SUBMISSION OF ADDITIONAL COPIES OF TECHNICAL PROPOSAL

• Document Type: CREST
• Collection: General CIA Records
• Document Number (FOIA) /ESDN (CREST): CIA-RDP78-03424A000500010017-9
• Release Decision: RIPPUB
• Original Classification: S
• Document Page Count: 12
• Document Creation Date: December 22, 2016
• Document Release Date: April 10, 2012
• Sequence Number: 17
• Publication Date: August 24, 1956
• Content Type: LETTER

25X1
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Au t2a 9
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Very , t ]3 y s,
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Subject: Submission
copies of a Fi
-11
Reference: Contract RD-1'
Dear Sir:
In accordance with, a t*t by of your
organization,, three ( c ?~rigi l prop l for VLF
Antenna Development,e r. y k
. i eci for ovri is
01
,/Jg
Inc!
x K w ,minis ~..
M-MMUR, ~_
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 Declassified in Part - Sanitized Copy Approved for Release 2012/04/10: CIA-RDP78-03424A000500010017-9
CONF,IULIV I IHL
I PROPOSAL FOR VLF ANTENNA IiVELOmNT
6F
!ing 19 6 _
?
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I. SUMMARY
It is proposed to undertake an additional task on the Ferrite Antenna
Development Program presently underway. The present program is described in
a "Proposal for Ferrite Antenna Development" and designated by
dated January 1956. A continuation is described in "Proposal
for Additional Ferrite Antenna Development, Phase C" designated by
dated May 1956. Briefly the work under these proposals is as
follows:
Phase At (1) Design, construction and delivery- of twenty narrow
band units, tunable over a calibrated spectrum of 50 to 250 mcps.
(2) Design, construction and delivery of ton wide band (50 to
250 mcps) units.
Phase B: Investigation of the feasibility of a broadband ferrite
antenna covering the frequency spectrum 3 to 30 mcps.
Phase C: Investigation of the' applicability of ferrite antennas
to very low frequencies, nominally 300 to 300,000 cps.
The additional task described in this proposal is to run concurrently with
Phases A and B and will be of approximately three months duration. The addit-
ional task is to design and fabricate an antenna system which will provide a
bandwidth of 2 kilocycles and will operate at a center frequency of approximately
25 kilocycles. The operating temperature will be approximately 20 degrees Cen-
tigrade and the output impedance of the system will be 72 ohms. The complete
antenna will be designed to be readily demountable for a man-pack transport.
II. PROPOSED PROGRAM
Several methods appear to offer a possible solution to the problem of
designing a VLF'antenna system which will operate at a frequency of approximately
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25X1
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o A
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4W
0
CONFIDENTIAL
25 kilocycles and possess the properties of high sensitivity and broad band-
width. One method consists of fabricating an antenna system containing approx-
imately 20 separate demountable ferrite antennas, each having a bandwidth of
100 cps. Each antenna would be tuned to a slightly different frequency and
the combination would give a total bandwidth of 2 kilocycles. Attached to
the base of each antenna would be a low noise transistor preamplifier. After
preamplification a summation of the twenty individual signals would be obtained
by feeding the 20 preamplifiers into a summation amplifier. The transistor
amplifiers will be explained in more detail in a latter portion of this proposal.
Figure 1 illustrates how the overall Wstem will be connected, and Figure 2 shows
haw the output from each antenna is combined to give a total bandwidth of 2 kilo-
cycles.
Some problems requiring investigation in'a system composed of 20 narrow band
elements are as followsa (1) the construction of narrow band units having a Q
of as high as 200 at 25 kilocycles is marginal with presently available ferrite
materials so that it may be necessary to compromise on a Q of 100 and a 10
element antenna system, (2) the packaging of the antenna elements in such a
manner as to minimize the effects of mutual coupling, and (3) the selection of
transistor type and operating conditions to give a noise figure for the preamp-
lifier of as close to zero db as possible.
: D NTIAL
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Individual
Ferrite
Antenna
(20 to be used)
Goupl.ing
Network
FI,
reamp.7 i fier %
smplifie
L Summation Amplifier
i;-
rrea'npiifie~
7
*= 72 ohm line
vidual
Antenna
Response
L = ~ - 0 KC
14 Kc.
j1 V
~ay4 d Ufa ~T ago CPO
1 1 i
I
I
1 1 I
I
Frsq,1ency
amplifier
.._ Mak
- .707
.16 Kc
FJGURZ 2
-3-
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 Declassified in Part - Sanitized Copy Approved for Release 2012/04/10: CIA-RDP78-03424A000500010017-9
Other methods of designing a VLF antenna system will be considered. One
very promising method consists of adding a number of relatively low Q ferrite
antenna elements in series with one capacitor to resonate the total inductance
to the desired center frequency of approximately 25 kilocycles. The series
element method is illustrated in Figure 3.
? Antenna
72 Chm Line
LA
FIGURE 3
Ffl- Coupling
- - Network
Transistor Amplifier
Previous work on ferrite loops by other workers (1) has indicated the
desirability of maintaining a high length/diameter ratio in each element and
adding additional ferrite material by connecting windings of spaced rods in
series as in Figure 3. Since all elements contribute at all frequencies but
the Q is much lower (Q of approximately 12) an analysis or experiment is re-
quired to determine whether the first or second method gives a greater effec-
tive height for the antenna system.
(1) H. Blok and J. J. Rietveld, "Inductive Aerials in Modern Broadcast
Receivers't, Philips Technical Review, Vol. 16, January 1955.
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Still another method would involve an increase in the cross sectional area
of the antenna.
This could be accomplished by combining a group of smaller rods
into one large bundle as illustrated in Figure 4.
