The Truth About Antenna Gain
OR ISOTROPIC TO DIPOLE REFERENCE CONVERSION,
THE BOVINE FECAL MATTER DETECTOR FOR ANTENNA GAIN ADVERTISING!
    

Definitions and Assumptions:

There is nothing new about most of the antennas in common use by amateur radio operators, but there is a lot of confusion about their performance. This article is intended to unravel some of that for the new comer to ham radio. I refer to the test material that you studied, and use it for a Skeptic's Guide to Antenna Advertising. I freely quote (with credit) some statements that are out there on the internet. They said it well, and I encourage you to read the whole articles at the links from which the quotes come, to become better informed. Back in the 50s and 60s, when we did NOT have easily accessed information on the internet, hams generally built their own antennas, even beam antennas. Even NOVICE license hams did it. Nowadays, that seems to be rare. I encourage home brewing antennas because it saves money and you LEARN something. There are necessary theoretical flourishes to define what we are talking about, and how we measure relative performance of any antenna. Then we will discuss what antenna YOU might like to use. So here it is:

An S unit is assumed to be 6 dB. CAVEAT: The common 50 microvolt S9 is not a meaningful standard, because no one states what antenna is used. A well tuned Hammarlund HQ-170 can have 15 uV for S9. The upper dB ranges are very compressed. My NC-303 is about right in lab measurements. Most all radios have lousy accuracy on their S meters. But it is still a useful measure of relative signal strength, and everyone uses and understands its meaning.

IEEE: It is assumed in this standard that an antenna is a passive linear reciprocal device. Thus, where a definition implies the use of an antenna in a transmitting situation, its use in a receiving situation is also implicit, unless specifically stated otherwise.

SIMPLER DEFINITION: Due to reciprocity, the gain of any reciprocal antenna when receiving is equal to its gain when transmitting.

VOLTAGE GAIN BETWEEN TWO MEASURED VOLTAGES (V1 AND V2) IS DEFINED AS: Gain = 10 × log10(v2/v1)
You had this on your amateur exam. Now you are going to USE IT to select antennas. THE MOST IMPORTANT TAKE AWAY IS THE PUBLISHED ANTENNA GAIN IS RELATIVE TO SOMETHING ELSE. If you do not know what "something else" is, you don't know anything useful.

RELATIVE GAIN TO AN ISOTROPIC ANTENNA IS DEFINED AS: Gain with reference to a non existant theoretical antenna that radiates equally in all directions. This is not just in a horizontal plane, it is in all 3 axes. Put differently, if you put an infinitely small point size dot light bulb in the center of a sphere or globe, it would light up all the sphere equally.

"Free space" also assumes there is no ground anywhere nearby. So if you mount your antenna to a rocket and launch it into orbit, it might approximate this model. Such a model does not exist in reality. But here it is anyway:

The gain of a dipole in "free space" (launched into orbit) is: +2.15 dBi.

In other words, a perfect lossless dipole antenna, launched into orbit, has a gain of +2.15 dBi. So this ignores loss in the coax connectors, wire resistance, and losses in the lousy voltage balun that you should not have used, regardless of what the sales guy said. dBi is often what is used for gain specs on antennas, because it results in bigger claimed numbers.

WHY DO YOU GIVE A CRAP ABOUT ANY OF THESE THEORETICAL STATEMENTS?

Antenna manufacturers publish gain figures for their products. People who are buying their products normally will buy products which claim bigger numbers for gain. Also, the buyers often do not understand that gain is "relative to something" which in many cases, is a non existant isotropic antenna. They do that because it results in bigger numbers. Which sells more antennas to the uninformed. YOU SHOULD GIVE A CRAP BECAUSE IT MAKES YOU A SMARTER ANTENNA BUYER. This is expressed mathematically:

G dBd = G dBi - 2.15 dB But this is only true for a dipole in "free space" or orbiting in space.

WHY DO I CARE ABOUT THIS MATH?

Subtract 2.15 dB from all published figures, to get the gain relative to a dipole in "free space". Wait a minute, you told me that I have to launch my dipole with a rocket to get that. This dBd is for free space, not something you find in a ham's back yard.

