Cables

RF cables are , for frequencies higher than HF , almost exclusively coaxial cable ( or coax for short , derived from the words of common axis ) . Coax cables have a core conductor wire surrounded by a non-conductive material called dieletric , or simply insulation . THe dielectric is then surrounded by and encompassing shielding which is often made of braided wires . The dielectric prevents and electrical connection between the core and the sielding . Finnally , the coax is protected by an outer casing which is generally made from a PVC material . The inner conductor carries the RF signal , and the outer shield prevents the RF signal from radiating to the atmosphere , and also prevents outside signals from interfering with the signal carried by the core . Another interesting fact is that high frequency electrical signal always travels along the outer layer of a conductor : the larger the central conductor , the better signal will flow . This is called the "skin effect" .

eVEN TRHOUGH the coaxial construction is good at containing the signal on the core wire , there is some resisance to the electrical flow : as the signal travels down the core , it will face away . This fading is known as attenuation , and for transmission lines it is measured in devibels per meter ( dB/m) . The rate of attenuation is a function of the signal frequency increases , so does its attenuation . Obviously , we ned to minimize the cable attenuation as much as possible by keeping the cable very short and using high quality cables .

Here are some points to consider when choosing a cable for use with microwave devices .

1. The sorter the better : The first rule when you install  a piece of cable is to try to keep it as short as possible . The power loss is not linear , so doubling the cable length means that you are going to lose much more than twice the power . In the same way , reducing the cable length by half gives you more than twice the power at the antenna . The best solution is to place the transmitter as close as possible to the antenna , even when this means placing it on a tower .

2. "The cheaper the worse !" The second golden rule is that any money you invest in buying a good quality cable is a bargain . Cheap cables are intended to  be used at low frequencies , such as VHF . microwaves requires the highest quality cables available . All other options are nothing but a dummy load .

3. Always avoid RG-58 . It is intended  for thin Ethernet networking , CB or VHF radio , not for microwave .
4. Always avoid RG-213 . it is intended for CB and HF radio . In this cas he cable diameter does not imply a high quality , or low attenuation .

5. Whenever possible , use Heliax ( also called "Foam") cables for connecting the transmitter to the antenna . When Heliax is unabavailable , use the best rated LMR cable you can find . heliax cables have a solid or tubular center conductor with a corrugated solid outer conductor to enable them to flex . Heliax can be built in two ways , using either air or foram as a dielectric . Air dielectric heliax is the most expensive and guaranttess the minimum loss , but it is very difficult to handle . Foam dielectric heliax is slightly more lossy , but is less expensie and easier to install . A special procedure is required when soldering connectors in order to keep the foam dielectric dty and uncorrupted . LMR is a brand of coax cable available in various diamter that works well at microwave frequencies . LMR-400 and LMR-6000 are a commonl used alternative to Heliax .

6.  Whenever possible , use cables that are pre-crimped and tested in a proper lab . Installing connectors to cable is a tricky business , and is difficult  to do properly even with the proper tools . Unless you have  access to equipment that can verify a cable you make yourself ( such as a spectrum analyzer and signal generator , or time domain reflectometer ) , troubleshooting a network that uses homemade cable can be difficult .

7. Don;t abuse your transmission line . never step over a cable , bend it too much , or try to unplug a connector by pulling directly the cable . All of those behaviour may change the mechanical characteristic of the cable and therefore its impedance , short the innner conductor to the shield , or even break the line . Those problems are difficult to track and recognize and can lead to unpredictable behavior on the radio link .