Radio frequency range

Stepped transmission line transformer (TR2) with a frequency range 40-220 MHz is shown in Fig. 3. The rf power is fed from this broadband transformer through a symmetrical 100 Q cable to the coil in a modified E235 Varian large sample cavity and from the latter to two 50 Q loads. The four rods in the cavity are connected to form a Lecher type cable with a line impedance Z == 100 Q. With this anangement a VSWR of 1/1.2 between the rf transmitter and the loop in the cavity is achieved in the frequency range 70-220 MHz. The rf field at the sample position was measured to be 0.03 mT t. 

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Electromagnetic earthquake forerunners show themselves like the electromagnetic phenomena, including electromagnetic emission (EME) in a radio frequency range. This emission caused by collective exiting of the set of local mechano-electrical transformers (MET). The excitation mechanism inside the crust is determined by the fact that elastic tension ranges up to the threshold level within the source area. 

If energy of the proper frequency is supplied, a transition between these quantum states occurs with the absorption of an amount of energy equal to the separation of the states. The frequency of the absorbed radiation lies in the radio-frequency range and depends on the local magnetic field at the atom in question. 

Radio frequency interference (RFI) EMI in electronic equipment caused by radio frequencies, ranging typically from 10 kHz (104 Hz) to 1000 MHz (109 Hz or 1 GHz). 

Theory. If two or more fluorophores with different emission lifetimes contribute to the same broad, unresolved emission spectrum, their separate emission spectra often can be resolved by the technique of phase-resolved fluorometry. In this method the excitation light is modulated sinusoidally, usually in the radio-frequency range, and the emission is analyzed with a phase sensitive detector. The emission appears as a sinusoidally modulated signal, shifted in phase from the excitation modulation and partially demodulated by an amount dependent on the lifetime of the fluorophore excited state (5, Chapter 4). The detector phase can be adjusted to be exactly out-of-phase with the emission from any one fluorophore, so that the contribution to the total spectrum from that fluorophore is suppressed. For a sample with two fluorophores, suppressing the emission from one fluorophore leaves a spectrum caused only by the other, which then can be directly recorded. With more than two flurophores the problem is more complicated but a number of techniques for deconvoluting the complex emission curve have been developed making use of several modulation frequencies and measurement phase angles (79). 

NMR is a spectroscopic technique that relies on the magnetic properties of the atomic nucleus. When placed in a strong magnetic field, certain nuclei resonate at a characteristic frequency in the radio frequency range of the electromagnetic spectrum. Slight variations in this resonant frequency give us detailed information about the molecular structure in which the atom resides. 

Radio-bridge — A bridge circuit similar to the - audio-bridge except the signal source produces frequencies in the radio frequency range (e.g., 50 kHz-50 MHz) and a radio receiver (or even an oscilloscope) is used to determine the condition of null. 

The dense clouds mentioned in the first paragraph cannot be investigated by means of the X 21 cm line, or any optical transition, but only by means of molecular lines in the radio frequency range. This is why interstellar molecules are so important in modern astrophysics. Molecular lines contain much information on the physical state of these dense and cool clouds. Moreover, the abundance of interstellar molecules itself can shed light on the physical conditions, including the radiation field in dense clouds, once we more fully understand... 

Radiation intensities are referred to the radiation of a black body. In the radio frequency range it is convenient to express line intensities in equivalent line brightness temperatures, since the surface brightness Bv of a black body... 

Because of the high precision with which the frequencies of the interstellar lines can be measured (better than 1 part in 10s) there remains usually little doubt about the positive identification of the molecular species, despite the fact that only a few transitions out of the whole rotational spectrum of any one given molecule have been observed to date in the radio frequency range. Confirmation is obtained from observations of other rotational transitions, or from the detection of possible fine-structure components, or from observations of corresponding transitions of isotopically substituted species. However, some uncertainty still remains in the identification of formic acid, HCOOH, whose 1 io-ln transition is located in between two 18OH resonances. An independent search for the l0i — 0Oo transition for formic acid was negative (Snyder and Buhl, 1972). Similarly the identification of H2S and H20 still rests on only one observed interstellar radio transition and awaits further confirmation by the detection of other transitions. 

The C nucleus has /=j, like H, and the C-NMR spectrum of a compound can be observed using a different radio frequency range (in the same magnetic field) to that for H. The spectrum will give peaks for each different type of carbon atom in a molecule but the properties of the nucleus give some important and useful differences in the spectrum obtained ... 

A. Frequency. The apparatus can be operated over a wide range of frequencies from AC to the radio frequency range because of the use of internal electrodes. We have employed frequencies of 60 Hz (AC), 10 kHz (AF) and 13.56 MHz (RF). 

The operative selection rules allow transitions only between adjacent energy levels. It is the frequency of this radiation, which is in the radio-frequency range that is measured in an NMR experiment. 

Working with an NMR spectrometer is slightly more difficult than, for instance, with an electron microprobe. As the NMR frequencies are all in the radio frequency range, NMR spectrometers are computer operated radio transmitting and receiving systems controlled by a computer. A receiver and transmitter antenna surrounds the sample, which is located in the cavity of a super-conducting cryomagnet. 

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