Ham stackexchange

Nyhetsflöde med senaste 30 inläggen från forumet

1. Considerations for UK / USA operation

   I'm just getting into this. I'm currently based in the US and am studying for my ARRL Technician exam. I'd like to start listening in, so I'm in the market for my first radio.

   One wrinkle in that I'm originally from the UK, frequently travel back, and may move back there at some point

   Without getting into recommendations for specific products...

   Are there any considerations I should keep in mind when shopping for a radio, if I want to be able to operate it in both the US and UK?

   • Power feels like the obvious one, but it looks like DC PSUs are fairly normal.

   • I would imagine that the bands etc are necessarily compatible, but perhaps that's not a fair assumption?

   And I'm sure there are other factors I've not even thought of yet.

   What should I watch out for?

2. Has anyone else compared a Magloop to a Vertical?

   I built a 3' diameter mag loop using rg8 and had a cap at the top. I compared it to a 17' vertical EF on my van and beside my house. Both times the Magloop did a good transmit signal into assorted Websdr's around the us, australia and hawaii. But it doesnt compare to a bigger antenna for receive. It just doesnt intercept as much magnetic waves as a bigger antenna. Has anyone else done receive tests with various Loops and Mag loops?

3. How To Program Radioddity's QB25 Manually (Without Software)

   I want to know how to program a Radioddity QB25 by hand. I have tried their instructions

   https://www.radioddity.com/pages/radioddity-qa-bank

   for programming using the QB25 using the microphone's keypad.

   Although I have tried the exact and several variations of the instructions (below). I cannot hit the repeater from my location, and my ht-s can.

   I would love more detailed instructions than what's on their web page. (The following is directly pulled off the web page in case the link falls short.)

   Save Channels With Offset Without Using A Computer It is difficult to save an offset to memory when programming the QB25 without a computer.

   The following steps will allow you to save a complete channel with offsets and any signal tones necessary. Delete channel (if necessary)

   • Menu 45 Switch to VFO Enter RX Frequency (enter zeros if needed to fill all blanks. Example 146.7000) Enter RX Tone (if needed) Save to empty channel - Menu 44

   Enter TX Frequency (enter zeros if needed to fill all blanks. Example 146.1000) Enter TX Tone (if needed) Save to the same channel as RX frequency - Menu 44 (Hold the Right knob in until it returns to the operating screen) Change to Memory, Select Channel, and check operation.

   It appears that this problem got resolved by deliberately removing squelch by pressing the Midi button, but that appears not to be the case. Therefore, I am leaving this open in case someone runs across a solution.

4. Do highly sensitive radio receivers experience less fading

   I need to know whether radio receivers that are highly sensitive, like having sensitivity less than 0.0224 micro volts for AM reception, experience less fading than less sensitive radios.

   Which specification of a radio receiver is responsible for mitigating fading?

5. TS-520S Transmit Issue

   I'm new to amateur radio. I've just passed my foundation exam (UK) which allows me to transmit at 25W.

   I inherited my dad's Kenwood TS-520S and have an AT-200 and have been following this procedure to prepare to transmit: QSL.net TS-520 Tune-up Procedure

   I can get through steps 1 to 11 and see the power into my dummy load on the AT-200 while in tune or CW mode. As soon as I switch to USB or LSB, the power meter on the AT-200 has no movement. Is it possible that the transmitter is faulty?

6. K4KQB was my call sign 70 years ago. Is it still valid?

   K4KQB was my call sign 70 years ago. I had a general license Is it still valid valid?

7. Safety concerns with this Antenna in close proximity?

   Brand new amateur radio license holder, and I (still) know next to nothing. As I posted in a previous thread, I recently got my mobile radio: Anytone D578UVIIIPlus. This base station accessory bundle that I also got, came with this vertical antenna: AnyTone Tri-band Mobile DMR Antenna.

   Today, I put together my antenna and attached it to the magnetic mount in the above bundle and I've attached that mount to a metal cookie sheet. It's currently on a desk just a couple of feet away from me and my radio. Stability looks good.

