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I am setting up a HAM antenna (50 ohm) on my house terrace. The cable length is going to be around 25 meters from terrace to my transmitter. Which type of co-axial RF cable should I choose with low signal attenuation (practical) in mind? For example, RG-6, RG-8, LMR types...

Frequencies - 145MHz and 433MHz. Maximum power that I would transmit is 500W. Budget is not a problem but please do not suggest superconductor based solutions ;)

PS: I am just asking for the type of cable to be used in general.

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    $\begingroup$ Hi enemra! Under your closed identical question on electronics.SE I made recommendations on how to avoid this being closed again due to it being a hardware recommendation question. You did ask for a type now, that's better, but it was already how the question was interpreted in the first place. You also delected crucial information (500 W), and didn't add the required information you've been specifically asked to produce (impedance). As a friendly reminder, the verbatim message you got from me was: $\endgroup$ Commented Oct 31, 2023 at 20:15
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    $\begingroup$ Hi enemra! This question is a product recommendation question, and these are off-topic on here. Also, your question is "impossible" in the sense that "lowest attenuation" would be achieved with a cryogenically cooled superconductor vacuum waveguide; not something you can buy, unless you live in a sci-fi novel or are extremely rich :) but even on the side of things you can buy today, there's always better but more expensive material. Your question should probably be rephrased to "what are the options for 500W transport over 25m at 145 MHz and 433 MHz, and what are the factors that should … $\endgroup$ Commented Oct 31, 2023 at 20:15
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    $\begingroup$ … affect my decision? What are the operational trade-offs?". Also, it'd probably be a good idea to ask that more differentiated question over at our sister site, ham.stackexchange.com, not here. (Note that they have the same rules where you cannot ask for specific recommendations, but they will love to explain the trade-offs and how to make that choice yourself.) It's always a good idea to be specific about what you want to connect – for example, knowing the characteristic impedances of your radio and your antenna would be concern #1! $\endgroup$ Commented Oct 31, 2023 at 20:15
  • $\begingroup$ Product recommendation questions are off-topic here; perhaps revise the question to explain what parameters you're taking into account when comparing coax, and ask what people would recommend you look at. $\endgroup$ Commented Nov 2, 2023 at 15:47

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Your original question isn't really answerable, so I'll answer a different question:

What factors should impact my transmission line selection for VHF/UHF use at 500W

In no particular order:

  • What is acceptable loss: For ex, if 40% loss at UHF isn't acceptable, you may want to eschew transmission line in favor of an auxiliary link: perhaps VoIP over wifi/ethernet or even an amateur radio low-power auxiliary link.
  • Does the radio need to be in your shack, or can it be moved to the attic and operated remotely? Moving the radio to shorten the transmission line may open up a broader selection of media to you.
  • What type of 500W matching devices and RF chokes you have available if using coax transmission line.
  • What type of RF Exposure Survey concerns exist at VHF/UHF at 500W and how do various transmission line media contribute to Survey results?
  • What kind of regular access and/or maintenance ability will you have once installed? How critical is it to get it bulletproof-right the first time?
  • At such high power, surely this isn't to feed a 1/4 λ vertical. What is the geometry of your antenna system and will it be fixed, rotatable, or otherwise remotely adjustable? Transmission line media or mechanisms may need to accommodate motion.
  • Impedance matching devices will need to be appropriately rated for your application and power.
  • etc etc etc

ISM limits are 1 W, so what's w 433 MHz?

The general character of your question coupled with your call-out of 433 MHz make me think you may not yet be licensed... and that's ok! Always good to ask questions. Obviously, no transmitting at these frequencies and power are permissible in that case.

Even if you are licensed, please be sure to work with someone who is experienced and knowledgeable before attempting your installation: high power at this frequency can cause damage to members of your household and/or neighbors.

Hope this is helpful!

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Simple, you use whatever size coax it takes to keep insertion loss to 2 dB or less at the highest operating frequency. You have a 25-meter-long coax operating at 433 MHz. Hit the coax spec tables and find a coax's type (size) to give you 2 dB or less insertion loss.

The last step is to check the coax's power rating and ensure it is rated equal to or larger than the amount of power you intend to operate at. This is not much of a real issue unless you have very short coax runs. For example, 5 feet of RG-58 meets the insertion loss at 433 MHz but would overheat with 500 watts.

Do that, and you will find LMR-400 (.405") barely fits the ticket, and LDF4-50A (.5") is a better fit. Let me know how it work$ out.

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