I am very new to everything connected to radios and am trying to build a antenna like in this guide. They use Coax cable as an antenna. I have noticed that Coax cable is often relatively expensive (as it is quite complex for being just a cable), and therefore wonder if it would matter if you buy a 10$ one from a European shop, or cheap one off eBay (like this one). I know it might be a very basic question, however I would like to be sure, as the antenna is a vital part of the project.
Of course the quality matters, but you would need to perform careful field tests to tell the difference, if any.
Ok, of course thats true, but what I meant was more: Is the "quality" expensive europan cable made in the same factory as the cheap chinese one?
Seems a shame to buy a coax length with connectors on both ends, only to cut the cable. Also consider the coax from China may take a month to get to you.
Cheaper coax usually has fewer wires in the woven shield, even though both may have the same "RG" designation. For the length of coax needed for the antenna, doesn't make much difference.
Paul
bestanamnetnogonsin:
Is the "quality" expensive European cable made in the same factory as the cheap Chinese one?
It is quite possible , hopefully a European vendor will provide the tested performance of the cable though the Chinese vendor may not be able to do this though and may supply an inferior product direct.
You definitely want proper coax that is to the relevant spec (characteristic impedance, losses, velocity
factor). There are various specs, like RG58, designated for RF use, and you need to understand the
difference between 50 ohm and 75 ohm cable and use the correct impedance. The thing to absolutely
avoid is shielded cable not designed for RF.
Coax cable is sold per unit length normally and you solder or crimp on the relevant connector.
For SMA connectors buying a pre-made cable is a good idea, they are small and fiddly to solder/crimp
reliably yourself.
In my experience the chinese SMA cables I've bought are OK (a few short jumper cables only, but
look pretty flat on my spectrum analyzer to 1.5GHz).
For higher frequencies and longer cables the loss per unit length gets to be important.
BTW that guide gives the length for 458MHz ISM band, not 433MHz as claimed. Use 173mm for
433MHz
For bench use, pre-assembled cable assemblies bought from eBay (China) are cheap and good enough, just. The crimping tends to be poor however and eventually the connectors fall apart.
For anything that needs to be reliable, whether it be SMA, NType or BNC, I would crimp them myself.
As has been mentioned for short lengths, the co-ax with less copper in the screen is probably OK, but the co-ax with a high copper content does crimp more securly.
The picture is the antenna arrangement from a small satellite. Forced to use high quality PTFE co-ax as PVC is no good in space. The antenna is a steel tape rule dipole on 437Mhz, note the toroid as a choke balun by the antenna.
For higher frequencies - up to ~10GHz - semi-rigid eg RG402 or RG405 is good.
Much above that waveguide is used.
Allan
srnet:
Forced to use high quality PTFE co-ax as PVC is no good in space.
Interesting - I know PVC is never used as the core insulation in coax as its moisture-absorbing and lossy,
but is PVC no good for space due to temperature range or something?
MarkT:
Interesting - I know PVC is never used as the core insulation in coax as its moisture-absorbing and lossy,
but is PVC no good for space due to temperature range or something?
It outgasses and goes brittle in a vacuum.
So in the end I think this one should work right? Its an RG174 so I guess the diameter is the same as in the guide and it should work with 433 MHZ... Am I correct in assuming that?
Yes, maximum frequency is listed as 1GHz, and it seems to be a cable with copper-coated steel inner conductors
so should be pretty robust. At RF frequencies only the outer layer of the conductor carries current so you don't lose out by having steel core.
bestanamnetnogonsin:
I know it might be a very basic question, however I would like to be sure, as the antenna is a vital part of the project.
If it is a vital part of an important project.... then quality can matter. But it depends on how we define quality. Like.... materials used in the cable... manufacturing tolerances.... degradation rate etc. Testing should help to give details of the performance over a long time.
What I do wonder is, how rigid are these cables? I kind of doubt that they are self supporting and the straws the guys from the guide do not look very reassuring (considering the extreme environment they are meant for)
RG402 is pretty rigid - 1.14 inches od, solid copper outer, ptfe insulation, silver plated steel inner.
Good stuff - but not cheap.
Allan