Posted in: New Hams Corner

New Hams Corner: Wire/cable selection and use

Wire/cable selection and use

Too many new hams have no electrical background and no one to turn to with questions. One problem that can be avoided is easy if you understand a basic principal…bigger is better! In most cases the larger the wire or cable the more current or RF power it will handle.

Let’s start with DC cables. Most come with a red and black wire. The red is plus and black minus. When hooking up to a power supply with terminals match the colors and you are good. When connecting in a vehicle the center pin is positive and the shell is negative (-). This is pretty much standard in the US. If you go abroad anywhere could be different colors And might be the opposite polarity. This is where it is nice to have a small VOM to measure The voltage before connecting anything!

I hope that most people realize that AC voltages can kill you. Before you open up your Power supply or other equipment that plugs into a wall outlet make sure you pull the plug and disconnect from other equipment. Remember that capacitors will hold a charge long after unplugged so if it is a large one you might be wise to short it before working. PLEASE if you don’t know what you are doing, work with someone who does! AC cords tend to be big enough for the rating of anything that they are attached to. Check the tag to see what current rating you have and make sure the cable is large enough.
Most are 14 or 12 gauge so there is plenty of capacity.

Now the tricky stuff, RF cables. There are quite a few common cables that hams will use. RG-8, RG-58, RG-8X, RG-213 and RG-214. Use one of the charts to determine how much loss per hundred feet of cable. Then you can make your selection. Most magnetic mounts come with RG-58 cable and the length is usually under 20 feet. The loss is not going to really hurt you for that short a run. Now when you are looking at running cable from a basement to rooftop you might have 150’ so you need to account for lost signal.

A quick way to see if you are losing too much is to add the losses and gains to see where you will end up with respect to RF power. Transmitter output is 20 watts @ 145 MHz you have a Hundred feet cable of RG8X (loss 4.5 db/100). Your antenna is very good and you have a gain of 4.5 dB @ 145 MHz In theory you will have about 20 watts Effective Radiated Power or ERP. This would be a unity gain system…not bad, not great. By playing with cable alone you can Increase this number but the cost and cable size might not make it worth it to you. If you are running up a tower you also want shielding from other radios using the tower. So that could be a defining factor. As always, lots of variables and many choices!


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