RE: TOP LOADED VERTICAL ANTENNA RADIATION RESISTANCE
I have modeled top loaded verticals over perfect ground with two
different loading configurations. Radiation resistance turned out
to be approximately linear between 20% and 80% of full size (1/4
wavelength) and independent of the configuration of the top loading
capacitance. My calculated radiation resistances are indicated
in the following text for the lengths discussed.
de Tom N4KG
On Wed, 30 Apr 97 23:42:13 -0400 "Joe Subich" <W8IK@ibm.net> writes:
>In <33680F16.3505@ix.netcom.com>, on 04/30/97
> at 10:33 PM, Ed Goodman <n5nug@ix.netcom.com> said:
>
>>Joe, to answer your question, the frequency you ask in your question
>>is as always, 1.850 Mhz.
>
>The 31' antenna would be about 21 degrees. Based on the chart in the
>NAB Handbook, it would have a radiation resistance net of any matching
(MY MODEL SAYS 5 OHMS FOR A 21 DEGREE TOP LOADED VERTICAL OVER PERFECT
GROUNG - N4KG)
>network (folded diploles, linear loading, stubs, etc. are matching
>networks) of around 5 Ohms. With the heavy top loading that might
>increase to around 10 Ohms (without looking it up, I believe W2FMI
>showed one could approximately double the radiation resistance of a
>short vertical with heavy top loading.
>
>>What if I told you that a 39' antenna would produce a 292 millivolt
>>per meter field at 1 Kw at 1 kilometer? Would you believe that? The
>>FCC did, and has type certified the CTSVR for commercial AM broadcast
use. (39 FT IS 26.5 DEGREES AT 1850 KHZ AND
EXHIBITS A RADIATION RESISTANCE OF 8 OHMS OVER PERFECT GROUND WITH A
RESONATING TOP LOAD - N4KG)
>
>292 mV/M is within about 3% of the theoretical maximum for an
>infinitessimal monopole. 39' is, again, about 21 degrees. It looks
>like the station had an excellent ground system and good ground
>conductivity in the direction of the monitor points. Assuming an
>"optimum" ground system (ground losses in the neighborhood of 1/2
>Ohm),
>10 Ohms net radiation resistance and good conductivity, 292 mV/M is
>within tolerance of the measuring equipment.
>
>>Another installation is for a station in Alabama and is now under
>>test. The FCC has reinstated the operating license and issued a STA
>>for the station, The FCC required the transmitter power level be
>>reduced from 1 KW to 600 watts due to the increased field produced
>>by the monoband CTSVR. The original antenna was a 72 degree tall
>>structure tuned for 1,450 Khz. The radial system was the 120 radials
>>you mentioned in your response for Rauch.
>
>A 72 degree structure would show a net radiation resistance of about
>20
>Ohms. Sounds like the matching/tuning network for the original 72
>degree tall structure was very inefficient. That happens more often
>than most non-broadcast engineers realize - particularly if the tuning
>
>network included a low-Q series inductor and had high circulating
>currents for some reason (mis-tuned?).
>
>>Mr. Rauch has maintained the Los Alamos labs did not test the antenna
>>as we have announced. We sent him what we thought was proof.
>
>I know nothing about Los Alamos, does it have a reflection free low
>frequency antenna test range? In any case, top loaded antennas have
>been used nearly forever in AM broadcast service. I know of one very
>strange antenna farm, a six tower array, in Columbus, OH in which no
>two towers are identical and two (used in the night time DA) of the
>six
>are heavily top loaded. That array has been in use - with the top
>loaded towers - for at least 30 years.
>
>Getting back to 160, with the 17' (11 degree) top loaded antenna and
>a (EXTRAPOLATING MY DATA, 11 DEGREES LOOKS LIKE
LESS THAN 2 OHMS RADIATION RESISTANCE OVER PERFECT
GROUND - N4KG)
>typical amateur ground system of 6 or 8 60' ground mounted radials, I
>would expect a measured field strength (1 KW @ 1 KM) somewhere around
>60 to 75 mV/M at 1850 KHz, mostly due to ground loss. Because of its
>higher radiation resistance, a 90 degree radiator will not lose
>nearly
>as much in the ground system. With the same typical amateur ground
>system, I would expect the taller radiator to generate a field
>strength
>somewhere around 150 to 200 mV/M. Note: I'm estimating the net
>radiation resistance of the 11 degree top loaded antenna at 8 Ohms
>and (WAY TOO HIGH AN ESTIMATE...CLOSER TO 1 OR 2 OHMS - de N4KG)
>the ground resistance at 22 Ohms.
>
>A well designed top loaded vertical will perform as well as it can
>given
>the limitations of its ground system. No antenna can magically
>overcome the laws of physics and "fix" a bad ground system as some on
>here have claimed. The theory shows that an infinitessimal monopole
>is
>less than .5 dB inferior to a full 90 degree antenna OVER PERFECT
>GROUND but in the real world, ground losses in normal amateur
>installations effect the short monopole much more severely than a
>taller radiator.
>
>73,
>
> ... Joe Subich, W8IK ex-AD8I
> <W8IK@ibm.net>
> <jsubich@ibm.net>
>
>
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