Post by darklordchris on Jun 16, 2010 16:57:25 GMT -6
i need some help you guys. since last year i have occasionally tryed to build a couple of radio related projects,....but it seems like know matter what i do they never seem to work. other then a kit i haven't had any success.....am i missing something here? are their some very basic but special things about transmitter building that i should know? do all the parts have to be made out of some special material or something? is it just that you can't use certain equivalent parts? i mean when a transistor or a diode says its equivalent to the part you need is their some reason with radio thats not true?
like i've been trying to build 2 field strength meters for a week. all the parts are good and their it doesn't seem like i'm getting dead solder joints. it passes all the conductivity tests i do. all the parts are , or at least they say their equivalent to the ones in the diagrams. and i now its getting power through it and my circuits are on broken.......but the things i've been trying to build never do anything !!!!!!!!
Keep in mind to that transistors and diodes have an upper frequency limit. RF circuits will have to be built with parts that can handle those frequencies used.
The higher the frequency worked with the closer and more shielded parts should be. When working at VHF and UHF it gets tricky for the average experimenter. This is why working with stuff down in the AM or shortwave broadcast band range is best for beginners.
A schematic of your meter would help if you can post it. Did you test it around a good transmitter? A CB radio or even a wireless remote for an RC car should show you if it's working or not.
Keep in mind to that transistors and diodes have an upper frequency limit. RF circuits will have to be built with parts that can handle those frequencies used.
Agreed!
Also the Ft (the 'transition' frequency where the transistor's voltage gain =1) of the transistor can determine the maximum frequency it can be used at for gain. For example, if you want a transistor to have a gain of 10 in a circuit at 30MHz, you need a transistor with a minimum Ft of 300MHz. Given losses, stray capacitance, etc, a transistor likely need an Ft of 400MHz in my experience to have a good solid gain at 30MHz.
That's why UHF amplifiers typically have a gain of only 4-5 for a single stage preamp.
Post by darklordchris on Jun 23, 2010 3:25:23 GMT -6
i built 2
the first one i build i used a silicon diode and possible the wrong choke. thats why it didn't work. i will try to post a schamatic. its from this book. thats the one i plan to rebuild.
That's pretty much an "RF sniffer" for a VOM. It needs a LOT of field strength to get the needle to move without direct contact.
If you use a 4.7 or 10p capacitor, you can directly connect it to the RF output for a general reading if RF is there or not, but you aren't getting anywhere a true reading in RMS volts or anything.
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