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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 ...

6

Your reasoning is not too far off. Say you attach a signal generator to an antenna, and then probe the magnetic and electric fields at many places around this antenna in a test chamber. The ratio of the electric field strength to the magnetic field strength is called the field impedance. For any antenna, several wavelengths away (in the far field), this ...

4

A "shielded loop antenna" is a very misleading name. It isn't an antenna, with a shield. It's an antenna made from a shield, with a feedline inside it. It's commonly stated that the shield blocks electric fields and not magnetic fields. But that's false: it's physically impossible. That's not to say that a shielded loop antenna has no merit beyond an ...

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 ...

3

"self-resonance" sounds like not quite the way to describe the issue. The problem is more simply that if the circumference is not very small relative to the wavelength, the antenna is no longer a small loop. As a rule of thumb, electrical engineers consider anything smaller than $\lambda/10$ to be "small". When things are "small" the analysis can be ...

3

VK1OD compared several small transmitting loop antenna calculators. The author concludes that there are serious errors in the ARRL analysis which have made their way into some (most?) of these calculators. He buttresses these claims with a detailed NEC model to estimate the structure, capacitor and ground losses which are so crucial to proper calculation of ...

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

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 ...

2

This is extensively and accurately covered in a great page by W8JI. It starts: Small Receiving Loops Small loops are often referred to as "magnetic radiators". Folklore claims a small "shielded" loop antenna behaves like an electrical sieve or filter, sorting "good magnetic signals" from "bad electrical noise". Nothing is further from the truth! ...

2

FORGET the third horizontal (yellow) loop antenna! I finally disabled mine because it caused me no end of trouble with noise pickup and thousands of spurious signals sent to the BT (Blitzortung) servers in Europe. Although it appears in the official documentation, I have not been able to find anyone who is using one. Most are using just two vertically-...

2

The lightning detection antennas are not used in any sense to triangulate or to direction find through any other method such as phasing. The three antennas are there simply to provide quasi 360° detection of lightning events. The actual direction finding is accomplished by analyzing the timing of detected lightning events from multiple sites using GPS time ...

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 ...

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

The short answer is that the shield becomes the actual antenna. It's addition makes it easier to construct a balanced loop which has deep nulls broadside to the loop. These nulls would obviously be helpful in fox hunting / direction finding. The long answer follows: First of all the picture included from the 1988 ARRL Handbook has been shown to be a ...

2

An ungrounded antenna element is subject to precipitation static (P-static) buildup that can result in arcing. At least one element of a dipole is usually without a DC ground reference while if one conductor of the transmission line to a loop antenna is grounded, P-static cannot build up to cause an arc. P-static can be caused by dust, snow, or rain and ...

1

As the circumference approaches 1/4 wavelength, it becomes increasingly difficult to tune it, as it is no longer a small loop and starts taking on properties of a a resonant loop, which is tuned differently and has a different radiation pattern. The primary principle of a small loop is that it is small enough that the RF in the loop is nearly all the same ...

1

I suggest you disconnect the loop from the central part of the variable cap . Instead connect to the other section...you then have a more efficient cap without the losses of the mechanical contact. By using both sides or both sections you convert the cap into a poor man's butterfly cap

1

I understand people like to use coax for antenna experiments but in this case I don't think its a good choice. That outer loop diameter is too small for 40 meters and it is a wire when it really needs to be a tube. What I would suggest is making this thing again but using copper tubing for the outer loop and simple insulated wire for the inner loop(inner ...

1

A third, horizontal loop, will give you an antenna which is horizontally polarized and omnidirectional in azmith, with nulls in the up and down directions. Would this benefit a direction finder? I wonder if there could be any advantage in placing loops several meters apart. Aren't directional nulls suffering from the close neighbour - other loop? I ...

1

The shield prevents (or greatly reduces) influence of the electric field component of the received signal, making the loop effectively a "pure magnetic" antenna. Shielding makes the antenna "quieter" than an unshielded loop or whip antenna by preventing the capacitive coupling of localized noise sources. It is particularly effective in shielding against the ...

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