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Author Topic:   Can I Trim VHF Cable

Jimm posted 11-21-2002 06:28 PM ET (US)
My Shakespere VHF antenna has about 15 feet to go my radio which is three feet away. Can I cut off the extra or is the antenna tuned to that wire length?
JBCornwell posted 11-21-2002 06:49 PM ET (US)
Ahoy, JimM.
The coaxial cable is balanced, so it's length is not going to affect antenna tuning.
It is important, however, that if it is shortened the termination (connector) is correctly installed.
I suggest that you get a new connector (Radio Shack) and be sure you know how or get instructions on installing it.
Red sky at night. . .
JB
Dick posted 11-21-2002 09:57 PM ET (US)
Rather than shorten the cable wrap in a figure 8 patern and zip tie it out of the way. If you must shorten it use the Shakespeare no solder connector.
bsmotril posted 11-21-2002 10:03 PM ET (US)
The correct answer is it depends on the model of the Antenna. Call Shakespere to get a definitive answer. Some Marine antennas that are designed to function without a ground (like a boat application) use the coax as a tuned counterpoise to the antenna radiating element(s). If your's is that type of antenna, trimming the coax will have a very detrimental effect to its' operation. If indeed you find that is the case, a figure-8 coil with a loose center binding is better than a coil as another poster has stated. I know that on the Galaxy 8' model antenna, you can trim with no problem.
BillS
jimh posted 11-21-2002 11:20 PM ET (US)
The length of transmission line between transmitter and antenna is generally not part of the tuning of the antenna system.
Jay A posted 11-22-2002 12:17 AM ET (US)
Coiling or looping 10' to 12' of excessive cable(RG58U)will marginally effect your performance. It creates a magnetic field which may increase "noise" such as electrical interference from poorly shielded engines! What you need is a PL258 connector available at Radio Shack. There are two types, solderless and the solder type. Which ever you choose it is critical NOT to damage the grounding braid and allowing any part of it to touch the center conductor.
triblet posted 11-22-2002 12:38 AM ET (US)
At these frequencies, length isn't generally
important. Shortening will reduce losses
SLIGHTLY, probably not measureable.
Don't get the connector at Radio Shack. It
won't be marine grade. The Shakespere and
Center-Pin solderless connectors (same thing,
some slight differences, I think the
Center-Pin brand is a little better (and a
little pricier) are great.
Chuck
jimh posted 11-23-2002 01:47 PM ET (US)
In a well-matched antenna-transmission line system, there are no antenna currents flowing on the outside of the outer conductor of a coaxial (unbalanced)transmission line. The transmission line can be arranged into loops or coils without affecting the transmission line currents flowing on the inside of the outer conductor.
It is not unusual in high-power transmission lines for the outer conductor to be constructed of bare copper tubing. Even with power levels of 10,000 watts or more flowing in the (inside) of the transmission line, there is no current flowing on the outside of the outer conductor, and one can touch the bare copper surface without a problem. In fact these large transmission lines (6-inch diameter) are suspended from clamps and are not insulated from grounding.
These same laws of physics apply to 25-watt marine radios and the 0.375-inch transmission lines used in that application.
If the transmission line is of marginal quality and the shield coverage is low (as is often seen in expensive but poor quality "marine grade" transmission line) it may be possible that looping the line into a coil could cause it to be susceptible to pick up of stray fields. However, the antenna, usually a few inches away, is intentionally designed to pick up magnetic fields (that is how it receives radio signals), so the problem of noise induced from coiling of the transmission line seems to be not a significant one.
That said, I would not intentionally create a coil of transmission line and pass the spark plug leads through its center.
Because the matching of transmission line to antenna is often poorly accomplished in marine band antennas, they may be more suspectible to having some antenna currents flowing back toward the transmitter on the outside of the outer conductor. In this case it is possible to suppress these currents by creating a small coil of transmission line. To the currents flowing on the inside of the transmission line this coil is invisible. To the currents trying to flow on the outside of the transmission line the coil represents a small amount of reactance which tends to suppress the flow of such currents.
As a matter of fact, on my boat I have the excess transmission lined coiled up into a solenoidal winding for precisely this reason.

