THE ISOTRON REVEALED - PART 2 By Dave Cuthbert

Transcription

THE ISOTRON REVEALED - PART 2
By Dave Cuthbert, WX7G
Introduction: In part 1 of the Isotron Revealed
we explored the theory of the Isotron. Experiments
and simulations were presented to show the three
modes in which the Isotron can operate; Dipole,
Asymmetrical Dipole, and Top-Loaded Vertical with
Elevated Feed. The most important thing to remember is that in a standard Isotron installation, the
Isotron ground wire does most of the radiating—
not the Isotron. With this in mind we can continue
with our investigation and learn a bit more about
the Isotron and small capacitive antennas in general.
The Isotron E-field: The isolated Isotron E-field
was calculated to confirm that the field strength
measurement taken at 235 meters was in the farfield region. In the far-field region, which is generally considered to begin one-half wavelength from the antenna, the E-field field is proportional to 1/r. In the near-field region the E- field is a bit more complicated. The following
formula, from the Antenna Engineering Handbook, is used to calculate the free-space transverse (parallel to the antenna element) E-field due to differential current flowing in the antenna (refer to Fig. 1).
Idz = differential current element, current in amperes, dz in meters
r = distance in meters
β = 2π λ , lambda is 42.9 meters at 7 MHz
j = −1
E is given in V/m
antenneX ~ November 2002
The Isotron Revealed - Part 2 ~ Page 1
Fig. 1
The actual field strength measurements were taken at a distance of 235 meters with the Isotron 6 meters above earth ground
and the measurement antenna at ground level. The angle from the
Isotron axis to the measurement antenna is 91.5 degrees. We will solve
for an antenna current of 1 ampere. Substituting these parameters into
the formula, and solving for distances from 1 meter to 235 meters, we obtain the Electric Field versus Distance graph shown in Fig. 2. The E-field
decreases as 1/r beyond the 50 meter point, so we can conclude that at 235
meters the measurement antenna was well into the far-field region.
Fig. 2
The Isotron E-field and Safety: The E-field near the Isotron can be quite high. I shall refer
to the FCC OET Bulletin 65, Edition 97-01, Table 1, LIMITS FOR MAXIMUM PERMISSIBLE EXPOSURE, section (A) Limits for Occupational/Controlled Exposure. This document gives a Controlled Exposure Electric Field Strength limit of 1842/f , where f is frequency in MHz. Solving
this formula for 7 MHz gives a maximum time-averaged, controlled exposure limit of 263 V/m
or 168 dBuV/m. If we use Fig. 2, it would seem that a safe distance would be 1 meter from the
Isotron. This would be correct if you were perpendicular to the Isotron axis and midway between the plates. Perhaps the radial E-field is higher, so let’s solve for the radial E-field using
this formula from the Antenna Engineering Handbook.
 1
j 
−
cos θe − jβr
Er = 60β 2 Idz 
2
3
 (Br ) (βr ) 
The E-field magnitude is shown in Fig. 3 and is lower than what I expected. As a check, let’s
calculate the radial E-field based on the voltages on the Isotron plates using a formula from
Introduction to Electromagnetic Fields. We shall solve for 1 amp of antenna current, which produces a voltage of 2 kV across the 50 uH loading coil. The capacitance assigned to each plate
is 10 pF, and each plate will be treated as a point charge with a potential of 1 kV.
E=
ql
4πε 0 r 2
(2 cos θ + sin θ )
q = cv = (10pF)(1000v) = 10nC
l = distance between plates = 0.3 meters
ε 0 = 8.854 pF / m , the permittivity of free space
Fig.3
These two methods of calculating the radial E-field agree within 4 dB. With the FCC 7 MHz
time-averaged, controlled exposure limit of 263 V/m (168 dBuV/m), it would appear that a
safe distance from the Isotron would be less than 1 meter. These two E-field calculation methods do not take into account the finite size of the Isotron plates and the actual near field may
be higher. Just to be on the safe side, I would not stand closer than 3 meters from the Isotron
during high power operation. For more on the subject of RF exposure you can refer to the
ARRL website at arrl.org or the FCC at fcc.org.
Power Handling: The RF power that the Isotron can handle is limited by the loading coil.
