![]() ![]() ![]() Thus, you'll need to use the 3/4 inch ID ones. Mix 31 split beads are ideal for this application, but it takes 8 turns to equal 5k ohms. In other words, at least 5k ohms, and perhaps 2 or 3 times this in some cases (i.e.: short, stubby antennas and/or poor mounting schemes). Unfortunately, most are inadequate for the purpose because they are physically to small and/or the incorrect material (mix) and/or not enough suggested turns.Īs a rule of thumb, a choke should have an impedance at least two magnitudes greater than the impedance of the circuit it is applied to. Most antenna manufacturers supply ferrite beads or toroids with their antennas for use as chokes. Remember, everything before the choke is part of the antenna, and will both radiate and receive, adding to the background noise level we all have to content with. ![]() If you mount them inside, you will be plagued with RFI issues, including erratic controller operation. The chokes must be installed outside the vehicle, and as near to the base of the antenna as possible. It is not uncommon for the RF voltage to be as high as 250 volts, and in a few cases, nearly 1,000 volts! That's high enough to zap any solid state controller! The amount of RF present on the leads depends on several factors, especially where, and how the antenna is mounted. The motor and reed switch leads of all remotely controlled antennas operate above RF ground potential. If you opt for an automatic controller, make sure the controller manufacturer's instructions are followed closely to avoid damage to the motor. They exhibit several unforeseen consequences because of their diminutive size, not the least of which is their minuscule motor, which is easily damaged under stall conditions. Short, stubby, HF mobile antennas are very popular due to their diminutive size, light weight, and apparent ease of mounting (they're not necessarily less expensive). This said, if your antenna doesn't need matching to achieve a low SWR, you either need a better antenna, better mounting scheme, or both! The highlighted article explains how that is accomplished. Properly matching is also a prerequisite. If you fail to properly choke common mode, the coax cable will become part of the antenna system! The highlighted article explains how that is accomplished. How to properly choke them is covered below.Įvery single mobile installation will have some level of common mode current flowing on the coax. For best results, separate chokes should be used. If the reed switch (turns counter) is used, they too must be properly choked. First, the motor leads must be properly RF choked, and most factory-supplied (or described) ones are inadequate for the purpose. Whatever system is used, there are several prerequisites which must be performed to assure smooth operation. It may be as simple as a DPDT rocker switch, to a fully-automated system requiring minimal attention from the operator. Contents: Basics Proper RF Bypassing Other Caveats SWR Considerations How They Work, Reed Switch Type How They Work, SWR Detect Type Manual Controllers Stepper Controllers Odds & Ends Īll remotely controlled HF mobile antennas require some sort of device to control the motor. ![]()
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