GOOD CATCH. ANY DIPOLE has even more dBi than that, due to the effect of ground reflecting some of the dipole energy. In effect, there is another "dipole" under the ground by an approximate equal distance than the height ABOVE ground. The underground dipole can be thought of as a reflected mirror image, because that is what ground does.

At some radiation angles, the combined energy of the two "dipoles" ADDS to make a larger signal. This is a two element beam of sorts, which I will explain more about later. The de facto "beam" has one "reflector", the ground. What makes it nicer sometimes is that with LOSSY ground the reflector is further underground, in effect making the antenna appear to be higher in practice and in some modeling software.

The law of conservation of energy basically states that "There is no such thing as a free lunch". We defined that an antenna is a PASSIVE device, not a power amplifier. If we increase the radiation in ONE direction, it must REDUCE the radiation somewhere else to make up for it. By putting a reflector behind your headlights, it makes them brighter ahead on the road; not so much inside the engine compartment, where it does not matter.

Some radiation angles are more desirable than others. If our antenna, at a certain height, radiates more power straight up (Near Vertical Incidence) or NVIS, it is good for shorter distances. That is good for local rag chews or emergency communications. If our antenna radiates most of its power near the horizon, then it is better for DX.

GAIN for an antenna is specified for its best "lobe" or direction of maximum radiation.

REALLY REALLY IMPORTANT FUNDAMENTAL ANTENNA GAIN TRUTH

"Ground Gain" - The increase in signal level possible at elevation angles where the direct radiation and the ground image of an antenna are additive in phase. For horizontally polarized antennas, this can be as much as 6 dB. What does this mean?

Total "isotropic" dipole gain, dBi, over ground = 2.15 + 6 = 8 dBi.

The new dBd term means gain with respect to a dipole. But we still have a variable: "How good is the ground?" This can be perfect ground (maybe a rooftop of metal), sea water, good ground, or bad sandy ground. OW. My head is hurting. Put simply, a dipole can have a gain over an isotropic radiator of 5 to 8 dBi, depending on ground. Use that range of figures when you do any conversions. YOUR installation may vary from the theoretical published assumptions, but you will get a fair approximation.

So to compare ANY antenna to a dipole, convert its gain figure to dBd. THIS SEPARATES THE REAL GAIN ANTENNAS FROM LOSERS.

WHAT ABOUT THE COBWEB ANTENNA?

The cobweb antenna is an array of dipoles fed from a common feedline. Don't get sucker punched by the gain figure of "Up to 5 dBi gain will allow you to work DX easily -- even on QRP." There is a trade off for wrapping a dipole back on itself to reduce its size. A full sized dipole has more than 8 dBi gain (taking ground reflection into account), or gain over a non existent theoretical isotropic antenna that radiates equally in all directions and has zero dBi (referenced dB to isotropic). Keep in mind that 5 dBi of the cobweb antenna is only half an S unit down from a full size dipole's 8 dBi. This is potentially a good solution to your problem, if you have a small lot. Keep in mind that making a dipole smaller (loading coils, traps, or wrapping it back on itself) also reduces the band width. Band width is the range of frequencies that are below 2:1 SWR. However, if you have a small space to put up your antenna, this might be a good trade off. Could you homebrew a clone of the cobweb? Maybe. Would it be worth your effort? Only you can decide. If this is your first antenna, I would suggest you give the MFJ cobweb a try. I WOULD RECOMMEND THE COBWEB OVER ANY VERTICAL, ESPECIALLY FOR A NEWCOMER TO HAM RADIO. The only reason I bring this particular example up, is to demonstrate how to decode gain figures in advertising.

See this COBWEB antenna at: http://www.mfjenterprises.com/Product.php?productid=MFJ-1835.

VERTICAL ANTENNA GAIN

Theoretical Quarter Wave Monopole gain (perfect ground) gain is 5.14 dBi. This so called "gain" happens due to the antenna being shorter than a half wave dipole, and does not really exist. This is not fair because a ground is introduced in the quarter wave model, which the dipole in the dBi or dB gain relative to isotropic does NOT include. So we are comparing apples to hamburgers here.

