Where is the 40-60% claimed ground loss?
I get 4%.
On Dec 17, 2012 6:12 AM, "DAVID CUTHBERT" <telegrapher9@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
>
> 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 <k4kyv@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
>>
>>
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>>
>
>
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