# Tag Info

14

Google defines "linear" as "arranged in or extending along a straight or nearly straight line." Wikipedia tells me that "linearity refers to a function or relationship which can be graphically represented as a straight line". Such systems can be described by an equation of the form $y=mx+b$. In the case of RF amplifiers, the relationship is the input ...

13

What is dBm? dBm stands for decibels relative to one milliwatt. Decibels represent multiplicative factors, or ratios; by establishing a specific reference level they can instead be used as absolute values: 0 dBm is 1 milliwatt, 3 dBm is approximately 2 milliwatts, etc. How do I convert it to watts? Convert the decibel value to a scale factor and ...

7

Every linear amplifier designed to work with a transceiver has a bypass circuit which switches the antenna between the amplifier and transceiver. When transmitting, the transceiver connector is connected to the input of the amplifier, subsequently the output to the antenna. When receiving, the amplifier circuit is disconnected and the antenna is connected ...

7

The GS-35A tube is the liquid cooled version of the same tube. Some hams have fabricated their own version of a liquid cooling jacket to replace the fins on the GS-35B: The designer of this jacket correctly highlights the challenges of selecting and maintaining the liquid medium since it is exposed to the full anode high voltage. Deionized water will ...

6

Are there specific problems or equations that are easier to deal with in dBm vs watts? Decibel units, dBm being an example of such, provide a more intuitive measure of some property that responds logarithmically, like power frequently does. Consider, if you are transmitting now with 1W, and you add 1W more, you have doubled your transmit power. That's a ...

6

LM387 is just a really old, ordinary op-amp as far as I can tell. It has a funny pin configuration, so if you have a PCB already made you need to make modifications. I suggest cutting the traces, scraping off the solder mask, and soldering wires in place with the right layout. Look for datasheets on the internet for the pin configuration. For old stuff ...

6

Professional-level dummy loads generally have a "sampling" port that provides a reduced level signal for analysis. You should be able to create something similar.

6

T4 is just this idea: simulate this circuit – Schematic created using CircuitLab Sometimes it's done this way: simulate this circuit The primary of XFMR1 is equivalent to T4 from the question. Since T4 provides the collector load for the transistors, it's a factor in voltage gain. For a differential-mode input, the collector currents correspond to ...

6

If the frequency counter is measuring -5.3 dBm, then before 20 dB of attenuation the power was 20 dB more than that, so 14.7 dBm. "dBm" means decibels relative to 1 milliwatt. 14.7 decibels can be converted to a ratio like so: $$10^{14.7/10} = 29.5$$ So 14.7 dBm is 29.5 milliwatts.

6

If I understand the description correctly: simulate this circuit – Schematic created using CircuitLab If that's all it is, I wouldn't say it's any big deal. Whenever you have multiple paths to ground, current will split between them. It's these currents that keep "ground" approximately the same voltage everywhere: an essential assumption in the ...

6

Is it possible to do without the power amplifier at TX ... by using a high gain antenna for RX? It is. You need another 27 dB (37-10) of antenna gain in addition to whatever you included in your existing link budget. You can split that gain between the RX and TX antennas to avoid needing a very high gain antenna at one end. High gain antennas are also ...

5

DVB-T signals are OFDM (orthogonal frequency division multiplexing), which means that you take your symbols, put them in a vector, put that vector through a discrete fourier transform, and get the time-domain signal you want to transmit. Now, this is fine and all, but OFDM hence has a high Peak-to-average power ratio. Which means that your amplifier needs ...

5

There is nothing especially unusual about this situation: a modulator in most radios will output only some very tiny power which then goes through multiple amplification stages. You just don't have all the stages. Building a little amplifier to take your 50 mW and amplify it enough to drive a typical linear amplifier could be a fun little homebrew project, ...

5

The VSWR isn't the problem per se, it's just the impedance that appears at the transmitter's terminals. Take the load at the end of the transmission line, transform it according to the electrical length of the feedline, and put that equivalent impedance right at the transmitter's terminals and you will have the same damage. A particular VSWR can result in a ...

5

Let's work an example. Say the aircraft transmits at 1 watt, or 30 dBm, on 2.4 GHz. The aircraft is 500 meters away, giving a free space path loss of 92 dB. Between transmit and receive antennas perhaps there is 10 dBi gain, so the power received at the relay is: $$30\:\mathrm{dBm} - 92\:\mathrm{dB} + 10 \:\mathrm{dB} = -52\:\mathrm{dBm}$$ Meanwhile, the ...

