Antenna Installation

[i4110peter]

I am installing a new Shakespeare 5225-XP with a new ICOM M412 radio. The antenna installation instructions say to leave at least 3' of coax. Is there any advantage to having more than 3'? I know that in a CB, antenna tuning is very important. Is it the same with a VHF setup? Thanks for your help. Peter

[reelapeelin]

Found this...looks like there is some loss w/long lengths...


Coax Cable length. It is permissible to cut coax if it is too long, however, you must make sure the antenna is at least 3ft from the radio. This is an almost universal requirement from most radio manufacturers. At any rate, you probably won't want the antenna that close to you when you transmit. If you decide not to cut the coax, you can loosely wrap up any excess into a large coil if you wish. Coax cable does present some signal loss, and that loss is greater the longer the coax is, so you can improve the performance of your system by ensuring you do not have excessive cable. This loss is highly dependant on the coax type being used, but a good rule-of-thumb for the RG-58 coax that is commonly used by antenna manufacturers is about -3dB loss for every 50ft.
If you have not figured it out by now, a -3dB loss halves the signal. Therefore, 50ft of coax would present a halving of the signal, and if connected to a +3dB gain antenna - doubling the signal's ERP, and the result would be 0dB, or unity gain. However, the -3dB loss along the coax is a true loss in signal strength, while the +3dB gain is only an increase in ERP. Therefore, you must pay someh attention to the interconnection of the antenna to the radio. Some antennas come with lower loss coax, such as RG-8X, so if you have a long cable run, such as in a sailboat mast, you may wish to consider low-loss cable.



This was copied from a pretty detailed discussion re antenna installation and function...if you wanna read it all, click below


http://www.boat-project.com/tutorials/vhfant.htm

[macojoe]

I need to replace mine also. but I have always left what ever it came with

[Road King Cole]

The tuning is more for matching the wavelength to the antenna length. Excessive cable lenght would just cut the power.

As long as the cable that came with the antenna is not excessively long, I would use it as is. The factory connections would last longer than a well crimped\soldered homemade connection.

rkc