Effect of Earth Conductivity on a Buried Radial System

[R. Fry SWL]

The link below shows how the 1.85 MHz r-f ground connection loss and system radiation efficiency of a 10m monopole using a set of 32 x 10m radials vary with the conductivity of the earth in which the radials are buried.

http://i62.photobucket.com/albums/h85/rfry-100/10m_Vert32Buried_Radials.jpg

Richard Fry

[K5UJ]

I'd sure like to know how accurate that FCC ground conductivity map is.   It is older than dirt but as far as I know, it is the only thing we have to go on, for g.c.  I'd like to know the method they employed for determining the various g.c. contours.

Rob

[R. Fry SWL]

There is quite a bit of variability in the conductivities of those M3 chart zones depending on the season of the year.  And probably some of the boundaries have shifted around.

But I have measured the field of WJR (760 kHz, 50 kW omni, 24/7) about 100 miles from their tx site, using a calibrated F.I. meter.  The measurement (~ 2 mV/m) agreed very well with the FCC groundwave propagation chart for the conductivity of that path on the M3 map for the radiated power and frequency.  The measurement was taken in the month of November back in the 1960s.

Broadcast stations have been able to modify the licensed values for some installations based on groundwave field intensity measurements showing different conductivity values for certain paths than given by the M3 chart.

[k4kyv]

Something that has always bothered me about that contour map is how the conductivity frequently jumps two-fold or more across a boundary. As an extreme case, notice that the boundary between western Minnisota and most of N Dakota jumps from 4 to 30.  Those figures might be close near the middle of the contour zones, but anyone should know it doesn't abruptly jump from 4 to 30 as you cross a line on a map.  For specific locations anywhere close to the marked boundaries, I would say the conductivity is anyone's guess, probably somewhere between the two extremes, unless measurements are taken. 

[W8IXY]

FWIW,

Regarding ground conductivity and the FCC M3 map:

I measured WHAS in Louisville, KY, 50kw @ 840 kHz, about 15 years ago when we were applying for a power increase in Cleveland on the 850 kHz first adjacent here.   According to the official records, WHAS was listed as placing a 0.5 mv/m field intensity over (of all places) the Hara Arena in Dayton.  I measured from the actual tower itself on a straight line radial out from WHAS until I located their 0.25 mv/m contour.  The measured WHAS field intensity over Hara Arena was 0.25 mv/m.  We were able to "fit in" a 50kw signal on 850 Khz by proving the actual coverage of WHAS.  This is standard procedure if you want to prove that calculated data from the FCC's M3 conductivity map is different from the actual measurements.   Measurement always win out.

Another example is from a small radio station I have done work for for over 45 years.  In 1964, the measured field intensity in several directions from that station corresponded closely with the M3 map.  In the mid 2000's, one radial direction in particular, which had measured about an 8 in 1964, varied between a 1 and a 2 in the mid 2000's.  The signal was propagated quite poorly in that direction.

Here in Northeast Ohio, I have measured all the major AM signals over the years and the ground conductivity measured on all those AM stations has shown a gradual deterioration, resulting in significantly less field intensity in 2012 compared to the measurements from 30 to 50 years ago.

The higher you go in frequency, the more pronounced the effect of ground conductivity has in a groundwave signal.  Since most of our amateur radio propagation is via skywave, there is some, but much less effect on signal propagation than there is on groundwave.  In fact, much above the 60 meter band, the groundwave becomes very limited in usefulness.  There are other effects from ground conductivity from antennas such as from a beam on a tower that have been calculated and measured, and reported in other articles.

73
Ted  W8IXY

[KC2ZFA]

The FCC map is discussed here:

http://www.fcc.gov/encyclopedia/m3-map-effective-ground-conductivity-united-states-wall-sized-map-am-broadcast-stations

I think the original work is this:

http://tinyurl.com/7z5vynp

Peter

[R. Fry SWL]

The higher you go in frequency, the more pronounced the effect of ground conductivity has in a groundwave signal.

Here is a link to a short paper with graphics showing these effects over most of the AM broadcast band...

http://www.radioworld.com/article/am-coverage-frequency-vs-conductivity/23739

[K5UJ]

The f.i. reduction for the local 1-As has been a topic at hamfests around here amongst the broadcast hams.  Some do better than others but they don't bomb the way they did in the 60s.   The consensus seems to be that the problem is a combination of a noise floor rise (making the station seem weaker) but more important,  a reduction in g.c. around the tx sites due to urban sprawl type development.   

[K3YA]

AM broadcast is mainly concerned with ground-wave so the average earth conductivity over the entire signal path will determine how fast the signal drops off.  The conductivity well removed from the antenna is significant,

Ham radio is mainly concerned  with sky wave so earth conductivity away from the immediate vicinity of the antenna makes no difference.  Local conditions can very dramatically.  For example, conductivity down in the swamp may be vastly better then the rocky hilltop a mile away.

The conductivity map may provide a valid average that's useful in broadcasting, but it is not so helpful to us hams on shortwave.  For us, it's the local conditions that count.

[K5UJ]

We are interested in it because some of us operate on medium wave.  g.c. matters because it is a factor in earth losses vis a vis the ground system collecting and returning currents to the feedpoint. 

It is also important for 160 and 75 horizontal antennas if they are close to earth (< 1/4 wave height) because lossy earth will negatively affect antenna efficiency.

[KM1H]

Soil conditions can easily destroy a radial field over those many decades and should show up during regular nearfield testing.

In some cases copper thieves may have been involved.

Im also a bit surprised that the BC industry hasnt approached the FCC about all these RFI polluters in homes, etc that has reduced their effective coverage resulting in loss of revenue.

Carl

[W8IXY]

Soil conditions can easily destroy a radial field over those many decades and should show up during regular nearfield testing.

In some cases copper thieves may have been involved.

Im also a bit surprised that the BC industry hasnt approached the FCC about all these RFI polluters in homes, etc that has reduced their effective coverage resulting in loss of revenue.

Carl

I spoke with Bill Cross from the FCC at last year's and this year's Dayton Hamvention.  The (very brief) summation of his answer is that the FCC has neither funding nor manpower to get involved with consumer type RFI.  BC stations are concerned, but without FCC clout, can do little if anything about man made noise.

73
Ted  W8IXY