# Tag Info

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A cross-meter is capable of showing you three measurements simultaneously: Output Power Reflected Power SWR From this image by Axel Schwenke on Wikipedia, you can see that the needle on the left indicates forward power, and the needle on the right indicates reflected power. The observed intersection of the two needles can be used to indicate the SWR of the ...

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My trick is instead of cutting, rather extend the whip first, and see if the SWR gets better or worse. Extend it 1/2" with a bit of wire wrapped around it and sticking out. If the SWR gets better, then you made it too short in the first place. If it gets worse, you can start trimming. Remember you should only measure SWR when you are away from the antenna - ...

8

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 fixing the denominator to a standard reference — e.g. one milliwatt in dBm. But the most convenient thing about decibels is that, although they are ratios, because ...

8

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 materials. This will put you in the ballpark and likely more than close enough to make an educated guess about the application(s) the ferrite core would be ...

7

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.

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(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 fixes for a similar bridge-circuit-based Arduino analyzer originally published in QST, November 2017. I realize I am a newcomer and many have already figured out ...

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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 strangely. I can offer to perform another experiment - to measure the parameters of the RG-8/U coax cable using the analyzer and the AntScope and AntScope2 software....

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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.

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A properly calibrated cross-needle power meter such as e.g. the MFJ-842 actually tells you something more than just the forward and reflected power, which as you point out can just as easily be indicated by two separate instruments. The intersection of the needles gives you a pretty good indication of the actual standing wave ratio or SWR because the SWR is ...

6

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 $\mu_r$ (gut feeling: baking paper is nice as it is very "slippery" on flat surfaces) make as many turns as you want around that; you're building a coil now, with ...

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.

5

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'm pretty sure you're breaking the law. WiFi operates in the ISM bands, which sets a very strong limit on how much power your device might send in any ...

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If you take SWR measurements at a range of frequencies and plot them (or just watch the SWR reading as you change frequency), you should get a graph with one or more valleys of low SWR. Ideally, if you find the ratio between the valley's actual position and where you want it to be, and trim or rebuild-bigger the antenna according to that ratio (or rather, ...

5

There's no problem with what you're proposing, usually. Most of your other equipment probably has exposed metal connected to “ground”. If it's not causing trouble, this won't either. But if you have the problem known as “RF in the shack” — your transmitter's RF coming back to you on the outer shield of your feedline — then the metal will be RF-hot, but you ...

5

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 without adding a balun? Or would there be a risk of damaging the signal generator with signal reflections (it has 50 ohms and +10 dBm (0.01 Watts) output) ? A ...

5

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 those $25 LCR meters , it won't have the accuracy or precision you require, plus you won't be able to zero it. Another issue is that you'll have a wide variance ... 5 The US National Bureau of Standards (NBS) had a system for basic measurement of frequencies as early as 1911. They used very basic calculations to determine the resonance of an LC circuit. The math was greatly advanced in 1923 in an article in Radio Broadcast magazine entitled ""Reducing the Guesswork in Tuning". The method involved careful measurements of ... 5 [Discovered this via a comment on another answer.] According to their Wikipedia article: Lecher lines were used as frequency measuring devices until frequency counters became available after World War 2. The idea is to short a transmission line after some distance, forming what we now would call a resonant stub. The resonant reflections set up standing ... 5 Based on your description, I would suspect moisture damage. Moisture ingress in coax cables typically results in the corrosion of the copper braided wires in the shield. The oxide that forms increases the RF losses of the shield. A simple resistive test of the shield is not usually sufficient to detect this condition. If the center conductor is multi-... 5 RG-174 cable does not offer very good shielding. This can be a significant problem when you are attempting relatively high levels of attenuation as the cable leakage may provide more signal than the attenuator is allowing. Here is an illustration of coaxial cable leakage from Wikipedia: There will be losses in the "transport mode" that are due to the RF ... 5 How do I convert the above data to provide the impedance of my antenna, at this point, for this tested frequency? "Z (Ω) = 167.19 - j63.91" is telling you the impedance. The impedance of your antenna is$(167.19 - j63.91)$ohms. Then, how many ohms, increase or decrease, would be required to “transform” from the antenna impedance, so it becomes the 75Ω ... 5 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-circuits sells them, but they're tiny - the cores are say 2 x 4 mm and the wires hair-thin. Power handling is 0.25 Watts. Here's a photo/rendering - the pads ... 5 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 to eliminate the leak path, by shielding one or both ends. You'll get good results with aluminium foil: Power the HT from its battery only. Connect some of the ... 4 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 processing it. Octave or Python would work, or you could write your own. Then build a circuit like this, adjusting parameters to suit: simulate this circuit –... 4 One way would be to connect a HF transceiver to the SWR meter, and that meter to the antenna. Assuming proper licensing to broadcast on bands - tune to the middle of each amateur band and check the SWR while transmitting a tone (or whilst whistling). The lower SWR reading would indicate closest resonance to that band. Checking at various points around that ... 4 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 full range of the ADC. Still, that doesn't directly relate to power. If the measurement goes from -110 dB to -70 dB, that's a difference of 40 dB. That's a ... 4 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 difference in overall physical structure versus Fig. d is smaller than if the design used a smaller PCB. For monopole antennas, the ground plane acts as another ... 4 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 is proportional to the square of amplitude, power is$2^2=4\$ times the carrier power. So I think your CBer friend has it backwards. If you are looking at the ...

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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. As a practical matter, if you are supporting horizontal dipoles, there is very little current at the open end of each leg so very little radiation occurs in this ...

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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 inductance in order to avoid the losses in a typical matching network. Due to the close proximity of the two coils, the mutual inductance must also be considered. ...

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