Several good mentions have been made in the comments section by readers regarding the transmission line (T/L). Since this is the next logical item in the system to discuss, what follows is my take. Most of the information provided is pretty basic for low power (less than 100 watt) stations. You could spend a lifetime studying T/L and antenna theory, but I have better things to do, as I’m sure you do. So well keep it simple. If most of this information is old hat and you are curious about my setup, go to the bottom of the article.
The transmission line is the link from the output/input connections of the transceiver to the antenna. Actually, everything from the output of the power amplifier (PA) section of the radio, and the input to the RF amplifier section, to include a tuner if one is used, to the feed point of the antenna is part of the T/L. So, no, the T/L is not just a piece of cable. To complicate things a bit further, the antenna and it’s associated T/L are known collectively as the antenna system.
The perfect T/L would move all of the power transmitted from your radio to the antenna as efficiently as possible, which means no loss of power. If I am sending a radio message to one of my folks and my radio is set at 25 watts, I want all of that 25 watts to radiate from the antenna.
That same TL needs to move received signals from the antenna system back to the radio with no distortion or loss of signal strength. If my antenna picks up a transmission at 100 milliwatts, I want all 100 milliwatts to be present at my receiver section.
The ultimate T/L would be inexpensive, durable, easy to install and remove, and require very little maintenance. An added bonus would make it nearly invisible to prying eyes.
An important item to remember: most transceivers require a 50 ohm impedance at the output in order to be perfectly “matched” to an antenna. Any deviation in either direction, more or less impedance, causes loss of efficiency and in extreme cases can damage your radio, T/L and antenna.
There are several different transmission line setups available but the two most common are: ladder line and coaxial cable.
Ladder line (also called twin lead or open-wire) consists of two parallel conductors separated by an insulator. ( http://www.universal-radio.com/catalog/cable/3028.html ). The insulator is usually plastic or air. “Twin lead” was once commonly used with TV antennas and is identified easily as it has a continuous plastic strip separating the two conductors. “Window ladder” is used with amateur radio and is identified by the rectangular air gaps spaced at regular intervals in the plastic that separates the conductors. The characteristic impedance of twin lead is 300 ohms and window ladder is 450 ohms. Ladder line uses the air between the two parallel conductors as it’s dielectric and if installed correctly has very little loss.
The primary advantages of ladder line with respect to coax are:
1. Lower loss.
2. Can drive a balanced antenna (eg: dipole) without a balun.
1. Usually requires periodic mounting standoffs
2. Must be kept away from metal objects.
3. Can lose its low loss features when wet or icy.
4. Requires a tuner when used with unbalanced antennas.
Coaxial cable or “coax” also has two conductors, however one in located in the center of the cable while the other surrounds the center conductor for the full length of the cable. The conductors are separated, in most cases, by a plastic or foam insulating material and the outer conductor is protected by an insulating cover. The center conductor is normally a single copper wire while the outer conductor is a braided wire which is usually, but not always, made from copper. Depending on the coax, it’s characteristic impedance is 50 or 75 ohms. Much closer to the radio output requirements. Use a good quality 50 Ohm coaxial cable with appropriate power rating such as: RG58, RG8X, RG8, RG213, Belden 9913F7, Davis RF Bury-Flex. A coax with a dense (or double) braid is worth the money. Simple installation; ideally the coax will go directly from the antenna feed point to your transceiver.
The primary advantages of coax with respect to ladder line are:
1. Most transceivers are equipped with coax connectors. If using ladder line, a balun or tuner is required.
2. Coax is not effected by nearby metal objects.
3. The impedance of coax doesn’t change when it rains or snows.
4. Ease of setup and tear down.
I’m a coax guy. Just keep-it-simple-stupid. Rolls up tight and unrolls right, every time.
The transmission line (T/L) for my semi-permanent base station consists of a 30′ section of RG-8 coax with soldered Amphenol 83-1SP silver plated PL-259 connectors. The tuner is an ICOM AT-100. (LDG AT-100 Pro as a backup). The antenna is the Buckmaster 300 watt 7 band (offset center fed) OCF dipole. My backup antenna is a Carolina Windom 300 watt 8 band OCF dipole.
Why 30′ of coax? That’s the distance from my transceiver to the antenna feed point with a few extra feet of coax for slack. Don’t make it any longer than necessary. Why use coax instead of ladder line? I have found that coax is far more durable in the environment that I operate in. In addition, I can attach it directly under the metal roof of the building my radio room is in and make “fairly” sharp twists and turns that I can’t make with ladder line. The entire system can be pulled down, coax rolled up and thrown into a Pelican case and be ready for transport in about 5 minutes. If you don’t move your station on a regular basis, then something like ladder line might work better for you. Last but not least, the area I live in has an environment that is nearly considered rain-forest. Ladder line does not behave well in wet and icy conditions.
Why RG-8? My loses are pretty negligible with only 30′ of coax, especially since I keep my power out well below 100 watts, and only operate this system on the lower ham bands. It is a cost vs. performance issue. I chose to forego the extra expense needed to mitigate the small losses that I would probably encounter by not using a more expensive coax.
Why not use crimp together fittings instead of soldered? Less power lost due to heating and potential arcing in a loose fitting. If you don’t know how to properly solder and seal your fittings, then you could buy cable made to length with fittings installed. Best answer, stash another arrow in your quiver and learn how to solder and build your own cables (as well as other stuff). Go here and learn how to do it correctly: http://www.k3lr.com/engineering/pl259/ . The Amphenol silver plated fittings are just about bullet proof.
Why do I use a tuner when this antenna works without one? Since the Buckmaster is a OCF and is almost an exact copy of the Fritzel antenna, it’s characteristic impedance of 300 ohms is matched by the balun (6:1) at the antenna feedpoint to that of the coax (50 ohms). Buckmaster has tested and recommends running the center of the antenna at 30′ and the ends at 10′ to avoid high SWR. However, I run my antenna about 15′ above the ground in the NVIS mode, and this will lower the input impedance of the antenna to somewhere in the 100 to 75 ohm area. So, being the cautious guy I am with my equipment, I run a tuner to protect the transceiver. And even though my SWR is good at the output of the tuner, I should probably perform an antenna analysis and change out the balun. Something for the future.
With my field radio system everything needs to fit into a small molle bag attached to my ruck.
I use RG-8X in the field because of it’s small diameter and weight. It also handles the frequencies up to and including 10 meters, loss wise, much better than regular RG-58. Since the KX-3 runs only about 12 watts max, that’s important. I keep two 25′ sections with a connector to join the two together if I need 50′ of coax.
The tuner is integral to my Elecraft KX-3.
I use “FLEXWEAVE” 14 AWG 168 bare copper wire or “Hot Rope” 0.133″ on a spool, purchased from TheWireMan.com. I determine what frequency I am going to use and “cut” each 1/4 wave dipole antenna to length. Using un-insulated antenna wire rolled up on small spools allows me to unroll it to the length needed without actually cutting the wire. Any wire left unused and rolled up on the spool is electrically shorted together and only adds the width of the spool to the antenna length. If one were to use insulated wire on a spool it would act as a balun. Care must be taken not to short out the wire against objects including yourself.
No balun is used, I just connect a bnc-to-binderpost adapter ( http://www.amazon.com/Parts-Express-Binding-Posts-Adapter/dp/B000LFWQH4 ) to the antenna wire and to the coax. A dipole at 10′ usually has a characteristic impedance of between 75 and 50 ohms, just what the radio is looking for. The tuner does the rest.