Since the current is low at the ends of a halfwave patch and increases in magnitude toward the center, the input impedance zvi could be reduced if the patch was fed closer to the center. A lot of hams used the 4400 in grounded grid by simply strapping both grids together and running the tube as a high mu triode. Building your own tubetype hf amplifier, doing it the. Sm 5 bsz tuning the grid circuit of high power amplifiers. There are some grounding techniques that apply to both types of. Note that with all patch setups the channel the guitar is plugged into will be the. This means two things influence gain of a grounded grid stage.
The class ab1 6dq5 and the groundedgrid 8 were carefully adjusted for optimum operation to get a fair comparison. The input impedance of the patch antenna varies with the feed location, thus, the. The input impedance of the antenna is the impedance that is seen at the terminals. For a square patch antenna fed in the manner above, the input impedance will be on the order of 300 ohms. Me, i always went for the cheap surplus tubes like the 8. The impedence varies with plate loading and goes to a very high value during the nonconducting part of the cycle. Plate current is common through both cathode and anode, and only dc plate voltage is not.
Some of these approaches determine only one single grid impedance for all three lines andor identify the phase impedance by using the virtual star point calculation method. The input impedance would no longer be 50 ohms since the grid circuit is no longer loaded by any grid current class ab, with 55 volts on the grid. Tuned input circuits for class ab2 cathodedriven groundedgrid. Pair of 8s in grounded grid configuration put out about 600 watts pep with nominal 2,200 volts on the plates. Making a test on the sound quality of capacitors is a difficult task. In many cases this is true, but in some cases it is not true. How do we calculate the input impedance of a patch antenna from.
I meassured the input impedance of the tubes with about 120 to ohms which gives. If a groundedgrid amplifier is used with the object of providing a reasonably good match for a coaxial line without a tuned input circuit. This method leads to errors in case of asymmetries in the grid impedances at higher. For best signal voltage transfer from guitar to amp you want a low impedance from the guitar and a high impedance for the amp at least 10 times more impedance for the amp is a guide called the rule of 10. The input impedance at node 1 is known and given as zin1z1z3z5z2z4. In any scientific work we would as precisely as possible describe the conditions under which the experiment was performed because our work doesnt hold any credibility unless others are able to reproduce and possibly confirm our results. I already did some modifications vacuum relays, fan control, 9 band operation 8010m and now i am looking for some ideas for an input circuit. The plate can also control the flow of current, as increasing the cathodetoplate voltage n i. Interest in the groundedgrid power amplifier has been high because the circuit offers one way to make use of otherwiseexcessive power output from a driver. After the model, with model excitation is completed, a grid system is applied to the. The grounding impedance characteristics of grid configuration. This grounded grid gg preamplifier will not make any artifacts to the sound and play 100% originally and naturally. The static input impedance of the cathode in a grounded grid circuit can be calculated if the valve characteristics relating to the specific circuit are known. The q is low, however, and once adjusted an input circuit requires no further attention for.
Using richmonds reaction integral equation, an expression is derived for the input impedance of microstrip patch antennas excited by either a microstrip line or a coaxial probe. It runs nicely with 700 to 850 watts from 80 to 10m. Many people think grounded grid hf pas do not need neutralization. Line to ground voltage monitoring on ungrounded and impedance grounded power systems december 2010at301 by reza tajali, p. While reasonable impedance matching can be important for proper exciter operation, the tuned input circuit of a groundedgrid or cathode driven radio.
The input impedance and length of the patch antenna decides the optimal input impedance point. Since the groundedgrid amplifier does not invert the input signal, the output signal at its plate is in phase with the input signal, which is then passed off to the cathode follower, which, in turn, delivers the inphase signal to the groundedgrid amplifiers grid. As we can see, the grounded load impedance z5 appears in the numerator the whole circuit is symmetric and we can now write down the input impedance as seen from the right into node 5. An rf power amplifiers tuned input circuit is more than just impedance matching. Determining input impedance of patch antenna in hfss.
The high efficiency, high output, and excellent linearity characteristics of the circuit are shown by the graphs. If this high impedance resonance happens to occur at a frequency range where the anode path to ground is parallel resonant, the tube can act like a tunedplate tunedgrid oscillator. Ive read a number of writeups recently about the impedence of a grounded grid ab1ab2 linear amplifiers input input. Power required to drive an 8 in grounded grid linear. The dynamic input impedance during working conditions is often higher. This can lead to thermal runaway phenomena within internal regions of the semiconductor junction. Guy i know had a contest amp which ran over 4 kv and somewhat less under load on a quad of 8s. As an example, the real and imaginary parts of zin z11 have been shown in the figure below. Grounded grid pentode rf amp the radioboard forums. Discussion in homebrew and kit projects started by n4hz, aug 16, 2011. The input impedance, like the fedthrough power, depends on the plate loading.
I used the steps in balanis antenna theory and design, page 821 zin 1yin 12g1 where g1 w120. Input impedance and fedthrough power in groundedgrid amplifiers. The microstrip patch antenna calculator determines the length l and width w of a rectangular microstrip patch antenna for a given resonant frequency or vice versa. I runs nicely with 700 to 850 watts from 80 to 10m.