   Eventually, I'll try it in my attic, but I was thinking of running some tests with the antenna on my desk where it is, but I'm not sure of the safety implications with it being that close. I know RF burns are a thing. But assuming I'm careful not to touch it while transmitting etc, are there any other safety concerns with the antenna being that close?

   Thanks in advance!

8. QYT KT-8900D programming 70cm

   Bought this radio about 5 months ago.

   Programmed some channels into memory for 2m Repeaters - works well. Now have some 70cm repeaters here and try to put them into memory channels.

   But for 70cm no shift, not tone is stored.

   For example: 439.225 / 431.625 tone 88.5 with 12.5kHz

   In VFO Mode, the 70cm repeater works.

   Has anybody had this issue?

   I have ordered programming cable - maybe in programm mode I am able to set.

   Done a radio reset: Now also 2m repeaters cannot anymore stored into memory.

   How do I restore the ability to store 2 m repeater and 40 cm repeater information in my radio?

9. Using UHF female cable connectors

   I am building an antenna that will have an LMR-240 coax pigtail to feed it. Past antennas I built used SO-239 panel mount connectors at the antenna field point (i.e. no pigtail). For mechanical reasons I can't do that with this antenna. So in place of the SO-239 I want to put a female connector at the end of the coax pigtail, for connecting the antenna to the rest of the coax feedline.

   Female UHF connectors are a little difficult to find. I found one where you solder a pin on the center conductor, insert that pin into the connector housing, and then crimp the housing onto the cable (braid) using a ring, much like I do with a male PL-259 connector.

   It is not clear to how that pin makes contact with the actual female part of the connector - I think it is just a pressure fit.

   My questions...

   Do you know how well that pin connection works and whether it is reliable, over time, outdoors?

   Do you think it would be better to just put a male PL-259 connector on the pigtail (where the center conductor is soldered directly to the connector pin), and then use a barrel adapter as needed to connect it the rest of the coax feedline?

10. Antenna Selection and Placement for high altitude rocket ground link

    As a part of my rocketry club this year, I am building our rf ground link system. I have the radio selection and design mostly finished, with the final hurdle being that of antenna choice and mounting location.

    System Details

    434Mhz radio transceiver with +30.2dbm (1W) of output power. with a maximum required distance of ~10 miles

    The rocket airframe will be entirely carbon fiber except for the nose cone. With internals containing multiple thick metal bulkheads

    Nose cone will always be pointing up (away from the ground station)

    The ground station will be on the ground near the launchpad (so roughly always directly beneath the rocket). While I would not be opposed to moving the ground station more away from the pad to achieve a better angle, I would perfer to stay near the pad as thats where the infrastructure exists

    Potential Options

    I do not have a lot of experience with antenna design/emission patterns so my concern is that our signal will experience a lot of interference / deflection from the carbon fiber body.

    The first antenna design I thought of would just be a standard 1/4 wave monopole antenna facing upwards in the nose cone, however, it seems that the radiation would be spread mostly outward instead of downward and would be largely blocked by the airframes

    The second antenna design would be a strip of copper tape (or something similar) mounted to the exterior of the rocket (running down the length of the rocket). In my mind this seems better as it would potentially interfere less with the airframes, but it still seems to me that the radiation would be mostly pointed outwards.

    There's some debate on the team about which of these two options would be better, and I am wondering if a better design exists. Of course, we are limited by aerodynamics and space, so anything outside of the rocket has to be relatively flat and secure, anything inside has to fit within the nosecone.

    Any help would be appreciated!

    Thanks, Ethan

    Edits

    Additional Rocket Details

    Rocket length: 124s inch

    Rocket diameter: 6inch

    Nosecone length: 42inch

    Fin size:

    The rocket is a solid propellant rocket

    Additional Radio details

    our radio is a custom-made PCB utilizing a CC1200 transceiver chip as well as a FEM to boost the power. (If anyone wants to take a look at this info I can send schematics)

    we will be manufacturing 2 of these identical boards and using one as a receiver on the ground station and one as a transmitter on the rocket

    The plan is to have the capability to do both an uplink and a downlink but the system will most likely just be a downlink

    Our ground station will be using a 70cm YAGI antenna (not yet chosen), with a highly sophisticated tracking system (someone's hand).

