silverbullet02

Still undecided on what I'm gonna run for an antenna just yet...still wondering how I'm gonna mount it, that's why I'd like some advice on what works well and what doesn't. Already got a Cobra 148 NW ST sittin here...still kinda unsure...don't really know a lot about these things yet. Thanks for the help guys,

Brian

edwinsmith

One of the best antennas is a 5/8 wavelength base loaded whip mounted dead center of the cab roof. It gives a good ground plane and pulls some of the energy which would otherwise go up and pushes it out. Like a figure 8 laying on it's side and flattened out some.

Edwin

greatwhite

I'd agree with Edwin on this. Twin truckers may or may not look "cooler", but the best way to get out in these trucks is with a roof mount. Not enough seperation between the twins to get the phasing correct.

I run a base loaded 54" Valor that I've had for many moons and am impressed with the range and quality. Waaaayyy better than with my dual setup.

DJ

I've been looking for a way to rig one on a slide in camper and havent come up with anything that gets out at all. Without the camper a magnet mount in the center of the roof works good.

Buckshotmckee

I'm running the Wilson Super Trucker. No complaints. I got pics in my gallery on were I mounted it. I also painted to match.

andyr354

Run a setup like that on my Jeep (big metal roof on the cherokee). I always can talk farther than guys with their fancy firesticks.

ds1rider

What you need is a whip antenna on your Trans Am.

tool

O.K Bandit.

silverbullet02

Actually............I do have a magnet mount antenna for the T/A...got the little cobra self contained cb...works real nice for the car where I don't really have room for a big cb...but yes, I do have an antenna on it...sometimes...lol...but thanks for all the good answers guys, I appreciate it. I'm more concerned about functionality than looks on the truck..but looking decent is always a plus...thanks,

Brian

Den

I'd go with the Wilson antennas. Over the years i've had the 500, 1000 and 5000 and the 5000 trucker. All worked great and the higher ones can really handle the power

No matter what you choose, make sure you tune it (SWR) or have it tuned by somebody. Makes a big difference in how they perform and is good insurance for your radio.

Den

edwinsmith

Ah yes! The good ol SWR. It is a ratio between what is going out the antenna to what is coming back and heating your output transistor. Essentially if the antenna is tuned properly it will radiate ALL of the power it gets from the transmitter. It also helps on receive. I could go deeper into antenna theory for those who are interested.

Edwin

vinny

Howdy,

YEAH! What he said...firesticks suck!

I have one on my Spirit R/T and it works so-so for CB,and really bad for FM reception. I wouldn't call it a comprimise...it just plain sucks.

Hey Edwin,riff away my man...I'd love to hear everything you got.

I know much less about CB's than I'd like to (read that as um...nothing).

I picked-up a SWR meter years ago...but I don't even know how to hook it up,OR adjust the antenna...(DOH!)

Thanks in advance,

Vinny

phox_mulder

Of course it sucked.
You were trying to use one antenna for two different frequency ranges.

Antennas are made for specific frequencies, use the wrong one, and performance suffers.

CB antennas are for CB's
FM radio antennas are for FM radio.
Shortband for shortband, VHF for VHF, etcettera etcettera.

Here's a great page for a days worth of reading.
http://www.firestik.com/Tech_Docs.htm

All that information at their fingertips, yet they still sell the combination antennas

That being said, my dual firestiks work just fine, and look great as well.


phox

edwinsmith

OK, well there are several things to consider for an antenna. An antenna is simply a conductor which has distributed inductance, capacitance and a fundamental impedance which is a voltage/current ratio expressed in ohms.

The simplest antenna is a 1/2 wave dipole which is two wires each 1/4 wave long stretched out in opposite directions and fed in the middle with usually twin lead. The characteristic impedance of a 1/2 wave dipole is 150 ohms.

A 1/4 wave whip which is usually found on vehicles is usually fed from the center conductor of a 50 ohm coax cable however it's impedance is ALSO 150 ohms. The other 1/4 wave of the antenna is made up of the vehicles body.

Since the coax is 50 ohms and the antenna is 150 ohms there is a mismatch of 3:1 and some device must be used to MATCH the two impedances. Usually a delta match is used and it is aptly named. Quite simply the coax center conductor and shield are gradually spread out in a "Y" fashion until it comes to the antenna. The old motorola 150 mhz whips on police cruisers etc... had a good example of the delta match in which the shield soldered to a metal tube which had a funnel shape where it exited the roof and the antenna touched an eyelet soldered to the center conductor of the coax.

