# Tag Info

18

Clearly capacitance is the key Capacitance is just one part of it. The gamma match in your question is three things: A sort of folded dipole, performing an impedance step-up A parallel shorted transmission line stub, adding shunt inductance A series capacitance An equivalent circuit is: simulate this circuit – Schematic created using CircuitLab So ...

17

Mostly, they used RF Ammeters in series with the antenna. The first ones were hot-wire ammeters which were completely mechanical devices. One end of a thin nichrome wire (or other wire of sufficiently high resistance) inside the meter was coupled directly to the pointer shaft (often wrapped around it); the other end was anchored to the meter case. As the ...

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

A gamma-match serves a triple purpose: As a small diameter wire parallel and in close vicinity with the main radiating element, it will carry only a fraction of the main element current while being exposed to the same electrical field strength. This turns it in an effective up-transformer of the antenna input impedance. It also forms together with the main ...

10

The situations described, schematically: simulate this circuit – Schematic created using CircuitLab When the source and load are matched (Rantenna = Rreceiver), you will be measuring the voltage in the first case through a voltage divider such that the measured voltage will always be half the open-circuit voltage.

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 ...

10

Intersymbol Interference (ISI) — or analog equivalents, like ghosting — are relevant only when the difference in time of arrival between the primary and reflected signal are significant compared to the symbol duration. For example, PSK 31 has a symbol rate of 31.25 baud, meaning each symbol is 32 ms long. If the difference in arrival time is significantly ...

9

The canonical solution to this problem is a Wilkinson power divider. There are of course other power dividers, but the Wilkinson is easy to fabricate, provides good isolation between the output ports, and is lossless when the output ports are equal and in phase, which is true for an ideal antenna array. If you were to make one of these yourself, you'd ...

8

My trick is instead of cutting, rather extend the whip first, and see if the SWR gets better or worse. Extend it 1/2" with a bit of wire wrapped around it and sticking out. If the SWR gets better, then you made it too short in the first place. If it gets worse, you can start trimming. Remember you should only measure SWR when you are away from the antenna - ...

8

You'll be fine to start without an additional SWR meter. An SWR meter doesn't provide any protection. With or without an SWR meter, you'd want to start on a new antenna on low power, then increase power only after measuring the SWR. Don't worry too much. If transmitting at much less than maximum power you won't damage anything even with the worst possible ...

8

First, a general statement: the antenna analyzer has one set parameter, the frequency, and one measured parameter, the impedance (which is a complex number and therefore requires two real numbers to display). Everything else can be derived, one way or the other. Why for Z & Zpar are there 2 numbers, one with a j in front. What do those mean? $Z$ ...

8

If nothing about the physical situation changes, then the SWR will not change. But something might change in the long run, perhaps: Damage to the feed line (water intrusion, sun damage to insulation, chewing by small animals, etc.) Damage to the antenna (corroded junctions, parts broken off, wire fatigue). Plants growing near or on the antenna. Your ...

7

Your TS430S has a solid state final and no internal antenna tuner. As such it is designed to work into a 50 ohm load at all times. This means you can disregard this note in your antenna tuner manual. If the tuner was used with a radio with tube finals then you would need to heed this cautionary note. If the tuner is used with a radio that has a built in ...

7

The electrically shortened antennas often found on HTs are not simple pieces of wire, but coils: By Shootthedevgru at English Wikipedia, CC BY-SA 3.0 The coil adds inductance over the length of the antenna, making it appear electrically longer than it is physically. Without the inductance, the antenna would need to be approximately a quarter-wavelength ...

7

The SWR is related to the reflection coefficient $\Gamma$: $$\Gamma = {Z_L - Z_0 \over Z_L + Z_0 }$$ $$\text{VSWR} = {1+|\Gamma| \over 1 - |\Gamma|}$$ where: $Z_0$ is the feedline impedance, usually 50 ohms, and $Z_L$ is the load (nominally, the antenna) impedance. The reflection coefficient is a complex number and thus takes into account the phase ...

