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(Note: I'm a beginner in antenna theory)

When browsing for low cost antennas online, there seems to be class of monopole antennas that have a coil or two built into the monopole, and are advertised to be suitable for operation in the GHz range despite being 200~300 mm in length.
For example:

From my understanding, the coil is meant to be a loading coil that adds inductance to the circuit to cancel out capacitive reactance from a shorter length antenna (e.g., when the monopole is shorter than a ¼ wave).
But in this case, the antenna is a lot longer than a ¼ wave for its operating frequencies (~75 mm for 1 GHz), which usually means the antenna is inductive rather than capacitive?
How does the loading coil work in this case? Is it even a loading coil at all?

There's also a few other components in the monopole whose purpose isn't clear to me:

  • Small cap at the tip of the antenna. I assume this is purely for mechanical reasons, and doesn't have any significant electrical role?
  • Thicker section of the antenna. No idea what this is for. Maybe mechanical for wind-loading reasons?
  • Loading coil-like helix as mentioned above
  • Pedestal which I assume acts like the ground plane, but it sure is tiny

Photo of antenna, with parts mentioned above highlighted

Image adapted from: AliExpress - OpenSourceSDRLab Store - 700MHz-2700MHz SMA Antenna for HackRF One


Overall, my questions are:

  • What are the highlighted components of this monopole, and what are their functions?
  • How does such a long monopole operate in the GHz range efficiently?
  • Are there any general design rules for such an antenna, such that I can tell the antenna's operating frequencies/gain/etc from a few quick measurements, or build one for a specific frequency?
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2 Answers 2

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advertised to be suitable for operation in the GHz range

Marketing will literally claim anything to sell an antenna; that's very much their job.

An antenna that doesn't come without a plot of efficiency over the relevant frequency ranges (or S11 or VSWR whatever sensible measurement relevant to you) should simply not get bought, no matter how cheap – anyone can sell you a piece of wet wood and claim it's an antenna for the 50 Hz – 12 GHz range. Promise!

Notice how even the reputable quectel antenna is sold as an antenna for frequencies up to 5 GHz – in whole bands this thing never gets better than 30% efficiency, see the image from the datasheet you linked to below. It's an antenna in these bands, alright, but is it a great antenna? No. Might a much smaller antenna have worked better, at least one band in isolation? Yes.

VSWR plot from linked datasheet

The radiation patterns, even for the lower bands, are surprisingly anisotropic, considering the device's symmetry.
My bet is on "used a rectangular ground (10 × 15 cm) plane sufficiently smaller than a wavelength for it to have effect on the antenna pattern"; that of course raises questions on the qualification of the tester.

xy plane at 900 MHz

From my understanding, the coil is meant to be a loading coil that adds inductance to the circuit to cancel out capacitive reactance from a shorter length antenna (e.g., when the monopole is shorter than a ¼ wave).

Let's look at the one band that the quectel antenna is actually good in (VSWR < 2): ca 1.8 GHz to ca. 2.3 GHz. Wavelength at 2 GHz is 15 cm, quarter-wave monopole would hence be 3.75cm long. This antenna is much longer than the wavelength it works well at. So, for that band, the coil can't be used to electrically enlarge the antenna.

OK, so let's look at the lowest frequency they advertise, 700 MHz. Wavelength would be 42.9 cm, quarter of that would be roughly 10.7 cm. So, the antenna, being 28.5 cm long, is still much longer than that.

So, no. This is not an electrically short antenna.

The coil might be a mechanical element – to make this outdoor (and probably automotive-mounted) antenna resilient to wind load and vibrations.

Of course, the coil affects the behaviour of the antenna, and thus the electrical design was done around it. Or maybe it wasn't; after all, the antennna really isn't that great for a lot of the bands it gets advertised for. But I think it was – the antenna doesn't behave really badly anywhere in the intended band, and achieving that is also not easy.

So I think what happened here was a design process where the coil was included for mechanical reasons, and to make the antenna have different electrical lengths for different frequencies, and then the number of turns and the position of the coil was optimized to give an acceptable performance over the whole frequency range.

Small cap at the tip of the antenna. I assume this is purely for mechanical reasons, and doesn't have any significant electrical role?

yeah

Thicker section of the antenna. No idea what this is for. Maybe mechanical for wind-loading reasons?

that, or maybe some higher-\$\mu_r\$ material to, like the coil, make the antenna seem shorter for high frequencies, in order to achieve some "generally acceptable" design.

Pedestal which I assume acts like the ground plane, but it sure is tiny

The pedestal goes onto the ground plane, which the user has to supply, e.g. in the form of a metal car roof. (typically, the outer conductor of the coax feed is slightly coupled capacitively to that ground plane, but that makes surprisingly little difference in practice, in my experience).

How does such a long monopole operate in the GHz range efficiently?

It doesn't, aside from that one range, where the frequency-dependent behaviour of coil, "???" component and the monopole rod kind of compensate each other well.

Are there any general design rules for such an antenna, such that I can tell the antenna's operating frequencies/gain/etc from a few quick measurements, or build one for a specific frequency?

Since you have very few parameters to optimize:

  • length of antenna
  • turns of coil
  • position of coil
  • position of ??? component

you'd probably just go into your microwave design tool of choice, set it up to run a simulation over a few hundred sensible combinations of these parameters (VSWR of a single rod is not going to take extremely long to simulate in e.g. CST), come back the next morning, pick one or two of the best results, and if you feel like it, optimize that a bit finer.

Note that it's not easy to rule out that specific parameters where given in a non-electrical manner. For example, this being an automotive antenna, there might be end-user preferences for an antenna that looks like a high-quality antenna to someone who's not studied electrical engineering, i.e. a long rod, with something that looks like a coil.

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  • \$\begingroup\$ The coil might be intended as a reflection/phasing device to adjust the radiation pattern. \$\endgroup\$
    – John Doty
    Dec 4, 2023 at 13:06
  • \$\begingroup\$ @JohnDoty I hadn't considered that, but then again, do the measurements from the data sheet look like that? \$\endgroup\$ Dec 4, 2023 at 13:57
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    \$\begingroup\$ The measurements do look like that, with a fair amount of radiation perpendicular to the antenna even when it's quite long in wavelength. On the other hand, they are a mess. A so-called monopole is only half an antenna, and the nature of the rest of it matters. What was the counterpoise? \$\endgroup\$
    – John Doty
    Dec 4, 2023 at 14:42
  • \$\begingroup\$ stated on page "13/21", it was a 10×15 cm plate of unspecified metal. \$\endgroup\$ Dec 4, 2023 at 15:23
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    \$\begingroup\$ That's what somebody thought the counterpoise was, but clearly there must have been radiating current flowing on the feedline and whatever it was connected to also. \$\endgroup\$
    – John Doty
    Dec 4, 2023 at 16:51
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How does the loading coil work in this case? Is it even a loading coil at all?

At the intended frequencies, this "coil" would have immense inductance, if you considered its inductance alone.
I have seen analysis of similar-looking antennas where this particular element was described more as a phasing section that allows the straight sections above & below to radiate constructively in the horizontal plane. If done correctly, radiation is enhanced toward the horizon at the expense of radiation in other directions...at one frequency.
Is it possible for this "coil" to achieve the goal described above over a wide frequency span?...very, very unlikely.

An example omnidirectional long antenna is shown below that can achieve a similar result:
phasing line on long antenna

Testing an antenna for gain is a complex procedure that few can pull off, making claims for gain difficult to verify.

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