New answers tagged

1

This sort of questions is probably best answered by modeling an antenna using either a NEC2 or NEC4 model. There are a variety of such packages available for this for DOS, Windows and Mac. That makes for much easier experimentation rather than having to cut and raise a lot of wire!


2

Practically speaking, the size and winding direction of the loading coil doesn't really matter. Unless the coils are monstrously huge compared to the wavelength involved, there will be a negligible effect on radiation. As for winding direction, if the coils are near the feed point of the dipole, you might see a difference if they are close enough to interact ...


1

Another example of an antenna without a connected feed point is a broadcast AM radio enhancer (MF band). A multi-turn loop of wire with a capacitor to tune it can be placed around or near, but electrically disconnected from a small AM pocket radio. The received signal will become stronger when the disconnected loop antenna circuit is tuned to (or very near)...


-1

An antenna that is small in relation to the wavelength always has a dipole radiation pattern. Electric dipole or magnetic dipole or a combination of both. For transmit small antennas have a narrow bandwidth because they have to be power matched to the transmitter. Currents become very large in transmit so efficiency suffers. Small antennas may have a wide ...


5

You could view an antenna as a two-port device. One port is the feedpoint, and the other port is "free space". The antenna's job is to transform the free space impedance of 377 ohms to the specified feedpoint impedance such as 50 ohms. Ideally we want an antenna with scattering parameters like so: $$ \begin{bmatrix} 0 & 1 \\ 1 & 0 \end{...


2

Not really answering the question but requesting more clarity in terms. From the IEEE definition of standard terms for antennas: I redraw the diagram of power flow in an antenna, (ignoring the polarisation mismatch branch) Where: $P_A$ = power available from the generator $P_M$ = power to matched transmission line $P_O$ = power accepted by antenna $P_R$ = ...


0

Effective coupling can result in either higher gain (into some feed point impedance) or to higher heating losses (due to the antenna's elements own real impedance == resistance). Either way, energy is being removed from the RF field (to maintain conservation of energy). I hypothesize (needs an NEC model to confirm the current maxima?) that a resistor of ...


-2

There are at least 3 reasons for using "special geometries" for RF antennas: resonant frequency, impedance match, and pattern. If the antenna is of a length and geometry that is resonant at the frequency of interest, then the antenna will be more efficient capturing energy from the airwaves at the frequency of interest. If the antenna geometry ...


3

A wire antenna that doesn't have a specific geometry may be called a random wire antenna. Such an antenna will correspondingly have a random performance with a random radiation pattern. In these cases, typically we care more about getting any signal out at all, rather than doing it efficiently or in a specific direction. A random wire antenna might be ...


1

There is a very simple answer regarding the diagram "Additional loss in dB caused by standing waves." It shows the fraction of the power sent into the cable that is converted to heat. (The rest is delivered to the antenna.) We could measure voltage, current and phase to compute power at both ends. There would be an additional loss if the ...


2

The question addresses a persistent confusion which is widespread especially in the ham radio community and can be tracked down to some published material (here no names!) and has survived since many years. However, a clarification can be straightforward and does not require complicated math. This answer starts from the “Total Feedline Loss” equation that ...


0

Yes, you can if the feeder length is right. It should be around 1/4 of the operating wavelength or any odd multitudes of 1/4 (i.e. 3/4, 5/4, and so on). For example, if the feeder length (measured from the feed point to the point where it touches the ground for the first time) is in the range of 0.2 - 0.3 wavelength, you can do quite OK without a current ...


2

This model makes two simplifying assumptions: losses are uniform throughout the transmission line, and there is a lossless tuner between the transmitter and the feedline adjusted such that the transmitter sees a matched load The first assumption isn't directly relevant to your question and is discussed in more details in the answer you linked. Now about ...


-5

The difference is that a (perfect) transmitter is trying to drive some (oscillating) current into or out of the feedline. Any reflected voltage coming back will bounce off a source trying to drive that junction to a different voltage, if it's out of phase. Or be amplified if it meets an in phase source. A receiver's passive impedance just eats the incoming ...


4

Let's say the antenna impedance on a given frequency is 100 Ohm, the feedline is lossless 50 Ohm, the transceiver input impedance is 50 Ohm. Between the antenna feed point and the feedline SWR = 2, 11% reflected power. This is a bit of dangerous thinking, because the distance between the antenna feedpoint and the feedline is zero. As such there can be no ...


2

I think you've answered your own question. In case a, with an antenna with 2:1 SWR and an otherwise lossless system, 11% of the power will be reflected and re-radiated by the antenna, so 89% makes it to the receiver. In case b, all of the power will be delivered to the receiver. Antennas are reciprocal, so all losses in transmit are the same as losses in ...


1

If the antenna isn't lossless (e.g. not superconducting), then some of the received RF energy reflected back into the antenna due a feedpoint mismatch will eventually be dissipated as heat in the antenna, or re-radiated as RF. But for received signals this is possibly only a loss of nanoWatts or picoWatts. However, not only is reflected signal energy ...


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