Antenna Ignorance

[Chaos703]

I'm in desperate need of a new mobile antenna. Two problems: 1) I don't know what antenna to get and 2) I don't understand the principles of grounding an antenna.

First the antenna:
After doing some reading, I keep coming back to the Austin Spectra. It's expensive but seems to have the best multi-band offerings. I'm in fine shape on 800 but I'm tired of listening to the crap static for everything else. Additionally, I really do enjoy listening to OHP low band (where you can find it). So I don't mind paying for it if it actually works. Has anyone had good or bad experience with either the Spectra or something else?

Next the grounding:
I hate asking this because I feel I'm admitting that I failed Scanning 101, but put simply, does the woven copper shield in the coax have to be grounded to the body of the car or should it be terminated? I've always assumed the way it's SUPPOSE to work is that the shielding in the coax connects with the mounting base which is grounded to the body of the car creating a ground continuity through the antenna, scanner and the car. I've tried it both ways and have always seemed to have better success by NOT connecting the base to the metal of the car with the exception of a mag mount which I ASSUME connects to the metal of the car—but I could be totally wrong about that. And then there's glass mounts that confuse me even more.

And if that wasn't enough, I'm also confused about the relationship between electrical grounding and the ground plane. Is the ground plan established by physically "grounding" the coax or is it a totally different deal?

Can anyone offer any help?

 

[russellmaher]

I have used the Spectra on my cars since the fall of 2006 and am a huge fan of it. You're right that it is an expensive antenna but it's worth it (IMHO) and it delivers for me on the freqs I monitor.

I monitor VHF-HI, UHF, and 800 MHz. The Spectra will do VHF-LOW and to get the best reception you can trim the top whip for whatever low band freq. you need to monitor. When you buy it, there should be a chart that tells you what to trim it to.

Since I don't want to drill holes in my vehicle, I use the NMO Trunk Lip Mount and find it gives great results. Some prefer to mount it on the roof by drilling the hole and they claim this way will get you the best reception possible since this creates the perfect ground plane. They may be right.

Here's the Spectra on my car so you can see how it looks.

Russell

 

[N5TWB]

I've known a couple of scanning enthusiasts in OKC that like their Spectras very much. Since I drive an SUV and like to park in my garage at home, I generally limit my scanner antenna to the Antennex tri-band scanner antenna but I do occasionally use a larger custom concoction that has an NMO base. However, I can reliably report that I've received low band OHP transmissions using just a VHF-Hi 1/4 wave magmount and it also received 800 TRS just fine, too. Outside of the cost, antenna experimentation is part of the interest in the hobby for me.

As to your grounding question, I think you are giving yourself a clue about the need for grounding with your comment regarding "crap static". I'd recommend you do some reading on the subject on the following excellent website: http://www.k0bg.com/ It is oriented to the mobile ham but you can learn a lot about mobile operations and installs for even scanning. Alan is also really good about answering questions on eham.net and QRZ.com on the mobile operations forums on those sites.

Electrical grounding and RF grounding are definitely two different things. Electrical grounding creates the flow of electrons to power your system. In a transceiver set-up rather than just a scanner, poor RF grounding can cause the transmitted RF energy to flow in undesired ways by returning on the outside of the coax to get back to the radio. In both transmitting and receiving, RF grounding is part of the antenna system to ensure the RF signal is used to its best advantage in the system.

 

[HogDriver]

I have an old magnetic base with a 30 inch steel wire that I have found provides the best overall reception for my particular application. I experimented with different lengths to find the best. I park in the garage so I can't put anything on the roof so it sits atop my "truck box". I pick up Tecumseh, Norman and Lexington just fine with it from just south of Del City.

 

[Chaos703]

Thank you all for your input! I've done a good bit of reading on k0bg's site. It's a lot to digest for an apparent novice, but it's still been extremely helpful. I hacked together some basic principles from the website just for testing purposes and have had some noticible success already. If nothing else, reading the site made me realize that I'm far from the only one who doesn't "get" antenna technology.

 

[HogDriver]

RF grounding is just a simple principle that the flat surface of your vehicles roof tends to amplify the signal. Case in point...back in the days when I had a CB radio I had a base loaded whip in the center of the roof on my station wagon. I was in eastern Oklahoma County and was talking to a base station in SW OKC. The signal was weak but when I turned the car toward the west the signal was as if down the street. By mounting it toward the back side of your roof, the larger area of metal forward of the antenna amplifies signals in front of you. Mounti ng in the center is more evenly spread around you.

As far as magnetic mounts go, they will be "electrically" grounded where it attaches to your radio. There is no electrical connection where it sits on the roof but as long as your vehicle and radio are bonded electrically, it works just as well as an antenna permanently mounted. Once a year check the connections in your antenna and make sure there's no corrosion setting up inside. That's a definite signal stopper! I had rain water that would collect inside the coax connector where the antenna would screw onto the base so I filled it with silicone grease. No more water issues!