Z Technology, Inc.

ANTENNA CALIBRATION TABLES

Application note

By: John Purdy

Application note

Z Technology, Inc.

July 1997


Congratulations on your purchase of a Z Technology Field Strength Meter with built-in Antenna Calibration Table. This feature will save you time by doing some of your work for you. You will no longer have to manually add Antenna Correction Factors and Cable Insertion losses to your Field Strength measurements. Load the correction factors into the meter's Antenna Calibration Table and it does the work for you.

In this application note, you will find a general introduction to antenna characteristics relating to Field Strength measurements. A description of how an Antenna Calibration Table is used, how a human readable text file representing the Calibration Table is edited, and how to download the file Calibration Table text file to the Field Strength Meter.

  Introduction Your Z Technology Field Strength Meter's built-in Antenna Calibration Table allows the meter to incorporate a correction factor determined by you and/or your antenna and cable manufacturer. It corrects for the characteristics of a particular antenna and antenna feed system, allowing your meter to accurately display dBuV/Meter on the Signal Level readout.

When shipped from the factory, the meter's Antenna Calibration Table generally is blank and disabled. The Signal Level readout displays dBuV. When you order a calibrated antenna and a calibrated cable through Z Technology, you may request the meter be factory set for that antenna and cable combination. A factory set meter has the meter's internal Antenna Calibration Table programmed with a best-fit correction over frequency for the system you order. The Antenna Calibration Table is enabled, and the meter's SIGNAL LEVEL readout displays dBuV/Meter.

Thirty entries are available for antenna correction over the frequency measurement range of the instrument. The entries may be grouped tightly to cover a small frequency range of a particular antenna and cable, or loosely to cover the entire range of the instrument. The Antenna Calibration Table is enabled or disabled via the front panel using Function F12.

A secondary use of the Antenna Calibration Table is to uniformly subtract 107 units over the entire frequency range of the meter so the Signal Level readout displays dBm instead of dBuV.

As stated in the meter's User Manual dBuV and dBm are related by the equation

dBuV - 107 = dBm

Note: Both dBuV and dBm by convention, are units of power related to the signal level across a 50 ohm load. dBuV/Meter is the unit used in measuring Field Intensity captured by an antenna.

The Antenna Calibration Table stored in your Field Strength Meter is easily changed or updated by editing a human readable text file that acts as the source of the table. The Z Technology LOAD_TBL.EXE program is used to read the text file, translate the information into a form usable by the meter, then download the information to the meter's Antenna Calibration Table. Generally, the source text file is named ANTxxx.TXT where xxx is the last three digits of the antenna serial number. ANTxxx.TXT and LOAD_TBL.EXE are located on the ANTENNA CALIBRATION diskette that came with your Field Strength Meter.

  Using Antenna Calibration Tables

To use Antenna Calibration Tables you'll need to know about the following:

  1. Antenna characteristics.
  2. Antenna Calibration Table interpretation.
  3. Editing an Antenna Calibration Table source text file.
  4. Using the ANTENNA CALIBRATION diskette to download tables.

  1. Antenna characteristics An antenna can be thought of as an impedance matching device to convert a signal propagating in free space, to a signal traveling along a coaxial cable. The Z Technology Field Strength Meter measures signals within the coaxial cable. An antenna is therefore necessary to convert a signal in free space to a measurable signal on a coax cable.

Due to physical and theoretical limitations, real-world antennas are inefficient conversion devices. This means, even when properly used, a significant amount of frequency dependent loss (or sometimes gain) will occur. For instance, consider a tuned dipole antenna with active elements cut to the correct lengths, using a properly matched feed system for the frequency of operation. Such an antenna operating within a specific field intensity at 100 MHz, will generate a much different power level on a coaxial cable then will a similarly constructed tuned dipole antenna cut for and operating at 1000MHz. This is true even when the second antenna operating at the new frequency is exposed to the exact same field intensity. In fact, there will be approximately 22 dB of difference in power level generated in the coax.

Significant design work has gone into making antennas as efficient as possible. Narrow band antennas designed to receive just a single frequency or a very small range of frequencies can be very efficient. Broad band antennas designed to receive a large range of frequencies trade off efficiency for bandwidth. Some antennas, incorporate a small amplifier designed to minimize the inherent inefficiencies of the antennas response over frequency.

