Cross pointer swr meter relationship between forward reflected and

Power/SWR Electronic Meters | eBay

critical frequency • cross modulation critical frequency For a particular layer of the propagated wave is no longer reflected back to the earth. critical inductance In a show forward power, reflected power, and standing-wave ratio (SWR). 2. A two-pointer meter used in aircraft to show the position of the aircraft, relative. Properly tuned, the ATR safely operates on all bands with RF power levels ( CW or SSB) of 3 power, reflected power, and SWR are displayed on an illuminated cross-needle meter. . observing the point where forward and reflected meter pointers cross. A ground post is provided for an RF ground connection. SX/SX cross needle SWR & POWER meter with a couple of meters indicating forward and reflected power respectively. It provides the direct read out of the SWR and output power value from the crossing of two pointers. SETUP.

Some of the RF energy reflected back towards the transceiver is lost as heat due to the cable loss of the coax cable. When the RF reaches the antenna tuner or the transceiver final stage, some energy is absorbed as heat but almost all of the energy is reflected back towards the antenna again!

Contrary to popular opinion the RF power reflected back even from a very bad match such as an open or shorted feeder will NOT damage the final of your transceiver.

SWR myths and mysteries

I will explain what can cause your rig serious damage later, so keep reading. The energy reflected back towards the antenna joins the energy generated by your transmitter and the energy being reflected from the antenna, but again the phase is different and this affects the standing wave.

The effect is small because having lost energy on the way to the antenna, and then on the way back to the transmitter the RF power of the now twice reflected wave is quite small. When the energy finally reaches the antenna for the second time it has been attenuated by the cable loss again. Again some of the energy is radiated and some is reflected… go to step 3 and repeat until all of the energy has either been lost as heat in feeder loss or radiated by the antenna.

Here is an example using approximate values for a typical feeder loss of 1dB and an antenna with an SWR of 2: Just over 20W is lost due to the 1dB feeder cable loss so 80W reaches the antenna where due to the mismatch causing the 2: By the time the 10W reflected signal reaches the transceiver through the 1dB feeder cable loss it has reduced to around 8W which is reflected back towards the antenna.

When the 8W reaches the antenna it is down to around 6W. So out of the W you originally put into the antenna feeder, about Because of these multiple reflections, high SWR causes the feed line losses to be multiplied, so low loss feed line causes less power to be lost in a mismatched system than high loss feed lines. What is Return Loss?

Return loss is the difference, in dB, between forward and reflected power measured at any given point in an RF system and, like SWR, it does not vary with the power level at which it is measured. Return Loss has a fixed relationship to SWR but is expressed in dB which makes it easier to use in calculations. For example an SWR of 2: A perfect but unachievable 1: In the real world an SWR better than 1. Most SWR meters do not actually measure the voltage maximum and minimum nodes along the transmission line.

Jetstream JTW2HF Cross Needle Swr- Wattmeter 1.8-200 MHz 300 Watt SH

They actually measure the forward and reflected power, normally by rectifying the voltage across load resistors on a directional coupler, so in reality they are measuring the return loss. Cross needle SWR meters read forward power on one meter and reverse power on the other meter.

The intersection of the two meters where the pointers cross is calibrated to show SWR or Return Loss, or both. All antenna loads have either capacitive or inductive reactance in addition to resistance. The combination of the resistive load and the reactive load is the antenna complex impedance.

This means that the load as seen at the transmitter looks like a combination of a resistance and an inductance or capacitance.

USA - Vswr meter circuit using variable rf attenuator - Google Patents

This is called resonance and is usually arranged to be in the centre of the band or may be offset slightly towards the CW or SSB portions on the band. At the band edges the SWR is typically slightly higher, the resistance is slightly higher or lower than 50 Ohms and there may be a small amount of either capacitive or inductive reactance.

You can see from the earlier example that the 8W of reflected power that gets back to the transmitter will not cause any major problems so what can cause damage to your transceiver if you have high SWR? The problem is not the power reflected back from the mismatched load or antenna. But running the transceiver into a length of coax cable with a short circuit on the end, or a faulty antenna, non resonant antenna, or no antenna open circuit is exceptionally dangerous. The culprits are the standing wave and the complex impedance.

Remember I said that the standing wave results in high voltage nodes at regular distances along the coax feeder? The voltage at these nodes is the vector sum of all of the signals being sent and reflected back and forwards along the cable. The voltage peaks could be very high. If due to the particular load impedance, RF frequency, and length of the transmission line coax feeder cableone of the voltage peaks happens to occur right at the transmitter end of the cable, there could be excessive RF voltage appearing at the transmitter output stage.

This could cause flash over or RF breakdown across final transistors, inductors or capacitors. The risk of this is unpredictable you could get away with it times on one frequency and then blow the rig at a different frequency. A voltage null at the transmitter end of the cable would cause high current to flow which could also damage the rig, but this situation is less dangerous than the high voltage event because it would probably trip the SWR protection in the rig.

Standing wave voltage is not the real baddy though; the other nasty is the complex impedance. As I said earlier all antenna loads have either capacitive or inductive reactance in addition to resistance. In a well matched antenna the capacitive or inductive reactance is balanced out leaving a mostly resistive load. But in a poorly matched system such as short circuit, faulty antenna, non-resonant, or no antenna there will be a lot of capacitive or inductive reactance.

