Re: Topband: Current Distribution on Buried Radials Used With Vertical Monopoles
On Tue,3/10/2015 11:22 AM, Guy Olinger K2AV wrote: Laying on the ground could mean anything from laying directly on dirt to top of the grass 5 cm above the dirt. That is a wide swing in VF. 73, Guy K2AV Yes, and as W8JI has observed more than once, skin depth at 160M can be tens of meters, and in many locations is probably a variable. 73, Jim K9YC _ Topband Reflector Archives - http://www.contesting.com/_topband
Re: Topband: Current Distribution on Buried Radials Used With Vertical Monopoles
Laying on the ground could mean anything from laying directly on dirt to top of the grass 5 cm above the dirt. That is a wide swing in VF. 73, Guy K2AV On Tuesday, March 10, 2015, Eduardo Araujo er_ara...@yahoo.com wrote: A specific description from you about the ground used for radials would be very interesting. 73, Guy K2AV Guy and all, Unfortunately the only data about terrain is almost flat farm land conformed by humus and neutral PH. Before having the tower and just for fun, I decided to play with Rudy N6LF measurements, against an inverted L, and to my surprise, I got extremely similar results about shortening radials and measuring the current of each of them at the antenna Base. Also, I did FS measurements at ground level at about 300 mts which acompanied the increase in current at the base of the antenna. Also I repeated the test of having 4 x 1/4 wave radials lying on the ground, measuring its current and FS. Then, raising them up in 3 step up to 85 cm from the ground and the change in FS measurement related directly with Rudy measurementsa huge change. Finally a comment about lying on the ground that I shared with Rudy, if you have 20 or less radials (I didn´t do it with more) and shorten them for maximun current at the base, which in my case was accompanied by an increase in FS, they are quite sensitive to the distance from the ground specially the last 1 or 2 mts. What I found was that if the grass move your radial up let say 5 cm in the last couple of meters, and the next one is just lying on the ground, even though equal physical length will have different currents ergo different length to both have the peak at the antenna base. As the number of radials increase, the radial length increased and that sensitivity gets smaller. Good dx to all Eddie, LU2DKT -- _ Topband Reflector Archives - http://www.contesting.com/_topband
Re: Topband: Current Distribution on Buried Radials Used With Vertical Monopoles
Heading back to the states in a few minutes. Quickly, wondering if one could measure antenna currents in the vertical portion of antenna with a pickup loop at various heights and compare with measuring ground currents at a common feed point before radials start or along a tight string pulled at various radial directions off the vertical radiator? Off to the airport. -- David J Rodman MD Assistant Clinical Professor Department of Ophthalmology SUNY/Buffalo Office 716-857-8654 _ Topband Reflector Archives - http://www.contesting.com/_topband
Re: Topband: Current Distribution on Buried Radials Used With Vertical Monopoles
A specific description from you about the ground used for radials would be very interesting. 73, Guy K2AV Guy and all, Unfortunately the only data about terrain is almost flat farm land conformed by humus and neutral PH. Before having the tower and just for fun, I decided to play with Rudy N6LF measurements, against an inverted L, and to my surprise, I got extremely similar results about shortening radials and measuring the current of each of them at the antenna Base. Also, I did FS measurements at ground level at about 300 mts which acompanied the increase in current at the base of the antenna. Also I repeated the test of having 4 x 1/4 wave radials lying on the ground, measuring its current and FS. Then, raising them up in 3 step up to 85 cm from the ground and the change in FS measurement related directly with Rudy measurementsa huge change. Finally a comment about lying on the ground that I shared with Rudy, if you have 20 or less radials (I didn´t do it with more) and shorten them for maximun current at the base, which in my case was accompanied by an increase in FS, they are quite sensitive to the distance from the ground specially the last 1 or 2 mts. What I found was that if the grass move your radial up let say 5 cm in the last couple of meters, and the next one is just lying on the ground, even though equal physical length will have different currents ergo different length to both have the peak at the antenna base. As the number of radials increase, the radial length increased and that sensitivity gets smaller. Good dx to all Eddie, LU2DKT _ Topband Reflector Archives - http://www.contesting.com/_topband
Re: Topband: Current Distribution on Buried Radials Used With Vertical Monopoles