Manufactures of calibrated antennas provide a Calibration Table with each antenna. It specifies antenna factors at particular frequencies to be added to the meter reading to obtain accurate dBuV/Meter measurements. You will notice such tables have a rather large change in values over wide frequency ranges. The manufacturer generates this data by testing the antenna on a carefully designed antenna test site with calibrated instruments traceable to the N.I.S.T. (National Institute of Science and Technology.)

Antenna feed systems include any component between the antenna connector and the meter input connector. Generally, this is a single coaxial cable, but in complex systems, it may include other components. Antenna feed systems may also contribute to signal loss. Z Technology supplies calibrated coaxial cables. A calibrated cable comes with a Cable Calibration Table. The Cable Calibration Table specifies the performance of the cable at specific frequencies.

An Antenna Calibration Table is a list of frequencies and corresponding corrections called Antenna Factors, a Cable Calibration Table is a list of frequencies and corresponding corrections called Insertion Losses. For a particular frequency, the sum of these two corrections (both in dB,) at that frequency, is the number to be programmed into the meter's Antenna Calibration Table at that frequency. This produces a very accurate dBuV/Meter reading.

Below is a portion of an Antenna Manufacturer's Antenna Calibration Table.

10 METER CALIBRATION
GAIN AND ANTENNA FACTORS FOR TUNABLE DIPOLE ANTENNA. BALUN #3
MODEL NUMBER:
S/N :
DATE :
TDS-200/535
227
21-MAR-1997

FREQUENCY
(MHz)
ELEMENT
LENGTH (L/2 in.)
ANTENNA
FACTOR (dB)
GAIN
dBi
170 16 1/8 13.0 1.85
180 15 3/16 13.5 1.85
190 14 5/16 14.3 1.52
200 13 5/8 15.1 1.16
210 12 7/8 15.3 1.39
220 12 1/4 15.1 1.99
230 11 9/16 15.9 1.58
240 11 1/8 15.8 2.05
250 10 5/8 16.4 1.80
260 10 1/4 16.7 1.84
270 9 15/16 17.3 1.57
280 9 11/16 17.8 1.38
290 9 3/16 17.4 2.09
300 8 3/4 18.2 1.58
310 8 5/16 18.7 1.37
320 8 19.2 1.14
330 7 13/16 19.3 1.31
340 7 11/16 19.6 1.27

ANTENNA FACTOR TO BE ADDED TO RECEIVER
METER READING IN dBuV TO CONVERT TO FIELD INTENSITY IN
dBuV/METER. CALIBRATION PER ANSI C63.5 METHODOLOGY.

Notice the list of frequencies, and corresponding antenna factors. This particular antenna requires the elements of the antenna to be adjusted to a specific length for measurements at different frequencies, thus the corresponding list of element lengths.

Next, we'll look at a Cable Calibration Table below.

Description:10 Feet 50 ohm S300 Cable, Type BNC connectors
Part Number: CAB 211/30)
Calibration Table:

FREQUENCY
(MHz)
INSERTION LOSS
(dB)

50 0.40
100 0.50
200 0.66
300 0.90
400 1.05
500 1.25
600 1.35
700 1.50
800 1.70
900 1.80
1000 2.00

Here the table is simply a list of frequencies and corresponding insertion losses.

Using the two tables above one can calculate the entries to be downloaded to the meter's Antenna calibration table. Let's use 300 MHz as an example. We look at the tables above and find the following:

Antenna's
Calibration
Table

+

Cable's
Calibration
Table

=

Meter's
Antenna
Calibration
Table

Antenna Factor (dB) @ 300 MHz

+

Insertion Loss (dB) @ 300 MHz

=

Correction (dB)

@ 300 MHz

17.8 (dB)

+

0.90(dB)

=

18.7 (dB)

For frequencies at or near 300 MHz the meter's Antenna Calibration Table correction should be 18.7 (dB). Normally a meter Antenna Calibration Table will be designed for more than just one frequency. One has a choice when designing the meter Antenna Calibration Table, to design it extremely accurately for a small band of frequencies, or to design a 'best fit' over the entire frequency range of the meter.

  2. Antenna Calibration Table interpretation. During normal power up, and power down of the meter, the Antenna Calibration Table enable/disable state is maintained. When the meter powers up it checks the integrity of the Antenna Calibration Table. If an error is found the meter beeps three times, and the Antenna Calibration Table is disabled. Under this condition, it is possible to use the meter but the SIGNAL LEVEL readout will display dBuV. This is an uncalibrated reading.