Coupling
Network
FIGURE 4
With the configuration of Figure 4 the pickup by the antenna would be
greater due to a larger volume of ferrite material. The antenna would be a
low Q system and coupled to a transistor preamplifier or possibly directly to
a 72 ohm line by a suitable impedance transformation. The configuration of
Figure 4 does not appear to make as efficient use of ferrite material as the
arrangements shown in Figures 1 and 3.
The final design of the transistor circuitry will be dependent on the
antenna configuration found most suitable. In view of this, several amplifier
designs will be described. In the event that a number of separately tuned
high-Q antennas are used., it will be necessary to have a preamplifier for each
antenna. It has been found experimentally that a conventional grounded emitter
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transistor amplifier stage having a collector voltage of 5 volts and an emitter
current of 1' ma. (a commonly used operating point) yields a noise figure of 26 db.
If the collector voltage is lowered to 1 volt and the emitter current to 100 micro-
amps the noise figure is reduced to 3 db, which should provide a satisfactory pre-
amplifier. A diagram of a low noise amplifier stage is shown below:
-3.2 volts
36K ? 5 20 K
2x167
FIGURE 5
0.1?f
A brief descussion of the problem of smmning multiple preamplifier outputs
will be given. A vacuum tube is essentially a voltage amplifier and the input
or grid circuit consumes negligible power at low frequencies. The transistor,
however, is a current (and hence a power) amplification device and requires power
to drive it. The low noise criterion necessitates a minimum number of stages,
and thus impedance matching must be carefully observed to ensure maximum efficiency.
The matching of the outputs of the preamplifiers to the input of the summation
amplifier presents a difficult problem. If it is assumed that the outputs of
20 preamplifiers are connected in parallel for the purpose of signal addition
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and that each amplifier has an output impedance of Z0, then the combined output
impedance is Zo/20.
-:
Z0
Z0
r
Z0
ZL
FIGURE 6
The load impedance ZL must be low compared to Zo/20 or else a given amplifier
will see as its load the combined impedance of the other 19 stages and little power
will be transferred to the load. If ZL is made small, however, poor efficiency
results since the optimum or matched condition for any stage is Z0 . ZL. A similar
matching problem arises if the outputs of the preamplifiers are connected in series.
One proposed solution for this problem is as follows: Again assume that 20
stagger-tuned antennas are used. These antennas can be divided into five groups
with four antennas in each group. The four antennas in each group are chosen so
that they differ in frequency as much as possible. The groups could be: Group 1 -
antennas 1, 6, 11, and 16; Group 2 - antennae 2, 7, 12 and 17; Group 3 - antennas
3, 8, 13, and 18; etc. If the four antennas in each group are connected in series,
each antenna is essentially a short circuit for the other three antennas inasmuch,
as they are tuned to different frequencies. Thus the signal summation within the
groups can be achieved with little power loss. With this tgrstem the summation
amplifier need only add the signals from the five antenna groups and hence the
matching problem is considerably reduced.
Following the summation amplifier would be as many transistor amplifier
stages as are necessary to bring the signal power to the required level. The
final stage would be a grounded collector stage (analagous to the vacuum tube
cathode follower stage). This stage has a very low output impedance for matching
the 72 ohm line.
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The current drain for the combined amplifier (preamplifiers, summation amp-
lifier, and additional amplifiers) would be very small - 2 to 5 ma. This should
ensure long battery life.
A block diagram for the completed amplifier system follows:
7---' `7F "-F `-F
Amp
Pre-amps
72 Ohms
FIG 6 7
If the final antenna design contemplates a single low-Q unit, the transistor
circuitry reduces to a single amplifier with the special properties of low noise
and low output impedance. The low-noise amplifier stage described previously
could be used as the first stage of this amplifier. A grounded collector would
again be used for the output stage.
The following three-stage amplifier might be constructed for use in antenna
testing. It incorporates a low-noise input stage and the low impedance output
stage. This amplifier has a voltage gain of 45 db. which is reasonable consider-
ing that the voltage gain of the last stage is less than unity. A secondary wind-
ing of about 10 turns is used to match the antenna to the low impedance input of
the transistor.
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TIAL
+22.5 volts
0.5
11ol S 16K
1.0
9
0.l
1 1S
Figure 8
!8K
Proposed methods of testing the antenna will be discussed briefly. It is
assumed that the antenna will couple principally to the magnetic field component
of a VLF radiated wave, Therefore, a loop can be used to generate an induction
field which will simulate far field conditions, It is likely that a large loop
of perhaps 10 feet diameter will be necessary to generate the induction field for
testing. The large diameter loop will allow the antenna under test to be placed
? at the center of the loop in known field conditions? If ;it is desired to compare
the efficiency of various methods of obtaining a wide-band VLF antenna, the large
calibrated loop should be particularly useful. If the frequency and time of
operation of VLF stations can be determined it will also be desirable to set
up the antenna and a suitable amplifier in an'area sufficiently remote from 60
cycle harmonic fields.
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CONFIDENTIAL
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CONHOLN I IAL
III. MANPOWER REQUIREMENTS AND WORK SCHEDULES
It is anticipated that approximately 13 weeks will be required for the
development of a suitable VLF antenna. Tether the final deliverable model
can be completed within the 13 week period will depend upon the availability
of suitable quantity of ferrite materials and/or availability of shop work.
It is anticipated that two junior engineers working under the supervision of
a senior engineer will carry out the basic testing of proposed materials and
designs. A mechanical designer will work out the details of the demountable
antenna for man pack transport. Following manufacture and ass9mbly of the
final model, tests will be conducted to evaluate the performance and guide
final adjustments.
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