That depends! How well do you want to know the distance and loss of your system elements? The propagation velocity affects the distance that reflections are reported on the Site Master. Cable manufacturers often spec their prop v +/- 10%. So, the antenna at the end of your cable will have a +/- 10% error in distance. You can minimize this problem by sweeping known lengths of cable in DTF mode. You can 'dial in' the propagation velocity until the reflection at the end reads properly on the Site Master. The cable loss given by the manufacturers, however, is usually quite accurate. Using their published specification is sufficient. The Site Master cable tables offer both propagation velocity and loss values for many standard cables used in cell installations today.
There is not a a DX symbol, what you may bear seeing is the error message for Integrator Failure. It kind of looks like a "DX". If this is really what is happening then your Site Master may have been damaged by excessive RF or ESD being applied to the test port (most common cause). There could be other causes for this type of failure too. You should contact our Customer Service Department for more help.
The FCC regulates how much output power of any given transmitter. Since this is a fixed quantity, the system designers are forced to squeeze out every bit of performance out of the entire system.
Basically, the maximum distance that can be measured in the DTF mode is controlled by 3 factors, 1) the data point resolution, 2) the frequency span or F2 minus F1 and 3) the propagation velocity entered for the coax under test. Since the propagation velocity cannot be changed, you must change either the frequency span or the data point resolution in order to go further. If using a 'B' or 'C' model, raising the data point resolution will almost double the distance. If you are using an 'A' or earlier model or cannot change the data point resolution then the frequency span will need to be smaller. Be sure to re-calibrate if the frequency span has to be changed.
Typical absolute measurement accuracy for tower mounted transmission lines is within 1 foot. Please see our application note "Distance to Fault" (page 12) for a more detailed explanation. This Application Note can be found on the SMIU Education page.
Cable loss is displayed in the SM as loss/ft or loss/m. In the catalogs it is displayed as loss/100 ft or loss/100 m.
All cables have loss. If there is too much total loss, then not enough signal is reflected back to the Site Master for a valid measurement. The noise floor of the Site Master is ~40 dB, so if there is more than 40 dB of loss in the run of cable the Site Master will have trouble.
Here's an example: A typical cable spec'd as follows: 3.9 db/100 ft @ 2000 MHz
Let's assume we have 500 ft of cable. The first question I ask is: "how much loss is there with 500 ft of cable?" 0.039 db/ft * 500 ft * 2 = 39 dB (The * 2 multiplier is due to the round trip nature of a S11 measurement i.e. down the cable and back).
If the noise floor of the Site Master is about 40 dB and there is 39 dB of loss in 500 ft of cable, I would expect to have measurement limitations with the Site Master. So, for this particular set of conditions we should disregard anything passed 500 ft.
So now we can answer the original question properly.
The code uses the c able loss value you enter to adjust the plotted DTF measurement. Let's use another quick example: if you have a 20 dB antenna 100 ft down a perfect cable you would expect to see that on a 20 dB spike in your DTF plot. Now if the cable is not really perfect and you do NOT account for the loss of 0.039 dB/ft the actual antenna will show as 27.8 dB (0.039 * 100 * 2 = 7.8 dB). If you entered 0.039 as the cable loss, the code will subtract (0.039 * 2) for every ft on the screen, giving you the actual antenna RL. That's grand until your loss approaches your noise floor. Once you have subtracted loss equivalent to your noise floor, the actual noise floor starts to creep up on DTF display.
Don't forget that while the noise floor is generally a fixed quantity (40-45 dB), the cable loss in a system varies greatly. So, the maximum distance you can see down a cable can vary greatly as well.
Incidentally, there are 2 ways to "see further down a cable": Reduce your loss or improve yo ur noise floor. They might try sweeping it at 1000 MHz where the loss is usually a bit lower there. The other thing they can do is use a true bench top VNA. It typically can get 50-60 dB dynamic range so you can handle more net loss. Another option would be to use a S251C in Insertion Loss mode. Insertion loss measures the loss from one end of the cable to the other (instead of doing a reflected measurement where you get hit with the cable loss 2x). While this would not permit DTF plots, but it would allow for an average cable loss value calculated.
The calibration temperature window is different depending on the frequency range you are using. At the upper frequencies the window is +/- 5 degrees C from the temperature the calibration was done. In the first few minutes of operation there is a rapid increase in temperature inside the SiteMaster. This is why the Users Guide recommends a 5 minute warm up prior to calibration.
Yes the unit was calibrated in the factory. However to make accurate field measurements an OSL calibration should be made prior to the measurements. If the temperature changes sufficiently between the calibration and the measurement the Calibration Off display is a visible and a new calibration is required.
