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  1. Wireless signal strength explanation needed

    Could you kindly explain me or advise following please...

    I have a TP-link wifi router.... when i measure signal strength just in the vicinity of the router- literally i touch my smartphone (with wifi signal strength measurement application) to a wifi router antena - i get signal strength -15dbm. The manufacturer datasheet says - "wifi transmission power <30dbm".

    So why do i have -15dbm then just at immediate vicinity of the router ? Where does 45 dbm go? (from 30 dbm down to -15)

    Thank you so much.

  2. Is a magnetic loop antenna a good option for those in an apartment?

    Are magnetic loops a good choice for those living in apartments or condos where a permanent installation is often not an option and the operator still wants access to the HF bands? Why or Why not?

    Edit: Building is from the 1800's, the roof of this building is sheets of steel and the walls are just brick, I am just under the roof, walls are brick, I am on the 4th (top) floor.

    Edit2: I have enough options for portable stations i am only looking for an apartment solution at this time.

  3. Heterodyning complex values in gnuradio

    I have been working at educating myself on how SDR and GNU Radio works.

    In order to demonstrate heterodyning, I set up a flow-graph consisting of 2 signal generators, a Multiply block, and QT GUI Frequency Sink. The generators are connected to the Multiply block inputs, and the Multiply block output is connect to the Freq Sink, in order to heterodyne the two generator signals.

    The frequency of the generators are linked to a slider. One generator is slider_value, the other is slider_value + 2000. The generators are both outputting a cosine waveform.

    When setting all the inputs to float, as I move the slider I get the results I would expect:

    I see one peak always at 2000 Hz: slider_value - slider_value + 2000 = 2000. Ie, the difference signal.

    I see another peak at the sum of the generator outputs: slider_value + slider_value + 2000 = (2 * slider_value) + 2000.

    However, if I switch all the inputs and outputs to complex, I only see one peak which is at the sum of the values. I do not see a peak at the difference, ie, nothing at 2000 (except when slider_value = 0).

    Can someone explain to me why I don't see the difference signal when multiplying complex signals?

  4. Packet Error Rate measurement

    I am struggling with the problem of measuring the Packet Error Rate (PER) in my receiver. I have implemented BER in the receiver system, but I don't know how to add a function that will also test PER (I haven't seen any PER block anywhere, even in the Labview Modulation kit). In the attachment, I'm adding a QAM modulator and demodulator (transmitter and receiver). The problem is that I don't have a CRC calculator anywhere in my code and I think I will need it.

    From what I've learned, the easiest way to do it is to number the frames and check if they come properly. If the CRC is correct then we intercept the frame. All this is done on the basis of a clock or timer (I don't know how to do it exactly). Unfortunately from what I saw after the transmitter, the only things I send are bits, not frames, so I don't know how to do it (or how to change it).

    Does anyone have any ideas how to solve these problems?

    enter image description here

    enter image description here

    enter image description here

  5. How effectively do modern transmitters protect against high SWR?

    The following are fairly common knowledge:

    1. Transmitting at full power with too much of an impedance mismatch can damage your finals.
    2. If we do so inadvertently, modern radios drop the power significantly for self-protection.

    Yet, it's still very common to hear people suggesting that high SWR risks damaging your transmitter. How much of an issue is this with modern radios? Do we really still need to be concerned about damaging our finals, or is it outdated advice that refuses to die?

    In the extreme case, can we freely transmit at (attempted) full-power into an empty jack without shortening the lifetime of the finals?

  6. ALIEXPRESS Products - Fake or genuine? [closed]

    Good day,

    For those who purchased radios from ALIEXPRESS, do the vendors sell fakes or genuine products?

    I'm buying a Yaesu FT-8900R which lists for approximately $100 less.

    If you do not recommend them from whom would you buy a radio?

    I would very much appreciate your expertise and experience.

    Regards, R

  7. How do I connect a homebrew antenna using RX-8 to a Yaesu FT-70DR (SMA)?

    I made an 2m / 70cm tape measure antenna using RG-8 coax. That part was quite a bit of fun and went fine. But when I tried to connect the coax to my Yaesu FT-70DR, the copper core of the RG-8 was too thick to fit into the SMA adapter. How do I connect my antenna to my HT? Thanks in advance!

  8. rtlsdr - hearing an FM broadcast on different frequencies

    There is an FM station in my city which transmits at 105 MHz.

