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It’s hard to argue with that title, but in the case of 2M vertical antennas, the question one might ask is, “How much better?”   Generally, you can assume that longer (taller) antennas of this type have more gain.  But should you pay a lot more money for a 12 foot antenna than a 9 foot antenna?  Are there magical designs that squeeze extra gain from their height?  Is the manufacturer’s claim believable?  I will try to answer some of these questions, and give you you some real numbers that you can use to compare antennas.  Note:  When I talk about “height” here, I mean the overall vertical height of the antenna from base to tip, not the height above ground.

Gain in any antenna comes from reducing the sensitivity in some directions to improve it in others.  Energy is still conserved in this world, so what you take away from one direction has to appear somewhere else.  In the case of 2M vertical antennas for use with repeaters, home stations, and mobile units, we want an omnidirectional pattern, and we get gain by concentrating the radiation in the horizontal direction.  Unless you have friends in an airplane directly overhead, this is a good trade off.  And unless the friends are very rich, the airplane directly overhead will only be a mile or two away, so the antenna doesn’t have to be very good in that direction for them to hear you anyway.

Let’s start with a simple vertical dipole, fed at the center – it’s omnidirectional and has a gain of 2.15db over an antenna that radiates equally well in all directions, an “isotropic” antenna.   So, that’s called 2.15 dbi gain.  Let’s see what happens if we change the length of this dipole from the resonant half wave.  We’ll go both ways, shorter and longer, and we’ll ignore the fact that any length different from a half wave will need a serious, maybe even impractical or inefficient, matching network.  Here’s the result:

Gain of a center fed dipole vs. length

There are two curious things about this plot.  First, there is a minimum gain of roughly 1.8dbi, no matter how short we make the antenna.  This seems counter intuitive; how can an antenna that small (I stopped at 5 inches) intercept much signal?  The answer is that the fields around an antenna extend beyond the wires for a sizable fraction of a wavelength.  Due to it’s vertical orientation, it still concentrates power in the horizontal direction, so it still has gain.  It would not be easy to get a good efficient match to it – it looks like a small resistance in series with a small capacitor – but it still works and has gain.  The moral is, don’t try to estimate the “capture area” of an antenna unless it’s really big compared to a wavelength, and has a lot of metal involved, like a parabolic dish.  By the way, a really short dipole has a theoretical gain of exactly X1.5 over an isotropic source, and X1.5 is 1.76db.

The second curious thing is at the other end of the plot.  The gain increases nicely with length above  a half wavelength (about 40 inches), reaching a peak of 5 dbi at 100 inches, almost 3db better than a dipole.   But then it plummets and wiggles around.  What is happening above 100 inches is that the pattern is breaking up into multiple lobes.  At 100 inches, it’s very close to 1.25 wavelengths, a sort of magic number.  You could think of it as two 5/8 wave antennas end fed in phase.  It’s also sometimes called an EDZ, for “Extended Double Zepp”.  In any event, it’s the longest thing of this type you can have without the pattern going to heck.  For comparison, here’s the pattern of the 100″ antenna, and two small steps up, to 110″ and 120″.

100 inch dipole

110 inch dipole

120 inch dipole

Notice that at 120 inches, the main lobes aren’t even horizontal anymore, but point up and down at about 47 degrees.  Hint:  Click on the pictures to see them clearly.

Now if you wanted more gain and a sensible impedance, you might be tempted to try two half wave dipoles, one  above the first, and arrange to combine their signals.  You might also be tempted to reason that you will get twice the signal power (two antennas, after all), so you should net 3 db gain over one dipole for your effort, for a total of 5.15 dbi.  You would be right – sort of.  Here’s the situation as shown in my modeling program:

The blue lines are transmission lines which join the feedpoints of the two dipoles to a common feed point, represented by the blue circle on the right side of the drawing.  The feed lines are actually of equal electrical length, not unequal as the drawing might imply.

The gain of this particular arrangement, as reported by the modeling program, is in fact about 3 db better than a single dipole – but only when the dipoles are separated quite a ways apart vertically.  When the two dipoles are almost touching, tip to tip, the gain is only about 3.9 dbi instead of 5.15 dbi.  What is happening is that they are stealing signal from each other.  This is another example of the “capture area” being outside of the actual antenna structure.   Fortunately, this kind of interaction can also be of benefit; at about 120 inches total height, the gain is slightly better than 5.15 dbi, namely about 5.4.  The two antennas are messing with each other in a way that enhances the gain.  After that peak, as you increase the total height of the array, the gain just wobbles up and down a little around the 5.15 dbi value, the value you guessed might be right.  The next two plots show the gain vs. total height, and the pattern at 270 inches total height:

Gain vs. Total Height - 2 dipoles

2 dipoles 270" overall height

What the plots are telling us is, if you want to have a tall antenna to get more gain, you have to populate the vertical space with dipoles.  When the gap between two dipoles gets too big, you don’t get more gain, and you need to shove a third dipole in there.  Same with 3 dipoles; at some point as you spread them out, you need to add a fourth one.  So, let’s skip to the results for 3 and 4 dipoles, and put two, three, and four dipoles all on the same graph:

1, 2, 3, 4 dipoles

The black curve is one dipole, as previously plotted above, the red is two, green is three, and blue is four.  The obvious feature of this plot is that there seems to be a slightly lumpy curve of maximum gain vs. height.  Where 1 dipole leaves off, 2 dipoles pick up the trend, and so on.

