(Now a bi-weekly program)
December 11, 2012
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dB or Not dB ...
Decibels and the Homebrewer
The Use and Benefits of Working with Decibels in Ham Radio and on the Bench
(Note: This is the last episode before the holidays!)
Decibels (dB, dBm, dBW, dBc, dBu) are useful concepts for homebrewing hams to be comfortable with. When these terms come up in magazines and equipment measurement/evaluation articles, we sometimes have trouble focusing on the real issue if we are busy wondering whether 3 dB means a factor of 2 or 4 (or something else). And when it comes to measuring your homebrew creation on the bench, the dB readings become the universal language describing performance: sensitivity, MDS, IMD, gain, LPF rolloff, and more. It is well worth the effort to review these concepts from time to time and keep familiar with them ... and that's the nature of our CWTD show this time!
73, George N2APB & Joe N2CX
Audio Recording ... (Listen to the MP3 podcast)
<20:03:59> "Alan W2AEW": Yikes - the Rohde & Schwarz app
note is 35 pages!
<20:09:54> "oh2nlt": Even TeamSpeak volume control is shown in dB scale !
<20:10:55> "Alan W2AEW": That modern s-meter is from a TS-870S
<20:14:05> "George - N2APB": Ha! You are right Juha!
<20:15:01> "Alan W2AEW": it turns multiplication and division problems into addition and subtraction
<20:16:11> "George - N2APB": Ah yes, the R&S paper is indeed longer than the 3 pages I mentioned. Was thinking of an IMD Measurement paper I was thinking of. Look at it this way ... 30 more pages of golden information!
<20:27:19> "Alan W2AEW": dBc is very common of course
<20:32:53> "George - N2APB": Juha, what was the test scenario of your 8594E plot?
<20:38:01> "oh2nlt": left most spike is opposite sideband delta cursor show the attenuated carrier which is -47.5dBc
<20:40:21> "Joe N2CX": Actually the spreading out of the display is a function of the frequency span. Resolution bandwidth is filtering of the signals displayed.
<20:40:32> "John ZL1AZS": This session is a useful way of reminding me of what I don't know that I don't know :)
<20:43:09> "Alan W2AEW": actually, looks like it might have been a 625Hz test tone, and you see that is +625Hz above the suppressed carrier, and the delta marker is on the lower sideband which is -625Hz from the carrier, thus 1.25KHz from the test tone
<20:45:37> "Alan W2AEW": looks like a nice attenuator - what's the frequency range?
<20:46:00> "Alan W2AEW": oops, I see it is DC to 3GHz
<20:46:26> "Joe N2CX": Alan, good example of how you really have to watch the settings and values to get the right interpretation!
<20:52:16> "Alan W2AEW": I've yet to measure a rig that has a true 6dB / s-unit scale! Many are close at the top end of the scale, but much less per s-unit at the low end of the scale.
<20:52:35> "Terry WB4JFI": I'm trying to find a spreadsheet that I created a while back, that creates a one-page printout of dBm, power (Watts), Volts RMS, Volts P-P, and S unit in 1dBm stepss. If I find it, I will send it to George.
<20:52:54> "Clint-ka7oei": Are "dBs" DeciBels's?
<20:53:14> "Clint-ka7oei": Decibels's
<20:53:31> "Alan W2AEW": or, is it dB relative to some quantity "s"
<20:53:44> "Alan W2AEW": ;-0
<21:26:08> "Ray K2ULR": Jan 8th
<21:27:01> "Ray K2ULR": 73
<21:28:34> "Frank N3PUU": thanks guys, great session as always.. if I dont talk to you have a great holiday!
<21:29:16> "John ZL1AZS": Good Christmas and happy holidays to all!
<21:29:57> "John ZL1AZS": Is that Joe's workbench in the whiteboard?
<21:30:17> "Ray K2ULR": Merry Christmas to all.. and a happy new year!
SESSION NOTES ....Decibels and the Homebrewer
The decibel (dB) is a logarithmic unit that indicates the ratio of a physical quantity (usually power or intensity) relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities. A decibel is one tenth of a bel, a seldom-used unit named in honor of Alexander Graham Bell.
The decibel is used for a wide variety of measurements in science and engineering, most prominently in acoustics, electronics, and control theory. In electronics, the gains of amplifiers, attenuation of signals, and signal-to-noise ratios are often expressed in decibels. The decibel confers a number of advantages, such as the ability to conveniently represent very large or small numbers, and the ability to carry out multiplication of ratios by simple addition and subtraction.
A change in power ratio by a factor of 10 is a 10 dB change. A change in power ratio by a factor of two is approximately a 3 dB change.
