Do You Need an Antenna Tuner?
Do you need an antenna tuner?
Well, it depends..... How often have you heard that? If your goal is to be able to work multiple bands from a single antenna, then you probably need an antenna tuner. If you need to resolve bandwidth issues on an antenna tuned for a specific frequency, then you may need or want an antenna tuner. Let's concentrate on the first goal, using a single antenna to work multiple amateur bands. This is where an antenna tuner, or some form of matching network, is a requirement and greatly benefits the Radio Amateur.
Random length Wire (Balanced) Antennas
The vast majority of Amateur Radio Operators all share one thing in common, they all built their first stations using dipoles, long wires, delta loops and various other wire antenna configurations. Additionally, they all wanted to be able to work as many bands as possible using the same antenna. My first antenna was constructed of 9 gauge fencing wire (found everywhere in Colorado), several egg type ceramic insulators and an SO-239 connector. The antenna was cut for 40 meters, which also allowed me to work 15 meters with fairly low SWR due to the harmonic nature of a 40 meter antenna. I fed my antenna with 50 ohm coax, which my ham neighbor said was "close enough". The antenna worked ok, for what it was, but I was always curious why my CW straight key would periodically bite me!!! (See the section on common mode currents).
Random length wire antennas are a great choice for the new ham, and most old timers still have random wire antennas in their antenna farms for various reasons. Ideally, a random wire antenna should be cut to a minimum of 1/2 wavelength on the top band that you want to work. For instance, cut a random wire antenna for 1/2 wavelength on 80 meters and with the help of an antenna tuner you should also be able to operate on everything from 80 - 10 meters. This is considered a long antenna as compared to a short antenna which are generally shorter than 1/2 wave length for any given frequency. Suffice to say that, without going into antenna tuner heroics, average antenna tuners will be able to tune an antenna that is cut for 1/2 wavelength (or more) as compared to short antennas.
Why are short antennas less desirable than than 1/2 wave antennas, It's because the feed point impedance of a "short" antenna can become extremely low in resistance, but not necessarily reactance. To overcome low resistance situations large amounts of capacitance in the antenna tuner is generally required. Loading an 80 meter 1/2 wave dipole on 160 meters can (and does) happen all the time, but additional fixed capacitance in the tuner may be required to accomplish this. This is why we advocate for at least 1/2 wave length of wire in the air on the lowest frequency band that you want to operate on.
Here's an outstanding article by Lucia A. Lorona on antenna lengths and efficiencies. This is a particularly interesting article, especially from an antenna efficiency point of view.
Antenna and Feedlines
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As an example, my own 200 foot 80 meter dipole (random length, just over 1/2 wavelength on 80 meters) can be successfully tuned to 1:1 SWR on all bands 80 - 10 meters. Since the antenna is "short" on 160 meters, most of antenna tuners I have tried will not tune 160 on this antenna. An exception to that observation was a Nye-Viking MB-V-A tuner that could successfully tune the antenna to 160 meters, but it required large amounts of additional capacitance added into the network in order to find a match. This sounds great, but don't rush out to your local hamfest and buy the first Nye-Viking that you can find. There's a price to be paid for that matching capability that may be a hard pill to swallow for some. See the section on "Different Types of Antenna Tuner" for more details on the bitter pill.
The question is, when do you stop trying to "force" your antenna tuner to work before you turn your attention back to adjusting the antenna and transmission line? For me, the answer is clear, don't try to load 10 feet of coax connected to a lawn chair on 160 meters! Always try to have at least 1/2 wavelength of wire for the top band you're trying to work. It will save you time, hassle and money as you won't need a $1000.00 tuner to make it work.
Coax, Window or Open Wire Line?
For balanced antennas, I prefer window (450 Ohm) or ladder line (600 Ohm) also known as open wire line. Why? Because open wire line has a very good velocity factor (.98 on average) and extremely low losses, even over great lengths. This is not to say that you can't use coax to feed a balanced antenna. Definitely do your homework on loss factors for coax over medium to long distances. Coax is certainly easy to work with as it can be routed through your house, buried in the ground, strapped to the side of a metal tower (more on that later) and basically mistreated as much as you like. Ladder and Window line require a bit more planning when bringing the line into the shack (if you're going to run direct), but there's a very simple solution, if routing ladder line into your house is not an option. Specifically, you can use a current balun (1:1) to terminate the ladder line outside your house and then run coax from that point to your station. Here's what you need to keep in mind though, losses due to high SWR on coaxial cable (especially inexpensive cable) can be very high, while high SWR on a ladder line is really not a concern. Keep the coax from your balun to your tuner as short as possible to avoid losses.
Here's a link to an article by Lloyd Butler (VK5BR) that includes a discussion on the merits of open wire line vs. coax. Pay particular attention to the graph in
Figure #1, which illustrates attenuation losses for various types of coax over a 100 foot run with an SWR of 1:1. Additionally, Figure #2 shows what happens to
line attenuation as SWR goes up! Note that open wire line attenuation losses for 100 feet of wire only start to appear above 30 Mhz.
Unbalanced Antennas with Coax
Normally antennas fed with coax are designed to work on a specific frequency, or a selection of frequencies based on the use of traps and other tricks of the trade. Do you need an antenna tuner for this type of setup? It depends on how broad banded the antenna is on the specific frequency of interest. Antennas
have only one point of resonance (or near resonance) on a single band. The resonance point is generally designed to fall into the middle of the band with the hopes that the antenna system will be broad banded enough to give the operator a fairly low SWR on the edges of the band. This is most often visible on bands like 80 meters where retuning will most likely be necessary if you want to switch from the phone part of the band to the CW part.
This is where an antenna tuner can come to the rescue. Even very simple tuners (inexpensive) can be used to tame rising SWR on purpose tuned antennas and most hams have an antenna tuner in their shack for just this reason. The really good news is that most modern solid state rigs come with a built in antenna tuner. You may be thinking WOW! It's great that our manufactures were generous enough to add an antenna tuner to our radios so that we don't need an
external tuner. Well, the truth is a little less generous. In the old days, where tube final radios were the norm, the final output circuit or "tank circuit" was essentially as an antenna tuner! Modern solid state radios don't have tank circuits, their final output impedance is set and not adjustable. This is where the
internal tuner comes into play. This small tuner allows for the elimination of SWR on bands where the SWR might rise to about 3:1. This is great, but is that all we need? Probably not, especially if you're using an antenna that presents better than a 3:1 SWR ratio on parts of the band that you want to use. Additionally, if you plan on using an external amplifier, the internal tuner in your radio needs to be switched off. The amplifier or antenna tuner will do the job of properly matching the feed line impedance to the output impedance of your radio. Keep in mind, that the final output network of tube type amps are only designed to match minor changes in impedance and will not handle large SWR values that can found on wire antennas. In these cases an external tuner is a must.