    Today I discovered that I can also hear it quite well if I set the LO to 32.6 MHz and then shift the signal a bit. I'm using an rtlsdr with RT820T2 and gqrx.

    Some of the combinations that work are:

    LO: 32.6 MHz Broadcast: 32.4 MHz
    LO: 32.7 MHz Broadcast: 32.8 MHz
    LO: 32.8 MHz Broadcast: 33.2 MHz

    So, an LO increase by 100kHz shifts the signal by 400kHz. How is this effect called and what is the formula to compute the frequencies?

  9. Exactly what is the cause for the zero reactance seen in the impedance at the center feed point of a resonant half wave dipole?

    Lots of explanations i've read on the internet written by ham radio operators and in some text books state that a half wave dipole is resonant when the inductive and capacitive reactances cancel out.

    Exactly what is the cause for the zero reactance seen in the impedance at the center feed point of a resonant half wave dipole?

    The image below is from Wikipedia and shows the voltage and current distribution for the standing wave which exists on a half wave dipole at resonance.

    enter image description here

    Resonance is determined by the fact that the length of each antenna element is exactly 1/4 of the wavelength of the applied signal, and so the wave reflected from the ends is exactly 360º behind in phase which is the same thing as being in phase with the applied signal and the two add together to produce a resultant waveform with larger amplitude.

    Wikipedia describes this as "When an oscillating force is applied at a resonant frequency of a dynamical system, the system will oscillate at a higher amplitude than when the same force is applied at other, non-resonant frequencies."


    My understanding is that at resonance the voltage of the standing wave which is 90 deg out of phase with the applied current is always zero at the feed point at resonance. For an antenna which is longer or shorter the zero crossing point no longer occurs at the feed point and then the feed point has a non-zero voltage which is out of phase with the feed point current.

    So at resonance the voltage of the standing wave which is 90 deg out of phase with the current at the feed point is zero and it doesn't contribute any reactance to the feed point impedance.

    Is this correct ?

    If what i say is true then in the image above the voltage in the middle is not exactly right.

    See following some of the resources i used to try and find an answer to this question.

    Chapter 5 of Practical Antenna Handbook by Joseph J. Carr Fourth Edition.







  10. Capture Uplink GSM packets using 820T2 and SDR

    I am an extreme beginner with SDR. I am trying to use the 820T2 & SDR device to capture uplink gsm packets sent by my phone to the nearest Base Station. I have an Rx Monitor App installed on my phone to see my Uplink and Downlink channel frequency (904MHz and 949MHz respectively). I am using grgsm_livemon on a Ubuntu VM installed on VMWare to scan for packets. I am able to capture downlink data easily via gr_gsm. However, I see no signals on the frequency monitor of gr_gsm in the uplink frequency. I tried to move a few steps within the band to find any peaks, but unfortunately no result.

    I also tried getting a call on my phone and then I see some instantaneous peaks throughout the whole Uplink spectrum for unknown reasons. (This is totally opposite of what I thought, I believed a GSM uses TDM and FDM for multiplexing multiple users. And a particular user would be assigned a static frequency and one or multiple time-slots for communication, and hence I should see instantaneous peaks at my uplink frequency only). Also, I am unable to capture any packets from these instantaneous peaks.

    I would appreciate it if I get to know what am I missing to capture the uplink data or any tweaks to do it. In case you need any more information, Let me know. Thanks in advance!

    EDIT: So I am sharing a couple of screenshots to have a better understanding of the problem. a) Network Details normal. Here the Frequency hopping is off which is not seen in SS. UE Network Details normal

    b) grgsm output during Call. The peak is prominent but I couldn't take an SS as it's too fast. Frequency spectrtum during call

    c) Network details when I picked up the call UE during call

    d) grgsm DOWNLINK output

    downlink without call

  11. Long wire antenna end fed random length folded back on incline

    Trying to lengthen my 10mtr. end fed to 39 metres in length. To do this I need the first leg (10mtr.) to run at a lower height approx (5mtr)turning back on a dog leg, and the final 29 meters then gaining height to approx. 9 metres high to of Gable end of house. Observing the correct Unun and correct installation which I have already with the 10 meter wire. My question is would the fold back, although climbing in height interfere with radiating correctly.

    I can not find anything which suggests its unwise.

    Thanks for any input. CHRIS.

