Drip Meter? Whatz Dat?

A tale of the forgotten, undervalued yet useful (Grid) Dip Meter.

 

Perhaps you have seen one sitting forlornly on a table at a Hamfest with a perpetually diminishing price label – it may have never found an interested buyer, but why?

 

They are, as a group, less shiny than most, more recent, test devices. They offer very little in the way of flashing LEDs or digital displays – they are often characteristically odd shaped with of all things an analog meter at the bottom and some big dial taking up much of its front. They were very popular once upon a time, but now the only place that still makes them is MFJ (MFJ-201). You can still find all kinds of versions on eBay any day in the year. But, what do they actually do?

Grid Dip Meter

 

A Dip Meter was originally termed a Grid Dip Meter because the meter showed when the vacuum tube’s (yup, this was around long before solid state electronics) grid current “dipped” down when the gizmo as “coupled” to a resonant circuit! Long ago, they swapped the vacuum tube for a transistor, so the “Grid” part is now an anachronism.

 

A Dip Meter is, basically, tunable variable oscillator with a frequency determining coil exposed on top, where it can be inductively coupled to any resonant circuit you wish to test. Each coil has its frequency range and the better ones have a wider selection of working frequencies.When you have it coupled to a resonant circuit as your Dip Meter’s variable oscillator gets closer and closer to the resonant test circuit the meter will show that the resonant circuit is transferring the energy from you Dip Meter and the meter will be deflected to a lesser extent!

 

So, you have an inexpensive device that will be able to tell you if your equipment is resonant where you were hoping or it to be. How do you know if your antenna is “resonant” at the frequency you want to use it to transmit on? Nope, an SWR meter or even an Antenna Analayzer/Vector Network Analyzer will ever give you its complex impedance value. They can’t directly test for their resonant frequency! That’s a very useful ability, especially at the low low price that used ones are going for, but it doesn’t stop there!

 

You can use a Dip Meter to find the value of an unknown capacitor, or inductor, or the “Q” factor of a given inductor, the resonant frequency of a given LC circuit (like many antenna’s trap circuits), and even figure out what frequencies those old crystals were designed for!

 

So what’s the catch? Well, a Dip Meter is not a very accurate device. The tunable variable oscillator, at best, gives you only a rough approximation of what frequency it is really set for. If you need to have a more accurate frequency setting, you can always compare your Dip Meter’s frequency output to your much more accurate receiver (but keep I mind that the Dip Meter’s frequency can wander over time).

 

In order to do any test you have to couple it in some way. The most usual method is inductive coupling which is done by positioning the Dip Meter’s exposed coil parallel and close to the inductor in the resonant circuit being tested. But what if there isn’t an obvious inductor? Well, then you can try capacitive coupling by positioning the Dip Meter’s coil at a right angle to the test circuit’s leads (the dip will be less pronounced, but it will still dip to a degree). Then there’s the trick of link coupling. His method is done with a two foot length of coax with one or two turns of each end of the coax from the circuit under test, to the Dip Meter. Specific instructions on how to make adjustments depend on the model that you are using and although the instruction booklet might be long gone, they are probably still available somewhere on the Internet!

 

The easiest first use is to find the resonant frequency of an LC circuit. Place your inductor parallel to the LC circuit inductor and spin the variable oscillator frequency knob while watching for a dip in the meter reading. When you get an idea of where that dip occurs,move the inductor a little bit away from the LC circuit’s own inductor. This means that you are not loading the circuit as much or shifting its resonant frequency. The amount of dip will diminish,but it should still be noticeable. Either read the frequency off the Dip Meter, or find it by zero beating your receiver.

 

How do you find the value of an unknown inductor? For this you will need a few capacitors with known capacitance. A few fixed capacitors with 5, 20, 100 and 200μF should be sufficient. Place the inductor and capacitor in parallel, in a resonant circuit and use your Dip Meter as you did before to find the dip at its resonant frequency, then plug your numbers into the following equation:

Screen Shot 08-04-15 at 10.03 PM

Where

π≈3.14159
F is the frequency in MHz
C is capacitance in μF, and
L is inductance in μH

Plug in the numbers and out will come a good approximation of your mystery inductor’s inductance. Keep in mind that at different frequencies an inductor’s measured inductance might shift a little due to its distributed capacitance as well as other factors, so the best frequency to use is one near to the frequency that your circuit will eventually be used in.

 

How do you find the value of an unknown capacitor? For this you will need a few fixed inductors with known values. The use a very slightly modified equation that can be derived from the first one:

Screen Shot 08-04-15 at 10.07 PM

Where

π is still approximately 3.14159
F is the frequency in MHz
C is capacitance in μF, and
L is inductance in μH

 

Unfortunately the nature of a Dip Meter and the inductors that can be used limits these tests to capacitance of less than 1000pF in value.

 

Another thing you can measure with a Dip Meter is an inductor’s “Q” factor. What is a “Q” factor? Well, that could be a subject for an entire article all by itself, but one of its many definitions is related to its bandwidth – the higher its “Q” the narrower its bandwidth. The way to get an idea of your inductor’s “Q” in your resonant circuit is to find its resonant frequency which will be F, i.e. where its dip is the greatest. Now tune your Dip Meter to a higher frequency where the dip is reduced by 30% and call that F1, then tune the Dip Meter to lower frequency where it is dip is again reduced by 30% and call that F2. Plug it into the equation:

Screen Shot 08-04-15 at 10.08 PM

As long as you use something like a mica capacitor with a “Q” in the range of 1200 the ultimate “Q” of the resonant circuit should be primarily determined by the “Q” of the inductor.

 

You can take an old crystal and connect a few turns of wire at its base and use that to inductively couple you Dip Meter and find out what frequency it is resonant.

 

In a pinch you can even use your Dip Meter as a poor man’s signal source to troubleshoot your receiver. It won’t be anywhere as good as an RF frequency generator, but it certainly will be cheaper!

 

In addition to new and used MFJ models, you will find Heathkits, and Eico’s and Millen’s brand models on ebay. Try to get one that still works and HAVE FUN exploring!

 

– The Editor –