When making a measurement, the meter checks the tuned frequency, then scans its internal Antenna Calibration Table. The scan starts at entry number 1 and progresses to entry number 30. When a correction factor is found for the tuned frequency the scan ceases. The correction factor is added to the reading and displayed on the Signal Level readout.

The meter will not allow the Antenna Calibration table to be selected if it finds invalid data. The front panel function F12 will not operate. An attempt to enter Function F12 will result in three beeps, and the meter resumes Field Strength Mode.

When creating an Antenna Calibration Table source text file, it is good practice to:

Note: The DEFAULT.TXT Antenna Calibration Table source file template contains zeros for all values.

Note: The ANT227.TXT Antenna Calibration Table source text file uses entry number 30 to cover the entire range of the instrument from 3.0 MHz to 1000.0 MHz. It sets the correction factor for that range to be 0.0.

Below is a table demonstrating valid and invalid table entries of frequency.

 

Example Meter Antenna Calibration Table entries of frequency

Low Freq

High Freq

Validity

22.000

29.000

Valid

24.050

32.000

Valid, but only for 29.005 through 32.000

32.005

100.000

Valid

200.000

305.000

Valid, gaps permitted

309.000

308.000

Invalid, Low freq must be less than High freq

204.000

219.120

Invalid, 4th set above will be accessed first

10.000

21.995

Valid, sets do not have to be in ascending order

The LOAD_TBL.EXE program truncates table frequency boundaries to 5KHz steps. 254.333 MHz is truncated to 254.330 MHz, 47.999 MHz is truncated to 47.995 MHz. Correction factors may range from -200 to +200.

  3. Editing an Antenna Calibration Table source text file Included on your ANTENNA CALIBRATION diskette is a file DEFAULT.TXT. Use this file as a template to generate new Antenna Calibration Tables. Edit this file by using a text editor such as DOS EDIT. It is important to use a text editor that uses a mono space font because LOAD_TBL.EXE requires data to be in specific columns. DOS EDIT uses a mono space font. Windows text editors such as NOTEPAD default to a mono space font. Other Windows editors, that use variable space fonts, can be changed to use a mono space font such as Courier.

Lets look at a typical Antenna Calibration Table source text file, below is a copy of the file ANT227.TXT

#
#ANT227.TXT
#
#ANTENNA CALIBRATION TABLE FOR TUNABLE DIPOLE S/N 227
#
#
#
#
#
# Num Low freq High freq Corr
#234567890--1234567890--1234567890--1234567890--
1 20.000 29.995 0.4
2 30.000 34.995 -2.2
3 35.000 49.995 1.1
4 50.000 59.995 3.6
5 60.000 64.995 4.5
6 65.000 79.995 5.4
7 80.000 89.995 6.5
8 90.000 99.995 7.4
9 100.000 104.995 8.1
10 105.000 119.995 9.2
11 120.000 134.995 10.6
12 135.000 164.995 11.7
13 165.000 179.995 12.8
14 180.000 189.995 13.4
15 190.000 199.995 14.3
16 200.000 229.995 15.2
17 230.000 259.995 16.0
18 260.000 279.995 17.0
19 280.000 309.995 17.9
20 310.000 349.995 19.2
21 350.000 449.995 21.0
22 450.000 524.995 21.5
23 525.000 574.995 23.7
24 575.000 624.995 24.7
25 625.000 724.995 25.7
26 725.000 824.995 26.0
27 825.000 924.995 27.1
28 925.000 974.995 29.0
29 975.000 1000.000 30.0
30 2.000 1000.000 0.0

Notice, comment lines start with a pound sign (#.) There are four columns of data each 12 spaces wide. The last comment line shown indicates a spacing count for each column below it. The right most space of each column is the least significant digit of the value for that column.

The left most column contains the table entry number. Moving to the right, the next column is the Low Frequency value in MHz for the table entry; followed in the next column by the High Frequency value in MHz for the table entry. Finally, in the right most column is the Correction Factor in dB for the table entry.

The Correction Factor in dB added to the dBuV signal level yields dBuV/Meter. When the meter is used with a calibrated antenna and feed system, and the meter's Antenna Calibration Table is loaded and enabled, the meter's SIGNAL LEVEL readout displays dBuV/Meter.

By inspecting the above file, one can see that the table is made up of back to back frequencies spanning the frequency range of the meter. They need not be equally spaced frequencies. Notice 5 kHz steps in boundary definitions.

