27Mhz Antenna

watt swr meter

ادرس لینک جناب مهندس عاملی

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آنتن برای 14 مگا

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انتن برای باند 20  متر یا 14 مگا  بروش ساده تشکیل شده از :

طرح های مهندس دل قوی

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چندین عکس از کارهای مهندس دل قوی جهت اطلاع دوستان و تشویق ایشان:

multi band radio

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A better test

What the test above actually measures is the pinch-off voltage: the highest voltage that can be put on the MOSFET's gate without it starting to conduct. The circuit below shows a better way.

There are two push switches, one is a changeover switch, the second is a push to make (normally open). I use a couple of microswitches. It uses the diode test of a multimeter, or you could use any power supply, or a 9v battery, with a resistor in series with the MOSFET to limit the current.

With both switches in the normal position, the capacitor C1 charges up to the open circuit voltage of the diode tester. The capacitor's value is not critical, 10n-100n is fine. When Sw1 is pushed, the charged capacitor is disconnected from the meter leads and reconnected to the MOSFET gate. The MOSFET should turn fully on, so the diode tester will indicate a short circuit.

Release Sw1. The MOSFETs gate is still charged. Only leakage will discharge it, so the MOSFET should still stay conducting for quite some time.

Push SW2 to short the MOSFETs gate to source, to discharge it. The meter should show an open-circuit.

Alternatively, connect the capacitor to meter +ve via a diode, to allow the capacitor to charge, Now when the capacitor is connected to the gate, the MOSFET will conduct but the diode will stop the conducting MOSFET from discharging the capacitor.

Meter Check of a Transistor (JFET)

 http://www.allaboutcircuits.com/textbook/semiconductors/chpt-5/meter-check-transistor-jfet/

Testing a JFET with a multimeter might seem to be a relatively easy task, seeing as how it has only one PN junction to test: either measured between gate and source, or between gate and drain.

Testing continuity through the drain-source channel is another matter, though. Remember from the last section how a stored charge across the capacitance of the gate-channel PN junction could hold the JFET in a pinched-off state without any external voltage being applied across it? This can occur even when you’re holding the JFET in your hand to test it! Consequently, any meter reading of continuity through that channel will be unpredictable, since you don’t necessarily know if a charge is being stored by the gate-channel junction. Of course, if you know beforehand which terminals on the device are the gate, source, and drain, you may connect a jumper wire between gate and source to eliminate any stored charge and then proceed to test source-drain continuity with no problem. However, if you don’t know which terminals are which, the unpredictability of the source-drain connection may confuse your determination of terminal identity.

A good strategy to follow when testing a JFET is to insert the pins of the transistor into anti-static foam (the material used to ship and store static-sensitive electronic components) just prior to testing. The conductivity of the foam will make a resistive connection between all terminals of the transistor when it is inserted. This connection will ensure that all residual voltage built up across the gate-channel PN junction will be neutralized, thus “opening up” the channel for an accurate meter test of source-to-drain continuity.

Since the JFET channel is a single, uninterrupted piece of semiconductor material, there is usually no difference between the source and drain terminals. A resistance check from source to drain should yield the same value as a check from drain to source. This resistance should be relatively low (a few hundred ohms at most) when the gate-source PN junction voltage is zero. By applying a reverse-bias voltage between gate and source, pinch-off of the channel should be apparent by an increased resistance reading on the meter.

HF COMM FACTS