# Tag Info

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 ...

8

Just about anything connected to your feedline makes an antenna. A lot of the concerns in transmitting antennas (low SWR, high efficiency, etc) are of diminished importance for receiving antennas, so it's surprisingly easy to make a perfectly good receive antenna with very little design effort. You don't necessarily need the antenna to be resonant or "tuned"...

7

The advantage of Litz wire —lower loss than solid wire— is realized below about 1 MHz. It is of little use above 160 meters. From https://en.m.wikipedia.org/wiki/Litz_wire Litz wire is a particular type of multistrand wire or cable used in electronics to carry alternating current (AC) at radio frequencies. The wire is designed to reduce the skin effect and ...

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 ...

6

There are plenty of ways to make antennas smaller. Unfortunately, all of these things also make the antenna less efficient. Economics provides a compelling proof: if efficient, small, low-frequency antennas could be realized, why do AM broadcast stations erect huge, expensive towers to support their enormous antennas? Antenna inefficiency isn't necessarily ...

5

The choke should go at the transition between unbalanced and balanced, which is where the RG-6 meets the ends of the loop wire. You talk of coiling coax: I suggest you coil it around a ferrite core and not just air. The trouble with air-core "chokes" is they have no resistance, only reactance, in common-mode. Depending on what the common-mode impedance is ...

5

Summary: substantial advantage can be realized for every meter increase in the height of the loop. Other parameters, including the metal roof, are much weaker factors in performance. I simulated a 3.8MHz 1-$\lambda$ hexagonal loop (no wire loss) mounted 3-m above medium-conductivity ground, surrounding a 5m x 20m grid of wires as shown here: Because the ...

4

One way would be to connect a HF transceiver to the SWR meter, and that meter to the antenna. Assuming proper licensing to broadcast on bands - tune to the middle of each amateur band and check the SWR while transmitting a tone (or whilst whistling). The lower SWR reading would indicate closest resonance to that band. Checking at various points around that ...

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

So, yes, to tune a resonant antenna, you'll need an adjustable capacitor or inductor. You'd usually go the capacitor route, since adjustable caps are smaller, cheaper, and more exact, usually. Bonus: There's electronically variable capacitors! They're called varactors or varicaps (or just variable capacitance diode), and they're a very mature (read: old &...

4

Forget about air-core chokes. G3TXQ's page that Phil links to in his excellent answer here shows just how much superior ferrite chokes are than air core choke baluns. Please visit k9yc.com and download rfi-ham.pdf. Jim Brown there on page 36 recommends a choke balun made by wrapping 8 turns of the coax at the antenna feedpoint through four stacked 2.4" ...

4

Does this mean this antenna can't be tuned to the 160M band, or does this just mean that the impedance is so different from 300 ohms that the SWR is useless? A little bit of both. If you get an off-the-shelf matchbox, which is usually advertised as being good for a 10:1 range or even less, then you can expect that it won't be able to make this antenna do ...

4

Small loops and full-wave loops are very different antennas. There isn't a well-defined distinction between "small loops" and "large loops", but a typical rule of thumb is a loop is "small" when its diameter is less than 1/10th of a wavelength. At such a size, the phase delay around the loop is negligible and thus the current ...

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

Great idea. Why not just make a single two-wire transmission line, called an open circuit stub, and cut it to length with a knife. $Z=-jZ_0Cot(\beta l)$ or $C=1/{\omega Z_0 Cot(\beta l)}$ where $\beta=2 \pi/\lambda$ and $\omega = 2 \pi f$ and $Z_0 ≈ 100 \Omega$ for a pair of 1 mm lines 1 mm apart, but use an online calculator to find a more accurate ...

3

Very unlikely. 100W SSB over 9,000 miles is already a challenging path, but these antennas are very inefficient which will make it nearly impossible. Let's simulate with VOACAP. This is 100 W SSB, with 0 dBi antennas at each end, and a quiet noise floor. If the receiver is near a city, man-made noise will make the situation much worse. Your best chance is ...

3

Antennas don't have ranges. The maximum range of a link is determined by many factors in addition to the information you have about the antenna, among them: the power output of the transmitter (not the power rating of the antenna) the minimum usable signal of the receiver the strength of interfering signals the amount and type of obstacles (e.g. trees, ...

3

Copper indeed would be better. Silver would be better yet. These materials, however, are heavier and less rigid than aluminum. So there is that. To aluminum's advantage, it can be laser cut at very high accuracy. Copper has to be water-jet cut, at lower accuracy and higher cost. Beryllium copper alloy would be stiffer, but it's a health hazard and no shop ...

3

In a traveling plane wave the ratio of the electric ($\vec{E}$) and magnetic field ($\vec{H}$) is always the wave impedance of the medium (377 $\Omega$ for air or vacuum). Further, the fields are always perpendicular to each other and to the direction of the wave ($\vec{k}$) This is a very fundamental property of electromagnetic radiation. The wave you ...

3

[This is not my best area of knowledge, but I figured I'd give answering a try; please correct me if this is wrong.] Electric and magnetic fields are only usefully distinguished from each other in steady-state (DC or static charge) or “near field” (distances comparable to the wavelength) conditions. Propagating radio waves are both electric fields and ...

3

That will work a little, but you might want to consider adding a variable capacitor to get more performance out of it by making the antenna resonant at the frequency you are interested in receiving. Alternately add a high impedance amplifier at the antenna, such as in this DIY hula hoop antenna. Note that this will only by good for reception, though. A ...

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

"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

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

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

I'm not familiar with the practical design of such antennas (so consider this answer a placeholder until someone more knowledgeable comes along), but here are some speculations: The current density in the capacitor plates is much lower than in the connections to the capacitor, because (in a typical air-variable) there are many plates, and within each plate ...

2

Given the shape and desired band, I think what you want is a small loop antenna. This would simply be a coil of wire wound in the shape of your cylinder, preferably with a capacitor wired in parallel to make it resonant at the desired frequency and match impedance to the receiver. Small loops have the type of radiation pattern you are looking for (in the ...

2

Rules of Thumb ah... I can answer that higher is better for DX (more lower angle lobes) lower is better for NVIS (more higher angle lobes) longer is better than shorter, but much longer than a full wave on lowest desired frequency is not needed shorter feed is better than longer feed to the tuner to minimise feed-line losses resonance is not needed if you ...

2

Well, I'm a little out of my depth, but this does seem to have all the usual parts of a gamma match: the matching section hanging off the main loop is a balanced transmission line, and the part connected to the BNC center is spaced off it, so the coax is coupled capacitively into that transmission line. There are two transmission lines, and two gamma ...

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