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My favorite HF bands are 20 meters and 10 meters.

I am currently using a multiband vertical antenna, but really it's not so great for DX'ing. I am looking for a good upgrade.

I'm still learning alot about antennas in general.

I would like to try a 3 element Yagi antenna instead to improve my performance.

From some research, beam antennas only seem to come as monoband antennas.

If I have a full size Yagi antenna resonant for 20 meters, shouldn't this also be very effective on 10 meters, due to the relationships between the wavelengths at those bands? ( 20 meters being twice that of 10 meters) . Or am I completely wrong here?:) Thanks in advance.

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    $\begingroup$ A 20M antenna is likely resonant on 10M, but the feedpoint impedance at the center is completely wrong, likely many kiloOhms rather than anywhere near 50 or 73 Ohms. Usually traps are needed to shorted all the 20M elements when driven in the 10M band. But Yagi-Uda's with traps for 20/10 are made, not sure by who, but my club has an old one they used for field day. $\endgroup$
    – hotpaw2
    Jul 9 at 21:30
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    $\begingroup$ @hotpaw2 Comments are not for answering questions. But this would make a great answer. :-) $\endgroup$
    – Mike Waters
    Jul 9 at 23:05
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From some research, beam antennas only seem to come as monoband antennas.

Not true — actually 20/15/10m tri-band beams are very popular. Multi-band yagis come in two basic flavors.

  1. Trapped. Works on the same principle as a trap vertical to alter the effective electrical length of the elements at different frequencies, except there are way more elements to trap. A 20/15/10 monopole needs two traps; a 20/15/10 3-element beam needs 12. Looks like this.

  2. Interleaved elements. Basically like multiple beams sharing the same boom, with the driven elements fed in parallel. May be advertised according to the total number of elements, e.g. a 24-foot yagi with 3 elements on 20m, 3 elements on 15m, and 4 elements on 10m is a "10-element" tribander and looks like this.

Each design has its pros and cons (the first one will have some trap losses, and has to use a compromise element spacing that works okay for all three bands; the second one might have some pattern distortion or SWR oddities caused by interaction between bands, and will be physically larger) so it's up to you to decide which one suits you.

A third option is a log-periodic antenna. While they look a lot like Yagis, they're electrically different, and offer continuous coverage of a frequency range of an octave or more. HF log-periodics for 20/17/15/12/10 do exist, although my impression from limited research is that they're bigger, heavier, and have more elements, for less performance than a smaller, lighter tribander.

If I have a full size Yagi antenna resonant for 20 meters, shouldn't this also be very effective on 10 meters, due to the relationships between the wavelengths at those bands?

As others have very well said, no. A 20-meter monobander won't be good on any other ham band, and even if you operated it on a harmonic where it was resonant, the pattern would be pretty useless.

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    $\begingroup$ There is also the SteppIR yagi idea, which has variable-length elements; the elements are metal tapes in tubes, and the elements are made shorter or longer by reeling or unreeling the metal tapes with stepper motors as directed by an electronic controller. The element spacing is fixed on the boom, which is an engineering compromise, but they are effective and popular, although expensive. $\endgroup$
    – rclocher3
    Jul 12 at 15:48
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Engineer999.

The driven element of a yagi antenna is normally a half wave dipole, and it's true that a dipole antenna is resonant on multiple harmonically related frequencies. An ideal 10 m long dipole for example is resonant on the frequencies where its length is an integer multiple of one half wave length for each frequency ie: 15 MHz (.5 λ), 30 MHz (1 λ), 45 MHz (1.5 λ), 60 MHz (2 λ), etc.

However the distances between the elements along the boom of a yagi required to provide forward gain and reverse rejection must be matched exactly to suit the wavelength of the frequency of operation, and unfortunately these required distances aren't harmonically related in the same direct way as for element length and dipole resonance and don't follow the same rules.

To explain in more detail, the distances between the elements and the element lengths are precisely chosen so that RF energy induced in the elements from re-radiation from the other elements adds in the forward direction, and subtracts in the reverse direction, which gives the antenna forward gain and a front to back ratio. The interaction between element lengths and positioning is complex and only works at the design frequency of the antenna.

Another problem is that the impedance of the antenna will only be correct at the design frequency. At 14 MHz, if the impedance is perfect at 50 ohms, then the SWR will be good. But when operation is changed to 28 MHz, the antenna impedance will change and the SWR will be bad.