  12. Calculating satellite passes without an internet tracker?


    You want to know when a satellite will pass in order to receive an image transmission or to work an amateur satellite. However, you cannot access internet for whatever reason, and therefore can't simply look up a pass prediction chart or live tracker.


    Can you/how would you calculate approximately where a satellite would be at any given time, or calculate when the next pass would be? Do satellites always follow a scheduled path to repeat and cycle through? What basic orbit information should I collect about each satellite I need beforehand?

    Is the satellite always actively kept in this exact timing, or does it slowly "drift" from its scheduled path? I ask this because if it does drift, this would eventually make any planning useless without direct connection to the satellite, if that makes any sense.

    A simple path/time map (like below) would be best, but (offline) software also works.

    enter image description here

    Edit - Do the satellites transmit data about their position that I would be able to receive and decode?

  13. How did SMA connector size become standard?

    I'm working on a project in microfluidics and it has become apparent that a significant challenge to overcome is a lack of consensus in connector design.

    I have been thinking about my time working with microwave filters and the fact that the vast majority of devices were connected to the VNA with SMA connectors.

    My question is, what was the driving force behind that design becoming the standard?

    Any insight or history would be appreciated.

  14. Transmitting Video Underwater

    My brother and I are considering designing and building a remotely controlled submarine. As far as experience, I have some knowledge on RC systems, and I am almost done with my General license. My brother hasn't completed his license yet, but shares much of my experience.

    The problem we're facing is getting a live video feed/FPV from the craft while under fresh water. We don't need great range, just enough to get down 10-15 feet vertically, maybe (though more would be much better, of course). Our goal is for it to be completely wireless, avoiding having some type of umbilical cord.

    The question is, how low frequency can you go so that it is not absorbed, but intelligible enough to maintain (visual) control the craft at as low as around, say, one frame per second? I don't need amazing reaction speed like you'd need with a quadcopter.

    Next would be finding what frequencies you can legally transmit this on (I think this would be classified as "remote vehicle control"?), and where to get equipment for it. (We haven't even gotten here yet, but when we do, I'd like it to stay under $500.)

    Edit - so far, there are a few options I'm keeping in mind, along with others' advice (I'll accept the answer that I think most resembles the original idea):

    • Physical cable
    • Surface floating "buoy" antenna
    • long antenna to surface (similar to last option)
  15. Use (TV?) Log-Periodic antenna for VHF

    I found this old log periodic antenna that I assume was from a TV. Could I, or how would I, use this on the amateur radio bands? My main focus would be VHF, with UHF being secondary and HF would also be nice. I'd like to keep it as cheap as possible.

    enter image description here

    Largest: 4' 5" (134.62 cm) - seems to be electrically connected to form a 8' 10" element?

    smallest: 5" (12.7 cm) (unfolds to be parallel with another 5" element)

    "Other" smallest: 6" (15.24 cm) (by itself)

  16. How can you receive ISS broadcast using a Baofeng?

    I have a Baofeng UV-5R with stock and a Nagoya 701 (maybe fake?) antennas and wonder how I can receive voice from ISS astronauts? Do they have a schedule for going on air?

    I live in Romania so probably I have more chance receiving signals for Russian headquarters.

  17. Adapt old AvermediaTV DVB-T tuner as SDR

    I want to adapt the DVB-T AverMedia TV Pilot" tuner as a software defined radio, but it is not built on a RTL chip, so the question arises, is there some kind of driver to use my tuner as a radio?enter image description here

  18. horizontal EFHW antenna attached to gutter

    I'm brainstorming methods of mounting an EFHW. Unfortunately, I have no trees, so I'm considering a horizontal gutter mount which would afford me about 70 feet. Is this workable or should I consider a telescopic mast like a spiderbeam instead?

  19. Adding and subtracting dB/dBm/dBi values

    I've got a question and any clarification on that would be greatly appreciated. For calculating the pathloss in wireless communication we usually use the following formula: PL = pathloss (in dB) + shadow fading (in dB) + antenna gain (in dBi) + noise floor (in dBm) + penetration loss(in dB)

    My question is: do I need to convert dBm to dB and then add all of these value together to calculate PL. Would be the PL in dB?

    For example if I have a scenario in which path loss modeled as

    $$ \text{Pathloss(dB)}=148.1+37.6\ \log_{10}(d) $$

    shadowing is equal to 8dB, antenna gain is 10dBi, penetration loss is 5 dB and noise floor is 54 dBm, what would be the PL and what is its unit?

