retiredengineer:Finding channels missed by tuner scan, Part 4 of 4
The Sadelco 719E signal level meter has an elegant simplicity for channel hunting: turn it on, tune for channels, listen for an increase in noise as you tune through a digital channel (the digital signal sounds like white noise), and watch the needle of the panel meter for signal strength.
That type of meter is now hard to find, so I tried some channel hunting with my Sadelco DisplayMax 800 meter. This is what CH42, that was missed during my tuner scan, looks like:
As you can see, the top of the signal is not very flat, which probably indicates a multipath reflection problem. That's not surprising, because the antenna is aimed in the opposite direction and only reflections off the objects in front of the antenna are picked up; there is no direct signal from the transmitter.
When the 800 meter is in the single channel mode it scans across the channel making 43 readings in about 10 seconds, which is a lot like what a spectrum analyzer does. Those readings are averaged and a correction factor of 6.8 dB (according to the Sadelco tech) is added to give the final signal power in dBmV. In this scanning mode, the lower limit is -20 dBmV. If any of the 43 readings is below -20 dBmV, then the meter reads Ur
for under range.
Although this single channel mode allows you to see the signal shape, it is not useful for signals that are below the limit of -20 dBmV. For the above scan I used my CM7777 preamp to make the signal just strong enough for a scan. You can use a preamp before the 800 if you don't have any strong signals that would cause preamp overload. If you do have signals that strong, then you wouldn't be able to use a preamp anyway for reception, because the IM products produced in the preamp would mask your weak signals. See the charts by holl_ands
that deal with preamp overload and SFDR (Spurious Free Dynamic Range).
When aiming an antenna you can switch from the single channel scan to the frequency mode by pressing the F1 key just below CHN/FREQ of the display. You then will have immediate feedback readings of signal strength (at center channel) to help you aim the antenna. As a bonus, this mode goes down to -35 dBmV, which is -107.8 dBm at the antenna terminals with a 24 dB preamp. The reading doesn't include the correction factor, so you must switch back to the single channel mode for that. The freq mode looks like this:
Just for fun, I added a second preamp of 15 dB in series to give a higher reading. It worked, but the tuner was no longer able to lock on to 2.1 on CH20, because the SNR was reduced by the noise that was added by the second preamp.
The day after I made the above measurements, I went across the street and setup a 2-bay UHF antenna, my meter, and a preamp. I was able to get a nice scan and a stronger signal with the antenna aimed at the transmitter for CH42.
Interestingly, when I moved the antenna a few feet left or right, without changing the height or azimuth, there was a big difference in the signal strength and scan quality. This is most likely because of the tree line in front of the antenna about 200 ft away. Calaveras, in post # 14990 further down, mentions that he has seen the same problem with trees when using his spectrum analyzer.
Using the pilot:
Originally Posted by Calaveras
I've had my spectrum analyzer over to several locations where the antenna was looking through trees and the signals can be pretty ugly and vary dynamically with tree movement. Some signals make it through while others are destroyed. UHF is much more affected than VHF in most cases.
You previously asked about measuring the pilot signal. The 800 tunes in 125 KHz steps. You can go from the center freq to the low end by pushing the left arrow key many times, but it is easier to enter the freq of the low end and then push the right arrow key a few times to go by the pilot, which is 310 KHz above the low end.
In the case of CH42, you would step from 638.000 MHz to 638.125, 638.250, 638.375 to take readings. The pilot is at 638.310, which is between 638.250 and 638.375. The measurement bandwidth is 280 KHz at the 3 dB down points, so the last two frequencies in the series should show a higher reading.
In the single channel mode, the scan starts at 638.375 MHz, which is the first measurement of 43 across the 5.3 MHz center part of the 6 MHz channel.
The ads at the right in the new Forum software have squeezed my post to make it narrow (at least on my monitor.....maybe not on a widescreen monitor), so the above chart has been reduced in size from 648 x 568 to 414 x 363 pixels. For the full size chart go to:http://i496.photobucket.com/albums/rr328/rabbit73_photos/MeasuringCH42pilot2.jpg
The pilot at the left end of the scan, Figure 5, looks good.
Even though the scan in Figure 5 looks good, you must be careful not to conclude that the signal quality is also good based only on the scan. It is possible to have a good-looking scan, but not be able to receive the signal as Trip in VA found out.
It is the tuner that determines if the signal quality is good enough to decode.
My 8-inch Audiovox TV in the car was able to pick up CH42 WCVE; the 22-inch Sony was too big to bring along.
If you want to look at the manual for the DisplayMax 800, you can download it from the Sadelco website. They also have refurbished meters.retiredengineer
The measurement method that I have described will do what you want with the 800 meter.
Please let me know how it works out for you.
Other related posts and links:retiredengineer asks about pilotmy answerPart 1Part 2Part 33 types of overloaddBmV vs dBmOld TV Field Strength MeterTrees and UHF Reception in UK
The rabbit will be QRT for now.
(That's a ham Q signal)Edited by rabbit73 - 8/5/12 at 4:03pm