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Author Topic: antenna and transmitter questions  (Read 6175 times)
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kb3ouk
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« on: November 21, 2009, 07:37:52 AM »

First the antenna question:I know that adding inductance lengthens an antenna electrically, and that capacitance shortens an antenna. Using the two together allows you to tune an antenna over many frequencies. Now, if I had a piece of wire that is longer than 1/4 wavelength on 160 meters, say somewhere between 160-170 feet long, then would I only need a variable capacitor in series with the antenna and feedline to tune it for the bands from 160 and up?

Second:I know that produce 100 percent modulation from a plate modulated amplifier you need a modulator output equal to about one half the amplifier's input power. So say I had a trasmitter capable of 100 watts out with 140 watts in. To get 100 percent moulation I would need 70 watts of audio. But, what would happen if  I modulated that transmitter with a modulator capable of 300 watts of audio power?

Shelby KB3OUK
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N2DTS
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« Reply #1 on: November 21, 2009, 08:39:04 AM »

I will pass on the antenna question, but for the modulator, 1/2 the input power is the bare minimum you need, PLUS losses in the mod transformer.
Its best to have as much audio power as rf output power or more.
All my rigs have more audio power than RF output power.

Going wild could give you big power spikes, but 2x the output power should not be a problem, you just dont drive it as hard, which makes the audio cleaner than running something a full bore.

With a lot of audio power, you want to control the audio levels carefully, having just enough audio power makes things a bit fool proof, someone could crank things all the way up and not cause problems.

Like a lot of horsepower in a car, its nice to have, but you have to control it.

Brett
 
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kb3ouk
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« Reply #2 on: November 21, 2009, 08:57:39 AM »

The thing would be to limit the negative peaks to 100%, but allow the positive peaks to go beyond 100%.

http://amwindow.org/tech/htm/obmod.htm

This is how I was thnkng of connecting the modulator to the transmitter when I get around to building the transmitter I want to build. Got to start gathering parts.
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K3ZS
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« Reply #3 on: November 21, 2009, 10:14:47 AM »

I'll take a shot at your antenna question.     The series cap for a long antenna could be used to tune out the reactance.     It is use sometimes on a slightly longer than quarter wave end fed antennas.    At other frequencies you would have to match the impedance even if the reactance was tune out by a series cap.    I am not an expert so probably there are better explanations.
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kb3ouk
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« Reply #4 on: November 21, 2009, 10:28:21 AM »

OK that makes sense. If you took a coil and had taps on it for each band you wanted to use, then all you should have to do would be tune the cap for the frequency and set the coil to theright position. On 160 all that would need adjusted would be the cap and the coil could be taken out of the circuit.
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KM1H
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« Reply #5 on: November 21, 2009, 07:33:58 PM »

Dont mix up C and L with Xc and Xl.  If an antenna has Xc you need Xl of the same amount to cancel it out and provide a purely resistive load; and vice versa of course. This pure R can be any number and is usually not 50 Ohms in the real world of antennas. Now you need L and C (there are other methods) to transform say a 25 Ohm load to 50 Ohms so the TX will see a 1.1 VSWR.

Carl
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ke7trp
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« Reply #6 on: November 21, 2009, 09:04:14 PM »

I agree on modulator at least equal to Transmitter power. My Transmitter is 1000 watts output. But the modulator is 1650 watts output.

Clark
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KX5JT
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John-O-Phonic


« Reply #7 on: November 21, 2009, 09:21:58 PM »

I agree on modulator at least equal to Transmitter power. My Transmitter is 1000 watts output. But the modulator is 1650 watts output.

Clark

So won't you have an audio gain control to ease it back until everything looks perfect? 
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ke7trp
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« Reply #8 on: November 21, 2009, 09:26:01 PM »

Hell no.. I run it FB! all the time.. nothing but FB!!!!!!!!!!

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flintstone mop
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« Reply #9 on: November 23, 2009, 03:45:11 PM »

Hell no.. I run it FB! all the time.. nothing but FB!!!!!!!!!!



And that's the only way to make things happen on the airwaves.

Fred
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Fred KC4MOP
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