Thanks, I need to digest this.
mike va3mw On Thu, Oct 26, 2017 at 8:31 AM, [email protected] <[email protected]> wrote: > This issue does indeed keep coming up, so I am posting my response (of 3 > years ago) again for K7EG, N7WS, VA3MVW, W0MU, and others. The shortened > radials are still in use here, with 235 countries worked on 160. The Vp > shortening depends entirely on soil type, so math will only get you so > close. Better to actually measure them in place, in your ground. Only two > radials are necessary to do that. > > Brian K8BHZ > > ----- Forwarded Message ----- > > From: [email protected] > To: [email protected] > Sent: Friday, December 19, 2014 12:23:52 PM > Subject: Topband: Fw: Shortened Radial Experiments > > From: Brian Mattson > Sent: Saturday, October 14, 2006 7:56 PM > To: [email protected] > Subject: Shortened Radial Experiments > > In response to Eddy's (VE3CUI) question about anyone using "shortened > radials", I have been using these for two years now, with very good results. > > Coming to TopBand after decades on VHF, I was confused by the myriad of > opinions on radials. Comments like "resonance disappears after a few buried > radials" and "longer is better" were often seen. As a degreed electrical > engineer, I was puzzled by the abandonment of the laws of physics once a > radial was buried, or laid along the ground. Sure, the velocity factor & > loss factors change significantly once a radial gets near, or below, > ground, but basic electrical laws must still apply. > > As I first got on TopBand in the dead of winter, I used the single > elevated radial as discussed in "Low Band DXing". Pointed towards Europe, > and about 5 feet off the ground, it worked surprisingly well. However, when > it came time to upgrade the ground system, confusion set in with all the > conflicting opinions I read. Fortunately, I ran across Rudy Severns' (N6LF) > article on "Verticals, Ground Systems and Some History" in QST (July 2000). > ( As an electrical engineer in the switching power supply industry, I have > learned to listen when Rudy speaks!). One comment that really caught my > attention was on page 41: "For the 0.1 wavelength high (vertical) antenna, > if we have a good ground screen out to a distance of 0.1 wavelength, we'll > eliminate over 90% of the ground loss!". The lightbulb came on right then. > I could instantly visualize an Electrostatics Fields class representation > of a ground referenced hemispheric field intensity bubble with a radius of > the vertical height. I use > a phased pair of inverted L's for my transmit antenna, and each has > around 50 feet of vertical rise, so a system of enough 50 foot radials > should suffice. But the nagging thought of resonant length still bothered > me. Time to experiment (play). > > The inherent beauty of a quarter wavelength radial is in it's impedance > transformation properties. Basically, the higher the impedance on one end, > the lower the impedance on the other end. As the far end of the radial is > open circuited, the antenna end is as low as possible, and it is > non-reactive. Two opposing radial elements look suspiciously like a dipole, > so that's where I began. All my measuring was done at 1.83 MHz, so a > free-space dipole would be about 269 feet & have an impedance around 73 > ohms. All my experimenting was done with #14 solid insulated THHN copper > wire. > > My first experiment was to construct a full size dipole and lay it on the > ground. The resulting dipole was well below the lower operating frequency > of the MFJ analyzer, so pruning was in order. I finally achieved resonance > with a length of 182 feet! Rs was 130 ohms. So the velocity factor was > thus: 182/269 = 0.677. So Eddy, don't take the 0.5 number from "Low Band > DXing" as gospel, as it depends a lot on the type of soil you have. My soil > is sandy (almost like beach sand). Note too that the ground proximity has > increased Rs substantially. Next, I buried the dipole in a slit trench > approximately 6" deep. Again, the dipole was way too long. To prune the > buried dipole, I found it easiest to have the ends bent up so that they > protrude just above ground & place a bright colored "wire nut" on the end > (so I could find it again!). The resonant length of the dipole was now 107 > feet! Rs was 148 ohms. The buried velocity factor was: 107/269 = 0.398. > Note that burying the dipole has add > ed even more losses to Rs. > > The result of experimenting thus far resulted in a resonant radial length > (in my soil) of 53.5 feet (half of the dipole). With my 50 foot vertical > inverted L's, I was ecstatic. But how many radials would I need? > > I constructed another buried dipole of 107 feet length, at right angles to > the first, and so their centers were coincident. This gave me four radials. > I tested the second dipole as a separate entity, and it's numbers were very > close to the first. Next, I connected the two dipoles together (two > adjacent wires as one node/ the other two adjacent ones as the other). I > was astounded when the resonant frequency plummeted!! I almost gave up at > this point. As a VHFer, I knew that whether a ground plane has two or four > radials shouldn't make much difference. Indeed, some conicals feature solid > sheet ground screens. In any event, the quarter wavelength dimension > shouldn't change much. After stewing on this for a few days, I realized > that I had constructed a "Fan Dipole" which greatly increased the > capacitance to ground, thus lowering the resonant frequency. I then came up > with what I consider to be my only "original" contribution to this > experimenting. By connecting up opposing pairs > of radials as one node, and the other two opposing pair as the other > node. sanity was restored. I was pleased to see that Rs dropped almost > exactly in half (75 ohms), as two parallel impedances should. The basic > laws of physics were still intact! For want of a better name, I refer this > connection scheme as cross-connected dipoles. Realizing that with many > additional radials being added, the "cross-connection" scheme could easily > get lost. The solution was to have TWO connecting rings at the radial > junctions. The radials are then alternated from one ring to the other, so > that each ring has half the radials, but with NO adjacent ones. For > operating, the two rings are both connected to the coax shield, but for > testing, the two rings are separated to connect to the analyzer. One > curious effect was noted when the resonant frequency dropped slightly > (about 36 kHz). Pruning the radials by 6" restored 1.830MHz numbers. > (Radials now 53 feet each). This slight (second order) effect is > probably due to increased capacity, even with the cross connected > configuration. > > I then doubled the number of dipoles to four ( 8 radials), the > cross-connected dipoles again dropped to half Rs (now 38 ohms). Again I had > to prune the radial length (now 51'). > > Then the number of dipoles was doubled to eight (16 radials). Rs was now > 15 ohms. Elements again trimmed (now 49'). > > Finally, the number of dipoles was doubled to sixteen (32 radials). Rs was > now 7 ohms. Elements now trimmed to 48'. > > Please note that all the Rs readings were cross-connected dipoles in the > ground and NOT the antenna impedance. > > I then added my resonant vertical (50 feet vertical, rest in the top-hat). > > The antenna measurements were: 56 ohms with two radials. 43 ohms with > four radials. 30 ohms with eight radials. 26 ohms with 16 radials. And, 24 > ohms with 32 radials. > > One great feature of short radials that everyone seems to agree on is that > FEWER of them are required. From the antenna measurements, you can see that > doubling the amount of copper (& labor!) resulted in only 2 ohms > improvement from 16 to 32 radials. My second antenna only has 16 radials. > > My 48' or 49' radials are an efficient match for my 50 foot verticals, but > if I were to have a full-size (135 foot) vertical, I would still go to > resonant tuning. In this case, in my soil, the 3/4 wavelength radials would > probably end up around 3X48' = 144'. (possibly slightly shorter due to the > second order effect). > > Thanks to Rudy for his inspiration! > > Sorry for the long message, but I think it's sound. > > Best Regards, > > Brian Mattson K8BHZ > _________________ > Topband Reflector Archives - http://www.contesting.com/_topband > > > > > > _________________ > Topband Reflector Archives - http://www.contesting.com/_topband _________________ Topband Reflector Archives - http://www.contesting.com/_topband