The loading coil has a loss resistance of 20 ohms and a winding surface area of 20 square
inches. A coil dissipation of 40 watts will cause a temperature rise of approximately 80 degrees Celsius. The 40 watt dissipation is achieved with an RF current of 1.4 amps, which is
what is present when a 100 watt rig is running CW. Radiotelegraph and SSB normally have
duty cycles of less than 50%, so 200 watts should be fine with these modes. The Bilal manual
says that 500 watts can be used, but that the SWR should be watched to alert the operator to
coil overheating. I checked the PVC pipe that is used for the Isotron coil, and it has safety
agency markings that appear to indicate that it is a “self extinguishing” PVC. This means that,
in the event it catches on fire, the fire will go out when the external source of heat (RF) is
removed. This is nice to know, as I once had a homebrewed antenna mounted on my house
burst into flames and continue to burn after the RF was shut down.
Baluns and the Isotron: The Isotron depends on the “ground wire” for efficient operation,
since the ground wire, or counterpoise, really does most of the radiating. However, certain
ground wire lengths can result in a high feedpoint impedance, and coax shield current can
bring RF into the shack. Ferrite beads and coiled coax baluns are a good way to tame these
problems by controlling where the antenna current flows. To simplify things, the following
simulations are based on the Palomar Engineers FB-102 ferrite bead that has an O.D. of 1 inch.
These ferrite beads are available from the antenneX Shopping Shack in the BA-8 balun kit. A
FB-102 ferrite bead was measured on an MFJ-259B antenna analyzer and found to exhibit an
impedance of 50 +j85 ohms at 7 MHz. Each FB-102 is simulated as a 50 ohm resistor in series
with a 2 uH inductor.
Fig. 4
Fig. 5
The typical commercially built amateur vertical antenna has an adjustment for resonance
and a fixed matching circuit that provides a reasonable match to 50 ohms.
And most homebrewed verticals have both a tuning adjustment and an impedance matching adjustment, such as a tapped coil. On the other hand, the
Isotron has only a tuning adjustment and no means to adjust the impedance.
The Isotron input impedance is dependent on the length and routing of both
the ground wire and the feedline. When installing the Isotron you might get
lucky and hit a 1:1 VSWR, or you might have a situation where you just can’t
get the VSWR below 6:1.
The first step in installing an Isotron might be to decouple the coaxial cable
shield, at the Isotron, by placing several ferrite beads over the feedline. A
simulation was run with the Isotron mounted at a height of 20 feet over perfect
ground. The feedline is ½-wavelength
long and is run horizontally for 47 feet
Fig. 6
and then down to the ground. The
ground wire runs directly to ground.
Notice (in Fig. 4) that the feedline
shield current is much higher than the
current along the 20 foot ground wire.
What we have here is an inverted-L of
sorts. Most of the radiation is from the
vertical section of the feedline with very
little radiation from the ground wire.
Now let’s place five FB-102 ferrite
beads over the coax near the Isotron.
These are represented in Fig. 5 by the
violet box near the right-hand end of
the horizontal feedline and have a combined impedance of 250 +j425 ohms.
The results are shown in the currentantenneX ~ November 2002
temperature graph in Fig. 5. The 20-foot ground wire current is now a bit
higher than the feedline shield current. This installation will operate as a vertical antenna with the ground wire doing most of the radiating. Ten ferrite
beads would do a better job. Or a feedline choke can be implemented by
winding about fifteen turns of RG-58 on a 4.5-inch PVC form. Any winding
arrangement that yields an inductance of 25 to 50 uH will work.
Fig. 6 shows the Isotron mounted at 20 feet with the ground wire connected directly to ground and a feedline, without ferrite beads, sloping away
and connected to the same ground rod. The load at the base is a 45 ohm
resistor to simulate ground losses. Fig. 6 shows that the ground wire and the
feedline have nearly equal antenna
current.
Fig. 7
Fig. 7 shows the effect of placing
only three FB-102 ferrite beads over
the feedline near the base of the antenna. The feedline shield current has
decreased by a factor of two. The
ground wire and feedline shield currents were measured, using an MFJ-854
RF Current Meter, and the reduction in
feedline shield current was confirmed.
Ten beads would do a much better job.
The ferrite beads were then placed
over the feedline near the Isotron and
there was very little reduction in
feedline shield current. This also was
confirmed by measurements. To be effective the ferrite beads must be
placed in a section of the feedline that
exhibits a low impedance (would normally carry high current). This is where a NEC simulation or a feedline shield current measurement can be quite useful.
