It amazes me how the misconception of the G5RV being an all-band antenna continues to prevail. For years the G5RV has been described, sold to, and used by many hams as an all band antenna. It is not, and never was, designed as a multi band antenna. However, given a good external antenna tuner and low-loss* feedline any piece of wire is tunable to function on all bands.

Even Mr. Varney (G5RV) never intended this misconception to prevail yet it persists to this day despite the efforts of many knowledgeable persons to dispel the myth through the publications of ARRL, Ham Radio, etc. I believe it borders on criminal for vendors to sell this antenna knowing that the purchaser has been led to believe that this antenna has some magical properties permitting all band use to the extent that it is resonant and requires little matching on other bands. With that in mind let's discuss what the G5RV really IS!
 

The above chart shows the characteristics of the antenna elements on a G5RV style antenna. To describe what these numbers mean and why they matter I will describe the design criteria of the G5RV to put them into perspective.

Any wire antenna is generally based upon the concept of 1/4-wave elements. Remembering the graph of a sinusoid from high school, assume that graph represents the voltage along the wire. Label the center of the graph 0, with + and - 180 on the left and right ends. Since the end of the wire in our antenna is open to free space, there can be nowhere for current to flow. Hence, that end point must be a high voltage node. A high voltage node will occur at +/- 90 degrees from the center of the graph. Working our way back towards the center 90 degrees we find ourselves at 0, where the voltage is a minimum and coincidentally current is a maximum. Extending from -90 on the left to +90 degrees on the right yields the 180 degrees of a half wave dipole. It's that simple to understand. But these high voltage nodes also occur at 270 degrees from the feedpoint, and so on, at every odd multiple of 1/4 waves from the feedpoint. So we could have an antenna using elements of 1/4, 3/4, 5/4, 7/4 etcetera. Increasing the length tends to increase gain and impedance while reducing bandwidth slightly, but by and large the feedpoint is always a low voltage (current) node, primarily resistive, with little or no reactive component, and can easily be matched to most transmitters with little difficulty. Draw the graph of the sinusoid to help conceptualize this. (If you superimpose on the graph a Cosine wave you'll have the current graph as well.)

Using this premise, Mr. Varney (G5RV) devised an antenna that is 3/4 wavelengths per leg on 20 meters. This yields a resistive impedance of approximately 100+ ohms, easily matched to most transmitters, and yielding very acceptable performance on 20 meters.

"What about the matching section", you ask? Well, it isn't really a matching section at all. The purpose of the open wire feeder is to provide a connection to the antenna at convenient distance from the antenna. It could be any length if used all the way to the tuner, but to accommodate a connection to Coax Mr. Varney made it exactly (180 degrees) 1/2 wave long on 20 meters to specifically NOT transform the impedance of the antenna. Using any thing other than 1/2 wave sections of line would have changed the impedance, but that's another concept that I'll demonstrate further on. Going back to our sinusoid, you'll see that the magnitude of the phase angle is the same for any two points along the wave that are 180 degrees apart! This 1/2 wave section simple brings the antennas feedpoint impedance, unchanged, to a new location 1/2 wave away for attachment to a coaxial feedline, the length of coax is irrelevant since it is connecting to an impedance that is ideally resistive. That's what the G5RV is, no more no less. It is a 20 meter antenna, period! But it can, and is, used on other bands! How can this be? Well, for any other band, as shown in the table above, it acts as a random length dipole which can be tuned for all but the worst conditions with a good tuner. *However, if fed with coax, the losses will be significant for the higher SWR conditions that exist when the antenna's impedance differs greatly from 50 ohms. Using open wire line all the way to the tuner would largely alleviate this loss.

As you can see from the table for the full size G5RV, this antenna is probably tunable for most of the bands except 10. On 10 meters the feedpoint is almost 90 degrees out of phase, resulting in an extremely high impedance (close to a voltage node) that is very difficult to tune. However, this can be remedied by applying an inverse of Mr. Varney's "non-matching" section with a true matching section of open wire line that is an odd multiple of 1/4 wave lengths. This 90-degree phasing section will transform the voltage node at the feedpoint to a current node on the fed end, thereby yielding a tunable load impedance. Voila! But now this comprises a "band specific" antenna system doesn't it? Well, it was before too, and that's the point.

While I haven't work out all the complex impedances for different lengths of matching section, I suspect this antenna, using a 10-meter matching section, will still tune several bands with a good tuner. Although, once again, feeding any antenna with a high SWR through coaxial cable will be fairly lossy. Should you choose to use coax to connect to an open wire section on this antenna, or any antenna, a balun MUST be used to minimize feedline radiation from currents that WILL seek to flow on the braid. The open wire feeder is balanced, the coax is unbalanced, nothing will change those facts and a balance to unbalanced transformer (balun) should be used! Unless you don't care about RF on your feedline, in your shack, etc it's better to have a balun. Ignore those comments about balun losses; baluns don't dissipate any significant power unless you've got bigger problems with your antenna system's balance. But that's another story altogether.

Lastly, what about this 1/2 sized G5RV. Well, it's hardly a G5RV but let's not quibble, it's descriptively accurate. It is however a scaled down version of the same design and as such will resonant at 1/2 the wavelength. Looking at the table shows this fact on 10 meters where it exhibits a resistive load for 3/4 wave leg lengths on that band and similarly reactive loads for other bands

About the table:
The first column shows the physical length of the overall antenna in wavelengths for the different bands. I didn't include 75 meters since it's performance is so poor as to be pathetic, but it will operate on that band, just don't expect any great signal reports. The second column shows the wavelength of each leg (1/2 element) as seen at the feedpoint. The third column shows the feedpoint characteristic as being a resistive (node) or a reactive load. Following the characteristic is the phase error of these reactive components. A " -" indicates a "capacitive" type reactance and "+" indicates an "inductive" reactance. Each load has a different complex impedance comprised of a resistive and reactive (j) component, eg: approx. 103 + j48.6 occurs on 20 meters for a full size G5RV. Since the j component is relatively small matching is less difficult for a simple tuner. Some value of resistance would exist for the other more reactive conditions as well, like 298 - j1000 for 15 meters. In this case there's a lot more reactance to be tuned out and most internal units can't deal with it. And if fed with coax, the mismatch will contribute to much more feedline loss. The last column shows the total length in degrees for each leg. Subtracting from this the nearest divisible odd multiple of 90 yields the phase error in the previous column.

Check out this site, www.qsl.net/k2hq/g5rv.htm. His numbers are more worked out than mine since he used a computer based antenna-modeling program. If you look at this site you'll see that the SWR and impedances presented can get quite out of hand, over 3,000 ohms for 10 meters, as I suggested.

I hope this helps make some sense of the G5RV, as well as other antennas. Understanding the basic concepts of antennas will make you a wiser consumer. Remember, there is no magic in any antenna design; it's all based on the same basic physics which have been known for many years. Do not be duped into believing the claims of some of these ads. Smaller antennas generally yield smaller signals, narrow bandwidth doesn't mean bad, low SWR doesn't mean it's getting your signal out, etc. Read some of the ARRL publications, The Handbook is a good start, if you like you can delve deeper with the Antenna Handbook. There are lots of good references out there. It's interesting and a lot of fun to build effective antennas and tuners with junk parts. You'll be amazed what you can accomplish with $15 worth of wire and a good balun.