Electronic – Can 900MHz and 2.45GHz Antennas be differentiated by shape

There are hundreds of different antennas so for my simple attempt at an answer I'll concentrate on the "dipole" and I'm not going to go into formulas too much.

What physical factor affects the bandwidth of an antenna?

The impedance presented by the antenna is a major factor. If that impedance changes with frequency, then transmitted power will also change. Consider this: -

As dipole length becomes about half a wavelength the impedance becomes real and about 70 ohms. But go to 1 wavelength and the real impedance becomes infinite. This is a severe factor affecting bandwidth. Given also that the imaginary impedance change is quite severe, using a physical impedance (a series inductor for instance) to allow effective operation at slightly off exactly one half wavelength will result in a significant tightening of the bandwidth.

So, a short answer is that bandwidth depends on how you use the antenna as well as the choice of antenna.

There is a useful video at antenna.com on this page that might give a bit more insight. The same website goes on to discuss how thickening the dipole wire can make bandwidth wider: -

It now talks about VSWR (which is still all about the impedance presented by the antenna) and shows that a thicker wire produces wider bandwidth and, if the analysis spectrum were widened you would see much more significant effects: -

Antenna.com's conclusion below this is: -

Hence, this page can be summed up very succinctly: to get wider bandwidth, fatten up your antennas (this means: use more volume).

It's a really big subject but, for most regular antennas, the biggest decider of bandwidth is the driving electronics and impedance resonances. For instance, using a "short" dipole antenna antenna.com gives the following formulas and worked example: -

As an example, assume that the radius is 0.001 wavelength and the length is 0.05 \$\lambda\$. Suppose further that this antenna is to operate at f=3 MHz, and that the metal is copper, so that the conductivity is 59,600,000 S/m.

The radiation resistance is calculated to be 0.49 Ohms. The loss resistance is found to be 4.83 mOhms (milli-Ohms), which is approximatley negligible when compared to the radiation resistance. However, the reactance is 1695 Ohms, so that the input resistance is Z=0.49 + j1695. Hence, this antenna would be very difficult to have proper impedance matching. Even if the reactance could be properly cancelled out, very little power would be delivered from a 50 Ohm source to a 0.49 Ohm load.

For short dipole antennas that are smaller fractions of a wavelength, the radiation resistance becomes smaller than the loss resistance, and consequently this antenna can be very inefficient.

The bandwidth for short dipoles is difficult to define. The input impedance varies wildly with frequency because of the reactance component of the input impedance. Hence, these antennas are typically used in narrowband applications.

First lesson: Never trust Aliexpress product listings. That's an imperative.

Second lesson: Generally, the larger an antenna is relative to the wavelength, the more gain it can have.

That's why your home satellite dish isn't as highly directive as let's say this one:

so, first thing not to believe in the Aliexpress listing is this:

is this the real bandwidth? 700 MHz to 2700 MHz is a bandwidth of 2 GHz, at a center frequency of 1.7 GHz. In other words, this would need to be an ultra-wideband antenna (we can currently hardly build any antennas that can a bandwidth that is much larger than 1.1 of the center frequency). Yagis aren't ultrawideband.

So, what they don't tell you is that this antenna might work well for some of the bands they list below, but not as good for others. In fact, I wouldn't know how a yagi, whose design depends very much on the relation of wavelength to element spacing, could work for such non-integer-factor related bands. This sounds like a scam, in fact.

Now, aliexpress antennas for some reason (cheapness cost, of course, and seller competence) don't come with a measurement of efficiency over the specified frequency range.

If anything, that gain would be achievable at the upmost end of the frequency scale – again, gain scales with relation of antenna size and in a Yagi design, the number of elements that help reactively "focus" a delayed version of the excitation from the last element. Even so, I'd heavily doubt the 28 dBi.

So, rest assured, that, the workings of mythical antenna smiths nonwithstanding, your dish will outperform such a 30€ antenna scam.

As I said, often, legislation limits the Effective Isotropic Radiated Power. That is, you have to subtract the gain of your antenna from your transmission power. Hence, in transmit, you gain nothing by having a high-gain antenna.

Whether or not that's the case in Norway in the cellular bands is unbeknownst to me.