Enlighten me please :)

[holyshipmon]

Knowing nothing of antennas I have been throwing money away "experimenting" with different antennas and gain. I monitor 161.975 & 162.025 AIS VHF channels. Receive only(I decode the messages and plot the positions). I bought a 8' shakespere and tore it apart only to find about 8ft of RG58 run up the center with about 2ft of housing stripped. I am guessing this stripped area is where I receive the signal.

My question is would I do better running RG238 up the center and stripping about x amount of housing off the end?

Any suggestions or ideas to this or am I really lost in the concept of how an antenna works. If so please enlighten me.

Thanks,

Chris

 

[ka3jjz]

Whole books have been written on the subject of how antennas work - the ARRL Antenna Book (which I see has recently been reissued) is a must have if you're really interested in the science. You can also see the AC6V Antenna projects page - there are several links there relating to theory. Don't let the fact that the frequencies they discuss may not be the same as what you are looking for - a 2 mhz antenna uses exactly the same principles as a 160 mhz dipole (which sounds very much like what that Shakespere is, housed inside a radome to protect it from the elements). They are often applied somewhat differently but the science is the same.

However, you can do much better. A J Pole would, I have no doubt, be absolutely ideal for your application. As its angle of radiation is toward the horizon (you don't want to be listening to aircraft, for example - you want ships at sea), and there's a small amount of gain, I'd say it's worth investigating. If you live near the ocean, you may need to construct your antenna out of materials that are salt-air resistant to avoid the problem of corrosion. That page I just mentioned has a few links on J Poles. In addition, here's BuxComm's J pole page. The formula for the individual elements is about 2/3 of the way down the page.

73s Mike

 

[holyshipmon]

ok

Thanks for the help. I'll begin looking into antenna options and solutions specific to my application. I'll hit the library and the internet some more so that I may bring back some conversation of substance on the next post. Thanks for the links.

Chris

 

[DPD1]

The simplified answer is no... It doesn't work the same way the feedline cable works. Being 8 feet long, it should be a little more complex than what you say you saw in there. At that length it's most likely a collinear, which means that the 1/2 wave length for the band is duplicated more than once. You might be missing a couple details along the length of the element line that make it function the way it does. Anything that's shorter than that in an omni is going to be less, not more gain, because you're going down to just a single 1/2 wave element. You have to stack elements either horizontally or vertically to get more gain. There's no way around that.

You can get an even taller one (about 17'), which would bump you up to about 9 dbi... That's as good as you will get with an omni. If your targets are stationary, you would probably be better off with something directional. Even if you still got about 9-10 dbi, it wouldn't be quite as huge as the 17 footer... More like about 4' long on the boom.

Dave
http://www.dpdproductions.com
Antennas & Accessories for the RF Professional & Radio Hobbyist