[600MRG] NDB Antenna Calculator

Tue Oct 6 10:46:32 CDT 2015

Gentleman,

Thanks to all for the very interesting and informative posts!  They are much
appreciated!

I did quite a bit of broadcast AM work at one time. Radials matter.

73,

Dr. Tom.

-----Original Message-----
From: 600MRG [mailto:600mrg-bounces at w7ekb.com] On Behalf Of Warren K2ORS
Sent: Tuesday, October 6, 2015 07:00
To: Edward R Cole <kl7uw at acsalaska.net>; 600mrg at w7ekb.com
Subject: Re: [600MRG] NDB Antenna Calculator

Hi Ed,

    No. My point is that NDB antennas are nothing like most ham
installations. When an NDB antenna (as well as a broadcast and military
installation) is constructed the first thing they do is clear the land, the
antenna is not strung between trees next to a house. Coupling to trees etc
can introduce large losses. I tried to use an NDB antenna tuner on my 160m
3/8 wavelength inverted-L, the tuner had plenty of inductance to resonate
the antenna, but the antenna impedance was well outside the matching range
of the tuner. This is despite the fact that I have more radials than a
typical NDB. This is not a small effect, it would be analogous to
calculating ERP for a UHF station and neglecting to include 10dB of coax
loss.
   If you use a measured impedance and know the radiation resistance you
will be ok for calculating ERP, but if you get your impedance from a chart,
table or model you are likely going to be way off.
   The FCC doesn't require NDB or commercial stations to include this effect
as it typically doesn't apply.

73 Warren

On 10/6/2015 3:27 AM, Edward R Cole wrote:
> 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.
>>

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