    I have not made a link margin, and honestly, I have never heard of it. Correct me if I'm wrong but it seems to be a way to estimate the received power and plan antenna gains accordingly. My plan for this was to just field test the system across a lake or something similar.

11. Can an HT be harmed by transmitting near it with another HT?

    Title sums the question up fairly well. I was recently testing two HTs and it was not until after keying a repeater with one that I thought about the fact that the other was sitting (turned on) only about 1.5-2 feet from the one in my hand. I was transmitting at about 7 watts on the 2m band.

    Is it possible to have damaged the non-transmitting HT by doing this?

    Thanks in advance!

12. LoRa APRS using RaspberryPi 4B controlling an SX1278 transceiver

    Project Description

    I am building a tracking & telemetry system for a stratospheric balloon that broadcasts an APRS message via a 433MHz LoRa transmitter (SX-1278 chip). To test the components and architecture, I wrote some code to send an APRS message from a Raspberry Pi via LoRa to an APRS LoRa I-Gate at my house. I hear the device transmitting, but I do not see the message appear on APRS.fi.

    Some Background

    Automatic position reporting system (APRS) is a packet communications protocol for disseminating live data to everyone on a network in real time. At the link level, APRS uses the AX.25 protocol utilizing Unnumbered Information (UI) frames exclusively.

    The field specifications are (as directed by the APRS protocol):

    This information was sourced from the APRS protocol guide, page 12, accessible at: http://www.aprs.org/doc/APRS101.PDF

    The APRS network broadcasts on 144.39 MHz in the US. However, there is a growing community of operators using the LoRa frequency of 433.775 MHz (consistent across the globe) to send and receive APRS signals. The APRS network relies on a series of ground stations (digipeaters and I-Gates) that will receive and transmit these messages. The I-Gates will receive an APRS message and upload it to the APRS-IS service. A map of these stations can be found on aprs.fi and a list of raw messages uploaded are shown at: https://aprs.fi/?c=raw&call=

    To ensure I had the required physical layer to receive the APRS messages, I setup a LoRa I-Gate using a LILYGO TTGO operating at 433.775 MHz following the instructions listed here https://www.lora-aprs.info/docs/LoRa_APRS_iGate/quick-start-guide/ with my tracker visible here https://lora.ham-radio-op.net/views/search.php?imperialUnits=1&q=KW5AUS-1&seconds=0&center=30.3939,-86.5929&zoom=12

    Question

    Since the physical layer seems to be setup correctly, the core challenge is structuring the data appropriately and then transmitting it in the proper format to be recognized and decoded by the iGate.

    This takes us back to the start of our discussion: how do we use the sx1278 to broadcast a properly-formatted APRS message so that it is received? Is this code properly formatting the message? If not, what is missing? Is the SX1278 somehow interfering with the code?

    This is the code that I built: https://github.com/strobesactual/SABER_/blob/main/LoRa_APRS_3.py

    Finally, I am new to Python and coding in general so thank you for your patience!

    Update

    Thank you Hobbs for the recommendation to add the headers and then the text. I made the changes, but still am not seeing the message pop up on the I-Gate. Could this be an issue with the SX1278 library I am using (LoRaRF https://pypi.org/project/LoRaRF/) where it is adding Headers and sync words? Here is the code (please keep in mind that a transmission is happening but something is happening where the signal is somehow not formatted properly): lora_frame = ( f"{SOURCE_ADDRESS}-{SOURCE_SSID}>{DEST_ADDRESS},{PATH_ADDRESS}-{PATH_SSID}!{'Hello World'}" )

    byteframe = lora_frame.encode('utf-8')

    message_list = list(byteframe)

    LoRa.beginPacket() LoRa.write(0x3C) LoRa.write(0xFF) LoRa.write(0x01) LoRa.write(message_list, len(message_list))
    LoRa.write([tx_counter], 1)
    LoRa.endPacket() LoRa.wait()

