19

Dispelling the Myth To begin with, the typical HF SWR meter does not have the ability to separately sample the forward and reverse power, voltage, or current. Any description of the device or its circuitry that suggests this capability is flawed. We can show this empirically with two different experiments. Experiment 1 Connect a 100 ohm resistor directly ...


17

If an antenna analyzer shows 1:1, does that mean it's an ideal receiver as well? No. Assuming we're talking about a characteristic impedance of 50 ohms, a 50 ohm resistor (otherwise known as a dummy load) will show a SWR of 1:1, although it will almost certainly perform very poorly as either a receive or transmit antenna. The low SWR simply tells you that ...


13

I'm going to assume we are discussing ideal, resonant dipoles. Consider what's happening inside the dipole. Say you are transmitting a carrier. Say at one point in this carrier's cycle, the voltage is shoving all the charge carriers to the left. What makes the dipole resonant is this: those charge carriers get pushed down the wire. As the approach the end, ...


13

There are two different parts to antenna tuning: transmission line impedance matching and resonance. The antenna is at resonance when it presents a purely resistive load to the transmitter. That is, the reactive component of the load is 0: there exists no inductance or capacitance in the load. However, resonance says nothing about the value of the ...


13

With the information you've provided, there is no way to tell. Feedpoint impedance and VSWR have no relationship to antenna efficiency. See What is the relationship between SWR and receive performance? The feedpoint impedance also has no relationship to antenna efficiency. A "50Ω antenna" means the impedance at the feedpoint is 50Ω. The impedance at other ...


12

In theory, if you had lossless conductors in the antenna and a lossless matching network, your shortened, 1 foot dipole would have a gain of only ~0.7 dB less on 160 meters than the gain of a full size 1/2 wavelength dipole. But the world is far from perfect. The efficiency of an antenna is defined as: $$\text {Efficiency}=\frac {R_r}{R_r+R_l} \tag 1$$ ...


10

A cross-meter is capable of showing you three measurements simultaneously: Output Power Reflected Power SWR From this image by Axel Schwenke on Wikipedia, you can see that the needle on the left indicates forward power, and the needle on the right indicates reflected power. The observed intersection of the two needles can be used to indicate the SWR of the ...


10

Simplified answer: There is no relationship between SWR and receive performance. There is one condition for this simplification to be true: the received RF noise floor must be above your receiver's noise floor. Beyond this, anything you might do to increase the output from the antenna does nothing to increase the signal to noise ratio, which is a better ...


10

As you can see from the following equation, it is definitely not that easy. What I would do is draw a 10:1 SWR circle on a Smith Chart and assume that your tuner can match all of the infinite number of impedances inside that 10:1 SWR circle. If you don't know how to read impedances from a Smith Chart, it would be worth your while to learn how. The green area ...


8

How can I transmit without an antenna tuner? Simple. You use an antenna that's already tuned. There are plenty of radios that operate without any tuner. For example, basically every VHF radio. One reason for this is that most VHF antennas are purchased rather than manufactured by the amateur, and the antenna manufacturer has already tested and tuned the ...


8

Executive Summary Assuming: air dielectric (insulator) 50Ω characteristic impedance Then for round coax, make the inside diameter of the outside conductor 2.302 times larger than the diameter of the inside conductor. If the shield is square, and the inner conductor is still round, make the inside length of one side of the shield 2.134 times larger than ...


8

It's very difficult to predict the impedance of an end-fed wire, other than to say it's high. Usually it's determined empirically. You are looking for a theoretical formulation. Consider, the feedpoint is a voltage source which makes a difference in electric potential between to things. The end of the dipole, and...what? simulate this circuit – ...


8

As resistance is to DC circuit analysis, impedance is to AC (or RF) circuit analysis. Now let's take a closer look at what that means, from several angles. I'm going to not include any of the math, and just state without proof the various concepts' relationships. In simple, idealized circuit analysis of DC or digital circuits, we often assume that lots of ...


7

For many modulations, the modulation is very slow compared to the propagation delay of the feedline. For example, SSB is typically limited to no more than 4 kHz. That corresponds to a wavelength of of 75 km. As long as the feedline is significantly shorter than this, then the delay due to the feedline is negligible. It may be easier to understand ...


7

Once a ground plane antenna is 0.5 wavelengths or so above ground, there is no significant effect from the ground on the impedance of the antenna so it can be ignored. A close approximation of the real part of the feedpoint impedance in ohms of a 1/4 wave ground plane antenna in free space is given by: $$Z_{real}=20+\frac{2}{3}\theta \tag 1$$ where $\...


