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| guides:rf_cables [2016/04/01 15:08] – daveake | guides:rf_cables [2016/04/01 15:15] (current) – [Cables For HAB] daveake |
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| Loss is proportional to the cable length, so long cables should be avoided if possible. As an example, a 20 metre length of RG58 cable will attenuate 434MHz by 13dB, which is to say that the signal coming out is at about 1/20th of the level of what went in. In other words, not good. | Loss is proportional to the cable length, so long cables should be avoided if possible. As an example, a 20 metre length of RG58 cable will attenuate 434MHz by 13dB, which is to say that the signal coming out is at about 1/20th of the level of what went in. In other words, not good. |
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| The cable type also makes a big difference. In general, thicker cables have lower losses though there are other factors. As an example, RG213 cable which is 10.8mm diameter (vs 5mm for RG58) has a loss of just 3.2dB for a length of 20 metres at 434MHz. 3dB is a perfectly acceptable loss but 13dB isn't. | The cable type also makes a big difference. In general, thicker cables have lower losses though there are other factors. As an example, RG213 cable which is 10.3mm diameter (vs 5mm for RG58) has a loss of just 3.2dB for a length of 20 metres at 434MHz. 3dB is a perfectly acceptable loss but 13dB isn't. |
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| To help decide which cable to use, use a [[http://www.qsl.net/co8tw/Coax_Calculator.htm|Cable Loss Calculator]]. | To help decide which cable to use, use a [[http://www.qsl.net/co8tw/Coax_Calculator.htm|Cable Loss Calculator]]. |
| * RG-8X. Not as low loss (2.6dB per 10m at 434MHz) as RG213, but thinner (6.1mm) and more flexible. Cost is about £1/metre. | * RG-8X. Not as low loss (2.6dB per 10m at 434MHz) as RG213, but thinner (6.1mm) and more flexible. Cost is about £1/metre. |
| * LMR-400. Even lower loss (0.86dB per 10m at 434MHz) than RG-213, 10.3mm diameter, cost about £2/metre and is stiffer than RG-213. | * LMR-400. Even lower loss (0.86dB per 10m at 434MHz) than RG-213, 10.3mm diameter, cost about £2/metre and is stiffer than RG-213. |
| * LMR-240. Loss (1.7dB per 10m at 434MHz) is very similar to RG213, but the cable is thinner (6.1mm) and more flexible. Cost is about £1.40/metre. | * LMR-240. Low loss (1.7dB per 10m at 434MHz) which is very similar to RG213, but the cable is thinner (6.1mm) and more flexible. Cost is about £1.40/metre. |
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| As well as the resulting cable loss, also consider the cost of the installation, and how easy it is to run the cable - some cable types can be very stiff. Also, if you are using an aerial rotator, you will need some flexibility in the cable. | As well as the resulting cable loss, also consider the cost of the installation, and how easy it is to run the cable - some cable types can be very stiff. Also, if you are using an aerial rotator, you will need some flexibility in the cable. |
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| Something else to consider is the option of adding a pre-amplifier to your ground station. This limits use to receiving only, but can allow you to use fairly lossy (and therefore cheaper and easier to run) cable, and even then have an overall gain rather than loss. The amplifier should be fairly close to the aerial, to reduce the effect of any noise it generates, but should still be accessible in case you need to replace it. A good compromise might be to have the amplifier in the loft, with the aerial outside. | Something else to consider is the option of adding a pre-amplifier to your ground station. This limits use to receiving only, but can allow you to use fairly lossy (and therefore cheaper and easier to run) cable, and even then have an overall gain rather than loss. The amplifier should be fairly close to the aerial, to reduce the effect of any noise it generates, but should still be accessible in case you need to replace it. A good compromise might be to have the amplifier in the loft, with the aerial outside. |