    And the Transceiver setup: LoRa.setSpi(0, 0, 7800000)
    LoRa.setPins(22, 23)
    LoRa.begin() LoRa.setTxPower(17, LoRa.TX_POWER_PA_BOOST) LoRa.setRxGain(LoRa.RX_GAIN_POWER_SAVING, LoRa.RX_GAIN_AUTO)
    LoRa.setFrequency(433775000)
    LoRa.setSpreadingFactor(12)
    LoRa.setBandwidth(125000)
    LoRa.setCodeRate(5)
    LoRa.setLoRaPacket(LoRa.HEADER_EXPLICIT, 12, 15, True, False)
    LoRa.setSyncWord(0x3444)

    Thank you all again for your gracious help.

13. Can an audio amplifier be used to transmit or receive radio signals?

    Is it possible to make an audio amplifier transmit radio signals when connected to its output with a ferrite coil or any other loop antenna and make the signal receive with another audio amplifier?

    I have this question in my mind because of these reasons:

    1. An audio amplifier can respond to frequencies from 20 Hz to 20 kHz and when connected with a speaker, the coil in the speaker resonates as per the output frequency and if that can be connected to a loop antenna instead, it should radiate radio waves too.

    2. While experimenting with a tape recorder many years ago, I encountered one strange thing. While I was adjusting the screw of the tape head, the screwdriver accidentally touched one of the head connectors that was connected with the audio amplifier. And I was able to hear an AM radio station. I guess the screwdriver was working like a whip antenna and the tape head was working like a loading coil.

    So, based on these facts, I am assuming that a audio amplifier is able to transmit or receive radio frequencies without the need of any carrier wave. This would be done using direct induction just like how a matching transformer works between an audio amplifier and a speaker.

    

14. End Fed Ant w/o Tuner: Using 1:1 To Reflect

    Imagine I have a 34 foot length of 12 gauge stranded wire that is my end fed 20 Mhz antenna, set up as a sloper.

    I've attached a 49:1 at the feed point and a 1:1 Guanella after that, then 20 feet of RG8X coax with 10 small ferrites on the outside of the cable at the RF output.

    My thinking, is that any mismatch and reflection from the 49:1, will stop at the 1:1 and reflect back into the antenna.

    Thus, I will not need a tuner on this setup. Also, the shield of my coax will not be part of the antenna system.

    The draw backs will be energy consumed by the two transformers.

    Is this thinking correct ?

    

15. AM receiver with tubes: better reception quality than with transistors?

    Can an AM radio that uses vacuum tubes instead of transistors have a better reception quality?

16. Why can't I reach 2M repeaters, but can easily reach 70cm?

    I have a Baofeng 5RM. There are many repeaters in my area, both 2M and 70cm. I can reach many 70cm repeaters up to 10-12 miles, but can only reach 1 very close 2M repeater. I've tried with the longer of two antennas that came with the radio as well as tried with a homemade j pole and homemade vertical dipole.

    Any ideas of why I'm basically limited to 70cm?

    Thanks.

17. DX-70 Sideband leakage

    My radio Alinco DX-70 transmits on both USB and LSB, when I set it to transmit on USB. Signal is visible in USB as well as in LSB, but signal strength is bit less in LSB. It appears that radio is transmitting as DSB. Now my question how this can happen. What could be the problem that makes a SSB radio to DSB ? One more observation is that, when I transmit on LSB, then it is fine, there is no transmission on USB at this time. I need some help as how to diagnose this problem. At least if I can understand where is the problem, then I can determine if it repairable or not.

    Thanks

18. Why is the measured loss low in an antenna with high swr?

    I'm trying to understand the basics of antenna and feedline measurements with losses. I have a background in the sciences but not specifically in engineering. Nevertheless, I was doing some testing and wanted to either confirm or understand a specific outcome of my measurements. I'll preface this with that my measurements are not fully robust but do demonstrate basic antenna theory (plus I understand that high swr may damage my transmitter so no comments needed about that).