Under some conditions feedline antenna current can cause the rig can be “hot” with RF.
The symptoms of RF in the shack can vary from the SWR changing as you touch the rig to the
operator receiving RF burns when touching the mike or the key. Slipping ferrites over the
coax near the rig can sometimes help. In other cases, a ¼-wavelength counterpoise connected to the rig will help cool things off.
Ferrite Bead Heating: Ferrite bead heating can be a problem and should be checked.
The RF current will cause I-squared-R losses. In the case of the FB-102 ferrite bead, the resistive loss at 7 MHz is 50 ohms and a single bead can safely dissipate over 5 watts. A bead was
tested with 400 mA of RF and experienced a temperature rise of 80 degrees Celsius. Momentary current excursions to 1.5 amps did not change the impedance, which indicates the bead
was not saturating. If the ferrite beads get too hot, you can try increasing the number of ferrite
beads. For example, doubling the number of ferrites could reduce the RF by as much as 50%
and reduce the power dissipated, per ferrite bead, by a factor of four. The reduction in RF
current depends on the circuit impedance where the ferrite beads are placed. Or, use a coiled
coax balun. Although coax baluns can also overheat, the resistive loss of this type of balun
tends to be lower, allowing higher RF current to be handled.
To test ferrite bead heating, run the antenna at the desired operating power for a couple of
minutes, then switch the RF off, and touch the ferrite bead with a wet finger. If it doesn’t sizzle
then the temperature is all right. But usually the ferrites are mounted in a location that is difficult to reach. In that case, you might try testing them by keying the rig and watching the SWR.
If the SWR begins to change after some time, then the ferrite beads might be overheating.
Fig. 8
Horizontal Dipole Operation: The Isotron can be used as a shortened horizontal dipole
as shown in Fig. 8. In this simulation the Isotron is mounted 33 feet above Sommerfield-Norton
ground and the horizontal wire is 33 feet long. The vertical wire represents the feedline and
has not been decoupled with any ferrite beads. The 33-foot wire presents a lower impedance
than the 20 foot ground wire and so most of the current flows along the 33-foot wire. It is
interesting to note that the horizontal wire can be a random length and the Isotron should be
able to resonate it. In this case the use of ferrite beads placed over the feedline might be
helpful. In this model the feedpoint impedance is 30 ohms and the 50-ohm 3:1 VSWR bandwidth is 100 kHz. This could make an excellent NVIS antenna, and having the feedline originate at one of the antenna supports, rather than in the middle of the dipole, can be an added
bonus.
Multiple Counterpoise Wires: When more than one counterpoise wire is used, things
can get complicated. The radiation from the wires can add or subtract, depending on the
relative phase. A NEC model of your proposed installation can help you understand which
wires carry most of the RF current.
Fig. 9
Put your 40m Isotron on 80m: The 40 meters Isotron can be resonated on 80 meters by
increasing the top plate capacitance. The easiest way to do this is to simply add a wire to the
top plate. A simulation showed that a 21 foot wire will resonate the antenna (shown in Fig. 9) at
3.5 MHz and a 16 foot wire will resonate that antenna at 4 MHz. This antenna was actually built
and resonated at 4 MHz. The input impedance, using a single ground rod and two 30 foot
radials, was 52 ohms. A good simulation was achieved by using perfect ground and adding a
15 ohm resistor at the base to simulate ground losses. The simulation and the actual antenna
both showed a 3:1 VSWR bandwidth of 210 kHz. The simulated radiation efficiency is 28%. If
you simulate the 40 meter Isotron on 80 meters, change the nominal Isotron loading coil inductance from 50 uH to 30 uH to account for the drop in inductance as the coil is operated
further away from self-resonance.
Model the Isotron Yourself: Any NEC modeling program should be able to model your
Isotron installation. I use NEC-Win Plus+, which is available from the antenneX Shopping Shack
and can be downloaded and evaluated free for 30 days. Table 1 is a list of the dimensions I use
for the simplified top-hat Isotron model shown in Fig. 10. The dimensions are in inches and
each segment is 2 inches long. The blue row is the vertical antenna element and the other
rows are the top-hat spokes. In the vertical antenna element, place a voltage source and a
series RLC load consisting of 20 ohms, 50 uH, and 0 pF. Connect the ground or counterpoise
wire to the 0,0,0 point and adjust the inductance to resonate the antenna. The coaxial feedline
shield can be represented by another counterpoise wire connected to the 0,0,0 point. A series RLC load representing your ferrite beads or coaxial balun can be placed along the feedline,
if desired. The FB-102 ferrite bead can be modeled at 7 MHz as a series RLC of 50 ohms, 2 uH,
and 0 pF, which corresponds to the MFJ-259B ferrite bead measurement of 50 +j85 ohms. And
the Isotron can be translated to any height needed or mounted horizontally.
