[600MRG] NDB Antenna Calculator

[Edward R Cole]

Warren,

I guess if what you are analyzing is potential coverage of a BC or 
NDB system then you factor in all these effects, but my understanding 
was use of the NDB calculator for our purposes was to satisfy the 
calculation of simple ERP for FCC compliance.

Even seasonal change in soil conductivity is not asked by the FCC 
requirement.  Of course knowing more about the effects to 
transmission may help improve performance but most of those factors 
are just what you are handed with the environment that exists for 
your location (assuming you do not own a texas ranch covering 
thousands of square miles).

Most of us will have little choice in where the antenna is set 
up.  Choice of antenna type may exist if you have enough room and I 
suppose the field strength tests would be useful for deciding what to 
use.  My property is 252x300 feet (1.735 ac.) which is probably more 
than many have but much less that Rudy and others.  e.g. its too 
small for a Beverage.  My Inverted-L takes a linear area of 130-feet 
and ground plane area of about 100x100 foot.  About one third of my 
land is forested covering a 60-foot deep damp hollow which will never 
be cleared.  Also a private north-south airstrip borders me on the 
west property line so this limits safe antenna heights to be about 
ten foot over tree top (50-foot).

I live inside the Boreal Forest of the sub-arctic so not much I can 
do about that.  I am in a rural area so houses are spaced at least 
300-feet or more.  Fortunately utilities are buried so no interaction 
with utility wires.  I have two 50-foot towers at the ends of my 
inverted-L (the supports) which probably interact but may also 
radiate parasitically.  Actual RF measurements are not far off of 
those predicted with sw.

I do not have time to get into all the theoretical's of 600m as I am 
also very involved in other ham radio pursuits and running a small 
business at home.  Once I have my antenna tuned for best efficiency 
then its time to transmit.  I'll add a shielded loop to that for 
optional Rx and might I consider a flag type Rx antenna in the 
future?  Maybe.  WSPR tests may prompt future experiments; we'll see.

73, Ed - KL7UW

At 05:54 PM 10/5/2015, Warren K2ORS wrote:
>Coupling to trees, buildings etc will also introduce a lossy 
>resistance to the antenna impedance.
>
>See Bill Ashlock's article:
>http://www.geocities.ws/mike_staines/bill/loop1.doc
>
>   I can vouch for his conclusions from personal experience.
>73 Warren K2ORS
>
>
>On 10/5/2015 9:17 PM, Edward R Cole wrote:
>>Rudy,
>>
>>I assumed when you write ground resistance you mean for the ground, only.
>>
>>When I originally calculated my ERP in 2012, I used EZNEC to provide the
>>radiation resistance which was about 0.83 ohms and I measured ground
>>resistance with my modified MFJ analyzer by tuning the loading coil
>>until I got R +j0; R read 20-ohms.
>>
>>Rr/Rg is used in the calculation of EIRP.
>>
>>So if the variable given in the "NDB calculator" is Rg+Rr that should be
>>made clear instead of saying "ground resistance".    It seems this
>>imprecision in specifying variables is what keeps the analysis from
>>being accurate.
>>
>>73, Ed
>>
>>At 03:22 PM 10/5/2015, Rudy Severns wrote:
>>>Steve, he's going to have a real problem.  Rg depends not only on soil
>>>electrical characteristics and details of the radial system but also
>>>on the
>>>antenna over the radial system.  If you have NEC4 determining Rg from
>>>modeling is easy.  Direct calculation is also possible, I've done it, but
>>>it's complex because you have to start by deriving the E and H field
>>>intensities associated with the antenna in the near-field.  From that you
>>>can calculate the soil losses which determine Rg for a given Io.
>>>
>>>
>>>
>>>Too many people believe that a given radial system, at a given frequency,
>>>with given soil characteristics, has a fixed Rg.  This is false.  Rg will
>>>depend on all those things plus the details of the vertical.  This is
>>>carefully explained in section 3.2.3 of the 22nd edition of ARRL Antenna
>>>Book which shows among other things the dependence of Rg on the vertical
>>>height for a given ground system.
>>>
>>>
>>>
>>>73, Rudy N6LF
>>>
>>>
>>>
>>>
>>>
>>>_______________________________________________
>
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73, Ed - KL7UW
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