# Tag Info

12

Short answer: electrically small antennas have a relatively low radiation resistance. With less resistance, the resonance is less damped, meaning a higher Q factor and consequently less bandwidth. To expand on that a bit, consider a children's playground variety swing. It's essentially a pendulum, and depending on the length of the swing and the mass of the ...

9

If you're talking about placing the antenna indoors, there are no good options. A magloop might be your best option, but: It's still going to receive plenty of noise (stuff you may have heard about them being insensitive to E-field noise is overblown; it's only true for a specific range of distances, and in any case isn't enough of an effect to override the ...

8

The concrete is relatively transparent to radio waves of such large wavelengths (it attenuates, it doesn't reflect). However, steel bars within concrete typically convert that concrete to a solid reflector from the perspective of a wave with such a large wavelength. Basically, that effect scales: Just as your microwave oven's front door has a metal plate ...

8

In free space, the electric and magnetic fields are always in a fixed ratio, a physical constant called the impedance of free space, about 377 volts per ampere. The two are always in phase (and thus have identical wavelength) but in orthogonal angles. In fact, the magnetic field is explained by relativity to be the effect of length contraction of moving ...

6

Loops (active or passive) don't have the RFI rejection capabilities they are often claimed to have. RFI is electromagnetic radiation just like the signal, and it's not possible to design an antenna which accepts one but not the other. See Can I reduce RFI/noise at the antenna? Loops have the same directionality as a dipole, however their polarization is ...

5

From my current experience of using Cha F-Loop 2.0: Yes, you most definitely need an RFI choke to use a magnetic loop antenna, and a good one. Magnetic loops are not as good as you might think. Particularly they are very narrowband - you have to seat near it and tune the capacitor all the time. Or you need a remote tuning device. Despite the fact that some ...

5

You may be overthinking this. To make a long story short, just connect it and tune the capacitor to resonance and don't worry about the impedance of your pickup loop. When tuning the radio to antenna system for a transmitter, you have to impedance match the load to minimize reflections because power loss is typically a big deal. Reflections lead to high ...

5

Two factors explain the wider bandwith: Firstly, as the frequency increases, bandwidth will necessarily increase even if the Q factor remains constant. There are a number of ways to define Q factor, but the result is the same so consider this definition: $$Q = {f_r \over \Delta f}$$ rearranged: $$\Delta f = {f_r \over Q}$$ So as $f_r$ (the resonant ...

5

In, meaning Inside an apartment, a magnetic loop may not be a good idea for transmit, as the RF magnetic field can couple very strongly to any household wiring or appliances inside, window frames, etc., possibly causing hazards as well as severe pattern distortions and losses. For receive it might be Ok if the walls (stucco wire, metal siding, etc.) do not ...

4

Poor Loop Gain The small (<0.1 wavelength in circumference) loop antenna has very low gain. While the size somewhat lowers the directivity, the primary reduction in gain is due to inefficiency as a result the very low feedpoint resistance. When an antenna has reduced gain, not only are the desired signals reduced, so is any noise (QRN) since noise is ...

4

Small loop antennas require great attention to construction details. This is due to the (in)efficiency of a small loop antenna. By comparison, when constructing a simple half wave dipole, a few ohms of RF resistance has very little effect on its performance. This is because the radiation resistance of a dipole is around 73 ohms. So an additional few ohms of ...

4

You don't necessarily need a coupling loop. Nor do you need to worry too much about matching the antenna to 50 ohms. Firstly, because SWR is not very important for receive performance, and secondly because on a cursory search it seems the RTL-SDR at least when modified for direct sampling doesn't have a 50 ohm input impedance anyway. The simplest connection ...

4

I suspect they are built horizontally simply because they are easier to construct and support, especially at HF sizes. Due to constantly changing ionospheric conditions, the received phase of HF signals is random, so that's not a reason to prefer any particular polarization. Further, the ground does affect radiation patterns, but it's not so simple as "...

4

Under some circumstances, this can be true. What Leigh seems to be talking about is nulling out a local noise source, which a rotatable loop excels at. Between the loop and a vertical or dipole, in my experience the loop will often do what he says. But a loop compared to a beam? Not so much. I have always respected the author, Leigh Turner VK5KLT for his ...

4

I've been using magnetic loops for 20 years A magnetic loop can work well if it has two qualities: enough power handling (a 5 kV vacuum variable will handle 100 Watts, 15kV will do a kW); and a decent remote tuning setup. tuning is critical, as Eham reviews show the main dfficulty is getting the SWR properly dipped. Thanks to the popularity of robotics, ...