Calibration is absolutely necessary for good measurements. When you save a setup after calibration, assuming that you are using the "B" series Site Master, the calibration data is stored with the setup. As long as the temperature icon does not appear on the display you are in good shape. Now, if you listen to the design engineers, they are perfectly correct when they say that you may turn the Site Master off and then at a later day turn the unit back on and recall the saved setup with calibration. True statement. However, from a practical point of view, there is a temperature sensor mounted internally, which monitors the temperature of the PC Board. It is always good practice that when you turn the Site Master back on, that you allow it to warm up for about five minutes. Then recall the saved setup, re-calibrate and save it again over the same location. This eliminates the possibility that the temperature window was close to a limit. Especially in those areas where the outside temperature may change many degrees over the period of a day. Also, it is good practice to re-calibrate and save, when you have time to kill, such as while working up a tower. This may save you from suddenly seeing the temperature icon appearing as you make a measurement. Press "SYS" and "STATUS" to see the internal temperature range.
Connectors! Improperly assembled or installed connectors cause a significant number of cell site problems. Many do not exhibit any problems at the time of commissioning, but much later as the elements take their toll on the system. This can often be found early during a maintenance period. By making similar DTF measurements at both the installation and the maintenance, changes in connector performance can easily be seen. Other sources of cell site problems include crimped, dented, crushed or punctured cables. Check your cable vendor's specifications for installation to be sure you exercise proper handling.
After calibrating with the InstaCal, select measuring mode of Frequency vs. Return Loss. Connect a precision 10db attenuator and attach the short on the end of the attenuator. If the InstaCal calibration is accurate, the reading should be -20db.
The rule of thumb is that if you are in DTF (Return Loss Mode), then typically all connectors should be below -30, all cables should be below -40 as viewed on the vertical scale on the left hand side of the display. Again, depending on the cable run and antenna used (application); the antenna usually reads in the order of -15 to -20. However, application will dictate this whether you are operating in the 800 or 2000 MHz area or are using low band radio at 150 or 480 MHz. From this you can establish reference points which allow you to determine a connector pair at the junction of a jumper to the main feed line. Typically jumpers are about 2 meters long, so you should be able to establish your reference points from the length of the cable used.
RL and SWR are frequency-based measurements, while DTF is a distance measurement. A mathematical formula uses the propagation velocity to convert frequency to distance. If the value isn't correct, it will affect the outcome of the formula.
You may calibrate the Site Master with one precision load and make measurements with another precision load. Type "N" loads are specified to be better than 42 dB return loss, and 7/16 Din loads are specified to 45 dB. You can never make a measurement better than the quality of the calibration components being used. This is true for ALL test equipment.
The waveguide calibration uses a 1/8 wavelength offset short, a 3/8 wavelength offset short and a precision waveguide load for calibration. For coax calibration the open and short both causes high reflections back to the Site Master but an open on a waveguide causes minimal reflections to occur although they are about twice as much as the load. The 1/8 and 3/8 offset shorts are used to set the 0 (zero) reference of the Site Master just like the Open/Short for coaxial calibrations.
When doing a DTF, the unit is still calibrated to a particular frequency range, to check the frequency range that you are calibrated for: 1. Press the "Mode" Hard Key 2. Select "DTF" 3. Press the "DTF Aid" soft key 4. Look at F1 and F2, they represent the currently calibrated range of the unit. 5. If F1 and F2 are outside the frequency band of the antenna, it will look like a short, change the frequency range to the range of the antenna.
Based on the resolution chosen, there are a discrete number of data points that are created and displayed on the graph. The markers can only fall on one of these discrete points in the graph. To find out the frequency spacing of the points from F1, use the following equation: (F2-F1)/# of points.
1. You can use Site Master to verify your adapters. The User's Guides of all of our Site Masters have a procedure for Return Loss Measurements. 2. When you measure the return loss of your adapter after an Open Short Load (OSL) calibration, please pay attention to the frequency range of the termination (load) used. For example, 7/16 load works to 8 GHz, SM/PL works to 4 GHz, 28N50-2 works to 18 GHz. 3. Adding another adapter between the adapter under test and the load will degrade the measurement of the return loss of the adapter under test. Plus if one adapter is really bad, you won't know which one is the culprit. 4. Perform the OSL calibration at the Site Master test port. The OSL calibration needs to be performed at the same place you will be connecting the adapter to be tested. Then attach an appropriate load. You are now measuring the return loss of the adapter. 5. Ultimately you have to have known good terminations and a known good Site Master to be able to perform these measurements. If you have more than one unknown, then it becomes very difficult to tell where the problem is. Anritsu offers calibration services for Site Master and also for the loads as well as some of the adapters. If you still are having problems, please consider sending your equipment in for calibration.
The following limitation exists for most commonly used USB to Serial Adapters: The baud rate only goes from 9600 - 38400 kbs.