Fig. 10
Table 1
The Isolated Isotron: Let’s take another look at the isolated Isotron. The Isotron operating
by itself, with no counterpoise or ground wire, provides a good vehicle for investigating capacitive antennas. In fact, the isolated Isotron situation is what many designers of capacitive
antennas aim for. Remember that when operated in isolation the Isotron is simply a short
loaded dipole.
Radiation Power Factor: In the Small Antennas chapter of the Antenna Engineering Handbook, Harold A. Wheeler introduces the concept of Radiation Power Factor (PF), which is equal
to 1/Q. Radiation PF is preferred for describing radiation and loses because, unlike Q, it is
additive. Radiation PF is calculated by the following formula and is solved here for a lossless
isolated Isotron with the radiation resistance and reactance values taken from a NEC-2 simulation:
Radiation _ PF =
Radiation _ resis tan ce 20mΩ
=
= 10 ∗ 10
Antenna _ reac tan ce
2000Ω
The loss PF is calculated in a similar way; loss resistance divided by antenna reactance. The Isotron
loading coil loss resistance is 20 ohms, and the radiation efficiency of the isolated Isotron is found
by this formula:
Effective Volume: Harold A. Wheeler also states that the radiation PF is proportional to the
antenna volume and provides a formula to calculate antenna volume for a dipole with end
loading disks. Plugging the Isotron radius of 0.25 meters into the formula we get an antenna
volume of:
V =
4π 3 4π
r =
0.25 3 = 0.065m 3
3
3
(
)
The lossless radiation PF can be calculated from the antenna volume using this formula:
This radiation PF value agrees within 10% with the radiation PF calculated using the NEC2 derived radiation resistance and reactance. What Wheeler shows by radiation PF is that the
theoretical maximum antenna performance is a direct function of antenna size.
VSWR Bandwidth: The VSWR bandwidth of the isolated Isotron can be used to determine
the radiation efficiency. A short antenna like the isolated Isotron is a simple RLC circuit. The Q
of this circuit is X/R where X is the capacitive reactance of the plates and R is the sum of the
radiation resistance (Rr) and the loss resistance (Rloss). In the case of the lossless Isotron X is
2000 ohms and Rr is 20 milliohms. The Q is therefore 100,000 and the -3dB bandwidth is 7MHz/
100,000 or only 70 Hertz! Note that, at the -3dB points, the VSWR will be 6:1. The 3:1 VSWR
bandwidth, which is where I like to measure the bandwidth, is about one-half the -3dB bandwidth. Given the actual isolated Isotron Rloss of 20 ohms, and X of 2000 ohms, the Q is 100. The
3:1 VSWR bandwidth should be 7MHz/2Q or 35 kHz. When a counterpoise is added, the radiation resistance increases which causes an increase in the VSWR bandwidth.
The Capacitive Antenna Perfected? Short capacitive antennas, like the Isotron, are very
popular for amateur and even professional experimentation and construction. Professionally,
capacitive antennas have been well understood since before WWII. Many of the professional
design efforts have yielded huge VLF verticals around 0.02 wavelengths tall and have utilized
simple design techniques:
•
•
•
•
Make the antenna as tall as possible (to maximize the current-area Idz)
Minimize the top loading reactance (make the top hat really big)
Minimize ground losses (use a lot of radial wires)
Minimize loading losses (use a high-Q loading coil and matching network)
The three parameters that must be traded off in a practical short antenna are bandwidth,
efficiency, and size. This can be stated in a form similar to the familiar ohms law triangle, noting that BW
and EFF are linearly proportional, but BW and EFF
are not linearly proportional, to SIZE:
In spite of the predictable results obtained by
these design rules, and the associated theory and formulas, there is still a search on for a better short capacitive antenna. Some examples include the CFA, the EH, the Lal antenna, and numerous
amateur experiments such as the Biplane and the Teslavert. These antennas are certainly worth
investigating and have some pretty exciting performance claims.
