Topband: Ground mounted 1/2 and 1/4 wave verticals (was GAP)

Mon Dec 17 17:08:18 EST 2012

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" <zr at 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" <
> telegrapher9 at gmail.com>
> To: "Donald Chester" <k4kyv at hotmail.com>
> Cc: <topband at 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" <telegrapher9 at 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 <k4kyv at 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 at contesting.com
>>>>
>>>>
>>>
>>>  ______________________________**_________________
>> Topband reflector - topband at contesting.com
>>
>>
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