6

It looks like you're looking for an intuitive, practical understanding rather than precise definitions, so I'll see what I can do with that, with my own recent learning. The reason you care about impedance matching is that impedance mismatches cause the signal to be partially reflected — some of the energy is going the opposite direction than you want it to ...


6

If I use them as a dummy load for transmitters 144mhz and down, how bad would that be? It's just a mismatched load, and the consequences are the same as for a mismatched antenna. Check your transmitter's manual for the degree of mismatch it can handle and any applicable warnings. Modern transmitters typically have self-protection circuits and will not be ...


6

A properly calibrated cross-needle power meter such as e.g. the MFJ-842 actually tells you something more than just the forward and reflected power, which as you point out can just as easily be indicated by two separate instruments. The intersection of the needles gives you a pretty good indication of the actual standing wave ratio or SWR because the SWR is ...


6

The SWR meter helps you match the impedance of the radio to the antenna. If the impedance is mismatched, you lose power. If the impedance mismatch is large, you risk damaging your radio, particularly on the lower frequencies. Tube based transmitters and amplifiers have more leeway for mismatch than semiconductor based amplifiers. Lower power transmitters ...


6

Congratulations on earning your General license! You may get the antenna to work using the internal antenna tuner. Make sure the antenna is on a large groundplane. Try tuning it up with low power (< 10 watts) at first to see if the ATU can handle the impedance matching. If it doesn't tune up, reducing the length of the vertical element may help. Try ...


6

In most multi-band balun applications, there is rarely a need to maintain a perfect 50 ohm impedance within the balun. The feedpoint or input impedance is varying widely so another impedance bump in the mix typically has no detrimental effect. I highly recommend the use of coaxial wound over bifilar style for a 1:1 balun. Comparatively, the coaxial and ...


6

No, adding ferrite beads to choke currents on the outside of the shield of a coaxial cable does not affect its impedance or velocity factor. Impedance and velocity factor are determined by the inside construction of the cable: the outside diameter of the inner conductor, d, the inside diameter of the outer conductor (shield), D, and the magnetic permeability,...


6

For well-designed coax, the EM fields are confined to the space between the inside of the braid and the center conductor, i.e. the dielectric insulation region which affects the velocity factor. Therefore, those beads have negligible effect on the differential signals. They do have an effect on the common-mode signals on the outside of the braid which has ...


5

Your transmitter has something called an "output impedance" and expects to be loaded with this impedance. The standard is 50 ohms. You don't need to look up the output impedance, it was standardized and anything made in the last 50 years will expect 50 ohms. The 50 ohms the tranmitter expects is measured at RF frequencies. Many antennas look like short ...


5

Most radios with SWR protection don't detect SWR, they just detect reflected power. It's a much simpler (read that as cheaper) circuit to implement and it's what really matters anyway. The manufacturer will have established a threshold for reflected power, based on the MOSFET's tolerance. If the circuit sees more than this it will either clamp it at the ...


5

The antenna efficiency refers the ratio of radiated power and the power fed to the antenna. The rest of the fed power is transformed to heat by the losses in the antenna. Thus, the antenna efficiency is not directly linked to the radiation resistance alone, but also to the (resistive or dielectric) losses in the antenna. Higher radiation resistance does not ...


5

Glad to see this question posted! This has a really interesting answer which I'm not qualified to write a comprehensive answer to, so here's a rather unsupported answer to help you along until someone writes a better one. There are several reasons why coaxial cable is used in several different impedances. On basic principles: if a device on one end of the ...


5

RF Cafe cites quotes Harmon Banning of W.L. Gore & Associates, Inc.: In the early days of microwaves, around World War II, impedances were chosen depending on the application. For maximum power handling, somewhere between 30 and 44 Ω was used. On the other hand, lowest attenuation for an air filled line was around 93 Ω. In those days, there ...


4

Another phenomenon that might play into the impression that HF transmitters are more susceptible to damage from high VSWR is feedline loss. Feedline loss increases as frequency increases. So at HF frequencies the power reaching the antenna feedpoint is higher due to lower feedline losses, and for the same reason the signal reflected back from the antenna ...


4

Good questions. But, I think another question to add is: What is more important, SWR or resonance? Given that this is the "Amateur Radio" forum, I think answers that are most meaningful to amateur radio operators are more justified in this discussion. The reason I say this is that resonance and SWR as testified by several exchanges and questions are ...


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