7

Think of it first without the feedline: The half-wavelength of transmission line provides a 180 degree phase shift, and the (short) connection between the shield ends is "ground". That means it's at 0V relative to the environment, and also the voltage at the open ends of the coax center conductor will be equal in magnitude but opposite in sign relative to ...

7

There is a device that does this, but it's not a directional coupler. Rather, it's a circulator. A directional coupler isn't suitable since the forward and reflected ports merely "sample" the power in each direction. Most of the power just passes through the coupler, so at best you'd only reduce the reflected power reaching your transmitter by some ...

6

The source of the SWR limit on the transmitter end is the losses in the feedline. In general, the higher the matched line loss, the lower the maximum SWR that will be present at the transmitter end. Since the SWR limit is the result of losses in the feedline, the efficacy of the 'high SWR fix' must be considered. The mechanism has to do with the trips that ...

6

Firstly, at 900+ MHz, shorting out components to take antenna impedance measurements is not a good practice as the lead length can be a substantial portion of a wavelength. Stray inductance and capacitance must also be avoided. Secondly, your matching network appears horribly complicated for an impedance matching circuit. Generally, a simple "L" network (...

6

A low SWR bandwidth is a consequence of shortening the antenna for mobile use. SWR bandwidth is related to the antenna system's Q factor, the ratio of energy stored to energy dissipated per cycle. A higher Q means less SWR bandwidth. Decreasing Q will increase SWR bandwidth. That can be done by decreasing the energy stored, or increasing the energy ...

6

Many baluns will work just fine in either direction, though there isn't just one kind of "4:1 balun". This kind is wound on two cores, and works as a common-mode choke: simulate this circuit – Schematic created using CircuitLab A common-mode choke works in either direction, so it matters not which end is balanced and which is unbalanced (or if both ...

6

Reactance is the imaginary part of impedance. Together with the real part, resistance, impedance describes how a given load will respond to an AC voltage or current source. While for DC analysis resistance alone is sufficient for this purpose, for AC analysis we need to know not only the magnitude of the voltage or current, but the phase angle between them, ...

6

I would say in practice it is less important because hams use different types of antennas for HF and for VUHF. At HF, the same antenna is often used for many bands. The bands have large fractional bandwidth. Antennas are often simple wire, very thin compared to wavelength. And finally, because the wavelength is quite long, they are often too short for the ...

5

The main risk is damaging the radio with a high SWR. You can buy a meter that will measure the SWR, or your radio may have such a meter built-in. Or you can take precautions like reducing the transmit power. That said, you should be relatively safe with an HT. The transmit powers involved are usually low enough that it's economical simply size the ...

5

If you take SWR measurements at a range of frequencies and plot them (or just watch the SWR reading as you change frequency), you should get a graph with one or more valleys of low SWR. Ideally, if you find the ratio between the valley's actual position and where you want it to be, and trim or rebuild-bigger the antenna according to that ratio (or rather, ...

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

Given the matched loss of the feedline and the SWR at the transmitter, we can calculate the SWR at the antenna in three simple steps. First convert the SWR at the transmitter to the corresponding magnitude of the reflection coefficient (Gamma), or MRC for short within the context of this answer. The MRC is the magnitude of the complex ratio of the reflected ...

5

The MA5B is rated for 1,200 watts PEP. 200 watts is certainly well below that threshold. Things to check: Arcing, corrosion or moisture inside the match box A loose or corroded connection from the matchbox to the driven element Loose or corroded connections in the "X hats" A waterlogged, corroded or otherwise failed trap Loose or corroded connections in the ...

5

If the SWR was very poor with the whip installed, you could simply be hitting the limits of your analyzer. Inexpensive analyzers are not very precise when the load is very poorly matched. Alternately, something may be broken. Perhaps the analyzer, or the feedline you are using to connect it, the antenna mount, or the antenna itself. I suggest testing each ...

5

There is no need to "pin" an interstage impedance. You may directly transform from the native output impedance of one stage to the native input impedance of the next without going through an intermediate transformation or termination. When designing an interstage transformer, the general design rule is to ensure that the inductive reactance of each winding ...

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