Skip to main content
8 votes
Accepted

dBm signal minus dB

Decibels are all "ratios" at their core. A unitless dB is a simply a ratio of one number to another, perhaps input power relative to output power. We can also use decibels for absolute values, by ...
natevw - AF7TB's user avatar
8 votes
Accepted

How to measuring ferrite core efficiency vs frequency

To help identify a mystery ferrite core mix, the most common method is to take some measurements to determine the initial permeability (µi). Then compare your results to a table of µi for common core ...
Josh's user avatar
  • 138
7 votes

dBm signal minus dB

Think about it this way. 27 dBm means 27dB above a milliwatt. Take 6dB away. Now you have something that's 21 dB above a milliwatt. Or, 21 dBm.
mike65535's user avatar
  • 1,035
7 votes

RF Bridge Problem - K6BEZ Antenna Analyzer

(See updated information further down) I found an article to address the problems all in one place in QST, September 2019. The author addresses the same problems in the question, and explains his ...
Peter Buxton's user avatar
7 votes

What does an antenna analyzer tell me about these coaxial cables?

My name is Alex, I'm the head of technical support at RigExpert. This is actually an interesting question. Our engineers could not give an exact answer why the schedule in the second case behaves ...
Alexander Antonov's user avatar
6 votes
Accepted

Recommended way to analyze spectrum output of a high-power amplifier?

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.
Dave Tweed N3AOA's user avatar
6 votes

Ferrite rod or core permeability determination

Following up on @dfannin's excellent answer: The most intuitive way of dealing with this would be: wrap your ferrite rod in halfway stable paper or so, something that certainly doesn't have high $\...
Marcus Müller's user avatar
6 votes
Accepted

Signal power measurement

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 ...
Phil Frost - W8II's user avatar
5 votes
Accepted

Why is the insertion loss of my coax cable higher than expected?

Based on your description, I would suspect moisture damage. Moisture ingress in coax cables typically results in the corrosion of the copper braided wires in the shield. The oxide that forms ...
Glenn W9IQ's user avatar
  • 18.6k
5 votes
Accepted

How were 1920s hams able to measure megahertz frequency?

[Discovered this via a comment on another answer.] According to their Wikipedia article: Lecher lines were used as frequency measuring devices until frequency counters became available after World ...
natevw - AF7TB's user avatar
5 votes

How were 1920s hams able to measure megahertz frequency?

The US National Bureau of Standards (NBS) had a system for basic measurement of frequencies as early as 1911. They used very basic calculations to determine the resonance of an LC circuit. The math ...
Glenn W9IQ's user avatar
  • 18.6k
5 votes
Accepted

Ferrite rod or core permeability determination

you're on the right track to measure/calculate effective permeability. However, you need to use an inductance meter that can measure up to 1 nH accurately or so. If you're trying to use one of ...
dfannin's user avatar
  • 66
5 votes
Accepted

How to probe a dipole antenna?

First problem is how to connect the transmitter antenna to the signal generator. I will use a 50 ohm coax cable of small length. Since the waveform and directivity is not critical, could I get away ...
Kevin Reid AG6YO's user avatar
  • 24.6k
5 votes

Measuring energy density of 2.4 GHz and 5.0 GHz signals

I'm a math and computer science student, starting to mess around with using amplifiers and antennas to build long-range wifi hardware in my spare time. You're potentially breaking the law. In fact, I'...
Marcus Müller's user avatar
5 votes
Accepted

Cabling question between two devices

RG-174 cable does not offer very good shielding. This can be a significant problem when you are attempting relatively high levels of attenuation as the cable leakage may provide more signal than the ...
Glenn W9IQ's user avatar
  • 18.6k
5 votes

Determining Impedance With An Antenna Analyzer

How do I convert the above data to provide the impedance of my antenna, at this point, for this tested frequency? "Z (Ω) = 167.19 - j63.91" is telling you the impedance. The impedance of your antenna ...
Kevin Reid AG6YO's user avatar
  • 24.6k
5 votes
Accepted

UHF cubical quad VNA analysis interpretation

1. You need to re-think your balun. At UHF we don't use toroids and wire, it's always done with transmission lines of varying impedance, on PCB or maybe thin coax. There are transformers for UHF, mini-...
tomnexus's user avatar
  • 11.2k
5 votes
Accepted

How to properly measure sensitivity of the receiver?

