SearayPaul
Active Member
A couple of people have asked me over the last couple of weeks to post an article on VHF antennas.
My information comes from real world experince, not everyone will agree. I have bought enough antennas with my ham radio over the years to buy Gary's boat and antenna mast.
VHF is line of sight communication and there fore if you can't see it you can't talk to it so get the antenna as high and clear as possible. VHF looses a lot of signal going down the coax so buy a good low loss coax. No barrels, no splices, and no cuts should be used on the coax. Only a single run from antenna to radio.
Antennas commonly available are 3db, 6db, and for those with a lot of room 9db. The db moniker details the gain of the antenna which describes the electrical performance of the antenna. The higher the gain the more amplification of the incoming and outgoing signal and the easier it is for your radio to detect a signal and turn it into usuable information. The higher the gain the more concentrated you signal is in one direction. This is not a big factor in marine antennas but can be a big issue with a phased array. Gary needs one on his antenna mast. I really like his mast.
The gain is obtained by changing the wave length of the antenna from a 1/4 wave to a full wave antenna with the exact length to be determined by the gain of the antenna. If I remember correctly the wavelength is the inverse of the frequency, not sure about that formula. You can lengthen the antenna two ways.
You can use the calculated physical length of the antenna or you can put a loading coil in the antenna. A loading coil is nothing more than wrapping a wire of known length around a spool and feeding it with the coax. A piece of wire is then dropped down the coil to finish the antenna. To tune this type of antenna you move the outsided element up and down the coil and tighten it down when the antenna is adjusted for your frequency. This is usually done at the factory. Guys this is the simple version.
You can pay a little or a lot for an antenna and everyone asks me what you get. The higher priced antennas usually are better constructed than the cheap no name antennas to a point. Those 8 foot antennas are nothing but a piece of wire that needs to be supported by some method. There is no magic powder in the antenna to make it work better. This is about wave propegation and powder does not resonate. The fiberglass is there strickly for strength so that you can run into bridges without breaking the antenna off. The base loaded antennas all have a coil of wire with a covering to protect the coil and an external element. The quality of the coil and the stength of the external element determine the price.
A 3db should antenna should work as good as any other 3db antenna as long as the gain of the antenna has been determined correctly, the same is true for any antenna of similar gain. Spend what you want but 3db is 3db. Sales booklets are a lot like what PT Barnimum said many years ago. As long as you go with a name brand antenna you will be fine.
At a ham radio 2 meter contest we constructed an antenna out of 20 gauge wire, wooden dowels, and an electrical match. We then competed against a fellow with a very expensive commerical antenna. Care to guess who made more contacts? In the interest of fair disclosure we did tinker with that antenna for a long time before we got it to work correctly. I hope this helps someone.
My information comes from real world experince, not everyone will agree. I have bought enough antennas with my ham radio over the years to buy Gary's boat and antenna mast.
VHF is line of sight communication and there fore if you can't see it you can't talk to it so get the antenna as high and clear as possible. VHF looses a lot of signal going down the coax so buy a good low loss coax. No barrels, no splices, and no cuts should be used on the coax. Only a single run from antenna to radio.
Antennas commonly available are 3db, 6db, and for those with a lot of room 9db. The db moniker details the gain of the antenna which describes the electrical performance of the antenna. The higher the gain the more amplification of the incoming and outgoing signal and the easier it is for your radio to detect a signal and turn it into usuable information. The higher the gain the more concentrated you signal is in one direction. This is not a big factor in marine antennas but can be a big issue with a phased array. Gary needs one on his antenna mast. I really like his mast.
The gain is obtained by changing the wave length of the antenna from a 1/4 wave to a full wave antenna with the exact length to be determined by the gain of the antenna. If I remember correctly the wavelength is the inverse of the frequency, not sure about that formula. You can lengthen the antenna two ways.
You can use the calculated physical length of the antenna or you can put a loading coil in the antenna. A loading coil is nothing more than wrapping a wire of known length around a spool and feeding it with the coax. A piece of wire is then dropped down the coil to finish the antenna. To tune this type of antenna you move the outsided element up and down the coil and tighten it down when the antenna is adjusted for your frequency. This is usually done at the factory. Guys this is the simple version.
You can pay a little or a lot for an antenna and everyone asks me what you get. The higher priced antennas usually are better constructed than the cheap no name antennas to a point. Those 8 foot antennas are nothing but a piece of wire that needs to be supported by some method. There is no magic powder in the antenna to make it work better. This is about wave propegation and powder does not resonate. The fiberglass is there strickly for strength so that you can run into bridges without breaking the antenna off. The base loaded antennas all have a coil of wire with a covering to protect the coil and an external element. The quality of the coil and the stength of the external element determine the price.
A 3db should antenna should work as good as any other 3db antenna as long as the gain of the antenna has been determined correctly, the same is true for any antenna of similar gain. Spend what you want but 3db is 3db. Sales booklets are a lot like what PT Barnimum said many years ago. As long as you go with a name brand antenna you will be fine.
At a ham radio 2 meter contest we constructed an antenna out of 20 gauge wire, wooden dowels, and an electrical match. We then competed against a fellow with a very expensive commerical antenna. Care to guess who made more contacts? In the interest of fair disclosure we did tinker with that antenna for a long time before we got it to work correctly. I hope this helps someone.