Where is the 40-60% claimed ground loss? I get 4%. On Dec 17, 2012 6:12 AM, "DAVID CUTHBERT" <[email protected]> wrote:
> *Half wavelength vertical ground loss* > > Let's see if we can quantify the conduction losses of a 1.8 MHz half > wavelength vertical connected to average earth via a ground rod. This paper > by N6LF shows one skin depth at 1.8 MHz to be 6 meters. > > http://www.antennasbyn6lf.com/files/ground_skin_depth_and_wavelength.pdf > > Let's assume the current magnitude in the ground mirrors that of the > antenna. Driving the antenna at the base such that the current at the > antenna center is 1 amp, the ground current 40 meters away from the antenna > is 1 amp. The 1 amp of ground current passes through a section of earth > having an effective depth of of 6 meters. For a 1 meter radial length and > 40 meters from the antenna the section has dimensions of 1 meter X 6 meters > X 250 meters (250 meters is the circumference). Given a resistivity of 200 > ohms/meter the resistance of this section is 200/(6 X 250) = 0.13 ohms. The > loss in this section is 0.13 watts. Using NEC we see with the base current > set to give 1 amp at the antenna center the power into the antenna is 100 > watts. > > Closer to the base of the antenna the effective ground resistance > increases due to the smaller circumference. Closer to the antenna the > current decreases. Roughly Integrating the ground loss from the base to the > 80 meters away gives a total ground loss of 4 watts. The no-radial ground > loss is 5 watts and the antenna gain is reduced by 10LOG(100/96) = 0.2 dB > from the full radial case. > > How about ground loss due to the induced E-field in the ground? I believe > this is accounted for in the previous calculation. I ran a NEC simulation > to explore this. The two cases were a 266' vertical fed against thirty 3' > radials and thirty 133' radials. The radials are 0.05' above medium ground. > The NEC Average Gain was compared for the two cases and showed a difference > of 0.06 dB. > > Dave WX7G > > On Sun, Dec 16, 2012 at 6:42 PM, Donald Chester <[email protected]> wrote: > >> >> Then, why do broadcast stations that use vertical towers at approximately >> a half wavelength, purchase valuable real estate and spend thousands of >> dollars for the copper to install from 120 to 240 or more radials, each >> usually a half wave or more in length? >> >> See G. H. Brown: "Ground Systems as a Factor in Antenna Efficiency", IRE >> Proceedings, June 1937 p. 753. Brown demonstrated that the distribution of >> earth currents and ground losses is such that the region of maximum current >> and loss occurs at a distance of about 0.35 wavelengths from the base of a >> ground mounted half wave vertical antenna, which was verified >> experimentally. >> >> There is zero loss at the base of the antenna itself, since there is no >> base current because the antenna a fed at a current node. An rf ammeter >> inserted in the ground lead, as well as one inserted in in the antenna lead >> attached to the insulated base of the radiator will read zero. The ground >> losses occur farther out from the base of the antenna. Low effective earth >> resistance provided by a good ground system is ABSOLUTELY NECESSARY for >> vertical antennas of ANY height if one expects good radiation efficiency. >> The claim that no ground system is needed for a half wave vertical is >> nothing more than a long-standing popular misconception. >> >> This topic prompted me to dig out and review an anecdote I recall reading >> in my decades-old copy of CQ magazine's Vertical Antenna Handbook, by USNR >> Capt. Paul H. Lee, K6TS (1974). He reported receiving mail from a ham who >> had made the "discovery" that he could tune and operate a half wave >> vertical without a ground system, feeding it by a parallel tuned tank >> circuit whose lower end is grounded. Since an rf ammeter in the ground >> lead showed no current, he could dispense with the ground system and its >> loss. He suggested to the Capt. that he should "discover the new world of >> half verticals with no ground system". >> >> Quoting from the text (p. 84): >> >> "The correspondent's claim... is true ONLY IF HE IS CONTENT TO THROW AWAY >> FROM 40 TO 80 PER CENT OF HIS RADIATED POWER IN THE FORM OF EARTH LOSSES. >> (the correspondent) stated, 'The ZL's call ME, when I use my half wave >> vertical!' This is not surprising, in view of the fact that the half wave's >> vertical pattern has a lower main lobe angle than a quarter wave would >> have... However, he would hit the ZL's even harder if he would put in a >> ground system. Of course, the half wave vertical is not dependent on a >> ground plane, however lossy or efficient, for the condition of RESONANCE, >> since it is resonant in itself because of its half wave length. However, >> IT IS DEPENDENT ON A GROUND PLANE FOR ITS EFFICIENCY OF RADIATION, as is >> any vertical antenna...' >> >> >> Don k4kyv >> >> >> >> >Given that a half wave vertical has a base impedance of over 1000 ohms >> and a single ground rod in dirt is 100 ohms at most not a single radial is >> needed to obtain close to 100% radiation >efficiency. >> >> > Dave WX7G >> >> >> >> > And this statement is based on what? Publications, measurements, >> > modeling? >> > >> > I have built a number of 1/2 wave verticals without radials and compared >> > them to 1/4 wave verticals with radials. They are >> > indistinguishable in performance and certainly do not exhibit >> > substantial ground losses AFAIK... >> > >> > Rick N6RK >> >> >> >I can think of NO earthly reason,that makes ANY electromagnetic sense >> to me, as antenna engineer fo placing a radial system under the end of a >> vertical 1/2 wave antenna - "earth-worms" not >withstanding! >> >> >It's CURRENT that "warms the earthworms"! NOT electric field intensity! >> >> >...the ground system does NOT act as a "shield" from the "lossy earth" >> nor protect the "earth-worms"! There is absolutely NO reason to require a >> radial system under a 1/2 wave vertical antenna. >> >Such an antenna will operate just fine on its own in free-space. >> >> >Consider this - to deliver 1000 watts to a 1/4 wave vertical with a >> REALLY GOOD ground system and a driving point impedance of say 40 ohms >> would require 5 amps of RF current delivered to the >antenna system and >> ground. Todeliver that same 1000 watts to an end-fed vertical of 2000-4000 >> ohms real would require an antenna current, at the fed endof 0.5 -0.7 >> amps! It's the CURRENT >that produces the losses in the "lossy earth" and >> "warms the earth worms". At worst, for the 1/2 wave end fed vertical - a >> simple ground rodshould be just fine, and the earth worms should be >quite >> comfortable, and the antenna will work VERY well!! Of course it will be >> 250-260 feet tall! >> >> >Charlie,K4OTV >> >> >> _______________________________________________ >> Topband reflector - [email protected] >> > > _______________________________________________ Topband reflector - [email protected]