    So, the setup is as follows. I have an 66ft endfed antenna with a 49:1 unun and 50ft of RG-8X coax. The antenna is designed for resonance on 40m, 20, 15 and 10m. I wanted to measure my antenna's performance on 30m at 10.110 mhz. When measured directly at the end of the 50ft coax with a VNA the S11 return loss is about -2db or an SWR of 11:1. When a tuner is placed near the transmitter the S11 return loss is -30db when tuned (as measured from the transmitter side).

    Test 1 - Xmit Power and Measured Signal Strength: My antenna on 30m is mostly in an NVIS orientation and I have a websdr about 300 miles away that receives my signal well. I have the tuner setup at 1:1. At 40w CW the sdr receiver measures a constant S5 signal, at 20w it measures S4.5, 10w - S4 and 5w - S3.5. This clearly matches with theory that 1/4 of the power is a drop in 6db or 1 S-unit. I performed this multiple times and it reflected theory properly.

    Test 2 - Direct xmit with 11:1 SWR: Removed the tuner and did the same test transmitting 40w directly connected to the end of the 50ft coax to the antenna with 11:1 swr. I confirmed the transmitter was still measuring the output wattage. The remote sdr measured drop was only a bit less than 1 S-unit across 40w, 20w, 10w and 5w or about 5db. So 40w measured at S4, 20w at S3.5 etc.

    So, what is really going on here? I'm only losing 5-6db of signal with 11:1 swr. My assumption is that this answer -> What is the actual loss in a feed line with high SWR? is what is happening (or power reflecting back and forth from the transmitter to the feedpoint). I used the KV5R line loss calculator and it suggests about a 2db loss on 10.11mhz with 50ft of RG-8X at 11:1 swr (https://kv5r.com/ham-radio/coax-loss-calculator/). No idea what the higher swr effect in the 49:1 unun is - but guessing at least another 1db.

    Am I understanding this observation of high swr loss correctly? I know my measurements are not using proper equipment, but the external sdr measurements are consistent after many observations.

19. How To Convert Morse Key Input To Audio

    I am thinking of purchasing a TYT TH-9800 Plus. Its only drawback is there is no input for a CW device.

    What Equipment would be required to convert CW output into audio input?

20. How to determine if filter is well matched?

    Hello i'm in process of creating band pass filters for my ham radio. I calculated the filter for 80m band basing on this website:

    And recreated it in reality:

    Then I started measuring it with nanoVNA, and tuning the trimmers to get best frequency response:

    As You can see, the frequency response is nearly as simulated, but I'm not sure about the impedance matching. Is this how the Smith chart should look like? Is this filter good or bad? I'm concerned about that, because the rosponse is not 50ohm in whole passband, but only on the sides of it. Also, is this How logmag of s11 should look like?

21. Ferrites losses calculation

    I am new on RF design. I would like to know more about ferrites. I am looking for information about how to calculate ferrite losses and saturation points for a given application. I also would like to know what a good starting kit of ferrites for HF applications is.

22. How do I test if three turns are better than 4 turns?

    I built a class ab amplifier kit to use with my qrp radio. 2 watts in creates 50+ out.
    This was the pa 70 amplifier kit.

    It is a kit with no instructions, but a whole suite of internet videos and articles.

    I built mine using an MRF 9120 transistor. There was a comment on a thread somewhere that said use 4 turns on the output transformer instead of three when using the 9120 chip. That will help yield full power.

    How can I test which is indeed best? I would think more turns changes the output impedance, but by how much. I am wondering how to best analyze the setup I have.

    

23. Transmitting high-speed PWM over RF

    First, let me apologize in advance if this isn't the correct exchange to post this question on.

    I have a 50 MHz PWM signal (it's actually a 50 MHz 50% DC clock with a 1PPS clock embedded with PWM) that I would like to transmit over the 2.4 GHz ISM band with as low latency as possible. My original thought was to digitize the clock with a high-speed ADC, then stream the samples to GNU Radio and transmit via SDR, then recover on the receive side and playback the samples with a high-speed DAC. The problem I'm running into is that the only capture cards I've been able to find capable of running at the sample rate needed (>500MS/s) and streaming in real-time to GRC are prohibitively expensive ($8000+).

