+Jonah Miller is currently working on a multi-part series of articles on how computers work. In one of the installmetns he mentioned the good old triode vacuum tube with a cathode, plate, and grid. That brings up the subject of today's post, the grid dip meter, (GDM). If you wandered over here from the ham radio practice tests, and you want the shortest answer possible, then click here, or scroll down the page. If you want more information, read on:
The grid Dip meter schematic is shown in picture 1.
The main purpose of the circuit, at least for amateur radio operators, is to determine the resonant frequency of some other circuit or device, (like an antenna). Let's say you've built you're latest radio and you'd like to get all the power you can from the driving finals, into the antenna. To do that, you'd like to make sure that the antenna resonates at the same frequency you'd like to send and receive at. If the antenna resonates at a little bit lower frequency, you can always trim it's length, or if it oscillates at a little bit higher frequency, you can always make a longer one. To find out which I need to do, I'll use my grid dip meter to determine the antenna's resonant frequency.
The basic grid dip meter's circuit consists of a Colpitts oscillator with the inductor, L1, of the LC feedback loop exposed and protruding from the device, (see picture 2), it's the coil on the far left edge of the picture.
To determine my antenna's resonant frequency, I'll attach a small coil to the middle of it, (I tend to only use dipoles because I'm lazy). Next, I'll place the exposed coil of the grid dip meter near the coil attached to my antenna. Then, I'll tune the oscillator's operating frequency until I see the reading on the meter on the front face of the apparatus, which is denoted by G in the schematic, dip, (hence the name).
So,why does the current being supplied to the grid of the Colpitt's oscillator dip? At the resonant frequency of the device being measured, the magnetic field from the inductor begins to drive the coil on the device under test. In this case, the antenna radiates that energy away. The current that was driving the grid is now reduced in an amount related to the amount of energy that went into the device under test, (the antenna). Pretty cool!
The Answer to the Ham Radio License Exam Question
You want the two coils to be as loosely coupled as possible. If they are tightly coupled, (located very close to one another), then the inductance in the coil attached to the device under test will effect the resonant frequency of the GDM. The end result will be that the resonant frequency won't be the actual resonant frequency of the device under test, it will be an odd munge of the device and the GDM circuit. On the flip side, you don't want to couple too loosely either, or the grid current simply won't dip.
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