Wednesday, March 6, 2013

The Quest for Cable

As I've mentioned a few times, I'm looking to put antennas in my attic or otherwise in a high location. In order to really do that, I'm going to have to find some good quality cable. Cable is perhaps the unsung hero of amateur radio. You can have the worlds best antenna, and 1500 W out of your transmitter, but if your cable isn't up to snuff, then you can lose a significant portion of your power before it even hits the antenna.

The first thing I did in my quest to find the right cable was to look for guides to cable, as I really have no idea what I want exactly. I found one from pcs-electronicsham universe, and from the wireman. These guides all include spec sheets for the various types of cables that are in use, including signal loss, relative velocity, impedance, and some give some hints for the maximum power.

The bottom line is, you want to buy the best cable that you can afford, in general. You want to get the length right, with perhaps a few feet of wiggle room in case something happens. Ideally, you want to use only a single length of wire the entire distance, with exactly the right connectors for your needs.

I recommend the following rules of thumb. Later I will explain my recommendations.
  • Keep the power loss to no more than 3 db in the cable. 
  • If you can, use the same impedance as your antenna/transmitter, but always keep the impedance to within a factor of 1.5 of your antenna/transmitter.
  •  If you can, use only a single strand of cable. Also, use the right connectors for both ends.
  • Buy for the largest frequency that you think you will be using.
  • Get a cable that is slightly larger than you need, but not excessively larger.
  • The velocity factor only matters if you are trying to build a phase changing antenna.
  • There are devices where you can switch which antenna you are using, even remotely. Thus, if you buy one very expensive cable, it might be able to serve as a cable for multiple antennas.

3 db is equivalent to half of your signal strength. For the most part, stations won't notice this, unless you are on the very edge of connectivity. The only reason you would need better than this is if you are doing something like satellite contacts near the horizon, where even a few db lost can make a huge difference.

Most transmitters and antennas have an impedance of 50 ohms. The easiest way to explain this is to think of two hoses connected together. If the sizes are just right, then the water flows smoothly between the two hoses. As far as the hose is concerned it really is just one hose. But if there are differences, then the flow can change significantly, as is shown in the figure below. Let's say the source hose is larger than the destination hose.The water will reach the point of connection, and part of it will be forced to stop. This will cause some turbulence in the flow, losing overall effectiveness. As electricity flows in a loop, the same thing will happen when going from small to big. Bottom line is, get the same impedance if you can. If there is a small mismatch, it won't kill your signal, but you definitely don't want a 300 ohm wire for a 50 ohm antenna/transmitter! However, if you have a 75 ohm wire for the same set up, you probably will be okay, assuming you have a relatively new transmitter. Still, it is worth the extra money to get the same impedance cable as your antenna/transmitter!

Reflection of signal with different sized pipes

As to the single cable, it can best be explained by the same hose analogy, every time you connect a hose together, then you will lose some water (Unless you have a very tight connection). Adapters are really just short cables, so minimize their use if you can.

Cable performance changes significantly based off of the frequency. In general, the higher the frequency, the more signal will be lost. For me personally, the highest frequency I imagine using is 440 MHz, thus I look at the loss for the 440 MHz (Usually 400 is the closest).

A person moves slower in water than they do through air. Likewise, electricity moves slower in some devices than others. The velocity factor is how much a signal is slowed down from vacuum. Essentially, radio waves move at the same speed in air as in a vacuum, so... The only reason this would make a difference is if you are trying to use multiple antennas broadcasting the same signal at the same time. I'm not going to go into the details in this post, but you might want to shift the phase of the signal by some portion, which increasing the cable can in fact manage.

Lastly, if your situation dictates it, there are devices, called antenna switches, which will allow you to switch between multiple antennas using only a single cable. You can even do this remotely. Take a look at eham for more details.

I'm not going to go into great detail, but I will say there are ways to match impedance even if you aren't using matched cable/antennas. But they are a pain, and require some additional hardware, it is usually much better if you get the right thing from the beginning.

In summary, buying the right cable can be a difficult, confusing process. But if you do it well, it will help your station significantly. Choose carefully, and you will be well rewarded!

No comments:

Post a Comment