Ferrite rods, bars, slugs, plates and tubes are primarily used as magnetic cores in radio antennas, chokes, inductors and filters. In radio antennas, ferrite rods are useful from VLF spectrum to VHF spectrum. The advantages are small size, high Q and compactness.
Ferrite rods and bars are available in standard lengths up to 12" long. These rods are available from 0.25" to 1" in diameter. They are available in materials from permeability of 7.5ui to 10, 000ui in both Nickel Zinc (NiZn) and Manganese Zinc (MnZn) compositions.
The table below shows the material characteristics of the various types and of NiZn and MnZn ferrite rods, slugs, tubes, strips, plates and bars.
Rods with material 61 (ui = 125), 33 (ui = 800) and 77 (ui = 2000) are standard stocking items and can be purchased online from www.cwsbytemark.com. The direct link to the shopping cart is:
All other materials are custom manufactured and are readily available with lead time for delivery.
A partial list of Standard Stocking Rods available for immediate delivery that can be purchased through the online shopping carts is also shown below for reference. Please go to the shopping cart directly for a more comprehensive list.
Other dimensions and materials are available. Please call or email us at email@example.com for your other requirements.
Custom blocks of ferrites are frequently used in Antenna as well. Please visit www.bytemark.com/customblocks/html <<<link>>>for more information on custom blocks of ferrites and material availability.
For antenna application, the design objective is to get high Q at the center of the design frequency of operations. Ferrite Rods are widely used as loop antennas such as broadcast-band receivers, low and medium frequency direction-finder receivers, airborne navigation receivers, AM, FM and VHF receivers.
The material 61 (ui = 125) rods are mainly used in the 550 Khz to 1660 Khz commercial AM radio antennas.
Material 61 (ui = 125) rods are also used by radio amateurs in the 2 MHz to 30 MHz range.
The material 33 (ui = 800) rods are more suitable for lower frequencies in the 100 Khz to 1 Mhz or the low frequency (LF) ranges. The table above shows the recommended frequency ranges for different materials. As a general rule, the higher the operating frequency, the lower permeability of the rod cores are needed.
The material 77 (ui = 2000) rods are most suitable for very low frequency (VLF) receiver application. Material 77 is typically used in receiver application below 10 Khz.
Toroids cannot be used as receivers as they have closed loops. Rods, bars and plates have open loops and are thus suitable as radio receivers. However, the efficiency of the rod as antennas degrades as the rod overall length versus the diameter ( l/d) increases. When the l/d goes over 35, the rod behaves more like an toroidal cores, and the close loop effects kicks in making these rods ineffective as an antenna. On the other hand, for very short rods where the l/d is less than 5, the effectiveness of the antenna is also diminished. Therefore, optimum l/d ratio is from 10 to 25 for a rod antenna.
Loop antennas have a height factor called effective height, he (in m), which when multiplied with field strength, F (in uV/m), provides the loop-induced voltage (in uV).
It can be seen from the equation that the highest induced voltage occurs when the windings occupied the entire rod (when N is largest).
In a choke and filter application, the design objective is to get the highest impedance over the frequencies to be suppressed. Chokes and filters application is in a way, similar to the antenna application. As a general rule, the highest permeability rods are used to suppress the lowest frequency range.
The material 77 has a wide suppression frequency range. It is suitable for 1 Mhz to 30 Mhz in the amateur radio band.
The material 33 has a somewhat narrower band, and is suitable for the 3.75 - 7.5 MHz (40-80 meters band). Both material 77 and material 33 rods are also often used in speaker cross-over networks.
The material 61 is most suitable for the 7.5-30 MHz (10-40 meters band) range.
Due to the open magnetic structure of the rod configuration, considerable current can be tolerated before these rod cores will saturate.
There are several factors that have a direct bearing on the effective permeability of a ferrite rod, which in turn will effect inductance and 'Q', as well as the AL value of the rod and its ampere-turns rating. These are: (1) Length to diameter ratio of the rod, (2) Placement of the coil on the rod, (3) Spacing between turns and, (4) Air space between the coil and the rod. In some cases, the effective permeability of the rod will be influenced more by a change in the length to diameter ration than by a change in the initial permeability of the rod. At other times, just the reverse will be true.
Greatest inductance and AL value will be obtained when the winding is centered on the rod rather than placed at either end. The best 'Q' will be obtained with the winding covers the entire length of the rod.
Because of all of the above various conditions it is very difficult to provide workable AL values. The set of AL and NI values for various types of rods in the above table should be used as a guide only. These figures are based on a closely wound coil of #22 wire, placed in the center of the rod and covering nearly the entire length. Please note that there are many variables and that the inductance will vary according to winding technique.
EFFECTS ON 'Q'
Rods, tubes and strips can be custom manufactured in lengths up to a maximum of 18 inches. However, in most application requiring very long rods, multiple shorter rods are bonded together in a tubing to acheive length of over several feet long.
The dimensional tolerances for rods, tubes, and strips are approximately +/- 6%, and for plates are +/- 2%. All length tolerances are +/- 2%. The camber tolerance is 0.11 per inch.
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