Coax and Connectors, Who Knew There Were So Many Options
To build a HF station we need some essential components and some optional ones. Tonight I’ll discuss the feedline between equipment and the antenna which is commonly Coax cable and the common connectors used in Amateur Radio.
Regardless of whether you are operating at HF, VHF or UHF, the quality of your feed line is critical to your station. The feed line (also called the transmission line) is the RF power conduit between your radio and your antenna. All the energy you generate travels to the antenna through the feed line. By the same token, all the signals picked up by your antenna must reach your radio through the same feed line.
The problem with any feed line is that it isn’t perfect—it always loses a certain amount of the energy. To complicate matters, all feed lines are not created equal. The amount of loss at any frequency will vary considerably from one type of feed line to another.
The most common type of feed line is coaxial cable, or simply coax. It is called coaxial because there are two circular conductors positioned “co-axially” one inside the other. The inner conductor is usually called the “center conductor.” It is surrounded by a solid or multi-stranded outer conductor commonly called a “shield.” The shield is usually surrounded by an insulating plastic jacket. There is also insulating material between the center conductor and the shield. This insulator material can be hard plastic, foam plastic or even air.
There are many types of cable to choose from and it can be confusing to chose the best one. I’ll cover the most common choices of cable to get you started. We’ll focus on the most popular cables, with 50 ohm impedance to match the output impedance of our transceivers.
Here’s the really simple, short list:
RG-58 type is the Standard cable for mobile installations and basic HF requirements. It’s not good for long distances and not good for higher frequencies like UHF. It’s cheap but it’s the common starting point when discussing coax, RG-58 is the same diameter and appearance to the RG-59 cable found at your hardware store, or used by your cable company for patch cables for TV. Don’t get confused between RG58 and RG59, one is for Amateur Radio and is 50 Ohms, the other is used for TV and is 75 Ohms, don’t mix and match these two very similar coax cables.
RG-8X type is slightly larger than RG-58 and lower loss as well but still convenient for shorter cable runs and jumper cables between your radio gear. A lot of patch cables are made using RG-8X because it’s thing and flexible. Use it up to 50 feet in length for HF. I would use it up to 25 feet in length at VHF, and probably even shorter for UHF.
RG-8U type is about twice the diameter of RG-58 and RG-8X and it’s the general purpose coaxial cable, best for long cable runs in HF and VHF.
At one time, RG-58, RG-8X and RG-8U were military standards but now these terms are used rather loosely and refer primarily to the size of the cable. That’s why I used the term “type” to the to indicate that it’s not a precise standard.
All three of these cable types will handle 100W or more at frequencies including HF, VHF, and UHF, which covers most ham transceivers. If you are running more than 100W, you should check the power specification of the cable you are thinking of using.
Shopping for Coax for Amateur Radio can be very confusing with all the choices, brands, and model numbers commonly available. My suggestion is stick with these basic types to start with and as your knowledge evolves, you can try more expensive and better coax if the need arises.
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Signal Loss
All coaxial cables will attenuate the signal as it travels down the cable and the signal loss can be significant. For example, just 3 dB of signal loss means that you’ve lost half of the transmit power as it propagates down the line. This loss applies for both transmit and receive… you’ll have less power out to the antenna and less signal showing up at the receiver.
The cable loss will be determined mostly by the size of the cable (bigger is better), the dielectric used in the cable (the insulator between the center conductor and the shield) and the frequency of operation. As an example, consider a 100 foot run of cable for use at VHF 146 MHz, which is high enough in frequency and a long enough run such that we’ll see some significant losses. According to online calculators, 100 feet of RG-58 style cable produces a loss of 5.5 dB, which means that only 28% of the power gets through the cable. This is not good, so we would rarely want to use RG-58 for that long of a cable run.
Changing the cable to RG-8X drops the loss to 4.5 dB, which is only a minor improvement, about 36% of the power makes it through. However, using RG-8U type cable decreases the loss to 2.4 dB that’s 58% of the power makeing it through the cable), so clearly the larger cable size has an advantage.
Now let’s change to LMR-400. I’ve not mentioned this Coax yet but it’s a popular cable that has the same diameter as RG-8U but with a lower loss. It’s typically more expensive but has even lower losses. The 146 MHz loss through 100 feet of this cable is 1.5 dB, or 0.9 dB better than ordinary RG-8U. Now with this much better coax a loss of 1.5 dB means that we still lose 30% of the power.
Now let’s see what happens when we change the frequency of operation. If we use our 100 foot run of LMR-400 on the 20m HF band, the loss is only 0.5 dB. This means that 90% of our signal power makes it through the cable.
Other Specifications
There are a few other cable specifications that you may be concerned about, depending on application. Cables with solid center conductors are less flexible than those with stranded center conductors. The dielectric material and the outer insulating jacket can also affect the flexibility of the cable. For portable operations, make it a point to get cable that is rated “flexible” because it is easier to handle and deploy. Direct burial cable has a durable outer insulation that will withstand being buried in the ground but that is commonly very stiff. The type of outer shield used in a cable can vary widely, with some cables providing much more shielding and isolation than others.
Another popular type of feed line cable for HF use is ladder line. In fact, at HF frequencies it is the common feed line for random-length dipoles and other antenna designs. Ladder line consists of nothing more than two wires in parallel separated by insulating material.
Remember coaxial cable commonly used for Amateur Radio has an impedance of 50 ohms and is unbalanced because of the center conductor runs inside of the outer shield separated by the insulator.
Ladder line impedance's can vary from 300 to 600 ohms and is balanced since both wires are identical and are running parallel to each other separated by the ladders or windows.