The decibel symbol is often qualified with a suffix, that indicates which reference quantity or frequency weighting function has been used. For example, dBm indicates a reference level of one milliwatt, while dBu is referenced to approximately 0.775 volts RMS.
The definitions of the decibel and bel use base 10 logarithms. The neper, an alternative logarithmic ratio unit sometimes used, uses the natural logarithm (base e).
Some familiar representations of dB ?? ...
Don't worry everybody gets confused now and then when dealing with decibels.
Decibel: A unit used to express relative difference in power or intensity, usually between two acoustic or electric signals, equal to ten times the common logarithm of the ratio of the two levels.
Although the base 10 logarithm of the ratio of two power levels is a dimensionless quantity, it has units of "Bel" in honor of the inventor of the telephone (Alexander Graham Bell). In order to obtain more manageable numbers, we use the dB (decibel, where "deci" stands for one tenth) instead of the Bel for computation purposes. We have to multiply the Bel values by 10 (just as we need to multiply a distance by 1000 if we want to use millimeters instead of meters).
Incidentally 1 dB increase in sound level represents approximately the smallest sound change that the human ear can discern.
It may appear to be confusing but by having units defined in terms of logarithms, multiplication and division can be done using addition and subtraction.
For example, a 10 times increase in power is 10 dB, so if you have 1 Watt of RF and you amplify it 10 times, you have 10 times more power. If you attenuate it to 1/10th the original power, you decrease by 10 dB.
Taking this idea a little farther amplification by 100 times (10 times 10) results in an increase of 20 dB (10 plus 10).
Important thing to remember is that dB usage ultimately relates to either power ratios or absolute powers.
Commonly dB as the above definition says is that it is used to refer to power ratios.
What does dBm Mean?
Similarly, when dealing in RF, dBm refers to a power level relative to 0 dBm or 1 mW into 50 ohms, corresponding to 0.2236 mV rms.
At higher power levels it's common to refer to 1 Watt into 50 ohms or 0 dBW.
Table of common values showing dBm, dBW, rms voltage and rms power
Signal purity and dBC or dB relative to a carrier
Spectrum analyzer photos with a carrier and several harmonics
Note that spectragraphs and equipment displays often represent the signals with a vertical axis in dB. The major divisions are usually programmable, and more often than not they are in 10 dB increments such as shown in the image on the right of the HP-8561 spectrum analyzer.
On the left, one can see many harmonics of the 21.75 MHz fundamental frequency being measured, as well as other signals. The fundamental signal is usually the highest level displayed, and the signals to each side of the fundamental exhibit smaller levels (sometimes!), which are often indicated in terms of "dB down from the fundamental".
One the right, the spectrum analyzer is showing a view of the fundamental frequency (14,00070 kHz) being transmitted by an SDR Cube transmitter through the RF PowerCube amplifier. With a close-in view of the signal (resolution bandwidth = 100 Hz), we can see the opposite sideband level of the 1.25 kHz test tone showing as being -47 dB ... or "47 dB below the carrier".
Attenuators ... Essential in Test Bench Measurement Equipment
Signal-to-Noise Ratio (SNR
S- units and dB
An S-METER is calibrated by connecting a signal generator to the antenna terminal and setting the output power to 50uV, or -73dBm, and adjusting the S-meter calibration pot for a reading of S-9. Since the S-meter is usually derived from the receiver AGC line, it .is. relatively linear from about S3-S4 and upward (since a good AGC usually "kicks in" around -100 to -105dBm). This linearity is also due to the diodes used for the AGC detector, once they are conducting in the linear region (again, around S3-S4). Statements that "S-meters are totally worthless" or "a change in 2 S-units means nothing" are thus actually quite incorrect. An S-meter .is. a fairly good RELATIVE power indicator for received signal strengths and noise levels.(From NA5N ... http://www.gqrp.com/s_units.pdf)
A Typical (?) Test Bench
See how many places dB's are used in these tests that N2CX and N2APB were making for some SDR Cube IMD measurements ...
[Answer: 10 log (3765499 / 376549.9)]
Okay ... Now let's take a look at how all these components connect together ... with dB's in mind!
Antennas – dB gain relative to what?
DB gain statement is meaningless without an appropriate reference.
dBi sometimes used to refer gain to an isotropic antenna and this is ok for comparison of antenna gains in modeling but is often misapplied in antenna advertising as in
Excerpt from W8JI work:
dBd vs dBi
The dipole is the basic building block of many antennas. A dipole does NOT have 2.2 dB gain over an isotropic radiator when the dipole is placed over earth. The dipole has about 8.5 dB gain over an isotropic radiator! Always remember this when you see antenna models over earth given in dBi. If the model over earth shows a "gain" of about 8.5 dBi, the model effectively has the same gain as a dipole.
