[600MRG] E-Field Probe Antennas

Tue May 31 15:33:57 CDT 2016

See below for a copy of simulations I reported last year on how an 
"E-field" antenna really works at VLF/LF.  The "antenna" starts at the 
top of the "probe", continues down the outside of the coax feedline, and 
ends at ground level or at the level where a large ground plane is 
present and connected to the coax feed.  The high impedance E-field 
amplifier can be inserted anywhere top to ground. There is enough 
information to do the simulation.  Heff (mtrs) multiplied by the 
incoming field strength in V/mtr gives the output RF voltage of the 
simulation.

I just ran a NEC4.2 simulation of a 0.15 meter vertical on top of a 10m 
pole, and also for the same antenna on a 1m pole.  This is a rough 
analysis. It could be done with any version of NEC.  The "pole" 
represents the shield of the coax feedline of the voltage probe antenna 
and is connected to perfect ground at the bottom.  I placed a 1 megohm 
resistor between the antenna and the top of each pole, then noted the 
voltage drop across the resistor  for an applied 1 Volt/meter vertical 
Electric  field at each frequency. The received voltage is also 
(approximately) the Effective Height (Heff) of the antenna.  Incoming 
field strength X Effective Height (mtrs) = open circuit received voltage.
_Freq MHz  Heff on 10m pole   Heff on 1m pole_
0.1                      13.8m                       1.34m
0.5                      17.4                           1.75
1.0                      17.8                           1.84
2.0                      19.0                           1.89
4.0                      25.7                           1.91
6.5                      136.8                         1.92
7.5                      48.6                           1.92
30.0                    0.44m                       2.19

At LF and MF where the 10m pole is short compared to 1/4 wavelength, the 
gain in received signal strength is proportional to the relative heights 
of the 2 poles.  At HF, the 10m pole becomes 1/4 wave resonant near 
6.5MHz, causing a huge peak in received signal strength, and possible 
overload of the amplifier in the antenna. The probe on the 1m pole has a 
nice nearly flat response from 0.1-30MHz.
Conclusions?  I have a low frequency Voltage probe antenna on an 8m 
pole, grounded at the base.  It has a low pass filter that cuts in in 
the BC band, so HF is greatly attenuated.  The received signal level at 
VLF-MF are so high that no preamp is needed at the receiver.
If HF reception is also desired, then I would follow the AMRAD ideas and 
either mount it on a short pole out in the open, with a ground rod at 
the base, or up on a roof with a piece of ground screen.  It would be 
interesting to try grounding to a metal roof
Note that if you ground the probe right at its housing, the signal 
levels will be _very_ low because, as noted above, the true antenna 
height is from the ground point to the tip of the probe.
The NEC4.2 punch card format input is as follows (I don't use the GUI to 
input data)
CM 0.15 meter monopole on 10 meter grounded support over perfect ground
GW 1,101,0,0,0,0,0,10.1,0.002 ! 10.1 mtr vertical wire at origin, 4mm dia.
GE 1,0 ! Ground plane present, wire will be grounded to it.
LD 4,1,100,100,1E6,0 ! 1 meg resistor 9.95 mtrs up wire (at center of 
segment 100).
LD 5,0,0,0,58.1E6 ! All Wire is copper
GN 1,0,0,0 ! Perfect ground.
EX 1,1,1,0,90,0,0,1,1,0,1 ! 1V/m vertical E field along x axis.
FR 0,61,0,0,0.500,0.500 ! Freq 500kHz - 30.5MHz
CM FR 0,1,0,0,0.100,0 ! Freq 100kHz
XQ
EN
Note that if you run a ground wire from your roof to Earth, your results 
will be closer to the 10m pole above because the ground wire will be 
part of the antenna.
On 12/23/2015 3:52 PM, Frank Lotito wrote:

  The 600mrg bulletin board of Dec 20 and 21, 2015 had a few postings on E-Field (Probe) antennas.  Included were snippets recommending how and where to ground the coax feed line to the E-probe antenna's amplifier unit.  Definitely good food for thought, at least for me with my miniscule theoretical understanding of how antennas work.   However, etched in back of my mind is the September 2001 QST article by K0BRA, "The AMRAD Active LF Antenna," pages 31 through 37.  The author very briefly discusses the use of a ground screen directly under the feed point "to stabilize fields around the antenna and to reduce noise coupling," page 37, the caption directly under the color photograph.  [http://www.arrl.org/files/file/Technology/tis/info/pdf/0109031.pdf  ]

Could someone explain the merits (positive / negative) of the AMARD chicken wire ground screen vs the grounding methods suggested in this column's Dec 20 and 21 replies.  Has anyone published "A" vs "B" vs "C" comparisons of test results for different grounding methods? Published a theoretical analysis of an E-Field probe antenna, as to exactly where the vertical radiator is?  I would assume empirical data can be more conveniently obtained if the antenna was scaled for VHF instead of LF?

On 5/31/2016 2:07 PM, Frank Lotito wrote:
> In recent weeks the 600mrg bulletin board had a few discussions on E-Field Probe antennas.  I am still having trouble understanding how E-Field antennas work. I have a few questions regarding the effect(s) of the ground lead / ground stake / radial system / mast immediately below the probe.
>
>
> The length of the ground path to the ground stake / radial system / or grounded mast mast may be a few inches long, to many-many feet long, even longer than the probe antenna itself.  This ground is far from ideal, ideal being 0 +j0 ohms, my experience tells me that some sort of antenna still exists if we disconnect the E-Probe and replace the probe with a simulated antenna, but leave the ground system / matching transformer / FET follower with its transformer coupling in place.
>
>
> My gut feeling is if we disconnect the probe antenna (whip) and replace it with a simulated antenna, we are very likely not going to hear only the receiver's self-generated noise.  We will hear signals (maybe), atmospheric noise, or interfering signals due to man-made emissions.  The reason - Ground loops?  Buried antenna phenomena, alla well casting antenna and the like?   Can someone review what could be the reason(s) we still hear signals / atmospheric noise / etc. when the probe is replaced with a simulated antenna?
>
>
> Frank Lotito  K3DZ / WH2XHA
>
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