Amateur Radio transceivers are designed to work with an impedance of 50 ohms, so you must typically use 50 ohm unbalanced coax at the back of the radio.
Baluns inserted at the point when ladder line meets up to coax line is one method of converting 300 or 600 ohms balanced feedlines down to 50 ohms unbalanced coax. The name Balun stands for Balanced to Unblanced BAL-UN.
The other approach is to use a manual antenna tuner which we talked about last week to transform the impedance of the antenna system to 50 ohms for your radio. Many models of manual antenna tuners feature a built in balun for converting balanced antenna connections to unbalanced coax.
So which type of feed line should you use at your station? Fortunately, the answer is simple: You want the feed line that has the lowest loss at the highest frequency you want to operate.
As you probably guessed, low-loss feed lines are more expensive. So it’s also a balancing act with your budget and the length of feedline required. The shorter the line distance, the cheaper the feedline you can use. In a mobile installation, you can use an inexpensive feed line such as RG-58 because you’re only using a short length. As long as the SWR is low, the loss will be acceptable. However, if you have an antenna that is 100 feet from your radio and you are operating at, say, UHF, RG-58 would be an extraordinarily bad choice! For this installation you’ll need to invest in something much better—probably LMR-400 or even better.
For HF base stations which is what we’ve been talking about the last few weeks always use the lowest-loss coax you can afford. Since you’ll probably be using your feed line for several years or longer, you want something that can support your changing interests and you’ll be glad that you hopefully already have a low-loss feed line that’s up to the new task.
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So that’s my Coax notes for this evening. Does anyone have any questions before we move into the next topic of Connectors?
Connector use varies based on frequency band and also based slightly on power. The most common one that I see out in the field and on equipment are what we will talk about this evening.
First off is my NO CONNECTOR AT ALL Warning
Certain antennas and feedline, like twin lead or random wire or loop antennas on HF use bare wire connections with no traditional connectors at all. When working with feedline that has no connector just as a warning these wire connections may not be insulated, so don't touch the wire unless you're sure that the transmitter is off and disconnected. That warning should apply to handling any type of feedline in Amateur Radio, but it’s especially true with bare wires of any kind.
The UHF connector, or now often called the HF connector is properly known as the PL-259 (male connector) or the SO-239(female jack). When this connector was first developed before WW II the nomenclature of HF, VHF, and UHF was different than it is now hence the original name was the UHF connector. This connector is the most common connector in HF and is still commonly found on a lot of HF/VHF/and UHF equipment, it is a relatively large threaded connector. By today's standards it’s still perfectly suitable for HF use, but there are much better solutions for VHF and UHF. Keep in mind though if your equipment like a mobile dual-band radio uses this connector, then you’ll need to use the same connector on your antenna feedline even though it has a lower frequency range and higher loss levels than better connectors that have been invented since. The PL-259 can be used with acceptable loss from the lowest HF bands right up to 100 MHz, but is often used all the way up to 440 MHz UHF as long as the coax feedline is limited to 10-15 feet like in your vehicle mount.
SMA and the varieties. One of the smallest and nearly universal for portable handheld radios now, this is also sometimes seen elsewhere on smaller radio devices. It’s used a lot on VHF and UHF. The standard configuration has a male with a center pin and threads on the inside, and a female with threads on the outside. The reverse configuration (known as RP-SMA) has a male with a center pin but threads on the outside. SMA can be used with relatively low loss from the lowest HF bands, all the way up to 18 GHz.
Type N connectors - while certified up to 18 GHz like the SMA connector, the N type are often also used for UHF gear. I've seen them on commercial gear and ham gear. This connector has an extra "circle" of metal shielding the center pin on each end. It's slightly larger than even the old PL-259 HF connector. You’ll also see a lot of these on RF test equipment. On my multi-band multi-mode Yaesu FT-847 transceiver there are 4 antenna connectors on the rear of the radio, 3 are the traditional HF connector for HF, 6M, and VHF, the fourth is a N type connector for a dedicated UHF antenna.
BNC connectors - this is a quick-mate connector. It takes less than a half turn to mate/unmate this connector, which is great for anything that's being replaced often. Older handhelds used to use BNC connectors before SMA came along and QRP portable HF radios often use BNC for the speed of coupling and uncoupling in the field. BNC can be used with relatively low loss on HF, all the way up to about 4 GHz.
TNC connectors – are a threaded version of BNC. It's about the same size, but is rated up to about 11GHz. Not common at all, but you might see it. There is also a reverse version (like for the SMA).
Other connectors in the rarely seen category are the Type C connectors which are another variation on BNC, which is quick-mate, weatherproof and rated up to 11GHz. And finally the Mini-UHF more confusion abounds here! This connection can be rated well over 20GHz, unlike the much lower rated full size UHF or HF connector that it seems to be based off of. It’s similar to a standard polarity SMA connector. But whereas SMA uses 1/4" threads, Mini-UHF uses 3/8".
So in quick summary the grand daddy UHF connector, also known as HF connector or more accurately part name the PL-259 is VERY VERY common but not really that good except for the HF bands.
The variations of the SMA connector are found mostly on handhelds and when shopping for an after market antenna for your handheld, make sure you know which exact SMA you have on your radio before ordering that matching antenna.
BNC and it’s variations are great for quick connect and quick release, or an environment like test gear where you make those on/off connections a lot.
And finally N type which is common on new model repeaters, UHF radios, and lots and lots of applications that are higher than UHF well into the Microwave range.
So that’s it, Coax and Connectors. Does anyone have any questions.