The last entry is a catchall to cover any holes that may be left intentionally, or by accidental oversight.

Note: In this example, the first correction specified by entry number 1 starts at 20 MHz. From 3 MHz to 20 MHz a correction factor of 0.0 is applied by the last table entry, number 30.

 

4. Using the ANTENNA CALIBRATION diskette to download tables. The ANTENNA CALIBRATION diskette contains

  1. LOAD_TBL.EXE
  2. DEFAULT.TXT
  3. ANTxxx.TXT

LOAD_TBL.EXE is the program used to download Calibration Tables to your Field Strength Meter. DEFAULT.TXT is a template you may use to write your own Antenna Calibration Table source text file. ANTxxx.TXT is the source file that has been loaded into your Field Strength Meter at the factory based on the antenna and cable combination you purchased from Z Technology.

Using the ANTENNA CALIBRATION diskette, we will step through an example of downloading an Antenna Calibration Table source file.

Power-up your PC in a DOS environment. Running from a DOS window under Windows 3.1, Windows95, or Window NT may cause the download program to error.

Insert your ANTENNA CALIBRATION diskette into drive A: and change to that drive by typing A: [ENTER]

Start the LOAD_TBL.EXE program by typing LOAD_TBL [ENTER]

You should now see the following screen on your PC.

 

WELCOME TO Z TECHNOLOGY’S LOAD_TBL PROGRAM

This program will read an Antenna Calibration Table TEXT (.TXT
extension) file in the current directory, translate it
to a Meter compatible form, then download it to the Meter’s
Antenna Calibration Table. Following the download it will
reprogram the Meter’s Antenna Calibration Table checksum.

See the Z Technology Application Note:

ANTENNA CALIBRATION TABLES

for specific information about the text format required by this program.

 

Press [ENTER] when ready to continue

 

Note: This program requires the Antenna Calibration Table source text file to be located in the same directory as LOAD_TBL.EXE.

Note: After the table information is downloaded a new Antenna Calibration Table checksum table is generated in the meter automatically.

 

Connect the Meter RS-232 port to your PC’s communications port

Place the Meter in remote mode by pressing the FUNC Button

Adjust the TUNE Knob to F93 and push the TUNE Knob

Press [ENTER] when ready to continue

 

The screen above gives instructions to connect the meter and prepare for serial port communication by entering Function 93.

 

Default values are in brackets

Meter Port Address? [1]?

PC communications port, [1] or 2

Baud rate? 1200, 2400 or [9600] ?

Generate .LOG file? Y or [N] ?

 

The screen above inquires the communication parameters. The Default values are those programmed at the factory. The last question asks if you desire to generate a .LOG file. The .LOG file shows the translation of the Antenna Calibration Table source text file to the hex address and data used for downloading into the meter's Antenna Calibration Table. It is generally used only for finding any inconsistencies should a problem occur.

 

The following TEXT [.txt) files are available:

<1> DEFAULT
<2> A1NT225
<3> ANT226
<4> ANT227

 

Make Selection.....[0] to exit .....4

 

The screen above shows the .TXT files available in the current directory. Here we have chosen file <4> ANT227

 

Creating temporary file: ANT227.CFG

40 LINES TOTAL, 0 BLANK LINES, 10 COMMENT LINES, 30

ENTRIES Found in ANT226.TXT

Downloading Antenna Calibration Table file ANT226

..........................

Antenna Calibration Table download completed

30 table entries downloaded

New Antenna Calibration Table checksum programmed in Meter

Deleting temporary file: ANT227.CFG

Press [ENTER] when ready to continue

 

Above is the final screen. First, it informs the user it has created a temporary working file with a .CFG extension. Then displays information about the Antenna Calibration Table source text file. In this case, 40 lines were found, none were blank, 10 were comment lines, and 30 were table entries. Next, it downloads the table data. Dots accumulate across the screen as the download occurs. The number of Table entries successfully downloaded is displayed, and then a new Antenna Calibration Table checksum is programmed in the meter. Finally, the temporary .CFG working file is deleted.

A new Antenna Calibration Table is now loaded in the meter. Push the FUNC button and rotate the TUNE knob to F12, push the TUNE knob. Rotate the TUNE knob to select ON then push the FUNC button. The meter is now using the Antenna Calibration Table, the SIGNAL LEVEL readout displays dBuV/Meter.