A No-Power Field Strength Meter

-

 


 

Before you scoff, this meter does work, and it works well.  Yet it has no battery, no amplifiers, transistors, valves, or even magical elves.  There are just 4 diodes and a capacitor inside the box, and if you have basic hand tools, should be able to make one in about an hour.

The circuit is shown in figure 1, and it’s derived from my old RF Design textbook from the early 1980s.  A simple antenna is connected to the SO-239 connector.  A pair of diodes (D1 & D2 rectify the incoming voltage which charges C1.  The variable resistor, VR1 acts as a sensitivity control to allow you to take relative measurements.  Diodes D3 and D4 protect the meter from overloads.


Construction

There is no PCB or even board required.  D1 and D2 are connected directly to the SO-239.  C1 is soldered to the pins of VR1, while D3 and D4 are connected directly across the meter terminals.  To avoid short circuits, I covered D1 in heat shrink.  Also make sure all ground connections go back to a single point.

 

The antenna is very simple to construct.  I used a piece of welding wire and soldered one end to the pin of a PL-239.  To prevent short circuits, I filled the body of the plug with hot-melt glue.  This way I can always recycle the plug if need be.

How do I use it?

Using the FSM is simplicity itself.  Place it close to the antenna in question (1 -2m is a good starting point) and transmit a carrier. (Of course, you checked the frequency was clear, right?).  Now adjust the pot to get ½ scale on the meter.  Now you have a baseline for comparison work.

Other things to note

Component values are not critical.  You could replace D1 and D2 with germanium diodes to get a little more sensitivity, but I don’t think this will be much benefit.  The value of the pot could be changed if all the adjustment seems to be at one end of its travel.  I found experimentally that 10K was about right for my equipment.

Also, you may want to put a little piece of bright coloured heat shrink on the end of the antenna to stop poking yourself in the eye with it.


 


Comments

Post a Comment

Popular posts from this blog

The QYT KT-7900D

-
Image
But is it any good? As you might have noticed, I am now a freshly licensed radio amateur .  As such I do need a transceiver or two in order to get on the air.  This is the story of one of my new radios. I have a limited discretionary budget to use for my new hobby, and I have to say from the outset that my lovely wife has been very supportive.  As such, I want to try and get the best value for our money.  One evening I stumbled upon this little item on Banggood for a little over $A150 including shipping.  Covering both 2m and 70cm bands with a claimed 20W output it seemed almost too good to be true.  I bought one. About ten days later a tiny package arrived for me.  At just 98mm x 43mm x 126mm the photos don't convey quite how small it actually is.  The entire case serves as a heatsink so it is heavier than it looks, but certainly not bulky. The standard power cable comes with a cigarette lighter plug.  As I intended to run it from home using a regulated PSU, I just cut the plug o
Read more

Life with Deep Racer - Part 2

-
Image
 An Update on AWS Deep Racer OK, so AWS Summit Sydney has been run and won.  Not by us however, but there are few interesting tidbits to report: The console is now widely available This is actually pretty big news, because: The console ties together Robomaker, SageMaker and everything else you need in a convenient package There are actual examples of functions included It seems to stay running now There is new firmware for the car Also important; this includes a rolling update for the Ubuntu base as well as the Deep Racer specific bits.  WiFi connection seems more reliable now as well. Our car has a new look Since my employer (Versent) was generous enough to send me to Re:Invent last year (where I got the Deep Racer), I thought it appropriate to deck it out in the company colours, and it was well-received at Summit last week.
Read more

DIY Self-Driving - Part 9 - New Imaging

-
Image
Please Note:    Unlike most of my projects which are fully complete before they are published, this project is as much a build diary as anything else.  Whilst I am attempting to clearly document the processes, this is not a step-by-step guide, and later articles may well contradict earlier instructions. If you decide you want to duplicate my build, please read through all the relevant articles before you start. We have a lot of work to do in pretty short space of time now, with new competitive opportunities in the very near future. To be honest, the line detection has simply not been good enough, and this is leading to some of the poor performance I have been experiencing with the car.  Whilst it really does work, it is not accurate enough.  There are a couple of issues here which need to be dealt with: The core processor is just not powerful enough; and The optics aren't good enough Processing As I noted in my "Next Iteration" article I am looking at othe
Read more