TopBand: Re: McCoys response

[Joe Subich]

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 
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.

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
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
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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