# At what watt level is it safe to use metal strips for modulated signals?

In my working design, I have a transceiver TX on low power; Then boost it in a preamp phase in the back.

The way I'm sending the signals (RX and TX) through the radio is by a network of 1/8 to 1/4 inch wide copper strips.

The first network is from the TX side of the Transceiver to the input of the preamp in the rear.

The second network branches to the RX of the transceiver and a secondary RX chip.

Here then lays a problem before me, at what watt level will cause my TX network to start emitting and cause interference in my RX network.

Is that okay, or should I invest in something else or just use wire?

The reason why I'm asking about the khz and Mhz is that the transceiver chip is going to handle all that is offered in both the khz and Mhz bands so talking about one specific band is going to limit what i can do.

• What do you mean by "metal strips"? Since waveguides are used at microwave frequencies, I was going to add the microwave tag; however, you didn't give us much details here. Please edit your question and add more detail. As it is, it is unclear what you are asking. Jan 16 '20 at 22:04
• yes, I stressed the same thing under your last question. "High and low power" are relative terms and up to you to define. The frequency changes everything about the design of a radio circuit, so it's also required that you define that. Please ask complete questions! Jan 16 '20 at 22:48
• Still no indication of what frequency or band your'e building for. Without that information, no one can answer your question. Jan 17 '20 at 16:55
• +1 for the helpful edit.
– uhoh
Mar 2 '20 at 0:17

There is no such thing as a minimum power level at which a conductor will start radiating. If it radiates at all, then the radiation will be proportional to the power applied. Therefore, the maximum usable power in this circumstance is the power at which such radiation will interfere with (or damage) the receiver, and in order to know it you must test empirically or use electromagnetic simulation software.

However, there are two facts about radio design which you should consider before proceeding with the above principle.

First: You should not use brass strips. Not only that, you should not use wire either. In any RF system beyond the most basic, RF signals should be carried between modules or circuits through transmission lines, which prevent (or more precisely, greatly reduce) radiation away from the line. This prevents interference among the parts of your system.

(Within individual circuits, where transmission lines are neither possible nor relevant (since the point of the circuit is to do something other than carrying the signal unchanged), wiring between components should be short, to prevent radiation and other undesired effects. This is usually accomplished in modern radios using a printed circuit board and surface-mount components, but various techniques can be used depending on the desired operating frequency range.)

The most commonly known type of transmission line is coaxial cable, which is used to connect transceivers to antennas and also to route RF signals between boards/modules within them.

On printed circuit boards, carefully designed copper traces function as planar transmission lines (microstrip being a common design).

Besides preventing radiation, correctly designed transmission lines also support impedance matching, which is required to ensure radio signals are efficently transferred from the transmitter to the antenna; mismatches cause inefficiency and often cause destruction of the transmitter due to reflected high-power signals.

Second: Most transceivers do not transmit and receive on the same frequency at the same time. This is because it is extremely difficult to prevent the transmitted signal from being partly reflected back and overwhelming the receiver, even if you build everything correctly. Changes in the antenna due to weather, birds, etc. can disturb it enough to cause lots of reflected power.

Instead, a transceiver does one of the following:

• Transmits and receives at separate times, known as half-duplex operation. In traditional amateur radio this is accomplished with an obvious PTT button, but digital packet radio systems such as cell phones and WiFi switch back and forth many times per second.
• Transmits on a different frequency than it receives. In amateur radio this can be seen in repeater systems. This requires either using widely separated frequencies ("cross-band repeaters"), or using a and precisely adjusted "duplexer" which is a pair of filters adjusted precisely to the transmit and receive frequencies — and the transmitter and receiver must be well-designed with appropriate shielding and transmission lines so that the signal which goes through the air and bypasses the filter does not interfere.
• Surely an RF signal must at some point go through something other than transmission lines. A typical RF circuit consists of transistors, inductors, capacitors...all these components don't consist of tiny transmission lines, do they? And yet the signal must pass through them, somehow... Jan 19 '20 at 4:46
• @PhilFrost-W8II I reworded and expanded on that some; better? Jan 19 '20 at 6:37

More critical than power is length. As a rule of thumb, over distances less than 1/10th the wavelength, metal strips, ordinary wire, or anything you might use to conduct electricity is probably fine.

Longer than 1/10th the wavelength, and the "wires" start to look more like "antennas", leading to unintentional reception or radiation. In these cases it's better to use some kind of transmission line, which is some specific physical arrangement of conductors designed to have a controlled characteristic impedance and minimize radiation.

Power level is another concern, but not due to radiation: rather the concern is either overheating and efficiency. Thicker conductors will have a lower resistance and thus lower loss, thus generating less waste heat and being more efficient. The maximum power a conductor can handle depends on factors such as the insulation surrounding it, the maximum temperature of the surrounding materials, and your design's tolerance for loss. Due to the large number of variables involved I can't give a general answer here, but usually an online calculator or a manufacturer's datasheet can provide more specific guidelines for a particular situation.