    My question is, is anyone aware of any alternative methods of transmitting the 50 MHz PWM over ISM? I would be willing to entertain a different method of streaming to GRC or an entirely different setup. I don't know of any radios that could take the signal as input directly and transmit it, then receive it and output it directly, but admittedly I am new to the world of RF so it's possible there's a solution that exists that I'm simply unaware of.

24. What is the difference between antenna input impedance and its radiation resistance?

    I got to know that the input impedance of the halfwave dipole antenna is given as:

    Zin=1/Im* sin(β(H-|Z|))

    where Im is the maximum current on the antenna( when excited with a 1 volt supply),

    H is the half-length of the antenna,

    Z is the position on the antenna and β is wave number.

    Apart from the input impedance there is another parameter called as radiation resistance calculated by dividing the I2m from the power radiated by the antenna.

    Now my doubts are:

    1. Are both radiation resistance and input impedance one and the same?
    2. If I take a full wave dipole (β*H=π) then at Z=0 ( input terminal) the input impedance value becomes infinite which is similar to open circuit condition so does that mean there wouldn't be any radiation by the antenna?
    3. If I consider the current distribution of the antenna for the full wave dipole (as given in the figure, the current standing wave value becomes zero at the input end of the antenna how can there be a distribution of the current on the antenna if the input current is zero?

    

    I seem to have confused myself a lot. kindly clarify me on this doubt.Thank you in advance.

25. Motorola XT460 CPS cable pinout

    I want to connect a Motorola XT460 to PC. I also have the CPS software for it. Unfortunately, I do not have the cable; it seems impossible to order it.

    Accordingly to my research it based on a UART TTL controller.

    There is an option in CPS for the COM port and the station has 2 I/O contacts. Direct connection to TX and RX (in any combination) doesn't allow communications with the transceiver.

    Maybe someone has more information about CPS cable pin-out or even ideas how to program such devices.

26. Choice of PCB coil design for filter inductor at ca 20 to 400 MHz, 5 W

    Looking at the diplexer schematic from this nice question,

    

    I realized this can completely be done in PCB-formed elements (minus the connectors). (As a kind of counter-action to the original question's intent to get leaded capacitors, I'm not only trying to eliminate the leads, but also the discrete components.)

    • the 50Ω aren't actually resistors, they are the wave impedance of the cable / device you attach there
    • a 20mm × 20mm rectangular plate capacitor formed by patches on the top and bottom layer of a 0.8mm thick piece of FR-4 ($\varepsilon=4.7\varepsilon_0$) is about 20 pF
    • There's PCB coil designs that provice up to µH in inductance.

    Now, assuming I don't plan to work with as high power as the original question, but still substantial power (5W) in a relevant range of 20 MHz – 500 MHz:

    Would

    1. a planar coil antenna
       
    2. a coil formed through alternating between layers (so that the main magnetic flux is inside the substrate)

    be most effective?

    • I'm told (but I don't have access to the software to simulate that) that planars have low Q. I have no such information on the other choice.
    • Which design has lower self-capacitance?

27. WSJT-X V2: How to identify stations already logged?

    In V1.9 of WSJT-X, there was a feature that allowed one to change the color of stations already logged. This was necessary in knowing whether or not to respond to a given CQ.

    In version 2, I am unable to determine whether or not I have already logged a potential contact that is calling CQ.

    Is there a way to have WSJT-X V2 signify whether or not one has logged a given call sign?

28. Exactly why do some SWR meters give a changing reading depending on the length of coax used to connect to an antenna?

    Following is my understanding of some theory and the questions are at the end.

    Is this right ?...

    A standing wave is an AC waveform oscillating at a particular frequency which has a varying amplitude as time progresses (only in a sinusoidal fashion if the AC source is emitting a sine wave, which in our case it hardly ever is), but has a fixed amplitude profile in space (or length) along say in this case the inside of some coax, and is the interference pattern created when two AC wave forms traveling in opposite directions cross paths (and the superposition theorem explains this).