Here is an objective vertical antenna ground radial loss study: http://www.kn5l.net/GroundRadialStudy/.

The BEST figure for gain of a vertical he gets, with a maximum effort practical ground is (drum roll): 1.79 dBi This is a staggering - 6.2 dBd compared to a standard dipole at half wavelength height. But at very low angles, with a maximum effort, the dipole is not so good at extreme low angles. How does a vertical compare to a dipole in gain for DX?

See his plot comparing a Inverted V at 40 feet on 20 meters, beating ALL ground mounted verticals, even at low angles. He compares 8 short radials, 8 quarter wave radials, and 60 radials. Even a vertical buddipole 19 ft up beats ALL the ground mounted verticals.

SO THE OLD MAXIM ABOUT A VERTICAL RADIATES EQUALLY POORLY IN ALL DIRECTIONS,
WHILE NOT TELLING ALL THE STORY, HAS SOME "TRUTHINESS". BOTTOM LINE:

You are far better off on 20 meters and up (the best bands for DX) using an inverted V (semi-omnidirectional) in a 35 to 40 foot tree than using ANY aluminum pipe mounted on the ground, regardless of how much effort you put into the grounding. You are still better off with a dipole, even if you raise the vertical above ground, if you install the dipole at an optimum height of half a wavelength above ground or better. This is primarily important for DX work on 20 meters and up. If you do not have a suitable tree, consider buying a push up mast or fibreglass support of 30 to 40 feet. This may be less expensive than a pile of aluminum pipe and traps. If you want to work DX on 40 and below, plan on spending a lot of money, and digging up the lawn. Or get a 60 foot tower for 40, and proportionately larger towers for 80 and 160. I have a friend with a foursquare vertical array for 80, and he regularly works Pacific rim countries. He has real estate and a technical expertise. If you are a beginner with ZERO space, you might consider a vertical that is ground independent (no radials), and mounted at least 30 feet up. I would point you to the AV-620 for 20 through 10 meters (including WARC bands). Then install tuned dipoles for 40 and 80, possibly like the multiband dipoles shown elsewhere on this page. I am trying to save you money and get you started for a reasonable price.

OTHER UGLY FACT OF LIFE ABOUT VERTICAL ANTENNAS:

A vertical antenna does not radiate anything straight upward. This is VERY IMPORTANT for close in local ragchewing on 160, 80, 60, and even 40 meters. A VERTICAL IS TOTALLY USELESS FOR THOSE BANDS FOR NVIS LOCAL WORK. DO NOT BUY A VERTICAL TO TALK TO BUDDIES NEARBY OR CHECK INTO TRAFFIC NETS. THEY DON'T WORK. Close in operation is the most common use for those bands, for most newbie operators.

Back in 1959, I suffered as a Johnny Novice (JN) using a Gotham Vertical with a 4 ft ground rod. Read this ad for that antenna, now that you are more knowledgeable: https://www.w8ji.com/gotham.htm.

THE TRUTH ABOUT YAGI ANTENNA GAIN

Practical maximum gain limit of a Yagi-Uda parasitic beam is 20 dBi, regardless of number of elements.

The Yagi-Uda array in its basic form has very narrow bandwidth, 2 or 3 percent of the center frequency.

WC7I has some good discussion of Yagi antennas on his web site:

"EZNEC* claims this (3 el) antenna has around 11.9 dBi gain, but that seems far too high to me. I expect about 7 dBi or 8 dBi maximum."

"If (3 elements) really has 11.9 dBi gain and you put 100 watts into the antenna, it will behave like 1548 Watts in the forward direction."

"I suspect that 8 dBi is probably more correct, but even 8 dBi will mean that 100 Watts in will behave like 630 Watts in the direction the yagi is aimed."

