thoughts on a QRP field strength meter

Discussion in 'QRP Corner' started by KA9UCN, Dec 24, 2015.

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  1. KA9UCN

    KA9UCN Ham Member QRZ Page

    Over the years I have found a field strength meter to be an indispensable tool however at QRP levels a small field strength meter is a bit lacking on sensitivity so I have started this thread to see what others think on the subject.

    In its simplest form a field strength meter is just a meter and rectifier. This works well at higher power levels. My personal thought is an op amp and minimal filtering such as a simple low pass filter would do wonders for performance. A field strength meter can be complex but in this case I would like to know your thoughts for a simple sensitive design.

    Joe KA9UCN
  2. KI6J

    KI6J Ham Member QRZ Page

  3. W7CJD

    W7CJD Ham Member QRZ Page

    I realised Grid Dip Oscillators are still available.

    I think I like that NJQRP Sniffer.
  4. AI6KX

    AI6KX Ham Member QRZ Page

    I am going to start out with a basic circuit using a 500-uA movement and see how it goes for tuning up a wire with a T-match on QRP. If it isn't sensitive enough I'll add a DC amp.

    Steve JS6TMW
  5. KA9UCN

    KA9UCN Ham Member QRZ Page

    This is the type of field strength meter I have had in the junk box for years. They were made under several names for years but are all basically the same.

    The pros.

    Cheap, simple, small and indestructible.

    The cons.

    It is not very sensitive.

    I do like the sniffer KI6J suggest. It would appear to have everything a person could want in a field strength meter.

    I am surprised to find all the designs on the net and am in a bit of a quandary as what to post on the subject. My memory does not serve me well but someone posted on the QRP corner that this is QRP corner, not beginner’s corner. A very astute observation. QRP is fun for all levels but in the case of seasoned QRP personnel it is not a beginning but a refinement of acquired skills. In that thought please excuse me if I sometimes go further into detail than is required for such a person. On the other hand there are a lot of beginners in QRP who have not had the advantages of formal training and years of experience behind them. Please bear with me as I will try to keep simple at first.

    The field strength meter is basically no more than a crystal radio. At higher received power levels it can work with minimal effort. As I had mentioned. I am a novice at QRP although long seasoned in RF design.

    The simple detector pictured is OK at higher power levels were the primary signal of interest is the strongest. I would not expect a lot at QRP levels and it will be totally useless in an aria were signals other than the primary one of interest might not be the strongest. So let’s first work on sensitivity with simplicity in mind.

    One of the first improvements might be to forward bios the detector diode with a bit of DC using a potentiometer and battery. This will keep things passive and greatly improve sensitivity. It will not help with unwanted signals and noise.

    Next would be to consider the antenna. Nothing in equals nothing usable. The antennas on Theas meters are critical. The little 4 inch one used with some meters just won’t due for QRP applications. A better antenna will be a requirement.

    Next Thought might be some form of selectivity. This can be simple or complex. Again working on the simplistic approach. I would incorporate a low pass filter for HF work. If there are AM BC transmitters very near by then some consideration might be in a band pass filter set up to attenuate BC RF. In any case a LC filter network will improve performance.

    Up to this point I have discussed what happens before the detector in a passive network. Beyond this things get a bit more but not excessively complicated.

    With the thoughts of what is happening before the detector an active RF amplifier would be the next logical step. I personally like 2n2222a transistors but this is a topic to be considered. The RF amplifier class of operation is non critical as we are in no way looking at signal quality. This makes things a lot easier to deal with.

    After the detector there is not a lot to be considered. A DC amplifier would improve sensitivity. This is an easy and cheap method. Again signal quality is not a consideration and all after the rectifier is DC. Beyond that mentioned things can get a bit complicated.

    To me this is a very interesting topic and it just might be time to redo the old meter

    Joe KA9UCN

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    KU4X likes this.

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