    And remembering that the AC wave forms in question are in reality just the movement of electrons (which is current which is coulombs per sec) in sympathy with the potential difference of the output of the AC source, which hardly ever is a sine wave, away from the antenna during the periods of source output which have a positive voltage with respect to the AC ground, towards the antenna during the negative periods, the whole thing moving in space away from the source at the speed of light x the velocity factor.

    At any one particular point along a transmission line,

    VSWR is the the ratio (one divided by the other) of two things :

    1. The potential difference between the maximum value in volts of an existing oscillating standing wave on the outside of the inner conductor and the inside of the coax shield at that point.

    2. The potential difference between the minimum value in volts of an existing oscillating standing wave on the outside of the inner conductor and the inside of the coax shield at that point.

    I've noticed that there are a few different kinds of SWR meters, some use a Bridge and some use a Directional Coupler. The different kinds probably have advantages and disadvantages.

    However i want to point out that no SWR meters measure the Standing Wave Ratio directly right ? They either measure the ratio of peak voltage for two wave forms traveling in opposite directions (directional coupler) or measure the apparent change in impedance inflicted upon the source by the presence of the standing waves (bridge)... is that right ? So they indirectly measure SWR and even more indirectly measure the impedance match of the coax to the antenna. That's like a 3rd hand me downed measurement.

    Also it's probably worth noting here that the VSWR on a transmission line, excluding I2R losses, is the same at every point along the transmission line.

    If there is an error current flowing on the outside of the coax due to incorrect balancing of the coax to the antenna because the AC current flowing on the inside of the shield splits up between the antenna and the outside of the shield because the impedance that the outside of the coax presents to the AC signal is not negligible compared to the antenna impedance, then apparently according to various sources such as the ARRL Antenna Handbook this can affect a VSWR measurement because it results in a "Common Mode Impedance" which affects the measurement made by the SWR meter.

    Can someone tell me EXACTLY how the presence of current on the outside of the coax changes the VSWR reading ?

    Or, what are the other causes if any of a SWR meter changing it's reading as the length of coax is changed ?

    Oh and btw the answer i'm looking for is not : SWR meters are designed to work with a specific feed line impedance and if that impedance isn't the correct value the SWR meter doesn't measure correctly.

29. Detecting signals using statistics

    I have sampled a section of spectrum every few Hz. I have mean, standard deviation, and range data for every frequency sampled (and I can gather more statistics if necessary). I've sampled for several seconds before storing the spectrum statistics.

    I would like to be able to guess where man-made signals (i.e. channels) exist in this spectrum (as opposed to noise floor). This is proving to be a difficult task, especially since the noise-floor power sometimes changes (different antenna inputs for different bands, for example.)

    I doubt anyone has a ready solution for this, but what are some starting points, or tips, to help me begin to identify channels?

    In the below image 'normal' is really autocorrelation from Pandas

30. Why isn't twisted pair used for feedlines?

    My son was asking about some Ethernet cable (Cat 5 unshielded twisted pair), and since he recently passed his Technician exam and we both have "radio stuff" on our minds, I tried to make the analogy between coax transmission lines and the balanced pairs in the wire. I understand they both are used as RF transmission lines, essentially.

    This got me thinking: why don't we use twisted pair for antenna feedlines?

    As question T9B03 on the FCC exam covers:

    Why is coaxial cable used more often than any other feedline for amateur radio antenna systems?

    The correct answer being "It is easy to use and requires few special installation considerations" and not that "It has less loss than any other type of feedline" — the latter advantage going to balanced line, which is low-loss but does require special considerations particularly to avoid coupling to nearby metallic objects.

    Would twisted pair offer something of best of both worlds? Could it provide lower loss than coax, and still without "special installation considerations"? (Looks like Category 5 cable specifically has a 100ohm characteristic impedance, but that seems easily enough dealt with since you'd need a unbalanced-to-balanced transformer off of most ham equipment anyway, right?)