling several possibilities and using the result to TRY things seems like a pretty good approach to me. 73, Jim K9YC On Tue,8/11/2015 12:42 AM, Guy Olinger K2AV wrote: In order to estimate your hypothesis you need to create a building shaped cage of dielectric wrapped, resistively loaded, interconnected wires. It is necessary to bookkeep in the model that the roof, as conductor, is connected to a vertical conducting face on four sides and then to ground. These faces electrically are at least in the quarter to half wavelength range vertically and can radically effect the appearance of the roof as conductor. These faces can have significant radiation. These faces can be the majority radiator with the dipole serving mainly as a matching device to the building as majority radiator. _ Topband Reflector Archives - http://www.contesting.com/_topband
Re: Topband: Best wire antenna for roof top location
Hi Jim, I must respectfully disagree. And warn any reader that using NEC ground media to simulate a conductive roof is a very troubled procedure with significant issues. Using the first media to simulate an elevated conductive plane for any purpose is a problem. Using it to simulate radials is simply the ubiquitous recurring error (out of a class of errors) that W7EL specifically lists as the example in self defense. You are putting forth a rare invocation of this same NEC weakness. In order to estimate your hypothesis you need to create a building shaped cage of dielectric wrapped, resistively loaded, interconnected wires. It is necessary to bookkeep in the model that the roof, as conductor, is connected to a vertical conducting face on four sides and then to ground. These faces electrically are at least in the quarter to half wavelength range vertically and can radically effect the appearance of the roof as conductor. These faces can have significant radiation. These faces can be the majority radiator with the dipole serving mainly as a matching device to the building as majority radiator. It is not inconceivable that the combination could function more like a big dummy load. 73, Guy K2AV On Monday, August 10, 2015, Jim Brown wrote: > Actually, what he says is not use the high level media to model a RADIAL > system. > > While playing with this model, I changed the characteristics of the higher > level media to have far less conductivity. The result suggested that my > model was good for what I was trying to understand -- that is, the very low > far field lobe was unaffected by the conductivity of the high level media > (that is, the roof). > > 73, Jim K9YC > > On Mon,8/10/2015 6:48 PM, Guy Olinger K2AV wrote: > >> There is a rather stern W7EL caution in the EZNEC doc about using two >> ground media. He particularly nixes using the inner media as sea water >> to mimic a high conductive surface. >> > > _ > Topband Reflector Archives - http://www.contesting.com/_topband > -- Sent via Gmail Mobile on my iPhone _ Topband Reflector Archives - http://www.contesting.com/_topband