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So I've been following this guide on how to build an antenna for a High altitude balloon: link

In the guide they say to use RG174 wire, which is why I bough some cheap RG174 wire off Ebay. I guessed that not even the Chinese could mess something as simple as wire up. However the wire I got looks a bit different to what I expected. I suspect it might not be RG174 wire but some other Coax wire. The inner wire is very thin (with a diameter of maybe 0.4 mm or so). Is that ok? Does the thickness of the wire used affect the antenna?

I suppose it isn't that important that the wire is RG174 as long as it is something similar? So what do you think? Is this wire ok to use?

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    $\begingroup$ Not sure if it is OK to use for your application but I consulted one of the many tables of Coax parameters available on-line and the inner conductor diameter is listed as 0.41 mm. I also noted that the loss looks to be fairly high around 8 dB/100 feet at 100 MHz. This loss figure though may or may not matter per your application but it is something to note. But, this coax is also much smaller than the typical coax used in most ham radio applications so that may be the driving factor for your needs. $\endgroup$ – K7PEH May 21 '17 at 15:51
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    $\begingroup$ What's the outer diameter, and what's the diameter of the inner conductor (or the width of the dielectric)? RG-174/U is a standard, so if these dimensions do not match that standard, you can right out prove that it's not RG-174/U. And maybe get a (partial) refund; I don't like people selling stuff as something that it's not, and you should, imho, not contribute to that business model if you have something like ebay as a system for giving direct feedback. $\endgroup$ – Marcus Müller May 22 '17 at 8:39
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They are probably suggesting the use of RG-174 because it is very thin, and for its size it is fairly strong and relatively inexpensive. If you can imagine doing the same kind of antenna using RG-8 or RG-58 you would not get much of it up in the air, even though it would be lower loss than RG-174.

For short runs at HF, it's fine - sure it's not the highest-performance coax money can buy, but it is light and relatively inexpensive.

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In antenna build part it does not matter, as it is not used as coax cable but as wire material for radiating and ground elements.

For cable part, as long as you keep it short (few meters) it also does not matter.

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  • $\begingroup$ no, you can't simply use any coax. Different coax has different wave impedance, and for an antenna feed, that makes a heck of a difference. $\endgroup$ – Marcus Müller May 22 '17 at 23:26
  • $\begingroup$ You beat me to it. I was ready to say that cheap coax makes a great antenna. More broadband than tiny little wire, for a dipole. But cheap coax really does not work well as a feedline of any length. Upvote, @Pedja and this should be the accepted answer because it answers the OP quite specifically. $\endgroup$ – SDsolar May 27 '17 at 0:46
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So, two conflicting aspects to this:

  1. yeah, as said in the other answers, the choice of coax here has probably been made more for mechanical than for RF reasons,
  2. No, that won't work as nicely. Antennas can be understood as matching circuits between transmission line impedance (typically either 50 Ω or 75 Ω) and free space impedance (120π Ω).

Thus, when dimensioning an antenna, you'll make sure that the wave impedance of whatever is used to feed the antenna (and that is, in the case of this particular antenna, exactly the coax you've bought) is actually the same impedance as the rest of your system.

Now, if someone sells you some cable that's even cheaper than RG-174, chances are it's not 50 Ω. So, you get a mismatch at where you connect the antenna to your system. Yay. So, you build a nice antenna, and then throw away power for reflections.

So, make sure you actually didn't get what you paid for, and if you didn't, get your money back, and get the right cable. You're the weaker part in this business; don't make yourself more of a victim as necessary. Someone else will see "oh, this guy sold this 10000 times, must be good", and so you accepting non-conforming goods doesn't only hurt you – but also the ham community overall.

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