12
votes
Accepted
Why does an "electrically small" antenna have narrow bandwidth?
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 ...
- 51.6k
10
votes
Accepted
Why are there differences in antennas that depend on electrical component and magnetic component of a radio wave?
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 ...
- 51.6k
9
votes
Accepted
Do short waves pass through or reflect off concrete buildings?
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 ...
- 15.6k
9
votes
Accepted
Is a magnetic loop antenna a good option for those in an apartment?
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 ...
- 11.8k
7
votes
Magnetic loop antenna : loops vs selectivity
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, ...
- 51.6k
6
votes
Accepted
How do active loop vs. passive loop differ in terms of noise rejection?
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 ...
- 51.6k
5
votes
Magnetic Loop Antenna with RFI Choke
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. ...
5
votes
How to attach a magnetic loop antenna to the 50Ω coax of an RTL SDR?
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 ...
- 5,691
5
votes
Is a magnetic loop antenna a good option for those in an apartment?
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....
- 13.2k
4
votes
Optimising a small magnetic loop with 1:1 SWR but little radiation
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 ...
- 18.5k
4
votes
How do active loop vs. passive loop differ in terms of noise rejection?
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 ...
- 18.5k
4
votes
How to attach a magnetic loop antenna to the 50Ω coax of an RTL SDR?
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 ...
- 51.6k
4
votes
Real world performance of magnetic loop antennas?
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 ...
- 7,864
4
votes
Real world performance of magnetic loop antennas?
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 ...
- 141
4
votes
Accepted
Magnetic loop antenna : loops vs selectivity
The ideal length of a small loop is 1/10 wavelength to 1/4 wavelength.
Below 1/10 wavelength, the gain drops too much and the bandwidth starts to become unreasonably narrow.
Above 1/4 wavelength, ...
- 5,691
4
votes
High-Z RF amplifier for magloop RX vs Q-factor
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 ...
- 51.6k
3
votes
Why are there differences in antennas that depend on electrical component and magnetic component of a radio wave?
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 ...
- 1,646
3
votes
Real world performance of magnetic loop antennas?
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.
...
- 97
3
votes
Accepted
How to estimate or measure the max voltage of an air variable capacitor
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....
- 2,988
3
votes
Accepted
Loop Antenna - configuration?
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 ...
- 18.5k
3
votes
Accepted
Optimising a small magnetic loop with 1:1 SWR but little radiation
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 ...
- 51.6k
3
votes
Stealth HF station setup advice
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 ...
- 24.4k
3
votes
Using a loop antenna on a harmonic of the intended antenna frequency
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 ...
- 51.6k
3
votes
Is a balun really necessary for a small loop antenna?
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 ...
- 51.6k
3
votes
Is a magnetic loop antenna a good option for those in an apartment?
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 ...
- 3,905
3
votes
Accepted
Loop Antenna Construction
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 ...
- 51.6k
3
votes
What's the methodology of MDF magnetic field antenna?
It's a circular loop antenna with a built-in preamplifier for weak signals. when electromagnetic waves hit it, alternating current flows out the wires to a radio receiver that you plug it into.
For ...
- 907
3
votes
Accepted
What's the methodology of MDF magnetic field antenna?
The antenna portion is an inductor, likely a multi-turn coil of circular (or a polygonal approximation to circular) loops of some conductive material (such as wire). An inductor picks up changes in ...
- 13.2k
2
votes
Why does an "electrically small" antenna have narrow bandwidth?
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 ...
- 2,922
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