Coil Wire Length with Capacitive Antennas: The question of loading coil wire length for
capacitive antennas has come up in communications among several GARDS members. Some
GARDS have said that, in their experience, the loading coil wire length is about 1/8-wavelength, while others have used ¼-wavelength, as with the Teslavert. The loading coil inductance required to resonate a capacitive antenna will depend on the capacitance between the
antenna plates. Simulated loading coils were designed to resonate a 7 MHz capacitive antenna using plates from 5 to 80 pF. The coils use a 4.5 inch O.D. PVC coil form, #12 AWG wire
and a winding pitch of 4 T.P.I. Table 2 lists the coil parameters and Fig. 11 shows the coil wire
length versus plate capacitance.
Table 2
PLATE C
5 pF
10
20
40
80
COIL L
103 uH
52
26
13
6.5
TURNS
29
21
14
9
6
COIL LENGTH
7.00 inches
5.00
3.25
2.00
1.25
WIRE LENGTH
34.2 feet
24.7
16.5
10.6
7.1
WIRE LENGTH
0.24 wavelengths
0.18
0.12
0.75
0.050
Fig. 11
Fig. 12
Now let’s see how coil diameter and turns-spacing affect loading coil wire length. Ten 25
uh coils were designed with diameters of 2 to 6 inches using winding pitches of 4 T.P.I. and 8
T.P.I. The results are shown in Fig. 12.
The wire length does not seem to be affected very strongly by winding pitch or coil diameter, but the wire length is strongly affected by the plate capacitance. Fig. 11 shows that plate
capacitances of 5 to 80 pF will require coil wire lengths from 0.05 to 0.25 wavelengths. The 1/
8-wavelength wire lengths used by several the GARDS members could be due to the use of
plates in the range of 10 to 30 pF.
The question of the phase shift through the loading coil of a resonant capacitive antenna
has also come up in communications among the GARDS. The idea is that the current is timedelayed through the coil, much as current is delayed through a transmission line. Intuitively
this makes sense. Calculations were performed on paper and then using MicroCap-7 evaluation SPICE. The current phase shift through the loading coil was found to be zero degrees
while the voltage phase shift is 90 degrees. However, these methods do not consider the finite
length of the wire. This issue will be investigated, and the results will be reported a GARDS
member.
Summary: In this two-part article we have seen how Isotron-type antennas work and how
their performance can be maximized. Several formulas have been presented to predict the
performance potential of these antennas, and a method to accurately model them has been
described. The Isotron-type antenna is certainly a versatile and useful antenna, and it remains
for you, the antenna experimenter, to invent new and useful antennas based on the Isotron
concept
References
• Basic Antenna Modeling: A Hands-On Tutorial by L.B. Cebik
• Introduction to Electromagnetic Fields, Third Edition, Clayton R. Paul, Keith W.Whites, Syed
A. Nasar
• Antenna Engineering Handbook, Third Edition, Richard C. Johnson (“Fundamentals of Antennas” chapter, Henry Jasik)
• American Radio Relay League, arrl.org
• Federal Communications Commission, fcc.org
• http://www.harvel.com/PVCpipe.html
Brief Biography of Author:
- 1979-1988 Hughes Aircraft ~ Designed a wide variety of test equipment for
high-power microwave tubes including high voltage and RF designs.
- 1988-1995 Tektronix ~ Worked on microwave hybrids, PLL design, and inhouse test equipment design for the 2784 Spectrum Analyzer. Sustaining
engineering and switching power supply design for several oscilloscope
lines. Designed a 0.025 lambda monopole for a commercial control
device.
- 1995-1997 Advanced Energy ~ Sustaining engineering for multi-kilowatt
plasma power supplies.
- 1997- present Micron Technology ~ A Micron Fellow since 2001, Analog
and EMC engineering to support IC manufacturing.
- Five FCC licenses including a commercial radiotelegraph license.
- Certified NARTE (National Association of Radio and Telecommunications Engineers) Electromagnetic
Compatibility Engineer.
- As part of my amateur radioactivities I have built small antennas since 1972
antenneX Online Issue No. 67 — November 2002
Send mail to webmaster@antennex.com with questions or comments.
Copyright © 1988-2002 All rights reserved worldwide - antenneX©

THE ISOTRON REVEALED - PART 2 By Dave Cuthbert

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