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Messages - RobRich

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136
Equipment / Re: Sky loop...height vs diameter
« on: February 24, 2024, 0456 UTC »
Similar results with my current 148' LoG that has an 1:1 balun at the feedpoint. I could imagine low LF and especially VLF radiation resistance to be down into the low milliohms.

Interestingly my old 148' shielded LoG actually worked somehwat decently on longwave, though it was not using any balun or transformer at the feedpoint, so it might even have been common-mode on the feedline contributing to the low-frequency reception.

137
It seems to be a decent signal in recent weeks. Flex and I have logged it over on the SWBC section as well.

https://www.hfunderground.com/board/index.php/topic,125747.0.html
https://www.hfunderground.com/board/index.php/topic,125545.0.html

138
Received mine, but admittedly, it is still in the box. I might get around to initial testing over the weekend.

139
Equipment / Re: Sky loop...height vs diameter
« on: February 22, 2024, 0458 UTC »
Assuming a 300' loop, at low frequencies up to ~3-4MHz it will likely model as more of a blob shape with more "gain" (actually less loss) straight up than towards the horizon. It should be a decent NVIS antenna.

Around ~6-7MHz is going to shift the pattern more towards the horizon than straight up. Not exactly a big deal IMO, as NVIS tends to roll off above 7MHz, if even that most days, anyway.

As you go higher in frequencies the pattern towards starts breaking into multiple lobes, with the resulting gain and loses depending upon the distribution of those lobes.

Losses will increase as the loop approaches ground, especially as frequency drops. Works pretty much that way for most any horizontal antenna, be it a dipole, loop, etc. That said a low loop will likely have ground losses across pretty much the whole HF spectrum, thus the previous mention of "gain" in quotes.

Actually, lots of us have receiving loops placed directly on the ground. ;) Losses can be offset with a preamp if truly needed, though a preamp is often not a requirement at low frequencies, especially with large loop sizes. My 148' LoG typically suffices without a preamp from MW up to around mid-HF.

Summary? A large loop even at relatively low heights should suffice as a decent receiving antenna where losses at higher frequencies can be offset with a preamp if needed. Conversely, it not so much as a great transmitting antenna, though lots of people routinely work lots of contacts with horizontal antennas at low heights. YMMV.



Now as compared to an 80m dipole at 35'+ height? Ground losses still will be present at low frequencies, but that issue should improve around ~7MHz.

The dipole is still low enough to more favor NVIS at lower-HF frequencies. As frequency increases above ~7MHz the dipole should start having more gain at angles more towards the horizon than straight up.

As to pattern the dipole is going to become more bidirectional as frequencies increase, though opting for an inverted V deployment will somewhat improve an omnidirectional pattern at the sacrifice of some broadside directivity.

The dipole will start breaking into more lobes as frequency increases, too.



Personal thoughts? Given the feedpoint height differences present, I would likely favor the dipole with a feedpoint at 35'+..... though perhaps a different type of dipole with symmetrical balandce. An 90'+ doublet center fed at 35'+ as an inverted V should make for a decent HF antenna, especially from 80m to mid-HF. Coaxial feedline would suffice for receiving; just ignore the mismatches. If transmitting is desired, feeding a doublet with ladder line and using a tuner would be the (much!) better approach.

https://ftp.unpad.ac.id/orari/library/library-sw-hw/amateur-radio/ant/docs/Introducing%20the%20All-Band%20Doublet.htm

140
Equipment / Re: LoG receive antenna, Loop on Ground!
« on: February 20, 2024, 1132 UTC »
It is a magnetic loop design. Note the "Let's look at the ideal coupling method:" here:

https://www.w8ji.com/magnetic_receiving_loops.htm

An often-cited claim for a shielded loop is the design somehow blocking electrical fields to optimize reception of magnetic fields. Outside of maybe some slight near-field effects, a shielded loop does not really shield much of anything.

Instead the shielded design is more or less to help balance loop symmetry to potentially equalize differential currents and limit common-mode currents. Therein is why a "shielded" loop might improve reception if CMC is present on the feedline. If not, then there is not much, if any difference versus just a simple wire loop.

These days, I might would make some changes. I probably would use a mix 73 binocular ferrite with a 1:1 winding ratio at the feedpoint. Using 4 turns towards the loop and 4 turns towards the feedline should suffice in my experience. The result is an isolation transformer. You probably could just float the loop shield in such a config.