For many years, grounded grid tube preamp has been tested and verified by many worldwide diyers. Input circuit for grounded grid amplifier with 2x 8. The input resistors set the amps input impedance and they act as tube v1s grid leak resistors. Due to the groundedgrid amplifiers low input impedance, however, this coupling capacitor must be large in value, far larger than the usual 0. The primary usage centers around situations where impedance matching the input has value. Eimac 4a typical operating specs for grounded grid operation rare on. With no input connections all four input jack tips are grounded through the jacks. The ratio of r2 output load impedance at the anode plus input resistance to input resistance. Since this typically yields a high input impedance, we would like to modify the feed.
If you look at the schematic of a typical guitar amplifier, you will notice that there is a resistor in series with the grid of the first tube, usually around 68k or so, and there is also a resistor in series with the grid of the power tubes, usually 1. It so happens that because grounding the grid causes it to acshield the cathode from the plate, groundedgrid amplifiers can be very stable with little attention paid to circuit layout at hf and. Resistor r1 is acting as the input impedance resistor. Design and fullwave analysis of supershaped patch antennas. Build your own receive preamplifier for your 2 way radio duration. With about 35 volt rms on the control grid the output power is about 300w. Input impedance and fedthrough power in groundedgrid. The emitter is held very close to ground due to r3 being fairly low impedance and c3 shunting the desired signal frequencies to ground. The input and output circuit of a grounded grid amplifier are connected in series through the tube. In between the ground plane and the microstrip structure, a. Line to ground voltage monitoring on ungrounded and. The resulting plots for z11 in hfss will show us the correct input impedance of patch antenna.
Eimac 4a typical operating specs for grounded grid. In electronics, impedance matching is the practice of designing the input impedance of an electrical load or the output impedance of its corresponding signal source to maximize the power transfer or minimize signal reflection from the load in the case of a complex source impedance z s and load impedance z l, maximum power transfer is obtained when. On the typical hf grounded grid amplifier, you will need no less than 2 meter functions. The difference is that the grid is a highimpedance input and the cathode is lowimpedance input. I was able to tune the sb102 to where there was no apparent excess heating of the 6146s and plate current on the sb102 was ok but still the bad vswr. The commongrounded grid configuration it applies to tubes, bjts and mos has the following features.
This optimal point can be calculated from the standard impedance 50 ohm and length of the designed patch antenna. There will also be 14 the impedance in the cathode circuit, that is, if the input. However, to decrease the input impedance to 50 ohms often requires a. The thing looks like a pushpull version of the socalled cascode rf amp design that was popular for vhf converters in the 1950s. But 18 has used rigorous mathematical steps to calculate the total quality factor qt for an etmp without superstrate conventional, and there is. Resistors r3 and r4 in parallel are acting as the grid stopper resistors. The 4x1 tetrode is an anomaly when placed in himu gg triode service.
Other grounded grid amps will require you to experiment with the correct input impedance to simulate for varying values of grid current. Grounded grid preamplifier user manual analog metric. These values are identical to the normal hi inputthe only difference between the normal and bright channels is the. Recently, i had the chance to compare my audible illusions modulus 3a preamp to the transcendent grounded grid linestage. Oscillation in a grounded grid amplifier will occur at the frequency where the plate and cathode circuits are both resonant, not just below the resonant frequency where grid driven amplifiers oscillate. By increasing the width, the impedance can be reduced. Back when we measured power as plate input power and not rf output power, the fcc even had a rule similar to this. Some similar designs eliminate r3 and c3 altogether. Previously, the patch antenna was fed at the end as shown here. Also, thanks to the differential pairs of the input stages, it rejects most of commonmode noise and has high psrr. It has dual 800 vac500 ma windings connected in parallel and fed into a voltage doubler that is identical to the hb600.
The width w of the microstrip antenna controls the input impedance. Vswr between the input to the tube and sb102 was terrible. Image of voltage v, current i, impedance z curve along the length of patch. A pair would take a kilowatt input on cw with ease. The original designed frequency range of this thing is 2 32 mhz.
The output signal is taken from the cathode, the resistor is also connected to the cathode. Without any calculation, it is very easy to give the input impedance vs. Since the input and output signals are ground referenced, the emitter and signals on the emitter are common aka a ground or circuit return path to both the. The driving impedance of a cathode driven amplifier always varies some amount with power level and tuning. In what follows, q is defined as the input impedance of the pinetwork divided by the reactance of the input, shunt elementtypically a capacitor. I got a pretty old amplifier with 2x 8 tubes in grounded grid. A high power grounded grid amplifier and power supply. What is the input resistance formula of patch antenna. I find the following firmula refernced to balanis antenna theory and designbut still not found it in balanis as written below, z 90. In retrospect, i should have taken an additional data point at 390 ohms.
We can say the input impedance of a cathode driven stage is dynamic, with the tubes driving impedance changing with parameters like drive power and how the amplifier is loaded or tuned. In order to calculate the input impedance of an equilateral triangular microstrip patch etmp only two papers by biswas and guha 2 and lee et al. The input impedance of a grounded grid amplifier is highly dependent on the load impedance the tube sees on the plate, the class of operation, and the drive level. I have also built the grounded grid for a friend in europe, who asked for not only dualmono pots linear taper for improved lr balance, but also a master volume control log taper to be used for general, daytoday listening once the the. If the ratio lw is close to unity, the radiation pattern will be symmetric but may not provide a resonable input impedance. Two basic techniques of audio wiring and grounding will be discussed standard methods used in. The antenna is composed of a squarering patch that is shorted to the ground plane through four shorting walls. You did the same thing with them, strap all the grids together and make it appear as a high mu triode.
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