Carl, What we do in the near-field to control ground loss affects the far-field signal equally at all elevations. Therefore there is no need to measure far-field field strength at more than one elevation.
We have control of the near-field and anything we do in that region shows up as a change in input impedance. Dave WX7G On Dec 17, 2012 3:08 PM, "DAVID CUTHBERT" <telegraph...@gmail.com> wrote: > Carl. I quantified ground loss in the near field. Now it's your turn. > Numbers please, not adjectives or hand waving. > > Dave WX7G > On Dec 17, 2012 2:59 PM, "ZR" <z...@jeremy.mv.com> wrote: > >> Because youre still stuck in neutral and are measuring/calculating >> nothing of interest. >> >> The loss is determined at various elevation angles at a sufficient >> distance by field strength. >> >> Get a helicopter. >> >> Carl >> KM1H >> >> >> ----- Original Message ----- From: "DAVID CUTHBERT" < >> telegraph...@gmail.com> >> To: "Donald Chester" <k4...@hotmail.com> >> Cc: <topband@contesting.com> >> Sent: Monday, December 17, 2012 1:53 PM >> Subject: Re: Topband: GAP Vertical Question >> >> >> Where is the 40-60% claimed ground loss? >>> >>> I get 4%. >>> On Dec 17, 2012 6:12 AM, "DAVID CUTHBERT" <telegraph...@gmail.com> >>> 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<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 <k4...@hotmail.com> >>>> 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 - topband@contesting.com >>>>> >>>>> >>>> >>>> ______________________________**_________________ >>> Topband reflector - topband@contesting.com >>> >>> >>> ----- >>> No virus found in this message. >>> Checked by AVG - www.avg.com >>> Version: 10.0.1430 / Virus Database: 2637/5466 - Release Date: 12/17/12 >>> >>> >> _______________________________________________ It is undesirable to believe a proposition when there is no ground whatsoever for supposing it is true. — Bertrand Russell