Topband: GAP Vertical Question

[Donald Chester]

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