How much voltage do i need to transmit using a 1/4 wavelength monopole?

[patrickian01]

I created oscillators ranging from around 13MHz up to 25MHz and the voltage levels read from an oscilloscope range from around 800mV pk-pk to 1.2V pk-pk. I plan to transmit them using quarter wave monopoles over a short distance of around 8 meters. How much pk-pk voltage do I need to transmit these frequencies 8m across? I tried building a common emitter amplifier using a BC547 but for some reason, it didn't work. Any help on the amplifiers would be much appreciated as well, and I would like to know how much voltage or current is needed for the operation above and how is it computed? thanks in advance.

 

[betwixt]

It depends entirely on how much you want to receive. Personally, to cover 8m I would be looking at a voltage in the region of around 1 to 5 mV to avoid causing interference. Show us the schematic of what you have tried, it sounds like you may have a serious impedance mismatch.

Brian.

 

[patrickian01]

so basically, the voltage that the oscillator produced alone is enough to be transmitted already? from the pin of the oscillator, i would just use a jumper wire to connect it to the feedpoint of the 1/4wave monopole and i would be transmitting already? or should i use a 75ohm coax to connect the pin to the antenna?

The circuit I used is this:
I used a program to get the values of the components. I used a jumper wire to connect the output pin of the oscillator to the input pin of the common emitter and that's it. If it helps, the circuit used for the oscillator comes from two inverters in the IC 74LS04 and the power supply of the IC is different from the Power Supply of the amplifier. thank you very much for the help

 

[betwixt]

I see a number of problems:

1. The oscillator. A 74LS04 oscillator will produce a near square wave output signal. Square waves are very rich in harmonics which will cause interference on many other frequencies. A large proportion of your transmited signal will not be on the frequency you want. You really need a pure sine wave or failing that, a near sine wave followed by filters to remove frequencies above the one you want.

2. The Amplifier. The antenna will have a very low impedance, depending on design it will probably be less than 100 Ohms. The amplifier is a standard voltage amplifier designed to drive a high impedance load, as a consequence it will have very little, if any, gain. If you need an amplifier at all, it should be a power rather than a voltage amplifier, one designed to deliver it's voltage across a low impedance load.

The classic approach to this would be a single transistor oscillator followed by a buffer amplifier to isolate the antenna from the oscillator stage but not to provide amplification. There would normally be an impedance matching network before the antenna as well. Before going further, is your oscillator tunable or are you using crystals to decide the frequency?

Brian.

 

[patrickian01]

I am using crystals. I also tried using transistor oscillators but i could not get it to work. is there a circuit that you could suggest i try? with the components as well. the crystals i have are mostly overtone crystals (72MHz, 45MHz, 39MHz, 63MHz) and one fundamental (18MHz).

By the Way, for the buffer circuit, since I'm using frequencies above 1MHz, I wouldn't be able to use the standard LM741 Op-Amps so i should use transistors for this part as well? and as for the matching network, would a simple Balun or matching transformer work? thanks

 

[jiripolivka]

I think you miss reading some basics about radio techniques.
Fist, in radio we talk about transmitting POWER. Transmitting power requires that a source and load are MATCHED, best by equal or conjugate values of impedance, to optimize power transmission.
If it comes to antennas, again the antenna impedance must be matched to a source and a load (in the receiver), to make sure all power goes where we want it.
Talking about voltages makes no sense. One can measure a high voltage across a high impedance while the power is low.

To transmit a signal from one antenna to another one needs to know about wave propagation. One can feed a high power into an antenna while at a short distance the received signal power may be quite low.

Read a good textbook on radio principles otherwise you seem to be lost

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As you talk about using opamps at RF frequencies, you certainly need to study a lot. TRy the ARRL Radio Amateur HAndbook, there you can find a good basic education on your problems.

 

[Borber]

Length of two quarter wave antennas for 13MHz is greater than 8m. Why not use cable to connect TX and RX?

 

[patrickian01]

RF is not really my strongest subject and it sucks to have to do a project on it but i have to. I'm probably going to read the ARRL as suggested. the project is about radio transmission, the length of antenna does not really matter as long as we get it to work.