This is transformed by the transmission line feeder cable and appears to the transmitter as if you have connected an inductor or capacitor plus the resistive component across the output. Note that due to the length of the transmission line a capacitive reactance at the antenna may look inductive at the transmitter and vice versa.

The effect of adding capacitance or inductance at the transmit output stage is to de-tune the final. In a valve output stage like a linear amp this can cause the same sort of flash over events as a voltage standing wave peak. In a transistor final it can cause excessive current and burnout the transistors. Less severe detuning will cause mismatch between the transmitter output and the coax feeder, resulting in mismatch loss and reduced output power being delivered to the antenna.

Actually an antenna tuner in the shack does not tune the antenna it is really an antenna matching unit. It is not capable of altering the impedance or resonant point of the antenna.

The same would apply to a screwdriver antenna controller or the remote tuned capacitor on a magnetic loop. The pick-up coil is located in the electrical field between the inner and outer conductors of a coaxial transmission line and has a voltage induced therein proportional to the current I in the inner conductor, there being a mutual inductance M between the loop and the transmission line and the loop being positioned in the plane of the inner conductor of the line.

A series circuit of resistance R and capacitance C connected across the transmission line conductors will give a voltage across the resistance R proportional to the voltage E between the line conductors. In directional couplers and so-called reflectometers the arrangements mentioned are combined in a sampling circuit in which the resistor R is connected in series with the loop and capacitive coupling is provided as by capacitor plates or armatures on the loop and the inner conductor or by capacitance effects between the components of the sampling circuit and the inner conductor.

Considering the sampling circuit mentioned and using lumped impedances, it is apparent that M is either positive or negative depending upon the directional relation between the loop and the wave signal energy traveling on the line. The forward traveling wave has voltage Ef and current If while the reflected traveling wave has voltage Er and current Ir. Thus, if Zo be the characteristic impedance of the line and p the reflection coefficient: The components are selected so that: If we let e be the electromotive force when M is positive so that the voltage across R and the voltage induced in the loop are additive, and let e- be the electromotive force when M is negative and the voltages referred to are opposed, the former gives a maximum and the latter a minimum indication, thus: It is also feasible to measure power P being fed through the transmission line: EQU1 When it is desirable to build a power meter with variable sensitivity in order to measure ratio or VSWR directly, the usual method is to incorporate a control into the meter circuit between a rectifier diode and the D'Arsonval meter movement.

When it is desirable to build a power meter with variable sensitivity in order to measure ratio or VSWR directly, the usual method is to incorporate a control into the meter circuit between a rectifier diode and the D'Arsonval meter movement. The control varies the sensitivity of the metering circuit so that meter deflection is given by: EQU2 K2 cannot be factored out and brought to the left hand side of the equation, so the control K2 does not multiply the power of all points on the scale by the same constant.

The instrument of the present invention avoids the difficulties indicated above and affords other features and advantages heretofore not obtainable. Another object is to provide an RF VSWR meter circuit for an electronic instrument of the type described, which reduces distortion of meter scale shape due to diode turn on characteristics.

The instrument in which the meter circuit design is used comprises a coaxial line section adapted to be inserted in the transmission line and having a tubular rectangular conductive metal body and a coaxial center conductor electrically insulated from the body. An inductive pick-up coil is mounted for rotation about an axis normal to the axis of the line section for sensing and measuring either the forward RF voltage wave signal level or the reflected RF voltage wave signal level in the transmission line.

SWR- Standing Wave Ratio

A D'Arsonval meter movement for indicating on a meter scale the magnitude of the signal sensed by the pick-up coil is connected to the coil through a circuit embodying the invention. The circuit includes a rectifier diode and a variable RF attenuator electrically connected between the diode and the inductive pick-up coil.

Accordingly, the deflection of the meter movement means is a constant function of the induced RF voltage level sensed by the inductive pick-up coil and represents either the forward voltage wave signal or the reflected voltage wave signal level on the transmission line depending on the particular orientation of the inductive pick-up coil.


The ends 12 and 13 of the coaxial cable between which the instrument 10 is inserted have standard coaxial connectors 14 and 15 shown in dashed lines in FIG. The instrument is anchored in a housing 16 of non-conductive molded material, that supports a front panel 17 and a protective transparent window element 18 that protects a meter scale plate The instrument 10 includes as basic components a line section 20, an inductive pick-up coil element 30, a printed circuit board 40 with associated circuit components, and a D'Arsonval meter movement assembly The line section 20 comprises a tubular conductive metal body 21 formed of aluminum, for example, and defining a longitudinally extending cylindrical space 22 therein.

Located within the space 22 is a center conductor 23 supported by and electrically insulated from the tubular body 21 by annular insulators 24 FIG. Located at each end of the conductive metal body 21 are female-type coaxial connectors 25 and 26 that receive the male-type coaxial cable connectors 14 and The tubular body 21 has a relatively large transverse bore 29 formed therein, communicating with the cylindrical space 22 FIGS.

Located partly within the bore 29 and supported by the metal body is a conductive pick-up assembly 30 constructed generally in accordance with U. The assembly 30 has an inductive pick-up coil 31 at its inner end positioned within the space 22 and a mounting plate on which the coil 31 and associate circuit components are mounted.

The coil 31 and swivel plate 32 are mounted on a shaft 33 journaled in a bushing 34 secured in the printed circuit board