Re: VHF Radio Installation w/ Pics
Not sure what you mean by DC ? there is no Direct Current in the antenna line, it's RF. What your checking with the meter is for a short from making the the connections with the 259 connector and if the solder has shorted in the grounding. Chances are there will be extra cable so coiling will help make an RF choke for possable outside electral interferance. But then again, you could just plug and play
DC means direct current. The suggestion was to measure the resistance between the coaxial transmission line's center conductor and its shield, presumably by making this measurement at the PL-259 connector. Making a measurement of resistance like this is not going to provide much useful information about the condition of the antenna, the transmission line, or the connector, unless it just happens to be the case that the antenna in its normal condition presents an open-circuit at DC. Some antennas exhibit this property. If that were the case, then one could detect a short circuit in the connector, the transmission line, or the antenna if they measured a very low resistance at the PL-259. But many VHF Marine Band antennas show a DC short circuit as their normal condition, and, for those antennas, the measurement of the resistance at the PL-259 is not likely to be very informative, unless it happens to show a very high resistance.
The notion that you can make meaningful measurements about the behavior of the antenna at radio frequencies by taking a measurement of its input resistance at DC is misunderstood by many boaters, as evidenced by the confusion created by this suggestion. In order to make a meaningful measurement of the antenna at DC, you would have to know what to expect from the antenna. Usually the manufacturers give you guidelines about the DC resistance of the antenna. You can use those guidelines to make a resistance test.
As for whether or not forming the excess transmission line into a few loops is going to have immediate and significant benefit to the operation of the radio system, this is very difficult to assess in simple terms. The best answer (that is easily understood) is that the transmission line typically has a minimum bend radius, and by forming the excess into a coil of loops of a few inch diameter is a good way to neatly stow the extra transmission line without causing damage to it. It is much preferred to use a coil than to tightly bundle the excess transmission line into a back-and-forth looped bundle like the sheepshanks knot.
Whether or not the formation of a coil of unspecified diameter of an unspecified number of turns of the transmission line at a random distance from the base of the antenna will produce a measurable improvement in the performance of the radio system is quite difficult to assess, but let's just say it can't hurt.
It is extremely difficult to imagine a situation in which a coil will tend to suppress interfering signals. This claim cannot be made with any sort of reasonable basis. The general notion of forming a coil in a coaxial transmission line is to try to suppress the flow of antenna currents on the outside of the coaxial transmission line outer conductor from traveling back to the transmitter. The reason for this is to maintain a more uniform distribution of the antenna current and confine the antenna current to the antenna array, and in so doing to improve the pattern of the antenna. Suppressing antenna current on the transmission line prevents the transmission line from becoming part of the radiating portion of the antenna. The creation of a coil with coaxial transmission line can accomplish this, but if it is to be effective it should be placed very close to the antenna, right at the feed point to the antenna. Placing a coil five feet away is not likely to be of much benefit in suppression of antenna current on the five feet of transmission line between the coil and the antenna.
Again, these are highly technical aspects of antenna theory. I don't bring up these topics, but when others do and provide information that seems quite speculative or in some cases not very accurate, I think it should be discussed in more detail. In this way misinformation is not continually propagated to boaters.
If I may, I recommend reading my article which touches on all of these topics and provides some recommendations and explains the basis for them. The article is available from
http://continuouswave.com/ubb/Forum6/HTML/003311.html