  20. Unknown signal on 446MHz LSB (possible RFI?)

    I am attempting to hunt down many sources of RFI in and near the UHF amateur bands at my home, but I stumbled across a very specific one that I have definitely never heard before. This one sounds less like interference and more like an intentional signal of some kind. I have listened to every type of UHF signal on the http://www.sigidwiki.com and none of them line up with what I am hearing here. I set my SDR mode to LSB so that the audio would be easily discernible.

    I first discovered this with my Yaesu FT-60R in AM mode on 446MHz out in the yard ~100ft from my house. I pulled it up on the SDR to easily record the signal for posting here.


    I have also recorded audio of the signal in hopes it will help someone to help me ID this signal.


    If audio recordings are not allowed on this site, please forgive me. I am very new to this!

    So I guess my real question is.. is this RFI, or is this an intentional signal? If intentional, I'm curious what it is. If RFI, then I need to go digging through my house trying to find the source.

    73s Steve

    EDIT: Apologies, I forgot to mention. I am located in the US (NE Florida) and 446MHz is allocated as the national 70cm calling frequency.

    EDIT2: Here is a wider view of the spectrum around 446MHz. It appears that there is a constant spike around 445.993MHz, and then on each side there are numerous "copies" of the signal dancing back and forth that you can see in the waterfall. It seems that certain ones are dancing "backwards" from the others, as if the signal in certain spots is inverted. I officially have no idea.

    Wider spectrum view

  21. Ham radio repeater

    I am wondering if using a VHF/UHF ham radio for emergency (I get injured, become ill, run out of water/food, etc.) communication while hiking the Continental Divide Trail would be possible.

    I see that the area I am looking to go has repeaters along the route so my theoretical range would be extended. I see there is some sparse contact information, other than that, I have no guarantee that it exists beforehand short of verifying those few contacts. Then, I must be tuned to that frequency to have my transmissions repeated at a greater power. Additionally, if I were in an emergency, it would boil down to whether or not someone is actually listening to those frequencies.

  22. How to calculate of shielded loop antenna correction factor without extra antenna measurements

    A lot of article about 60 cm shielded loop antennas and how to measure antenna factor using antennas with already known antenna factor.

    But for example if I have one shielded loop antenna (construction http://www.rfmw.cmi.cz/documents/deliverables/Traceable_loop_antenna_calibration_VNA.pdf ) with unknown antenna factor. Is it possible theoretically find Antenna factor if I know the current ant voltage throw antenna, radius and antenna inner conductor length.

    Kind regards Francisco

  23. What length and number of radials should be used for a dual band vertical whip backpack antenna?

    The Situation:

    • Very new Ham
    • I have a 2m/70cm 48" Abree whip antenna, originally designed to be connected directly to an HT
    • I would like to mount this antenna to the side of my hiking backpack, likely using the built in MOLLE system.
    • It is my understanding that adding radials to act as a counterpoise would improve the performance because in normally HT use the user acts as the counterpoise.
    • My Instinct is to do a single 1/4 wave for each of the bands I want to use, running the shorter length along the waist strap, and the longer around the backpack and onto the waist strap as needed.

    The Question:

    How many radials would I need to happily balance this antenna for 2m/70cm, and how would you recommend hiding/weaving them into a backpack?

  24. Bidirectional motor control using MFJ-4116 with RCS-4

    I have the RCS-4 with two MFJ-4116.

    Prior to installing my motorized HF antenna system topside, I bench-tested the core of the assembly in my shack.

    The MFJ-4116 near my transceiver was injected with reversible 12vDC which was successfully decoupled at the MFJ-4116 near the motor.

    So I installed the antenna-side of the assembly topside, then came into the shack to apply a reversible dc.

    I expected to have bidirectional control of the motor to resonate the antenna. As I've done on the bench, by toggling reversible DC as I transmitted and watching for any SWR change.

    It didn't work - Apparently the RCS-4 is somehow impeding the process.

    Any thoughts, anyone?

  25. The HPBW of a helical antenna

    can someone tell me what are the formulas of HPBW for a helical antenna?