GRAPHIC: Take off angle 0.5 wavelength above ground: http://www.wc7i.com/TakeOffAngle.htm

TRUTH IN ADVERTISING ABOUT YAGI GAIN FIGURES
APPROXIMATE MONOBAND YAGI GAIN OVER A DIPOLE (DBD) FOR VARIOUS NUMBERS OF ELEMENTS
ELEMENTSGAIN
25.0 
37.5 
48.5 
59.5 
610.5 
711.5 

In our dimension, this is the way the laws of physics work. Any claims to the contrary are sales department puffery, not engineering department data.

Traps have losses, which impair the stated possible gain for a monoband antenna.

Yagi antennas are a collection of compromises between gain, band width, front to back ratio, impedance matching, and clean pattern (lack of side lobes). This also reduces claimed gain, but often results in a more useable antenna.

As an additional rule of thumb, once there are around four or five directors, each additional director adds around an extra 1dB of gain for directors up to about 15 or so directors. The figure falls with the increasing number of directors.

The director/s are used to provide the antenna with directional pattern and gain. The amount of gain is directly proportional to the length of the antenna array and not by the number of directors used. One popular VHF yagi was tested at a radio meet with good measuring equipment. The owner removed every other director for most of the forward part of the antenna. The gain was the same.

On the other hand, correct tapering of director length and optimum spacing can have very good effects on band width and a clean pattern without excessive minor lobes.

Most beginner hams do not have the budget or space for a full size HF Yagi, commonly a triband beam covering 20, 15, and 10 meters. There is another version of the Yagi which is light (cheaper rotor and mounting mast), compact, reasonably priced, and often covers even the WARC bands of 17 and maybe 12 meters along with the standard bands. Its gain figures are given in the range of 3.5 to 5 dBd. There are even kit versions of this pileup breaker. After you experiment with a dipole and are ready to move up in performance, you should definitely consider a hex beam, if you cannot acquire and install a standard Yagi antenna. Here are some links to read up on all the possibilities of the Hex Beam:

THE TRUTH ABOUT NVIS ANTENNAS

There are a number of sites out there that have information on NVIS or short hop local communication. Some of them are pretty good. Pay particular attention to the military based information. If someone says to bury your antenna underground, you should hear your bovine fecal matter detector sounding loudly. If you are interested in EMcomm operation, you should know about NVIS. Here are a couple sites to get you started:

HOMEWORK: Can you tell if you are getting correct information? Is it a model or measured actual data? Did someone tell you to get too close to the ground or actually bury an antenna? Does that make sense?

"In sandy or rocky areas that may be very far down, in which case the antenna can be laid on the surface or buried."

Here is a good military antenna book that covers multiband dipoles, end fed wires, and even how to hook up your BC-610 transmitter:

WIRE ARRAYS AND DELTA LOOP GAIN FIGURES

Use the techniques I have presented here to judge whether you want to play with various antennas for your station. You now have the tools.

I like loops. Some sites claim only 2 dBd for 80 meter delta loops fed in the lower corner, based on modeling software. Keep in mind the output data is only as good as the assumptions made in the software modeling. Anything that involves the effect of ground reflection, loss, or absorption is a theoretical model. The book "Low band DXing" by ON4UN claims otherwise based on actual in the field results.

If you have the space, a full wave HORIZONTAL loop is great for local work on 160 and 80 meters. At 20 to 40 feet high, it can give very good performance, regardless of claimed gain figures. I will leave it up to you to continue your discovery and experimentation. This is what ham radio should be.

T2FD FOLDED BROADBAND DIPOLE; DO NOT BUY THIS ANTENNA FOR YOUR HAM STATION, UNLESS YOU HAVE A GOOD REASON

This variety of broadband dipole uses a resistor and a voltage balun to achieve SWR of less than 2:1 over its design frequencies. The balun and resistor are lossy. That is why they tell you not to paint the balun or resistor; the paint will burn off due to heat. The gain is around -6 dBd at best, and often is worse, typically by 2 S units. Many people use this antenna for ALE (automatic link establishment) because an auto tuner cannot respond fast enough to band changes that occur during the chirp that tests the propagation. The military and some emergency communications (Emcomm) people swear by it. If you are trying to use it for normal ham operations, you will swear AT it. It is also expensive. Leave this antenna to the intended users.

