Bluegrass Volunteer Examiners
Information for Amateurs Completing the New FCC Form 605
Recognizing this need, here is the information you will need to read and must comply with. Section 97.13(c) reads:
c. Before causing or allowing an
amateur station to transmit from any place where the operation of the
station could cause human exposure to RF electromagnetic field levels in
excess of those allowed under §1.1310 of this chapter, the licensee is
required to take certain actions.
--Repeater stations (all bands) non-building
mounted antennas: height above ground level to
Limits for General Population/Uncontrolled Exposure
In complying with the Commission's Report and Order, amateur operators
should follow a policy of systematic avoidance of excessive RF
exposure. The Commission has said that it will continue to rely upon
amateur operators, in constructing and operating their stations, to take
steps to ensure that their stations comply with the MPE limits for both
occupational/controlled and general public/uncontrolled situations, as
appropriate. In that regard, amateur radio operators and
members of their immediate household are considered to be in a
"controlled environment" and are subject to the
occupational/controlled MPE limits. Neighbors who are not members of
an amateur operator's household are considered to be members of the general
public, since they cannot reasonably be expected to exercise control over
their exposure. In those cases general population/uncontrolled
exposure MPE limits will apply.
Near-Field Electromagnetic Radiation Probes for Amateur Radio
by David S. Forsman, WA7JHZ, and Michael D. Tarola, KB7CP
Build these simple 1.8 to 29.7 MHz electromagnetic probes to determine your FCC OET-65B near-field radiation compliance using a common 10 M ohm digital voltmeter (DVM). The DVM and probes are connected through special cables composed of twenty (each) 100 K ohm resistors spaced at 10 cm intervals, insulated with plastic tubing, and placed inside one-meter (1m) lengths of 1/2 inch PVC water pipe (see drawing below). These cables resistively isolate the probes from the DVM at radio frequencies, while also reducing their own parasitic antenna tendencies.
Pay careful attention to the use of the 1N34A germanium diode for the magnetic (H) probe and the 1N914 silicon switching diode for the electric (E) probe. The 1N34A diode gives excellent low-voltage sensitivity to the H probe while allowing it to measure in excess of 1.63 A/m, and the 1N914 provides low junction capacitance and high-voltage capability to the E probe, thus allowing it to measure a maximum (peak) level of 350 V/m. Avoid using substitute diode types, since they can severely alter the overall performance.
These probes measure both the magnetic (H) and electric (E) components of your antenna's near-field radiation in units of amps-per-meter (A/m) magnetic, or volts-per-meter (V/m) electric. The following formulas are used to convert your DVM's voltage reading (or Vdc) to initial H and E values: H (A/m) = 4(Vdc + 0.05)/f (f in MHz), and E (V/m) = 10(Vdc + 0.3). For example, if your magnetic (H) probe produced a Vdc reading of 1.5 volts on your DVM at 3.8 MHz, then your initial H field value would be 1.63 A/m (4 x (1.5 V + 0.05)/3.8mhz = 1.63 A/m). For the E probe, you would use the second formula--the E probe, within the amateur bands from 1.8 to 29.7 MHz, is not frequency dependent. We will cover the formula derivations in later paragraphs.
Like a dipole antenna, these probes are polarized, in that they produce maximum Vdc readings when their axes are in the same polarity as that of the near-field. But most practical antennas exhibit a complex near-field. Because of this, a probe's Vdc value might have more than one maximum as its axis is aligned within a given field (the H probe's axis is an imaginary line passing directly through, and at right angles to, the imaginary plane of its open center, while the E probe's axis is an imaginary line passing directly through the centers of its two aluminum disks).
For "quick-method" measurements, while transmitting a continuous-wave (CW) signal (not Morse code), orient a probe at the measurement point to maximize Vdc on your DVM. This will require experimentally rotating the probe's axis through several different vertical and horizontal angles until you find Vdc max. Then, after calculating the conversion value from Vdc (and possibly f), multiply it by 1.732--the square root of 3. For single-element vertical antennas, it is easy to find the Vdc maximum--but the quick-method often overestimates simple near-field values.
For a second approach, imagine a three-dimensional
Cartesian coordinate set (x, y, z) with its origin centered at the measurement
point (see drawing below); position and read a probe with its axis aligned once
with each of the imaginary x, y, and z axes at the measurement point; calculate
the initial conversion values (three total); and apply them to the
root-sum-squared (RSS) formula, sqrt(a^2 + b^2 + c^2). This process adds the
three vector magnitudes of the mutually orthogonal initial probe values. The
result should fall between 57% and 100% of the quick-method value. This
procedure, unlike the quick-method, should minimize the errors associated with
complex fields, but it does requires more work.
On the lower-frequency amateur bands (160-75 m), where higher E values are allowed, the 350 V/m limit of the E probe can be compensated for by doing the following: lower the CW power output of the test transmitter until the E value is either equal to, or less than, 350 V/m; square the value of the ratio of the maximum allowable E field and the actual E field; and multiply this quantity by the new power level to determine the maximum power level at the maximum allowable E level (((Emax/Eactual)^2) x Watts = Watts max).
