Every antenna has a percentage of receiving frequency range. Now I have one question---what is the reception percentage of lower and upper frequencies for a ground plane monopole, 1/2 wave dipole, and discone antennas? Say, I have made a monopole for the SW center frequency of 14 MHz and I choose the whip to be 5 meters long. So, upto which lower and upper frequencies from 14 MHz it going to receive? I need to know the same for the other antennas but for the discone I would like to know for the VHF 100 MHz.
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$\begingroup$ Please also see this excellent answer to “calculating the bandwidth of an antenna” $\endgroup$– webmarcCommented Aug 18 at 21:41
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3 Answers
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In general, the bandwidth of an antenna is affected by the diameter of the elements in the antenna. The other answer covers this well.
The discone antenna is basically a dipole with half of the dipole having a variable diameter, which gives it an extremely wide bandwidth in terms of resonance. The lower frequency cut off for the discone is basically its design frequency. The upper SWR frequency cut off is somewhat limited by the way the antenna is constructed and losses caused by materials and feeds, not by the design of the antenna.
However, the radiation pattern of a discone (and thus the gain) is a bigger factor than its SWR bandwidth. As the frequency increases above the discone's base design frequency, the radiation pattern lifts up and approaches isotropic, and the horizontal gain decreases.
So, while the discone does have a SWR bandwidth limit (which you can increase even more by stacking discones), the parameter we care about instead is radiation pattern and gain. It's tricky to pick a threshold for this, as the antenna doesn't stop working, the signal just gets weaker. This of course can be somewhat overcome by the sensitivity of the radio and the transmit power, making it even less clear what the upper cut off frequency should be.
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$\begingroup$ well worthy of an upvote! I wouldn't compare a discone to a dipole with variable diameter – for that, the upper branch of the dipole would need to conduct in the same directions as the main axis – but as the rotational variant of a wideband horn antenna :) But that's really just how you "think" about the antenna (and the antenna doesn't care); and in the end, a discone is relatively wideband. I'm just not sure it is always helpful to think of antennas as modifications of simpler antennas, that very often tends to lead astray. Anyways, great answer! $\endgroup$ Commented Aug 18 at 12:22
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2$\begingroup$ It's often not whether your antenna is still an antenna, it also needs to work in the direction you need it to work in. $\endgroup$ Commented Aug 18 at 12:32
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First of all, you need to define what your "reception percentage" would really mean to you: is it the bandwidth at which the antenna is still at least half as eefficient as at its optimum frequency? That'd be the 3dB bandwidth. There's other measures, which often are better suited for any particular thing you want to do. For example, for transmitters, often the bandwidth at which the reflection coefficients drop e.g. below -10 dB is relevant (because you might not want to reflect back more than 1/10 of the power into the transmit amplifier, for example).
Having figured out what you mean with that:
That depends on the construction of these antennas!
- in the monopole case, the only "easily adjustable" variable is the radius of the conductor. The higher the diameter, the wider the bandwidth, but these things become pretty specific to things like ground plane conductivity. I'll be honest, a "whip" is not going to be straight enough to apply any closed-form formula well enough. OK, you can apply the formula, but your result will differ from reality enough that you could as well guess. So, measure the beast! VSWR over different frequencies is the usual measurement you make.
Here's a typical graph of monopole reflection over frequency, assuming differently thin conductor ($A$ is the radius). This is all the same monopole length!
Here, the lower the better: an |S11|of -16 dB, for example, means that only 1/40 of the energy put into the antenna at that frequency gets reflected back into the source. At 0 dB, all of the energy is reflected back and the antenna doesn't act as antenna at all.
Antennas are reciprocal: As good as it is for transmit, it is for receive.
Note that you need to multiply the $A$ from the graph by roughly seven to get similar results for a monopole for 14 MHz.
Nothing really changes the fact that you won't do much above maybe 15% of center frequency as 3 dB bandwidth for a monopole antenna.
- A discone for 100 MHz will usually be a shortened discone made from radiator elements. So, yeah, another thing that will be hard to calculate, but likely quite possible to simulate (discones are typically not dependent on external ground planes as much).
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$\begingroup$ I am not getting the right answer because you all are not understanding my question. It's not about the percentage of the signal strength But I am asking about the frequency range that the antenna would receive. A 75 cm whip antenna can receive 100 MHz but from 100 MHz to what upper frequency and lower frequency it will also receive? Can it receive down to 88 MHz or up to 108 MHz? $\endgroup$– user28409Commented Aug 25 at 6:32
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$\begingroup$ @Ofosot we understood that perfectly well. You must define first what you mean with "can it receive 88 MHz?". Is "receiving, but with 30 dB less gain than at 100 MHz" still "receiving"? We can't know how little efficiency at any given frequency still means "received" to you. So, we give you a graph, in which you can look this up. $\endgroup$ Commented Aug 25 at 11:15
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$\begingroup$ I don't need in dB but I need the value in percent. Usually, most FM receivers come with 75 cm whip. But a 75 cm whip would resonate best at 100 MHz. But it can still receive from 88 MHz to 108 MHz. But I need to know the formula or method to calculate the percentage of signal strength a whip would receive from the resonant frequency to the frequencies above and below it. $\endgroup$– user28409Commented Aug 31 at 14:49
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$\begingroup$ @Ofosot oh, you don't understand what dB means! Ah sorry, I didn't see that. You can directly convert percentage (of received power at the best frequency) to dB (of received power at the best frequency) directly; the formula for decibel is easy: ratio/(dB) = 10·log(ratio), whereas percentage is just ratio/(%) = ratio/100. $\endgroup$ Commented Sep 1 at 13:05
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If all you are concerned about is receive your best bet is to just build one and see if it meets your needs. For example I have G5rv dipole like antenna some 110 ft long. I hooked it to my sdr and it received all of the local FM broadcast stations just fine. Not near as well as an antenna specifically for FM but it worked adequately so that it just wasn't worth the bother to make a special antenna just to receive FM broadcast.
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1$\begingroup$ On what basis you made it 110f long? as per my calculations, a 1.5 m 1/2 wave dipole is enough for FM. $\endgroup$– user28409Commented Aug 31 at 14:42