4

Certainly it is possible to build a high-impedance amplifier. If you resonate the loop with a parallel capacitor (which perhaps includes the input capacitance of the amplifier), you've made a parallel LC circuit which at resonance has minimum current and maximum voltage. A high-impedance amplifier is what you'd want to best extract that signal. But if your ...

3

Whether an antenna is called an "electrical" or "magnetic" antenna, the generated and captured far-field electromagnetic photons of the same wavelength are identical. The difference in the two antennas occurs in the near-field where the electric field is dominant for electrical antennas and the magnetic field is dominant for magnetic antennas. The difference ...

3

Getting this antenna to perform, as with any electrically small antenna, is a question of minimizing losses. Without being able to test your particular design it's difficult to say exactly what the losses may be, but here are some possible culprits: Dielectric and conductive losses due to coupling into the wall and things inside it. Try moving it away from ...

3

From the ARRL Handbook... Spacing inches ___ V_peak 0.015 ___ 1000 0.02 ____ 1200 0.03 ____ 1500 0.05 ____ 2000 0.07 ____ 3000 0.08 ____ 3500 0.125 ___ 4500 0.175 ___ 7000 0.25 ____ 9000 0.35 ___ 11000 0.5 ____ 13000 Also note that these are mere recommendations. Actual high voltage you will see of course depends on the reactance and input power to ...

3

As it's name implies, a loopstick antenna is a magnetic loop antenna and is one of the best choices for an electrically short antenna; necessary because broadcast AM wavelengths are hundreds of meters long. Rather than a regular loop, a loopstick utilizes a large amount of (usually Litz) wire wrapped around a ferrite rod. This forms an inductor which is ...

3

Adding a smaller coupled loop will have a marginal result. The smaller, coupled loop seen on some small loop designs is there to provide impedance matching. If you wish to improve the gain of your loop antenna, consider adding a second turn to the main loop. Adding an additional turn to a small loop antenna will improve its gain. But the increase in gain to ...

3

The design of antenna called a magnetic loop has the disadvantage that it has a very narrow bandwidth, and the frequency is set by a variable capacitor mounted on the antenna. This means that changing frequencies requires you to go to the antenna and turn a knob. If your operating position is not next to the loop, this is very inconvenient. The loops in the ...

3

Dipoles are resonant on their fundamental frequency, and all odd harmonics. So a dipole resonant at 10 MHz is also resonant at 30, 50, 70 MHz and so on. At the fundamental frequency it takes one cycle for a wave to travel the full length of the antenna and back. At odd harmonics it takes 3, 5, 7, and so on cycles. Only the odd harmonics are considered ...

3

I use a magnetic loop as my main antenna since I am a renter. It is outdoors about 10 feet from the building and about 10 feet above ground level. It is approximately 1 meter in diameter. Performance: usable frequency range: 40 meters through 15 meters. 15 meters similar to dipole you can rotate. 20 meters similar to dipole you can rotate. 30 ...

3

You may be overlooking that the feedline is itself an antenna. If signals came only from the loop, then sure, there would be no need for a balun. But the feedline is also effectively a big vertical antenna in the common-mode, and is going to pick up signals as such. Again by the KCL, the currents picked up by this vertical, at the end of the coax where the ...

3

Definitely read Hobbs's answer re ins-and-outs of mag loops, spot on... I don't have anything to add on that part. Another option is to make friends with the building super. Depending on his/her disposition, you may be able to get permission to drop a line from the roof to your window. I had GREAT luck with this in one of the buildings I lived in (Washington ...

3

When I connect the probe from my oscilloscope, does the probe also act as an antenna? As it appears in your photo, yes. In some applications this may not matter, but in your case it sounds like your experiment relies on the loop having a predictable pattern for direction finding. In this case, having the probe (and also the scope chassis, and its power cord)...

3

When tuning can be achieved then the only variables you have to optimize is the coupling and the compensation of the feed loop inductance (because k < 1). Just modify the area of the feed loop by bending the wires to a smaller or a larger area. You will end-up close to 50 Ohm. But resonance of the loop does not exactly correspond with lowest impedance. ...

2

The prime reason that antennas electrically small in terms of wave length also have useful SWR bandwidths that are relatively small is due to the low value of capacitance of those small antennas to the radiation environment. That low capacitance results in relatively higher capacitive reactance across their feedpoint terminals, which changes more rapidly ...

2

Larger multi-turn loops with the turns too close together can be degraded by the "proximity effect": the turn in the front (or in the case of a spiral, the outermost turn) may have a certain current value at a given point on that turn; but the turn adjacent to it next to that point may have a current that is quite different. This can cause (among other ...

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