RF newbie here.

Lately I've been experimenting with RF. I built some simple oscillators, managing to get a tank circuit working as well as buffering the 1 megahertz output of a tin can oscillator. Great.

Then I attached a spool of wire and a listened to a nearby AM radio tuned to 1000 kilohertz. Upon connection of the wire antenna, it silenced (due to the lack of amplitude modulation from my little circuit). Wonderful.

But one thing is nagging me...how do I increase the power of the transmission? Now everything online says something about power, blah blah, watts, blah blah. But nothing really explains how it works. How do you increase the power?

I could make the antenna the right length. That would help.

More voltage or amplitude of the carrier frequency? Right now it's running on 5 volts. It'll probably have an effect to some degree, but I don't really think this is the right way to do it...

More current? This doesn't make sense. It's just a wire connected to an oscillator! Nothing is flowing into the wire. How would you increase the current to the antenna? This just does not make sense at all.

So how do you inject more power into the antenna? More current doens't make sense, so I guess the voltage must be increased. But...that doesn't make sense either. Those commercial stations broadcasting on kilowatts must use a extremely dangerous amount of volts!

Could someone explain this to me?

More specifically, how does radio power work? How would one amplify signal going into an antenna?


closed as too broad by Joe Hass, Leon Heller, Chetan Bhargava, Matt Young, Daniel Grillo Jan 7 '14 at 10:06

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • \$\begingroup\$ What you need is an amplifier, with gain at the frequency of your oscillator. As it happens, RF amplifiers are trickier than oscillators, mainly b/c at this kind of frequency, there are all manner of sneak feedback paths that turn amplifiers into oscillators. Be aware that if you attain more than maybe 0.1W, licensing regulations begin to apply. As it stands though your question is pretty broad, and is likey to be closed. \$\endgroup\$ – JustJeff Jan 7 '14 at 3:03
  • \$\begingroup\$ I don't understand this...if the signal swings rail to rail, how could you amplify it? \$\endgroup\$ – fuzzyhair2 Jan 7 '14 at 21:04
  • \$\begingroup\$ Voltage isn't everything. There's voltage amplification, and then there's power amplification. Maybe your oscillator has rail-to-rail output when the output isn't loaded, but as you increase the load (lower resistances) that voltage will sag. If the output impedance of your oscillator is high, it might not support much load at all. Enter the power amplifier. It essentially changes the impedance of your signal. You might even take a hit on the voltage, and still see a gain in power, as the power amp enables driving heavier loads. \$\endgroup\$ – JustJeff Jan 8 '14 at 4:21

Power is the product of current and voltage.

$$ P = IE $$

Impedance is the ratio of voltage to current (plus their relative phase).

$$ Z = \frac{E}{I} $$

Your antenna has some impedance. If this were an engineered antenna the impedance would probably be \$50\Omega\$, by convention. If your antenna is just a random spool of wire, its impedance is something else. It probably isn't even purely resistive. However, it has an impedance, and that impedance does not change unless you change the geometry of the antenna.

Your transmitter is a machine designed to deliver electrical power into a load, and that load is your antenna. Your antenna is designed to accept that electrical power and efficiently couple it to free space such that it radiates away.

If you want to deliver more power to the antenna, then you can increase voltage or current. But, the impedance of your antenna is fixed (unless you design a new antenna). Thus, the ratio of voltage to current can not change. Thus, to increase power, you must increase voltage and current such that their ratio remains the same.

Or, you can change your antenna. If you design a more efficient antenna, more of the electrical energy will be radiated away, and less of it lost as heat in the antenna and the transmitter.

And yes, high power broadcast stations do use dangerous high voltages.

high voltage

  • \$\begingroup\$ Thanks for the answer! How would you go about increasing the current? It's not like I can go measure it using a multimeter between the osc. and the antenna and say "aha! there's X mA going through here." Specifically which current do you increase? \$\endgroup\$ – fuzzyhair2 Jan 7 '14 at 1:44
  • \$\begingroup\$ @fuzzyhair2 the current to the antenna. The problem is no different than increasing the current/voltage/power to a resistor, but your load isn't a resistor: it's an antenna. \$\endgroup\$ – Phil Frost Jan 7 '14 at 2:07
  • \$\begingroup\$ Thanks, Phil! Could you point me to some resources where I could learn some design theory? \$\endgroup\$ – fuzzyhair2 Jan 7 '14 at 21:01
  • \$\begingroup\$ @fuzzyhair2 sounds like you've already found them, and interpreted everything as "power, blah blah, watts, blah blah". They say that stuff for a reason -- I suggest rereading it and coming to terms with exactly what about it you don't understand. \$\endgroup\$ – Phil Frost Jan 7 '14 at 21:24
  • \$\begingroup\$ @fuzzyhair2 You want a copy of "Experimental methods in RF design", maybe also a copy of the ARRL handbook. \$\endgroup\$ – Dan Mills Jan 14 '18 at 12:48

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