Here is the pattern of a 145-foot high copper wire dipole over medium real earth on EZNEC:
You can see the gain is 8.5 dBi, and it is just a simple dipole just over 1/2 wave high. Any antenna you model should always be compared to a standard like this!
The notion a dipole has around 2.15 dBi gain is only true for freespace.
Relative antenna test - hams set up impromptu antenna test ranges to compare antennas:
VHF/UHF antenna shootouts
Refs for antenna testing www.qsl.net/n1bwt/chap9.pdf
HFPack HF antenna shootouts ... http://hfpack.com/antennas/
The purpose of the HFpack Pedestrian Antenna Shootout is to try out a variety of pedestrian antennas, measure and document the antennas in a well-controlled manner, and report the results as a service to radio operators around the world interested in furthering the state - of - the - art for HF portable.
1. All antenna entries should be physically present to register at the HFpack Shootout site, Pacificon Convention, at 11am on Sunday, 20 October 2002.
2. The antenna shootout is for "Pedestrian Antennas". A pedestrian antenna is considered to be a lightweight antenna that can easily be carried to the shootout by one person in a backpack or bag. It is not necessary to walk around carrying the antenna while in operation. The shootout is not intended to include heavy antennas designed mainly for base station operation, thus, antennas exceeding 11 pounds are discouraged from entry.
3. The test transmitter has a feedline with a 1:1 balun/unun at the feedpoint. The test feedpoint (antenna mounting point) is elevated above ground level, and provides a BNC, SO-239, or 3/8-24
female threads, or terminal lug to connect the antenna to. The balun has at least 20dB of isolation at 14MHz.
4. During the RF measurement, the test transmitter delivers about 5 Watts CW into the entry antenna for about 30 seconds.
5. For vertical quarterwave whips, the feedpoint has a single quarterwave resonant "radial" counterpoise, sloping down at about a 45 degree angle in the direction of the test receiver site. No other
counterpoise will be used for quarterwave whips.
6. Since the objective is to remove any extraneous variables from the RF measurement, the standard antenna test fixture will be used for all antennas. So no other feedline will be used, and it is not necessary
of desirable for the entry antenna to include a support pole, since the support is provided by the test fixture.
7. Commercial antenna manufacturer personnel may not participate as part of the HFpack Shootout engineering measurement team, however they are invited to be present and help set up their antenna on the test fixture.
8. The only frequency of the shootout is 14.1MHz (+/-10kHz).
9. Antennas will be physically measured (inches), weighed (pounds), photographed (digital), and tested for radiated RF on the test range. The results of the tests, measurements, photographs, and description
of the antenna may appear on the HFpack website or other publications.
10. Antenna RF radiation will be measured by a calibrated test receiver using linearly polarized small antenna on an elevated support pole. The test receiver antenna will be operated first in vertical and
then in horizontal polarization. The entry may enter appropriately in either the horizontal or vertical polarization category.
11. Horizontally polarized antennas and vertical antennas not requiring a counterpoise will be elevated to 16 feet at the feedpoint or radiation center.
12. Antennas should self-match to 50 ohms nominal impedance.
13. Antenna owners who are not present may privately arrange for another person to enter their antenna.
14. A commercial antenna may be entered by anyone who owns it, however, multiple entries of the same type of commercial antenna by different owners is discouraged.
15. The general public, amateur radio operators, volunteer witnesses, and others will be present to view the shootout.
16. Other guidelines may be posted or verbally announced or personally advised at the shootout.
17. Requests to test an antenna using other than the standard test fixture and methods will probably be rejected unless there is a good and compelling technical reason to do so.
18. In the unlikely event of confusion or dispute, a panel of 3 HFpack members chosen by the HFpack members present at the shootout will attempt to resolve the issue in an amiable manner.
Mobile antenna shootouts: http://www.k0bg.com/shootout.html
Treatise by NA5N ... http://www.gqrp.com/s_units.pdf
Other S-unit vs dBm table ... http://www.astrosurf.com/luxorion/qsl-db-power-units.htm
Wikipedia ... http://en.wikipedia.org/wiki/Decibels
Mobile antenna shootouts: http://www.k0bg.com/shootout.html
HFPack HF antenna shootouts ... http://hfpack.com/antennas/
Antenna testing www.qsl.net/n1bwt/chap9.pdf
Decibel Measurements (R&S) ... PDF
An Excel spreadsheet that prints dBm, Power, Volts RMS, Volts Peak, and S units ... by Terry Fox, WB4JFI
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