Comment on My try for an apocalypse radio

• fullsquare@awful.systems ⁨4⁩ ⁨days⁩ ago

  that’s good, we’re not starting from zero then. take note that some solutions that work for 27mhz band don’t work on vhf/uhf, mostly balun and impedance transformer construction. HF baluns use multiple turns of coax or twisted pair through ferrite, on VHF and up it’s better to use single turns (ferrite beads) or transmission line baluns (folded balun or sleeve balun). it’s also more practical to use some form of transmission line impedance transformers (like quarterwave matching). on vhf and up lumped element tuners are not really a thing, antennas are much smaller so they’re just made to match

  The particular antenna I was asking about is something I couldn’t DIY

  i’m not familiar, if you could describe it in some more detail that can be probably figured out

  for 70 cm, i’ve suggested jpole because it’s compact (long and thin), you can probably package it in 30mm or smaller plastic pipe, haul it around, pull out and mount it with a clamp on top of (not parallel to) metal mast and be good to go. you can have metal mast couple cm below shorting bar of jpole, or even a bit away and parallel to for a short run and it shouldn’t matter. if needed, radiating part can be mounted parallel to metal mast but needs to be a good distance away (halfwave away or more unless you want to tune it in place). with nonconductive mast the only conductor nearby is coax. it would be 50cm-ish long so good for mobile setup. these are fast and cheap, the way i’ve made them the most expensive part was plastic box, can be made and tuned with nanovna in a single evening

  groundplane antennas would be even shorter but much bulkier, unless you detach radials or mount them on a kind of swivel mount, and you have to put them in the air, as in, bottom end of radials can’t be too close to ground. mast considerations are pretty much as jpole, as in, the hub can be mounted on top of metal mast but not parallel and right next to it. if folded, 2m version would be about 50cm long, so it’ll probably make sense on 2m and down. you can probably make a 70cm band quarterwave whip attached to magnet so that you can put it on roof of your car, but it won’t be as good because height is might here, and jpole has center of radiation half wave above mounting point just because of the way it’s built. just that little bit of extra height might clear some obstructions. straight dipoles should also work, but they’re T-shaped unless disassembled, and you’ll have to run coax perpendicular to dipole for some length to avoid common mode current problems -> not as simple mechanically, fine for horizontal not so much for vertical

  you could also make a small (5-7 el? depends on how long do you want it) 70cm band yagi and store it on top of your radio box, but you’ll need to put it on a mast then rotate it, or hold it in your hand, preferably standing on a hill or something when in use. stiff jpole would be probably fine for 70cm, 2m if you can accommodate the length, lower than that, 2m to probably down to 6m or 10m, rollup jpoles should work okay as long as you have a way to deploy them. below that it’s probably mast + some kind of halfwave dipole territory (efhw? doublet? series of center fed dipoles? ocfd? some type of vertical? you decide)

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  • tasankovasara@sopuli.xyz ⁨4⁩ ⁨days⁩ ago

    You got me thinking about a roll-up yagi: elements laminated on something that can be rolled up lengthwise and flown like a flag, but with channels at the top and bottom edge that take something to stiffen the ‘flag’ :)

    Will definitely also revisit the J-pole design. Come to think of it, so far the only store-bought antennas I have are on the car. Perhaps a little bit of ethos should be applied here – only DIY antennas for HAM activities. As for the radio, I’m going to try to never charge the battery on anything but the solar panel, so that all of my operating will be independently powered :D Right now it’s on a charger though, to get it to full capacity before the package is finished.

    Thank you for the inspiration!

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    • fullsquare@awful.systems ⁨4⁩ ⁨days⁩ ago

      i’ve seen someone made rollup j-pole on basis of fabric strip, with conductive fabric strips sewn on. so you could probably take an actual flag and do the same, because yagi elements need some thickness and flat strips are good enough. this looks a lot like yagi printed on pcb board, except that pcb board is stiff, and you’d need some rods to stretch the fabric. so there already is need to store long sticks. instead, you can make a regular yagi but with elements that can be detached when not in use. i bet somebody made 3d-printable clamps like that

      there are commercial antennas available, but you will pay an arm and leg

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