For example, suppose we have to reduce our transmitter output to 100 Watts to get 300 V/m at 1.9 MHz (160 m). Since we are allowed a controlled level of 614 V/m on this band, we square the ratio of 614 V/m and 300 V/m, and multiply this by 100 Watts to get 418 Watts (((614/300)^2) x 100 = 418 Watts). As long as we don't exceed 418 Watts peak output on 1.9 MHz, our E value won't exceed 614 V/m. Remember, also, that the uncontrolled E level at 1.9 MHz is only 433 V/m. This would represent a maximum peak power level of 208 Watts (((433/300)^2) x 100 = 208 Watts). Regardless of calculation, 1500 Watts is the maximum allowable peak power level, and some bands are even less. But, you may still be asking, "What makes them work?"
The H probe's characteristics are defined by Vo =wBs (from Faraday's Law), where Vo is induced loop voltage; w = 2(pi)f (f in Hz); B is magnetic field strength in Webers-per-square-meter (W/m^2); and s is loop area in square meters (Marshall 294). Since we prefer H, or magnetic field intensity in A/m units instead of B, we will use the definition B =uH, where u = 4(pi)x 10^-7 henry/m (Units 3-12, 3-14). This then gives us Vo =wuHs = 2(pi)f(4(pi)x 10^-7 henry/m)Hs. We can approximate this relationship as Vo = 7.896fHs, with f in MHz.
Solving for H, we get H = Vo/(7.896fs). For our 17.8 cm square probe, we get H = 4Vo/f. The resulting rectified DC voltage from the 1N34A is approximately 1.4 times the induced RMS Vo voltage, but the 4 M ohms of probe and lead resistance combined with the 10 M ohm DVM resistance give a voltage division of 1.4, so that Vdc could represent Vo, or Vdc = 1.4Vo/1.4. By substituting Vdc for Vo, we get H = 4Vdc/f. To compensate for low-signal 1N34A diode cutoff voltage error, we add 0.05 V to the Vdc value. This finally gives us H = 4(Vdc + 0.05)/f. Note, also, the importance of the H probe's 0.01 uF capacitor that attenuates unwanted E field response.
The E probe's characteristics are defined by Vdif = Ed, where Vdif is the voltage differential between plates and d is plate spacing in meters (m). In an E field, this probe is a capacitive voltage divider, where the capacitance of its plates (1.8 pF) is designed greater than the junction capacitance of the 1N914 diode (less than 1 pF). In an E field, where electric field intensity is given in volt-per-meter (V/m) units, you would find a voltage differential, Vdif, equal to the value of E between any two points spaced one-meter (1m) apart within the length of the field. Our probe is spaced at 10 cm (d = 0.1 m), so it intercepts one-tenth of the differential (Vdif = E x 0.1 m).
Solving for E, we get E = Vdif/d. For our 10 cm spacing (d = 0.1 m), E = Vdif/0.1 = 10Vdif. The resulting rectified DC voltage is approximately 1.4 times the RMS Vdif voltage, but, again, the 4 M ohms of probe and lead resistance combined with the 10 M ohm DVM resistance give us a voltage division of 1.4 so that Vdc can also represent Vdif, or Vdc = 1.4Vdif/1.4. By substituting Vdc for Vdif, we get E = 10Vdc. To adjust for the 1N914 cutoff voltage error, we add 0.3 V to the Vdc value, or E = 10(Vdc + 0.3).
The diode cutoff voltage error values for the 1N34A and 1N914 were made with empirical AC to DC voltage conversion measurements at 60 Hz. Using both a low-distortion audio oscillator and an adjustable autotransformer line voltage source, both diodes were individually fed with differing levels of 60 Hz voltage; their corresponding DC output voltages were then filtered by a 1 uF capacitor with the output coupled to a 10 M ohm Fluke model 75 DVM through a 4 M ohm resistor. This circuit models the dynamic conditions that the diodes would normally be subjected to in the probes, but at 60 Hz. Finally, the AC and DC voltage readings were compared to determine their voltage differences. The 0.05 V of the 1N34A and the 0.3 V of the 1N914 represent "worse case" error values that affect the accuracy--especially when measuring at low field intensity levels.
By giving our H probe only one turn, and using a 0.01 uF capacitor across its output, we avoid the need to electrically shield it from E fields. Though it does lack the sensitivity of multi-turn H probes, it is difficult to predict the characteristics of shielded multiple-turn H probes at higher frequencies because of the effects of distributed capacitance. By keeping it simple, our H probe (within limits) should better maintain the characteristics of its basic definition.
For the E probe, its simple fundamental design helps to overcome the detector diode's internal junction capacitance. An ideal E probe detector diode (or any diode) would have zero junction capacitance and cutoff voltage, plus infinite reverse voltage breakdown--but not yet possible. Some types of Schottky diodes have lower junction capacitances than the 1N914, but many have lower breakdown voltages. It should also be noted that neither probe can detect zero field intensity levels.
In conclusion, compare the measurements from your probes with probes of known accuracy--they are not replacements for certified laboratory grade models. At 950 KHz, with generally good results, these probes have been tested in H levels up to 1.55 A/m and E levels up to 742 V/m. Always follow good engineering and safety practices when constructing, and using, these devices. These probes are another valuable tool to help assure FCC OET-65B compliance.
"Units, Constants, and Conversion Factors." Reference Data For Radio Engineers. 5th ed. Indianapolis: Howard W. Sams & Co., 1973.
Site creation and design
CopyrightŠ 1998 by Harry Sparks, KN4S