5

Just to answer the second part of the question, you do not want an amplifier that has too wide a bandwidth, mainly to lower the complexity (and hence also the cost) of the amplifier. All users of radio spectrum (including radio amateurs) must not cause any interference to other users of the radio spectrum, and that means that out-of-band transmissions must ...

5

There is no need to "pin" an interstage impedance. You may directly transform from the native output impedance of one stage to the native input impedance of the next without going through an intermediate transformation or termination. When designing an interstage transformer, the general design rule is to ensure that the inductive reactance of each winding ...

5

and welcome to ham.stackexchange.com! Sorry, but the Elecraft KPA100 won't work at all with the Yaesu VX-8DR. The VX-8DR is a handheld that runs on the 6m/2m/1.25m/70cm bands; the KPA100 works on the ham bands between 80m–10m. What's more, the KPA100 is really meant to be part of the Elecraft K2, and isn't meant to be a stand-alone amplifier. You ...

5

120 volts times 20 amperes = 2400 watts. That would mean this amplifier would have to be more than 42% efficient to avoid popping the breaker. That's pretty unlikely. Check the specs for the amplifier to get its current requirements, then double it to account for the voltage conversion, and add some more for converter inefficiencies. A voltage converter ...

5

The problem is most likely the type 43 core material. It is quite lossy at 21 MHz and is a poor choice for the auto-transformer. Unlike a transmission line transformer, the auto-transformer relies on the coupling of the two coils to partially occur by passing flux through the core. The complex permeability of type 43 material indicates high losses for this ...

4

The tests they are referring to are for amplification characteristics like linearity and stability, not continuous thermal dissipation. That said, unless you've constructed the thing in such a way that the transistor's thermal dissipation capacity is severely derated, it is very rarely heat that kills finals, at least in the short term. While severe and ...

4

The SA602AN looks to be still available from many of the usual distributors. (Check octopart.com or findchips.com to see which distributors have them in stock.) The LM387 is apparently long obsolete. A quick web search didn't reveal any pin-for-pin compatible parts still being manufactured, so if you're using the printed circuit board from the kit, then ...

4

Yes, you'll need an external tuner, unless your antenna is already matched sufficiently well to 50 ohms. The objective of the tuner is to transform the impedance seen by some signal source, like your radio, or the amplifier. The amplifier will already be designed to have a 50 ohm input. Your radio, transmitting into that, is designed for a 50 ohm output, so ...

4

I entered your circuit into LTSpice and made some test runs to verify the following: In order to get reasonably accurate Fourier analysis, you need to simulate at least 20 cycles. I prefer 50. And, you need to add a SPICE statement setting plotwinsize=0 to eliminate data compression that compromises Fourier analysis. The "polarity" of V6 needs to be PULSE(...

4

I believe it's a misprint. Comparing the drawing showing the construction of the balun with its schematic representation, it does appear the dot on the lowermost winding (terminals 2 and 5) is backwards. If we build it like the schematic suggests, consider the application of a differential mode input on the balanced side: simulate this circuit – ...

4

A high SWR implies the load impedance is significantly different from 50 ohms, violating the design specifications of the amplifier. Yes, there is power reflected in a feedline. But it's not this power per se that causes problems, it's simply that the impedance seen by the transmitter isn't the 50 ohms it was designed for. In other words, the transmitter ...

4

The maximum power transfer theorem tells you how to design a load to extract the maximum possible power from a given source. It does not tell you that the source will survive this treatment, and it does not tell you how to best design a source given a load. If you apply the maximum power transfer theorem to a typical solid-state RF transmitter, it will ...

4

Simply put, the LNA amplifies everything that comes into it. If you think about it, it's actually kind of amazing. It's dragging its output value up and down in response to nanosecond-by-nanosecond changes in its input, reproducing signals at every frequency up to its bandwidth limit. A good amplifier is as linear as possible (meaning its output is ...

4

An anecdote: I had problems with my station which included computer-radio interconnections; some of my devices (RTL-SDRs) would lock up when I powered on my Yaesu FT-897. I eventually tracked this down to the power-on inrush currrent of the radio passing through coax shields and the chassis of the antenna switch that both were connected to. I solved the ...

4

Certainly it is possible to build a high-impedance amplifier. If you resonate the loop with a parallel capacitor (which perhaps includes the input capacitance of the amplifier), you've made a parallel LC circuit which at resonance has minimum current and maximum voltage. A high-impedance amplifier is what you'd want to best extract that signal. But if your ...

Only top voted, non community-wiki answers of a minimum length are eligible