    I've searched a lot and I saw that there are two different formulas of HPBW for the helical antenna,

    this one,

    $$ \mathbf{(HPBW)} = [\frac{52}{C} \sqrt{\frac{\lambda^2}{NS}}] $$

    and this one,

    $$ \mathrm{HPBW} \simeq \frac{52}{\frac C\lambda \sqrt\frac{NS}{\lambda}} \mathrm{degrees} $$

    so what is the difference between both ?

  26. For a half wave dipole antenna, what's the value of the reactances that cancel out when the antenna is resonant?

    According to a few descriptions of antenna fundamentals i've read recently including that in Wikipedia and the ARRL handbook, when a half wave dipole is resonant, the inductive and capacitive reactances cancel out, and that's why there is no reactance in the impedance.

    My understanding is that the amount of reactance seen at the feed point of a center fed half wave dipole is determined by the phase relationship in time between the applied AC electric potential at the feed point and the resultant AC current which arrives at the feed point after being reflected back from the ends of the antenna.

    So keeping in mind that a half wave dipole appears to be a series RLC circuit, what are the values of reactance that cancel out at resonance ?

    And why is an antenna resonant, because the voltage and current are in phase or because the reactances cancel out, or both ?

    See this question Exactly what is the cause for the zero reactance seen in the impedance at the center feed point of a resonant half wave dipole? which i have also posted which asks about the same subject.

  27. Is it possible to show frequency and channel name at the same time on AnyTone 878UV?

    On the AnyTone 878UV I can choose to display the frequency or the channel name or frequency (Settings->Radio Set->4 Ch.Name). But seems to me like there's enough screen real estate to be able to show both. Is there a way?

    I've also looked for a way to map a PF button to switching between Channel and Frequency being displayed, but so far have had no luck.

  28. UHF FM power amplifier design without expensive software

    I'm designing a small UHF data transceiver for the 70cm band, based on a Silicon Labs EZRadioPro Si4463 IC. Modulation is FM, 12.5kHz channel step. The concept is similar to SiLabs' AN656 application note but with a different power amp BJT as the NESG270034 and NESG250134 are obsolete.

    Design goal is 1W out (+30dBm) at 3V or 5V supply, from a +13dBm or +20dBm driver source. I figure that's about 17dB of gain. 500mW would also be passable.

    I've been looking at alternatives to the RF BJT, and have hit a small problem. Many RF BJT manufacturers only provide component models for expensive commercial tools like Agilent ADS or Microwave Office. For instance, Infineon provide S2P models (which I understand are usable with RFsim99) for most of their parts, but NXP only seem to offer S2P data for their high-power (25W+) amplifiers and not for their discretes (except some discontinued ones like the BFG135).

    Are there any free or low cost tools which can be used to design the matching networks for FM UHF power amplifiers using the ADS models?

    I'd also be interested in hearing suggestions on other UHF PA transistors I should look at but I think that might be better suited to a separate question!

  29. When and why does the size of a ground plane or radials matter?

    Consider these three cases:

    1. My understanding is that when building or installing an antenna designed to operate over a ground plane (e.g. a quarter-wave vertical), the size of the ground plane does not matter (e.g. the earth, possibly coupled using additional radial wires, or a car roof of unspecified size), provided that it is sufficiently large compared to the wavelength. This is commonly described as the ground plane acting as a reflector, so that there is effectively a second quarter-wave element “below the ground plane”. If this analogy is accurate, then obviously the larger the better.

    2. Designs involving self-supporting radials, flat or downward-sloping, may use a specific length of radials.

    3. A vertical resonant center-fed dipole must have a quarter-wave lower element — having the same symmetry as case 1 but physically instantiated.

    On the continuum of antenna designs between a ground plane antenna and a coaxial vertical dipole, why is it that sometimes there is a well-defined optimum size of the ground plane, or rather the antenna element which is not pointing upward, whereas sometimes it doesn't seem to matter much?

    This question is not about the size of ground-level radial networks, but about comparing designs which have “at least this big” ground planes such as a car roof or the Earth augmented with wires, with free-standing designs which have specific lengths of conductors.

  30. Using a Morse code key with a computer?

    Are there standard ways to connect a hand key (straight or paddle) to a contemporary computing device (PC laptop with no parallel port, Mac, Android, iOS device, etc.) for developing or improving ones Morse code sending skills?

    If so, can one use this setup to do hand-key QSOs, either using a remote transmitter if appropriately licensed, or directly over the internet?

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