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| guides:rf_cables [2016/04/01 15:05] – created daveake | guides:rf_cables [2016/04/01 15:15] (current) – [Cables For HAB] daveake |
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| === Impedance === | === Impedance === |
| Each type of cable has a [[https://en.wikipedia.org/wiki/Characteristic_impedance|Characteristic Impedance]], and this is almost always 50 or 75 ohms. Ideally we want transmitters, receivers, aerials, cables and connectors to all have the same characteristic impedance, and the standard for that is 50 ohms | Each type of cable has a [[https://en.wikipedia.org/wiki/Characteristic_impedance|Characteristic Impedance]], and this is almost always 50 or 75 ohms. Ideally we want transmitters, receivers, aerials, cables and connectors to all have the same characteristic impedance, and the standard for that is 50 ohms. |
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| Don't even think about using TV coax as (a) it's 75 ohms, and (b) it's usually of very poor quality anyway. | Don't even think about using TV coax as (a) it's 75 ohms, and (b) it's usually of very poor quality anyway. |
| Any cable will attenuate the signal passing through it, so what comes out it always less than what went in. This loss can be considerable with long lengths of cable, which is a shame as when receiving you want to get the aerial as high as reasonably possible, possibly resulting in a long cable down to wherever your receiver is sited. | Any cable will attenuate the signal passing through it, so what comes out it always less than what went in. This loss can be considerable with long lengths of cable, which is a shame as when receiving you want to get the aerial as high as reasonably possible, possibly resulting in a long cable down to wherever your receiver is sited. |
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| Attenuation is a factor of: | Attenuation is a function of: |
| * Cable length (double the length and you get double the loss) | * Cable length (double the length and you get double the loss) |
| * Cable type - some are much better than others | * Cable type - some are much better than others |
| * The frequency of the radio transmissions that you are interested in | * The frequency of the radio transmissions that you are interested in |
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| Loss if 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. |
