It was found that group delay is function of pressure in air dielectric coaxial cable. For example, 600-ft air dielectric cable will change phase 10 deg at 150 MHz when air pressure in cable changes from zero to 20 psi

Clements, P. A.

English

NASA Technical Reports Server

February 1975	 874-10295 
NASA TECH BRIEF 
NASA Pasadena Office 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Stable Group Delay Cable 
The problem: 
In distributing precision time and frequency 
standard signals from a control station to widely-
separated user stations, variable phase delays can 
exist along the distribution cables because of 
temperature variations and other physical influences. 
When the lines are long, these delays degrade the 
timing accuracy available from high precision 
standards, such as the hydrogen maser frequency 
standard.
The solution: 
It was found that group delay is a function of 
pressure in an air dielectric coaxial cable. For 
example, a 600-ft (183-rn) air dielectric cable will 
change phase 100 at 150 MHz when the air pressure in 
the cable changes from zero to 20 lb/in. 2 (1.4x105 
N/m2). 
How It's done: 
The figure is a block diagram of the operational 
controls of the system. A pilot frequency generator 
I
System 
Source 
Pilot
 DiPlex Freque Filter 100 M
rPhase 
I	 Detector
System 
Load 
Controlled Coaxial Line
Low Pass
Filter 
Dry Air 
Control
I I Dry Air! 
I Pressure I Motor	 Regulator j
	
[Desiccator 
U 
Dry Air 
437 
Block Diagram of Stable Group Delay Cable
Compressor 
(continued overleaf) 
This document was prepared under the sponsorship of the National 	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States 	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States 	 will be free from privately owned rights. 
https://ntrs.nasa.gov/search.jsp?R=19740000295 2020-03-17T14:18:46+00:00Z
sends a 100-MHz signal through the controlled 
coaxial line via a hybrid junction and a diplex filter. 
Since the coaxial line is terminated in a low pass filter, 
any reflection of the 100-MHz signal due to line 
pressure variation shows up as a phase shift in the 
phase detector. The phase shifter is adjusted to 
operate the phase detector near zero volts. The phase 
detector compares the phase of the 100-MHz pilot and 
the reflected 100 MHz from the coaxial line. An 
output indicates a change in the electrical length of 
the coaxial line. 
The phase detector dc output actuates a displace-
ment servo comprising a proportional control and a 
motor drive. Attached to the motor is a pressure 
regulator which meters the flow of dry pressurized air 
to the coaxial line in order to maintain a constant 
pressure. The motor regulator being used can produce 
phase shifts which correspond to changes in electrical 
length over a dynamic range of 8 to 12 cm per 500 ft 
(152.5 m) of cable. Additionally, it enables pressure 
regulation by venting excess coaxial line pressure to 
the atmosphere.
Note: 
Requests for further information may be directed 
to:
Technology Utilization Officer 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena, California 91103 
Reference: TSP74-10295 
Patent status: 
This invention has been patented by NASA (U.S. 
Patent No. 3,790,906). Inquiries concerning non-
exclusive or exclusive license for its commercial 
development should be addressed to: 
Patent Counsel 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena, California 91103 
Source: Philip A. Clements of 
Caltech/JPL 
(NPO-13138) 
Categories: 01 (Electronics - Components and 
Circuitry) 
B74-10295
	 05 (Life Sciences)


Stable group delay cable

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