Quite likely the signal generator, and the radio, are too leaky to do this experiment on one bench top. So the signal received by the radio is not as small as you think it is. The solution would be ...
tomnexus's user avatar
  • 11.2k
5 votes

How to make an RF probe for accurate power measurement at QRP power levels?

How to make an RF probe for accurate power measurement at QRP power levels?                     &...
Marcus Müller's user avatar
5 votes

Can an oscilloscope be used as an RF voltmeter?

One of the functions of an oscilloscope is actually to measure RF volts, just make sure to use the x 10 function to minimize loading on the circuit you are measuring, and keep in mind that some high ...
Andrew's user avatar
  • 3,685
4 votes
Accepted

Rudementary Impedance measurements for VLF antenna system

At 4 kHz you could use your laptop as a rudimentary two-channel oscilloscope. First find out if it has a useful two-channel input sound card, and find a way of recording a stereo 16-bit WAV and ...
tomnexus's user avatar
  • 11.2k
4 votes

Measuring dBs with software defined radio

Decibels express ratios. SDRs are often not calibrated to any reference, so when it says "-110 dB", that's relative to some arbitrary reference power. Often the reference is "full scale", meaning the ...
Phil Frost - W8II's user avatar
4 votes

Antenna design - Ground plane in simulation and in practice

The antenna is matched to be 50 ohm at A (Fig. a). So, to measure it in practice, the author designed a 50-ohm transmission line which connects A to B. Am I right? Yes. This choice means that the ...
Kevin Reid AG6YO's user avatar
  • 24.6k
4 votes
Accepted

Peak envelope power should be 2:1 or 4:1 on meter?

Say the unmodulated carrier amplitude (voltage) is 1. At 100% modulation, the envelope will vary between amplitude 0 and 2. So at peak, the amplitude is twice an unmodulated carrier, but because power ...
Phil Frost - W8II's user avatar
4 votes

Wire dipole antenna proving ground / test bench

Under ideal conditions you should strive to keep all conductive materials that are not part of the antenna system out of the near field of the antenna. This generally means non-conductive supports. ...
Glenn W9IQ's user avatar
  • 18.6k
4 votes

How to measure the power emitted from an HF antenna in near-field WPT?

WPT (wireless power transfer) is typically performed using flat, multi-turn coils (inductors) as antennas. In some cases, the coils are self resonant based on their parasitic capacitance and self ...
Glenn W9IQ's user avatar
  • 18.6k
4 votes

Determining Impedance With An Antenna Analyzer

Thanks for following up on your earlier question. All of the data you provide in your question represents the impedance the analyzer "sees" at the point you measured it. There is no need to convert ...
Brian K1LI's user avatar
  • 7,858
4 votes

Determining Impedance With An Antenna Analyzer

Firstly, an SWR of 2.62 is probably acceptable for a receiver, and no additional matching is necessary. See What is the relationship between SWR and receive performance? Also, the IP33 Mini Whip ...
Phil Frost - W8II's user avatar
4 votes
Accepted

Do External Signals Affect Antenna S11/SWR Measurement?

Yes, in my experience antenna measurements on a VNA are affected by external transmissions. They look like "noise" on the graph. Usually you can exclude them by eye - they're narrow and sharp while ...
tomnexus's user avatar
  • 11.2k
4 votes
Accepted

How to make an RF probe for accurate power measurement at QRP power levels?

A basic probe looks something like this: simulate this circuit – Schematic created using CircuitLab Ideally the diode has a low voltage drop, like a Schottky diode. But any ordinary silicon ...
Phil Frost - W8II's user avatar

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