If you have either an antenna which isn't resonant (to short or to long) or the match is messed up somehow you'll get part of the energy reflected from the mismatch which is usually the base of the whip.

Another popular method of matching is with a transformer. A 5/8 wavelength antenna is employed with part of the antenna in a whip and part of it in a coil in the base of the antenna. These are appropriately called base loaded. The transformer both matches the impedance and makes up the additional length of the antenna by replacing some of it with part of the coil.

There are also top loaded, center loaded and helical fiberglass whips which work well but still need the matching device at the base which for top, center or helical whips is probably a delta match. You can't simply hook one of these up to the coax and expect it to work right. The base hardware even though it looks like a bolt through some insulating washers forms the delta match. If you alter it you'll mess things up.

A whip antenna in the middle of a metal roof forms what is called a ground plane. Because the bottom half of the antenna is at 90 degrees to the whip the energy is pulled down somewhat from the vertical and is radiated outward instead. A typical ground plane will have 3 to 6 db gain over a 1/2 wave dipole.

Now if you have a lot of land you can get into other types of antennas such as a YAGI beam or a rhombic. A rhombic is a diamond shaped string of wire which is 10 whole wavelengths on each side. You feed it at one of the 30 degree ends and it gives you 20 db gain radiated in a line which bisects the 30 degree angles.

A Yagi is a beam type antenna which has a driven element, usually a 1/2 wave dipole and a reflector element and one or more director elements. The reflector elements are a bit longer than the 1/2 wave and the directors are shorter with each successive element shorter than it's predecessor the further you go out. You can keep adding elements until you reach a point of diminishing returns.

Then there are the phased arrays.

Basically if you place two antennas 1/2 wave apart and feed them each with the same signal "in phase" the energy will be concentrated in two lobes along a line perpendicular to a line drawn between the two antennas.

If the signal is delayed by 1/2 cycle to one of the antennas the opposite is true, the energy will be concentrated in a line parallel with the line drawn through the antennas.

In the past there was a CB antenna maker who sold an electronic beam antenna which employed 3-1/2 wave dipoles and a switch box to change the phasing among the elements to be able to switch the directionality of the antenna without having to use a YAGI and a rotor.

The coax is properly called a transmission line and as stated before it has a characteristic impedance. Being made of a distributed inductance and capacitance. To properly drive phased arrays you can most easily use coax cut to the proper length since the signal moves down the coax at nearly the speed of light. (Actually there is a characteristic "velocity factor" which is used to figure lengths. The signal always moves more slowly through a conductor than through air or a vacuum.)

TV twin lead is 300 ohms for those who remember it. The size of the conductors and it's spacing determines the impedance. Coax is usually 50 ohms as in RG58/U or 75 ohms as in RG 59/U which is used for TV cable. There are other transmission line impedances but these three are the most common.

An SWR meter simply measures the amount of power flowing in one direction and compares it to the amount of power flowing in the other direction. If you drop a rock in still water you see the waves radiate out in all directions, if they encounter a discontinuity (mismatch) some of the wave is reflected. The same thing happens in a coax. If the signal gets to the base of the antenna and there is a discontinuity or mismatch part of the signal is reflected back down the line. This is why a proper match and antenna resonance is so important. You want 100% of your power to be radiated and none of it to be reflected.

The same thing happens in reverse. When a signal is picked up by the antenna it goes to the base and if you have a mismatch then only part of that signal will go down the coax and the rest will be re-radiated. In fact, everything above also applies in reverse, if an antenna is properly matched it will receive the best it can. The directionality of beams etc... also works for reception as well.

A wavelength for CB at 27 mhz is about 32 feet which gives rise to the old standard 9 ft 1/4 wave whip. The higher you go in frequency the shorter the wavelength. When you get to microwaves the best transmission line is usually not a coax but a wave guide which is essentially a pipe or square tube the best of which will be silver or gold plated on the inside. Amature radio enthusiasts found out what frequency NASA was using for communications by estimating the length of the antennas on the space capsules from photographs and working backward to get the frequency. Yep they were using 1/4 wave whips.

Edwin

Den

Good post edwin!

Also remeber that the coax on the antenna should never be shortened or lengthened. It's a certain length for a reason. Also try to keep the extra antenna cable coiled loosely (not a tight coil) and without sharp bends or kinks.


Den