Hello Richard, and all Topbanders, I am not going to discuss the accuracy of what Brown, Epstein and Lewis found, only I am just curious about one thing.. Everytime I measured my radials with a current probe (home made), the total current entering the vertical is not equal to the sum of each radial current. Besides each radial even though being equal in length and lying on the ground, has not the same current each other. That happened with 16, 32 64 and 120 radials. So, I thought that probable because of ground differences, the current distribution on each radial was different and if so, I could not be measuring at the same current phase point. What I tried to mean in my last sentence is like as I was measuring at different points of each 1/4 wave of the other half of the dipole or kind of. When I read that they have measured the current in one radial and multiplied it by the number of radials, I wonder why even though I used simple instruments, got some different result..my method, my instrument or both perhaps?? Greetings to all Eddie, LU2DKT From: Richard Fry r...@adams.net To: topband@contesting.com Sent: Saturday, March 7, 2015 7:55 AM Subject: Topband: Current Distribution on Buried Radials Used With Vertical Monopoles For discussion... http://s20.postimg.org/kq2k6pox9/Current_Distr_on_Buried_Radials.jpg R. Fry _ Topband Reflector Archives - http://www.contesting.com/_topband _ Topband Reflector Archives - http://www.contesting.com/_topband
Re: Topband: Current Distribution on Buried Radials Used With Vertical Monopoles
On Sun, Mar 8, 2015 at 5:57 PM, Eduardo Araujo via Topband topband@contesting.com wrote: When I read that they have measured the current in one radial and multiplied it by the number of radials, I wonder why even though I used simple instruments, got some different result..my method, my instrument or both perhaps?? The Brown, Lewis and Epstein study (BLE) was most likely done over very uniform, and over pretty good dirt for RF. I would trust your measurements as far more typical of ham circumstances. A few years back a group of hams in the Raleigh NC area made many measurements of resonant frequency and R at that frequency and +/- 50 kHz placing a 151 foot dipole on ground (DOG) as if they were constructing a BOG. The point was to try to understand some things that were happening with BOG deployments. The 151 feet was chosen to give resultant resonances near/in the 160 meter band, given the kinds of velocity factors we were already measuring in preliminary stages. It also had an even equivalent in meters (46). The answer was that those figures were wildly variable, even in different spots in the same back yard. Sometimes there were large variations between one placement and another with same center and at right angles. Should note that it is not possible to model radial-by-radial strangeness in any NEC variant as the ground approximation schemes DEPEND on a monolithic uniformity around the compass. Some sites, due to the large essentially flat acreage, DO fit that requirement. Particularly commercial AM broadcast, and their propensity to site at low, large, flat sites. To defend BLE a wee bit, it contains some number of photographs of the site (figures 19-24) and equipment. The site depicted in the photos is very flat farmland. Figures 20 and 21 show the site to be flat at least to a distance of a mile by visual cues. Most people would call the site black dirt. Figures 22-24 have a built in calibrating grey scale indicator from clearly discernible black meter cases and white meter faces typical for the time. The photographs depict dark dirt, very flat dirt, presumably deposited over a very wide area by ancient alluvial processes, likely undisturbed at depth for millennia. Just looking at it, one would expect radials to behave the same all around, no obvious reason to expect otherwise. We do not know from the study text if they did prior experimentation that suggested/validated taking the single radial approach. In terms of time/expense to conduct the study, taking measurements on all the radials over the test circumstances included in their testing matrix would be an order of magnitude increase in expense. This is in contrast to many of our ham situations with no reason to expect uniformity of ground composition across a radial field. From various clues in my case, including excavations to flatten out US 64 Hwy when they four-laned it some years ago, the ground beneath contains red or grey clay hard pan, a varying depth layer of carbon/charcoal from an ancient forest fire, several varying layers of sand in areas draining to nearby creek beds, a layer of grey clay usually near the top, plus a varying depth of organic stuff. Probably the best space-wise site for traditional radials on my and my neighbor's property combined would have half the wire above a layer of sandy soil, and the rest over grey clay, with the sand going underneath the clay layer. That part of the property was a great site for the necessary septic field, further ruining any chance for uniformity at RF. A specific description from you about the ground used for radials would be very interesting. 73, Guy K2AV _ Topband Reflector Archives - http://www.contesting.com/_topband
