Antenna?????

It's certainly true that your antenna is key to any radio system but don't leave out an equally important part of what makes your antenna work.

That would be the quality or rather amount of ground plane area in very close proximity to your antenna mount.

The more area for a groundplane, the better the antenna works.
This is a constant for antennas operating below about 50mhz.
It's not theoretical, just Mother Nature.

And, as anyone who's been stuck in a DFW traffic snarl after a bean & burrito lunch will attest, Mother Nature is an immutable force.

Coax length & cutting to tune swr: Let's start by saying coax length should not play a part in the matching of your antenna to the vehicle..unless...you're dealing with one side of a co-phasing harness.

That type of line does need cut to a specific length. However, the line running to the radio from the splitter can be any length.

Secondly, with a fairly good swr, the system shouldn't care how long or short your coax is. The true swr stays the same.

Ultimately, if you can effect the swr reading by trimming coax length there's a couple possible reasons &/or problems.

One issue: Sometimes you can fool your swr meter just due to its location along the coax. In general, you want to have half wavlengths of coax between the meter & the antenna. This helps keep the meter in the right place in the line for an accurate measurement.

A halfwave in solid dielectric rg58 is aprox 12 feet.
The same halfwave in foam dielectric rg58 is aprox 14.5 feet.
These values hold true for the rg8x style cable as well.

Testing for inaccurate readings due to meter placement in the line is as simple as adding a short 3-5ft jumper between the meter & the antenna after your initial measurement. If the reading changes, you need to account for the reason.

Another issue: If you can change the real swr (the reading not influenced due to meter placement along the line) thru line length or physical layout of the coax, there's a strong probability that you have a poor groundplane to work over.

In this case the coax braid has become a part of the ground plane.
Not a good thing.
This leads to feedback from your stereo spkrs & ecm issues.
It also lends itself to producing a weak &/or distorted radiation pattern.

The cure is to "tie in" more of the body metals that are in close proxmity to the antenna mount. Things like mirror brackets, door jambs, body pillars etc all need to be electrically connected to the ant mnt grnd using the minimum of short heavy jumpers.

You keep the number & length of jumpers to a minimum by using the body metals as a conductor between the jumpers. Body members are typically wider & heavier than any jumpers you can make. This makes for much less inductance compared to a wire jumper.

It's unecessary to run a wire all the way to the frame or motor from the mirror bracket to get a ground. If the jumper is more than a foot or two, it won't act as a connection to ground anymore. It will become part of the antenna system just like the coax braid & radiate.

If the jumper gets close to 9 ft, it becomes a tuned radial such as you'd see on a base antenna. This generally doesn't help because that radial is directional & will skew the intended radiation pattern even though it made the swr look better.

When it comes to antenna systems, the only thing you want radiating is the antenna itself. You don't want your ground straps/jumpers/coax braid to radiate as that usually negatively affects the overall pattern you wanted.

Now you know...as little as I do.
bc

 

Good post Bob! Gave you a thanks!

 

what do u guy;s think of the firestik Dual Antenna Kit??