To elaborate further, for a dipole in free space, the fundamental resonant frequency is determined purely and only by the impedance of free space and the length and cross-sectional area of the antenna elements, and at resonance, the feed point will present a resistive impedance with zero reactance. If the dipole is an odd integer multiple of a half wave length (in your case it's 1 x half wavelengths at 14 MHz) the resistive impedance will be between about 70 Ω and a few hundred ohms, but if it's an even integer multiple of a half wave length (it's 2 x half wavelengths at 28 MHz), then the impedance will a very high value which is difficult to work with. Incidentally the exact value of impedance is related to the number of wavelengths and is independent of the actual frequency.

Having said that, without a matching network, the feed point impedance for a yagi at resonance is much less than 50 ohms, due to the presence of the reflector and director elements. To compensate for this, the length of the driven element is often chosen to be slightly less than a half wave length so that a matching network such as a gamma match can be used to cancel out the resultant inductive reactance caused by the shortened dipole while at the same time allowing an exact match to 50 ohms to be obtained. This arrangement is highly sensitive to the frequency and a correct match at 14 MHz definitely won't be correct at 28 MHz.

To complicate matters, the presence of the additional elements which surround the dipole driven element have an effect on it's electrical length, and this effect is frequency dependent. So if the antenna is tuned say for 14.000 MHz, if you double the frequency to 28.000 MHz, the antenna dimensions haven't changed, and the effect of the nearby reflector and director on the electrical length of the driven element won't be the same at the new frequency, and the impedance will change for that reason also.

So to answer your question, a 3 element yagi designed to work with good SWR, gain and front to back ratio on 14 MHZ, when used on 28 MHZ, won't behave like a yagi and will have little gain or front to back ratio, and the impedance will change and the SWR will be bad.

Hope that helps !

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A resonant 20 meter monoband Yagi will not work on 10 meters.

Have you considered a 3-band 10-15-20 Tri-bander? It is a beam, and definitely not a monobander.

There are also smaller tri-band directional beams, such as the Hexbeam.

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Dipoles are resonant on odd harmonics of their fundamental frequency. So a dipole resonant on 20 meters, or 14 MHz, is also resonant on 14×3 = 42 MHz, 14×5 = 70 MHz, and so on.

10 meters is (approximately) an even harmonic of 20 meters, so this does not work. At the even harmonics you get zero reactance in the feedpoint impedance, but the resistance is very high. You could say it's "anti-resonant".

Furthermore, although a dipole's feedpoint impedance repeats at odd harmonics, its radiation pattern is not the same at each harmonic. Yagi antennas rely on constructive interference between the driven element and the parasitic elements, but since the radiation pattern of these elements is different at harmonics the antenna may not continue to work as designed.

That said, there are directional antennas that work on multiple bands. They often rely on traps and/or combining several antennas together at the same feedpoint. For example, a "hex beam" is one such antenna which you can build yourself or buy in a kit. There are also some that look more like a Yagi just with unexplained extra elements.

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    $\begingroup$ Phil, the first part of your answer is incorrect, a dipole is resonant at every integer multiple of a half wave length, and you're getting resonance confused with impedance. A 1 wave dipole at 10 meters is exactly resonant and as such has zero reactance. The value of resistance or reactance in the impedance doesn't determine whether or not a dipole is resonant, only the electrical length and diameter of the antenna elements determine resonance. In a yagi, the other elements change the electrical length of the driven element and have the effect of lowering the resistive impedance at resonance. $\endgroup$
    – Andrew
    Jul 11 at 13:03
  • $\begingroup$ See en.wikipedia.org/wiki/Dipole_antenna which states "Thin linear conductors of length l are in fact resonant at any integer multiple of a half-wavelength" : resonant lengths l = n(λ/2) where n is an integer multiple. When you post answers like this you are propagating misinformation which is rampant amongst amateurs and the result is that nothing makes sense and everyone keeps getting confused. $\endgroup$
    – Andrew
    Jul 11 at 13:03
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    $\begingroup$ @Andrew IMO, at the very least here Phil is talking about what is practical. Please, do not say that Phil is propagating misinformation. FYI, he is one of the hams here who has an in-depth and accurate understanding of antenna physics. $\endgroup$
    – Mike Waters
    Jul 11 at 18:58
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    $\begingroup$ @Andrew Are you sure your definition of resonance is the only one? Are you sure whoever wrote that on Wikipedia is an authority on the subject? $\endgroup$ Jul 11 at 19:46
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    $\begingroup$ I don't think Wikipedia or @Andrew is wrong, it's just that there are a few possible definitions of a "resonant" antenna. In one sense, it just means there is some resonant mode at that frequency (why might not put a node at the feedpoint). In another sense, an antenna is resonant when the reflected wave is in phase with the applied wave at the feedpoint. By this former definition, even multiples are resonant, by the latter definition they are not. Either definition is fine, and it's usually pretty obvious which one is being used, but calling one of them "wrong" is just pedantry. $\endgroup$ Jul 12 at 20:13

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