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

Mon Dec 17 17:23:05 EST 2012

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" <telegrapher9 at 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" <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
>>>
>>>
>>> -----
>>> 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
>>>
>>>
>>