[600MRG] NDB Antenna Calculator

Tue Oct 6 12:42:12 CDT 2015

Ed,

    My whole point was that using a model or calculator designed around an 
NDB installation will give a gross overestimate for ERP for a typical ham 
installation. Amateur 500kHz antennas are typically very short electrically 
and generate high RF voltages and that field couples very strongly to 
objects in the near field .
   Trees adsorb and re-radiate a vertical field out of phase with the 
antenna, its a very lossy situation. Andy Talbot G4JNT did some measurements 
of the induced current in trees at 136kHz, very eye-opening stuff, I'll try 
to find his write-up.
Also, please read Bill Ashlock's article that was in the link I sent 
earlier.

 Of course I don't think that hams can clear a half mile around their 
antennas,  the point is to be aware of the difference between an NDB 
installation and an amateur installation. If using an NDB model be very wary 
of the results.

73 Warren

----- Original Message ----- 
From: "Edward R Cole" <kl7uw at acsalaska.net>
To: "Warren K2ORS" <k2ors at verizon.net>; <600mrg at w7ekb.com>
Sent: Tuesday, October 06, 2015 12:34 PM
Subject: Re: [600MRG] NDB Antenna Calculator

> Warren,
>
>
> At 04:00 AM 10/6/2015, Warren K2ORS wrote:
>>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.
>
> No my impression they are typically at or near an airport which has lots 
> of cleared landscape.
>
>>  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.
>
> I built pretty much a classic inverted-L except I ran three parallel lines 
> in the vertical and two parallel lines in the top horizontal which mostly 
> lowers Q.  I ran EZNEC-5 to obtain the impedance of base feed (0.83 - 
> j675) where I built a loading coil to supply the 675 ohms of inductance 
> needed to resonate the antenna at 500-KHz.  Once the antenna was up I 
> measured the impedance at 495-KHz and moved the coil tap until I minimized 
> the reactive reading (I use about 2/3 of the windings in my coil).  At 
> that point the MFJ analyzer showed Z = 20 +5J ( I could not get it exactly 
> to j0).  This is not 50-ohms so to match coax I used another tap near the 
> ground end of the coil to find Z = 50+j5 which was a reasonable SWR for 
> transmitting.
>
> I use a std Bird43 with 100H element for measuring fwd and ref RF at the 
> transmitter and it is a little more reactive probably due to RF on the 
> outside of the coax line.  I am using a NDB transmitter at 100w with RF 
> ammeter showing 1.4 A of RF current.  That corresponds with 100w on a 
> 50-ohm load.  The NDB did have a loading coil that I disconnected and the 
> output transformer can be tapped as low as 5-ohms or high as 50; I run it 
> at 50.  I use a 100w dummy load to check output with the RF ammeter and it 
> reads the same 1.4 amps leading me to conclude the match is good enough.
>
> The NDB loading coil is too small for my antenna and its intended that the 
> NDB be installed on the ground directly under the vertical antenna 
> section.  I chose to locate the transmitter in my shack and use coax feed 
> to my loading coil.
>
>>  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.
>
> Well, I think its unrealistic to expect every 600m station to cut down all 
> the trees on his land and live underground so there is no obstruction - 
> plus cut all the trees and bulldoze buildings for half mile radius (which 
> would be on somebody else's land).  Of course  if you own a large farm or 
> live on the prairie or in the desert trees/buildings are no problem.
>
> I do eme on 144 and 1296 MHz so well aware of the effect of foliage on VHF 
> and mw frequencies.  Typically HF hams ignore it.  I have about 50-foot 
> clearance of trees to the actual antenna but two towers support the ends 
> of the top wire and my house is underneath that.
>
>>   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.
>
> (see above)
>
>>   The FCC doesn't require NDB or commercial stations to include this 
>> effect as it typically doesn't apply.
>
> Why?  Are they using different physical laws?  Or do you mean they own 
> cleared land for antenna sites.
>
> 73, Ed
>
>
>>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.
>>
>>---
>>This email has been checked for viruses by Avast antivirus software.
>>https://www.avast.com/antivirus
>
> 73, Ed - KL7UW
> http://www.kl7uw.com
>     "Kits made by KL7UW"
> Dubus Mag business:
>     dubususa at gmail.com
> 