Now if you have been reading antenna ads, you might ask about verticals composed of multiple 5/8ths wave elements.  They are apparently magic!  Let’s add in combinations of 2, 3, and 4, 5/8ths wave elements to our plot:

It’s hard to see the additions – they just fit in with everything else.  If you click on the plot to make it bigger, two 5/8ths is magenta, three 5/8ths is gray, and four 5/8ths is cyan.  Obviously, no great magic here!  (Expect a future article on 5/8ths wave antennas!)

Lastly, let’s put in a theoretical antenna in which the current is constant over the entire length.  It’s called a “line source”.  It’s sort of as good as you can do, and can’t really be done in practice, but it’s a little reality check on the various models of dipole combinations.

The orange dots are the line source. As you can see, they also fit in nicely with everything else. What’s all this mean?

If you know how tall a 2M antenna is, the gain is going to be very close to the maximum gain in the curves above.

Yes, it’s possible to make a 10 foot antenna that has LESS gain, but it’s not possible to make one that has more, at least not by more than a fraction of a db.

Now, let’s look at some gain and height points for real antennas, gain as reported by the manufacturers.  In some cases they report gain as dbd – gain over a dipole.  This could mean they calculate free space gain (as everything above is) and subtract the free space gain of a dipole, 2.15 dbi.  It could also mean they actually put the antenna on a test range and compare the gain to a standard dipole.  This gives them the opportunity to fudge the result by where they place the dipole vs. the antenna under test.  They can also measure the gain on a test range (where the ground reflections add up to give more gain) and then subtract 2.15db.  In other cases, they just list the gain – relative to what, they don’t mention.  If it’s dbi, that gives them the highest number, even if it’s honest. If they don’t say, then you can maybe assume it’s dbi – or a marketing fairy tale.  So, here are some popular antennas, gain per the manufacturer, shown on the plot of the line source:

Hustler: squares
Green:  HS3-14448
Black:  G6-144B
Red:     G7-144B
Cyan:   HX6-14448

Comet: + symbols
Red:     F22-GF
Black:  CX333

Diamond: bullets
Cyan:   X30A
Red:      F23H

Hustler:
Hustler may take the prize for gain exaggeration.  The G6-144B is a very well built antenna, a veritable aluminum outhouse – but at less than 10 feet tall, it’s advertised 6dbd gain is just not believable.

The HX6-14448 lists the same gain, but at 20 feet tall, twice the height, that gain is realistic – at a cost of about $1000!  Most likely Hustler is playing games with test range figures over ground vs. theoretical dipole gain figures in real space.

The G7-144B is a slightly longer version of the G6, with only slightly less exaggeration in gain.

Comet:
The C333 is a popular tribander (2M, 220, 440) found at the White Box stations and many personal stations in the San Juans.  The gain shown here is only at 2M, and it’s not too far off from reality.

The F22-GF is a dualbander, 2M/440, which is also within about 1 db of reality.

Diamond:
The Diamond X30A takes the prize for truth in advertising.  Mike, N7TLL, has one of these and we borrowed it for awhile to use on the repeater.

The present antenna on Cady is the Diamond F23H plotted above, a 15 foot 2M only antenna which has greatly improved the performance of the repeater.  It’s advertised gain is about 1 db above the line source,  not terribly over rated.

CONCLUSIONS:

1. If you are looking at buying a VHF antenna, DON’T choose one based upon advertised gain.   Determine the likely REAL gain from the antenna height and the curve above for a line source.

2.  There is no magic antenna design that beats the gain vs. height curve.  If an ad says there is, remember what a famous ham Yagi designer said: “P. T. Barnum is alive and well, and writing antenna advertisements.”

3.  Once you have settled upon the gain (and therefore height) you need, choose the antenna based upon price, reviews of mechanical reliability, wind ratings and your location, etc.  Do NOT pay any attention to reviews touting the superior RF performance of antenna A over antenna B.  If they’re both about 10 feet tall, their performance should be virtually identical!  Anecdotal evidence is best reserved for things like flying saucer sightings.

4.  The info above is all scaled to 2 meters.  If you are looking at 220 or 440, , scale the info.  1 wavelength at 2M is about 81 inches.  At 220, it’s about 53 inches, and at 440 about 27 inches.  Figure the reasonable gain vs height accordingly.  Don’t try to scale to 6M or 10M or lower frequencies.  The ground becomes more important, and antennas 2 or 3 or more wavelengths tall are generally not practical on those bands anyway.

When in doubt, connect your VHF radio to a bent coat hanger, and get on the air!

Ed K.

One Response to “Longer is Better”

  1. Bryan says:

    Ed, Excellent!

    I learned Short Antennas can be OK for VHF mobiles losing only half a dB.

    and as I’ve said before All antennas of about the same length have about the same gain.

    As for antenna gain claims, when I worked in microwaves it was standard practice to include the test setup when giving any data as it influences the results. Antennas are no different.

    -geu

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