Field Strength Indicator teardown - Page 1
Forum section: electronics / general

author: sonnytiger
author_rank: Frequent Contributor
timestamp: August 05, 2013, 10:21:16 pm
content: http://youtu.be/W7__xz_5ZTAAnybody want to tell me some more about this thing? Everything else I know is in the description of the video or the video itself.Thanks!

author: Fraser
author_rank: Super Contributor
timestamp: August 05, 2013, 10:55:02 pm
content: its a very common model of SWR (standing wave ratio) meter used to CHECK how well an antenna is matched to an HF transmitter. It also has a very simple un-tuned field strength meter that utilises a diode detector feeding the meter. Its very insensitive.As an SWR meter it is used as follows:1. Connect transmitter to SWR meter.2. Connect Antenna to SWR meter.3. Set variable control fully CCW4. Set Switch to FWD5. Switch on Transmitter and transmit on the lowest power that provides a reading on the meter. This protects the transmitter PA stage.6. Adjust the variable control to provide a reading in line with the calibration point.7. Stop transmitting.7. Without adjusting anything, set the switch to the REV position.8. Transmit again.9. Take a reading of the SWR on the meter.10. Switch off the transmitter.The SWR reading shows the match of the antenna compared to 50 Ohms.To use the unit as a field strength meter, attach the supplied antenna and rotate the variable control fully CW. (This may or may not be necessary) Move the SWR meter towards the transmitter antenna and you should see the meter deflect indicating RF energy is being radiated.The strip-line directional coupler that sits between the antenna and transmitter sockets on the unit is relatively crude and designed for HF use. The rods on either side of the central transmission line are RF couplers that sample the RF energy flowing through the transmission line. The couplers are configured to provide the forward and reflected signal levels to simple diode detectors that feed the meter movement. Google directional coupler for more info.The other unit you alluded to is nothing to do with this meter. It is an antenna matching unit or antenna tuning unit that is designed to match a non 50 Ohm antenna to a 50 Ohm transmitter. When used, the SWR meter is connected between the transmitter and the matching unit and provides an indication of the SWR as the ATU/matching unit is tuned to the correct settings. Depending upon the design of the ATU/matching unit, it will either bring antenna onto resonance or will just make the antenna look like it is the correct impedance without improving its resonance and so efficiency as a radiator.

author: sonnytiger
author_rank: Frequent Contributor
timestamp: August 06, 2013, 09:32:19 pm
content: ah ok thanks! love these forums, always someone who can explain things!

author: xrunner
author_rank: Super Contributor
timestamp: August 08, 2013, 12:35:36 am
content: Ha! I used to have that exact model as a kid, when I played around with CB radios in the '70s.

author: sonnytiger
author_rank: Frequent Contributor
timestamp: August 09, 2013, 11:20:52 am
content: Awesome! haha, I figured it was pretty common, it was only like 10 bucks in 1975 according to the receipt that I got with the thing, and 10 bucks still wasn't all that much even in the 70s.

author: G7PSK
author_rank: Super Contributor
timestamp: August 10, 2013, 10:02:27 pm
content: I have what appears to be an identical unit that I purchased in the 1980's also have the instruction sheet that came with it, I will scan it at some point and put it up here if any one is interested.

author: G7PSK
author_rank: Super Contributor
timestamp: August 11, 2013, 01:59:35 pm
content: I shall inflict the instruction sheet on you lucky lot whether you want it or not.It contains a circuit diagram and for some strange reason I kept part of the carton it came in where the specifications were printed.

author: sonnytiger
author_rank: Frequent Contributor
timestamp: August 11, 2013, 08:02:44 pm
content: Thanks! cool stuff!

author: xrunner
author_rank: Super Contributor
timestamp: August 12, 2013, 12:29:48 am
content: Quote from: G7PSK on August 11, 2013, 01:59:35 pmOh I enjoyed that - I really did.