BTW, another often-cited LoG design consideration is software modelling might suggest a couple or so dB in directionality if feeding the loop at a corner instead of a midpoint along a side. Being realistic I kind of doubt any noticeable real-world difference.



Another popular alternative for a coaxial loop would be the NCPL design, which is basically a mobius loop. It might help a little with low-frequency performance for a typical-sized loop-on-ground antenna.

https://swling.com/blog/2020/04/diy-how-to-build-a-noise-cancelling-passive-loop-ncpl-antenna/
https://www.hamradio.me/antennas/airspy-youloop-lf-mf-hf-mobius-receive-antenna.html



All the above said, unless you have the coax available for experimenting, I would suggest first trying a simple wire loop-on-ground. 60' to 150' of whatever wire should suffice for MW to mid-HF. You might need a preamp towards upper-HF. Use an 1:1 balun or an isolation transformer at the feepoint, plus perhaps similar back near the receiver.

Case in point, my old 148' "shielded" LoG ended up buried under leaves, pine needles, and shifting soil/sand. A typical LoG is already well into negative dBs, and going underground can make the losses even worse. Mine was old enough to not bother with it anymore, so I eventually just deployed another LoG at a different spot. My current LoG is 148' of copper pet fence boundary wire with a LDG 1:1 balun at the feedpoint and a mix 73 isolation transformer near the receiver. The simple wire LoG seems to suffice for my purposes at the moment. YMMV of course.

141
Music at 0452z. SIO 433 trending 422. Airspy HF+D and 31' vertical. DSB sync tuning. Decent carrier at times, but overall it is trending more towards a marginal copy at the moment.

Thanks for the broadcast! :)

142
FYI....

Quote
When you receive your KiwiSDR 2 DO NOT, I repeat DO NOT, do any sort of Debian upgrade from the delivered Debian 11.8. Otherwise you may brick your Kiwi server (Debian should continue to run).

The Beagle guys have been at it again and have changed the way the SPI interface works. I need to understand the change and compensate for it in a later Kiwi release.

Thank you.

Source: https://forum.kiwisdr.com/index.php?p=/discussion/comment/17778/#Comment_17778

143
Shortwave Broadcast / BBC 25900 AM 1227 UTC 13 FEB 2024
« on: February 13, 2024, 1231 UTC »
French-language talk into music at 1227z. Off air at 1230z. SINPO 55555. Peaking ~S9+10 via Airspy HF+D and 31' vertical. Armchair copy this morning.

144
Order #60 here. I still need to test the linear power supply that I ordered awhile back, plus figure out what I am going to do about an antenna.

145
The first round of pre-ordered KiwiSDR 2 units have started shipping.

https://forum.kiwisdr.com/index.php?p=/discussion/3098/kiwisdr-2-shipping-status

146
North American Shortwave Pirate / UNID 6935 USB 2351 UTC 11 FEB 2024
« on: February 11, 2024, 2352 UTC »
2351z - "Welcome to the mental healthline...." into noise effects. SIO 433. Airspy HF+D and 148' loop-on-ground.
2323z - Off air

148
Ray, I suppose that might have been IR on your side of the big pond.

At my QTH the UNID station on 6935u is picking up considerably at 2319z.

149
Music at 2314z. OM at 2316z. SINPO 54243. Peaking over S9+10. Airspy HF+D and 31' vertical. USB ECSS tuning. Peskies on LSB.

Strong carrier, though strong to intense static crashing is making for a rough copy at the moment.

Thanks for the broadcast! :)


150
There is something under Radio Time Machine at 2250z. RTM has the stronger signal and modulation, while the other has a more prominent bass line. Airspy HF+D and 31' vertical.

RTM dropped down during the RTM ID at 2258z and again (RTM off air?) at 2301z, while the station with the strong bass was still there.

Looks like Ray might have it as well:

https://www.hfunderground.com/board/index.php/topic,125489.msg387942.html#msg387942



2303z - Music. Electronic with bass. SIO 433. Considerable to intense strong static crashing at the moment.
2304z - Modulation picked up. Good modulation with strong bass.
2320z - Ongoing music with bass.
2323z - Signal peaking S9+ at times.
2328z - CW noted. No decode, as I am horrid at Morse code.
2348z - Big signal drop. Down into the noise floor at times.
2351z - Another UNID noted. Switched to 148' LoG.
2355z - Signal picking back up somewhat.
0012z - SSTV then off air. Only managed a partial decode here. Anyone else? (thanks myteaquinn)

Thanks for the broadcast! :)

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