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Author	Topic: Can I Trim VHF Cable
Jimm	posted 11-21-2002 06:28 PM ET (US) My Shakespere VHF antenna has about 15 feet to go my radio which is three feet away. Can I cut off the extra or is the antenna tuned to that wire length?
JBCornwell	posted 11-21-2002 06:49 PM ET (US) Ahoy, JimM. The coaxial cable is balanced, so it's length is not going to affect antenna tuning. It is important, however, that if it is shortened the termination (connector) is correctly installed. I suggest that you get a new connector (Radio Shack) and be sure you know how or get instructions on installing it. Red sky at night. . . JB
Dick	posted 11-21-2002 09:57 PM ET (US) Rather than shorten the cable wrap in a figure 8 patern and zip tie it out of the way. If you must shorten it use the Shakespeare no solder connector.
bsmotril	posted 11-21-2002 10:03 PM ET (US) The correct answer is it depends on the model of the Antenna. Call Shakespere to get a definitive answer. Some Marine antennas that are designed to function without a ground (like a boat application) use the coax as a tuned counterpoise to the antenna radiating element(s). If your's is that type of antenna, trimming the coax will have a very detrimental effect to its' operation. If indeed you find that is the case, a figure-8 coil with a loose center binding is better than a coil as another poster has stated. I know that on the Galaxy 8' model antenna, you can trim with no problem. BillS
jimh	posted 11-21-2002 11:20 PM ET (US) The length of transmission line between transmitter and antenna is generally not part of the tuning of the antenna system.
Jay A	posted 11-22-2002 12:17 AM ET (US) Coiling or looping 10' to 12' of excessive cable(RG58U)will marginally effect your performance. It creates a magnetic field which may increase "noise" such as electrical interference from poorly shielded engines! What you need is a PL258 connector available at Radio Shack. There are two types, solderless and the solder type. Which ever you choose it is critical NOT to damage the grounding braid and allowing any part of it to touch the center conductor.
triblet	posted 11-22-2002 12:38 AM ET (US) At these frequencies, length isn't generally important. Shortening will reduce losses SLIGHTLY, probably not measureable. Don't get the connector at Radio Shack. It won't be marine grade. The Shakespere and Center-Pin solderless connectors (same thing, some slight differences, I think the Center-Pin brand is a little better (and a little pricier) are great. Chuck
jimh	posted 11-23-2002 01:47 PM ET (US) In a well-matched antenna-transmission line system, there are no antenna currents flowing on the outside of the outer conductor of a coaxial (unbalanced)transmission line. The transmission line can be arranged into loops or coils without affecting the transmission line currents flowing on the inside of the outer conductor. It is not unusual in high-power transmission lines for the outer conductor to be constructed of bare copper tubing. Even with power levels of 10,000 watts or more flowing in the (inside) of the transmission line, there is no current flowing on the outside of the outer conductor, and one can touch the bare copper surface without a problem. In fact these large transmission lines (6-inch diameter) are suspended from clamps and are not insulated from grounding. These same laws of physics apply to 25-watt marine radios and the 0.375-inch transmission lines used in that application. If the transmission line is of marginal quality and the shield coverage is low (as is often seen in expensive but poor quality "marine grade" transmission line) it may be possible that looping the line into a coil could cause it to be susceptible to pick up of stray fields. However, the antenna, usually a few inches away, is intentionally designed to pick up magnetic fields (that is how it receives radio signals), so the problem of noise induced from coiling of the transmission line seems to be not a significant one. That said, I would not intentionally create a coil of transmission line and pass the spark plug leads through its center. Because the matching of transmission line to antenna is often poorly accomplished in marine band antennas, they may be more suspectible to having some antenna currents flowing back toward the transmitter on the outside of the outer conductor. In this case it is possible to suppress these currents by creating a small coil of transmission line. To the currents flowing on the inside of the transmission line this coil is invisible. To the currents trying to flow on the outside of the transmission line the coil represents a small amount of reactance which tends to suppress the flow of such currents. As a matter of fact, on my boat I have the excess transmission lined coiled up into a solenoidal winding for precisely this reason.