24 March 2014

A sensitive Field Strength Meter

This weekend I had an interesting time experimenting with Field Strength Meters (FSM). I am interested in deploying a FSM in my garden in order to see if it will provide me a relative indication of the efficiency performance of different couplers for my end fed antennas.

I started out by measuring a range of different germanium diodes by simply measuring the forward resistance of the diodes using my Fluke 8024B multimeter set on the 2K Ohm scale. I am not sure how valid this is but I have read that it is used as a means to obtain a matched pair in the reference documents. I selected a 1N270 diode with the lowest resistance to use as a detector (ref notes below). I noted a significant difference in forward resistance between the 1N60P, 1N34A and 1N270 germanium diodes. Further experimentation needed. Presumably the lower the forward resistance is the lower the threshold voltage will be?

Based on a discussion with ZS6AJY who uses a FSM on a regular basis at his station, I started out by experimenting with a simple diode/capacitor circuit which would detect and rectify RF. I found that this was not sensitive enough. My aim being to use for low power (ideally +6dBm which is the power output of my MFJ259B antenna analyzer) for my antenna system measurements. I could get a meter indication at 5 watts when using a 500uA FSD ammeter. SInce I have a nice big 1mA FSD meter I looked for an amplifying circuit.

I then built a meter based on W1FB's circuit found on pg 147-148 of the W1FB QRP Notebook. This is an interesting circuit that uses 2 diodes as a voltage doubler. It also utilizes a tuned tank circuit to aid in improved peak performance. When I connected my 500uA FSD ammeter to the rectified output I was able to get a half meter scale deflection on the workbench with the MFJ nearby. The MFJ had a piece of wire about 5 feet long connecetd to its output while the FSM had a 3 ft antenna wire. Significant improvement in sensitivity was found by connecting a ground wire between the FSM and the MFJ generator. The circuit has a high Q and is very touchy as a result. This is not surprising since the tank circuit is lightly coupled to the detector using only a 10pF capacitor. I was unable to obtain a reading if the tank circuit was off resonance. I tried a number of frequencies from 2MHz - 20MHz and all were able to deflect the meter to FSD with the sensitivity control increased. The cut-over point between the 'HI' and 'LO' setting is somewhere in the 7MHz range. When the HI position is switched in the smaller inductor is switched in circuit parallel to the larger inductor, resulting in an inductance of 1.4uH. For the low range the ferrite core inductor provides an inductance of 22uH.

I built the DC meter with a 2N3904 transistor amplifier and with no emitter regeneration.  The original design uses a 1.5V battery supply. Since I only had a 3V battery holder on hand, I inserted a 1.5K Ohm resistor in parallel with the 1mA FSD meter. This is an old antique Weston meter that I obtained from ZS6AJY. It is nice to see it working again!

I installed the meter in the garden about 4 feet from the base of the end fed antenna. On 80m I was able to get a FSD easily with 1 Watt output. The meter is extremely sensitive and selective. The tank circuit has to be resonating on frequency in order to obtain a meter deflection. Once that 'sweet spot' is obtained then the meter is extremely sensitive. As I moved around the base of the antenna the meter clearly showed coupling going on between the wire and my body. I found I had to lie on the grass to minimize the effect. I also found that the FSM showed maximum deflection at a point where the SWR was in fact closer to 2:1. Was this due to the FSM picking up RF now being radiated by the coupler or was this in fact the point of maximum radiation from the wire? More experiments needed. On 7020MHz I found the environment even more sensitive and difficult to work in. One thing is for certain. This meter is extremely sensitive. More experimentation needed.

For me the FSM is a great learning tool. It gives one a 'feel' for RF and a visual indication of what is occurring at the base of an end-fed wire.

1-30MHz Field Strength Meter. LO~1-7MHz, HI~7-30MHz.

Sardine tin used for the FSM case.

Diode Measurements

Basic Meter tests lacked sensitivity. W1FB design with a tank circuit and voltage doubler/recitifier.

DC amp circuit design

Collector resistor design for a Vcc = 3V. 

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