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Author Topic:   vhf installation help needed

cc13 posted 03-09-2003 01:07 PM ET (US)
1972 16/7 Cohasset. Mounted vhf on top of console. Console has a rail running 2-3 inches above console. Bought a rail mount to place antenna on the console rail near the radio. Antenna arrived and is ready to be installed. 3 foot Shakespeare wire whip with 15' coax. Question? the antenna instructions say that cable can be shortened but "should" be at least three (3) feet long. I can run cable direct from antenna to radio with approx. 1 foot of cable.
In order to comply with the "should" language, I will have to remove console top and then coil the excess length inside the storage compartment. Extra work and somewhat bulky with antenna cable coiled in the storage compartment.
Can I safely violate the 3foot cable length rule/suggestion? If so, the installation will be much simpler, easier, and neater. Why do they recommend at least three feet of cable? Is it a safety/operational issue? If so, then I'll figure a way to comply. As mounted now the radio and antenna are less than a foot apart. Thanks for all advice.
Jerry Townsend posted 03-09-2003 05:51 PM ET (US)
The cable length must comply with the manufacturer's recommendation because the performance of the radio
"system" involves the impedance (akin to resistance) of the cable and the antenna. Apparently, there is enough adjustment of the antenna to allow a cable length of 3 feet. The impedance of the antenna of any short wave transmitter is adjusted (tuned) by the user to tune the entire "system".
I would suggest not going to the 3 foot length on the first shot - because all antenna's are not the same.
Similarly, the cable length on GPS systems with external antennas should not be cut - because there is no "adjustment" or "tuning" of the impedance of the antenna. ----- Jerry/Idaho
whalerron posted 03-09-2003 10:12 PM ET (US)
The instructions which came with my Standard Horizon VHF stated that the antenna and the radio must be separated by at least 3 feet of distance. I assume that if the 2 are any closer to each other, there will be feedback problems or the Standing Wave Ratio (SWR) will be detrimentally effected. I would expect this to hold true for all VHF setups.
jimh posted 03-09-2003 10:21 PM ET (US)
The feedline length does not affect the resonant frequency of the antenna, nor does it affect the VSWR other than by the effects of attenuation. The proscription against feed lines of less than 3-feet is probably motiviated by a desire to keep the radio away from the antenna so that the base currents don't interfere with the operation of the transmitter. Inside the transmitter is a sensitive audio preamplifier that could be affected by the presence of strong RF currents within a few feet. Hey--the radio sells for $150 and it is in a plastic case.
Most of the marine antennas being sold these days are variations of a 3/4-wavelength series-feed vertical operated without any ground plane or counterpoise.

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Author	Topic: vhf installation help needed
cc13	posted 03-09-2003 01:07 PM ET (US) 1972 16/7 Cohasset. Mounted vhf on top of console. Console has a rail running 2-3 inches above console. Bought a rail mount to place antenna on the console rail near the radio. Antenna arrived and is ready to be installed. 3 foot Shakespeare wire whip with 15' coax. Question? the antenna instructions say that cable can be shortened but "should" be at least three (3) feet long. I can run cable direct from antenna to radio with approx. 1 foot of cable. In order to comply with the "should" language, I will have to remove console top and then coil the excess length inside the storage compartment. Extra work and somewhat bulky with antenna cable coiled in the storage compartment. Can I safely violate the 3foot cable length rule/suggestion? If so, the installation will be much simpler, easier, and neater. Why do they recommend at least three feet of cable? Is it a safety/operational issue? If so, then I'll figure a way to comply. As mounted now the radio and antenna are less than a foot apart. Thanks for all advice.
Jerry Townsend	posted 03-09-2003 05:51 PM ET (US) The cable length must comply with the manufacturer's recommendation because the performance of the radio "system" involves the impedance (akin to resistance) of the cable and the antenna. Apparently, there is enough adjustment of the antenna to allow a cable length of 3 feet. The impedance of the antenna of any short wave transmitter is adjusted (tuned) by the user to tune the entire "system". I would suggest not going to the 3 foot length on the first shot - because all antenna's are not the same. Similarly, the cable length on GPS systems with external antennas should not be cut - because there is no "adjustment" or "tuning" of the impedance of the antenna. ----- Jerry/Idaho
whalerron	posted 03-09-2003 10:12 PM ET (US) The instructions which came with my Standard Horizon VHF stated that the antenna and the radio must be separated by at least 3 feet of distance. I assume that if the 2 are any closer to each other, there will be feedback problems or the Standing Wave Ratio (SWR) will be detrimentally effected. I would expect this to hold true for all VHF setups.
jimh	posted 03-09-2003 10:21 PM ET (US) The feedline length does not affect the resonant frequency of the antenna, nor does it affect the VSWR other than by the effects of attenuation. The proscription against feed lines of less than 3-feet is probably motiviated by a desire to keep the radio away from the antenna so that the base currents don't interfere with the operation of the transmitter. Inside the transmitter is a sensitive audio preamplifier that could be affected by the presence of strong RF currents within a few feet. Hey--the radio sells for $150 and it is in a plastic case. Most of the marine antennas being sold these days are variations of a 3/4-wavelength series-feed vertical operated without any ground plane or counterpoise.