G5RV ANTENNAS ARE NOT FOR THE BEGINNER; SEPARATE TUNER REQUIRED!

The G5RV is often sold to new hams. A separate, outside the radio, antenna tuner is REQUIRED for this antenna. It adds expense. It adds loss. It requires balanced line, which can be cranky if it gets wet or near metal objects. Many people also use a BALUN, introducing even more loss. On the upper bands, the G5RV pattern looks like a malaria germ, often not delivering on its promise of gain. Gain is only useful if it goes to somewhere you want to talk to. On 80 meters, the G5RV is a shortened antenna, and performs significantly worse than a standard dipole. The G5RV is optimized for 20 meters, and a compromise for everything else. You MUST erect the G5RV to a decent height to make it work on 20 meters, no less than 35 feet. The small amount of dBd gain of a three half wave antenna on just one band (half an S unit) is not worth the hassle. My opinion is that you are better off with a tame old dipole at a good height.

One G5RV installation that delivers on the promise: Use the specified doublet length for the horizontal wire and the open wire feeder length straight down from the antenna. Get the horizontal portion as high as you can, at least one quarter wave length on 20 meters or 35 feet. Higher is better. Ignore everything else about the balun or coax feeder. Install the very best remote antenna tuner you can afford at the bottom of the balanced feeders. Run the very best coax you can afford from there to the shack. This gets rid of a lot of the lossy components, and gets your rig happy with the SWR, for maximum RF output. The possible problems with this style of installation are: lightning and EMP. For lightning, you need a disconnect between the balanced feeders and the remote tuner. It should also ground the antenna, especially if it is erected high. Otherwise, nearby lightning strikes can damage the microprocessor inside the remote tuner. For EMP, consult the prepper and Emcomm sites. An EMP is what happens when Korea detonates a high altitude nuke over you, and burns out all things electronic. Read up on balanced doublets of 44, 88 and other lengths in the Cebik article referenced elsewhere on this page. You will get a cleaner pattern than the classic 105 foot G5RV.

Newbies can damage the finals while adjusting resonance of the external tuner. Internal tuners in most rigs will not work with the 10:1 or more SWR found on the G5RV. Most internal automatic tuners handle the common dipole, a multiband vertical like the AV-620, or compact hex beams just fine. QSY quickly and work the DX with the rig's internal automatic antenna tuner; or spend time tweaking an external tuner. If you include the cost of the external tuner for the G5RV, a dipole is cheaper.

LAST MINUTE UPDATE! A NEW ANTENNA MATERIAL: MFJANIUM

There is a new discovery for antenna materials that makes all previous antenna modeling work obsolete. This new metal is an outgrowth of cutting edge quantum mechanics and string theory research. The new element is stronger than Titanium. It is very light, owing to its anti gravity properties, eliminating the need for heavy duty tower structures. In addition, there are no losses when an antenna constructed from this new metal. In fact, the gain figures are increased dramatically. This is caused by MFJANIUM™'s super conductor properties, which amplify signals on transmit and serve as a low noise preamp on receive.

You should redo your RF safety survey (required by the FCC) in installing an antenna fabricated from MFJANIUM™ because of its properties. There are reports that a beta testing DX superstation employing this radical new material caused some HAARP-like effects in the atmosphere, causing a hurricane to appear.

OK, is your bovine fecal matter detector operating? Glad I could help. Seriously, I do not want to pick on MFJ. They make a lot of good products for ham radio use. I have the antenna analyzer and one of their new solid state amplifiers, and they work really well. Just be aware that EVERYBODY in the business will try to make the biggest sounding claims for performance. Now you know how to sort it all out and make the comparisons for the best choice for your station.

REFERENCES

GENERAL ANTENNA ARTICLES, WITH A GREAT ANIMATED GRAPHIC SHOWING HOW A BEAM ANTENNA WORKS: NVIS reference for military style HF antenna: Yagi designs: 1960s NOVICE OPS, BUILD YOUR OWN 2 ELEMENT BEAM FROM SIMPLE MATERIALS: Free version of EZNEC antenna modeling software.
  

73,
Janis
AB2RA
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