Introduction: Field Strength Meter Amplified Antenna
Finding other uses for a semi-obsolete CATV field strength meter.
Step 1: Acquired CATV Field Strength Meter
I recently acquired a Sumtel 5128a field strength meter. This unit is about 15-20 years old but I could see it's potential for use in the amateur radio hobby. This meter has a 75 ohm input and it was designed mainly for setting levels for Cable Television (CATV) and Master Antenna (MATV) systems. The unit has a lot of features which include the ability to scan and show on a LCD screen all channels from 2 to the top of the old UHF TV channels (roughly 54 to 900 MHz). It also has a spectral display which will show an 8 MHz span when a particular channel or frequency is entered. The direct frequency entry is a very nice feature which makes it especially useful for amateur radio. The meter measures in dBmV which is mainly used in the cable industry but can be easily converted to either dBuV or dBm using the appropriate charts found here: https://www.maximintegrated.com/en/app-notes/inde...
The spectral display screen has a moveable cursor that can be moved to any part of the display and the exact frequency can be determined. The sensitivity can be set with up or down arrows for stronger or weaker signals. The unit is of adequate sensitivity for directly hooking up to a cable TV or amplified antenna, but is too insensitive for making measurements out in the field directly attached to a small antenna. I came up with the idea of adding an amplified dipole antenna that increased the gain by 20-30 dB depending on the frequency.
Step 2: Coming Up With a Suitable Design for an Amplified Antenna.
Since there are so many cheap, well designed 75 ohm wide band amplifiers sold on ebay, catering to those that want to receive OTA (Over the Air Television) I opted to just purchase one and built the device around it. Using the arrangement shown, no shielding was needed and the device worked very well. I cut the arms to be resonant in the center of the UHF band and probably the most sensitive part of the amplifier's bandwidth. This is approximately 600 megahertz.
Step 3: Items Needed
1) Plastic project box at least 5 x 8 x 4 inches. (Radio Shack)
2) 2 pieces of 1/2 inch copper water supply pipe cut to 4 3/4 inches. (Hardware store)
3) 9 volt battery with connector (Radio Shack)
4) Miniature Single Pole Double Throw Switch (Radio Shack)
5) 20 dB TV antenna amplifier (ebay)
6) Hookup wire (Radio Shack)
7) Solder (Radio Shack)
8) Hot Melt Glue gun and stick (Hardware store)
9) 3 feet of scrap 75 Ohm cable with F connector on end. This would be free if you have any lying around, but if you must purchase it get the minimum amount that has a male F connector on the end. You might be lucky and the amplifier will have one attached.
10) Soldering gun and soldering iron (Radio Shack)
Step 4: Drill Holes for Two Sides of Dipole Pipes and One in Bottom for Coaxial Cable.
Drill holes in the side of the box to the same diameter as the copper pipes. Ream holes a little bigger if you cannot fit pipes into holes. Once two pipes are centered, made sure there is a 3/8 inch gap between them. Secure in place with lots of hot melt glue making sure there is lots of surface to solder wires to. Also, drill hole for coaxial cable in bottom of box. Tie knot as shown to keep cable from putting strain on amplifier board.
Step 5: Drill Holes for Switch and Led
Drill a hole for the power on Led and switch as shown. Mount amplifier on side and secure with small amount of Hot melt glue being careful to not overheat and damage board.
Step 6: Install Switch in Back of Project Box.
Install switch in back of box. It would be a good idea to solder everything together before mounting in box. The only exception are the wires that connect to the two copper pipes.
Step 7: Solder Input Wires to Amplifer
The input wires to amplifier should be soldered to amplifier before it is mounted in box. Once amplifier is in box, these wires should be soldered to pipes that make up the two arms of the Dipole. It would be a good idea to scrape and roughen up the surface of the pipes and tin them with solder. A soldering gun of at least 100 watts will probably be needed for this purpose. Use the soldering gun only to solder wire to pipes. For everything else, use a low wattage 25 watt, soldering iron.
Step 8: Wire Everything Together and Recheck Connections.
Once you have wired everything together you can recheck your work. Turn on with 9 volt battery connected and screw F connector into Field strength meter top female connector.
Step 9: Once Connected to Field Strength Meter Check for Signals.
Put Sumtel meter into spectral display mode like shown above. Turn off amplifier and set Sumtel meter to 100 megahertz. This is the center of the FM band. Some signals should be displayed as above. Using up and down arrows set to 10 dB as shown. Then turn on amplifier and see increase of signal strength of at least 30 dB at lower frequencies such as 100 MHz. Next set meter to the actual broadcast channel (channels usually have a different broadcast channel than they display, especially if they were not a UHF TV station before the switch over to ATSC) of local TV station. It should look like the second display of channel 22 in my area. You will notice at least a 20 dB increase of signal with the amplifier on vs it off. The TV signal will force you to set the sensitivity of the meter maybe as low as -40 dB due to the decrease in sensitivity at this higher frequency. Note the FM stations show narrow bandwidths vs the TV signal's large bandwidth that takes up the whole spectrum display. By moving the cursor to various points on a signal, you can read the peak signal strength at that frequency and compare it to other signals. This dipole antenna is very directional and you will find that the signals vary in strength depending on the direction of the transmitter. You can also experiment with horizontal vs vertical polarization by turning the axis relative to the ground 90 degrees. This setup can also be used to do various antenna measurements for ham radio and be used to look for stray signals. I recently did a scan of TV signals in my area and found a strong signal on channel 52 or 700 megahertz. There was no listing for channel 52 in my area so I checked the frequency allocations and found that frequency had been turned over to a cellphone company for special data services. After the antenna amplifier was completed, I measured the gain in 50 MHz steps from 50 MHz to 850 MHz. At 50 MHz the gain was 19 dB. The highest was 52 dB. Most of this discrepancy was due to the amplifier being designed to peak around 550 MHz. Surprisingly, the gain at 850 MHz was still 50 dB!
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