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The Hummer Knowledge Base

Accessories/Communications/Antennas/CB:


I need a military looking CB/10m antenna, any ideas as to where I may obtain such an item?

Mike

95 383-green diesel wagon


Mike.......

Barring being able to find a Mil Surplus antenna that could be tuned for the 10 and 11 meter band.......... You might consider an ICOM AH2b HF antenna......it's roughly the same length as a standard CB whip....the kind with the ball and spring mount commonly available at most CB shops and truck stops. ( I actually haven't measured the length for comparison purposes.)

The AH2b is one heavy duty antenna.......and it 'ain't cheap' either....but it is much more 'Hummer like' than any other whip I've run across.

I use mine in conjunction with an ICOM 706 HF transceiver, coupled through an SGC Smartuner........the combination works great on all bands from 80 meters through 10 meters.......including the 11 meter CB band.

(Please....no flames from any hams on the list that know I'm not supposed to use the 706 on the CB band......I'm aware of all the rules.... ;-> ....)

Cordially:
Rick Crider / KD4FXA
Monroe NC
95 diesel wagon.....


A few points to consider here:

  1. In general, the taller the antenna the better.
  2. In general, the closer to the center of the vehicle you mount the antenna the more even your transmit pattern around you. Meaning, if you mount the antenna on the right rear corner, your best transmission will be to your left forward. The antenna uses the car as a radiation mechanism.
  3. Contrary to popular belief, the 102" whip is NOT necessarily the best. It radiates its power in all directions except at its base. That means that a significant amount of power is being lost radiating toward the heavens. Fiberglass wrap antennae combat this problem.

    >   Sorry Perrone, this just doesn't make sence. A 102" whip is that length
    > because it's 1/4 wavelength long at 27 MHz and provided that it has a good
    > ground will radiate as such with a good pattern for a mobile antenna.
    > Anything loaded is going to be a comprimize, no matter what some of the CB
    > antenna manufacturers would like you to believe. (There's a whole lot of
    > hype in that industry!) This includes helically wound fiberglass antennas
    > which always have a smaller diameter conductor than a 102" whip, which is
    > only one of the reasons they're less efficient.
    

  4. Some fiberglass antenna can take a HELL of a beating, and some (like my k40) have a lifetime warranty. You break it (like I did Sunday) you bring it back for a complete replacement.
  5. NOTHING, and I mean NOTHING will make up for having a poorly tuned antenna. Get the thing tuned by a professional. It makes all the difference in the world. Don't be cheap and try to tune it with one of those Radio Shack SWR meters either. Get it done right.

    >   Can't agree more on antenna tuning. However, those cheap Radio Shack SWR
    > meters are based on a classic design that's been in use since the
    > beginning of time and works well. It doesn't matter if they properly read
    > what your output power is, which they may very well not read right. What
    > does matter is that they give you a good indication of relative reflected
    > power, which they do. Now, if you need a calibrated power meter, that's
    > another story.
    

  6. Second only to the antenna, the kind of CB you have makes the most difference in your ability to get good signals in and out.

Good info in the last two posts on this subject, and I stand corrected. Some of the info I got from my local CB Pro (who I trust implicitly) some I have gotten from various web sources, including one manufacturer of fiberglass antennae. Seems their info might have been biased. Anyway, I am VERY pleased with my installation and the installer.

-Perrone Ford


Allow me to address some of this. CB (11 Meters) is in the gray area between ground wave transmission, and line of sight. At lower frequencies, the signal hugs the ground, and changes between day and night. Around 30 MHz (give or take, depending on season, and sunspot cycle) we get into VHF propagation, where signals are line of sight. True, 25 MHz can act like VHF, and 50 MHz can act like HF, depending on many factors.

SO, getting the antenna up high (if that is what you are saying) IS a good idea. But, while putting it on a tower can work at home, it won't work mobile.

Longer is not better. Being resonate is important. The easiest (and shortest) length to match (to the feed line) is One-Quarter Wavelength.

Match: The transmitter is expecting 50+/-j0 Ohms. That is 50 Ohms resistive and no reactive component. For maximum power transfer, everything must be 50+/-j0 Ohms. The transmission line (coax cable) is 50+/-j0 Ohms and the antenna must match as closely as possible. The antenna (when on a good ground plane), will approach 50+/-j0 Ohms at 1/4 wavelength, and at 5/8 wavelength.

The middle of the band just happens to be Channel 19, which is 27.185 MHz. This is 11.04 Meters. One quarter of this is 2.76 Meters which is 108.6 inches. This is the ideal length.

The other ideal length, which has more gain, is 5/8 wavelength, which is 6.9 Meters, which is 271.54 inches. We cannot drive around with a 22.63 foot tall antenna mounted on our trucks. Our roofs are roughly six feet, so that is 28.6 feet -- FAR to tall for 13.5 foot openings under bridges!

Even a 108.6 inch or 9.05 foot antenna is a bit tall when on a six foot roof, which means such an antenna needs to be located lower, which causes other problems.

Still a 108" whip, mounted high would be the best antenna. And, yes, a metal antenna will out perform anything wound on fiberglass.

Regarding the comment that "Contrary to popular belief, the 102" whip is NOT necessarily the best. It radiates its power in all directions except at its base. That means that a significant amount of power is being lost radiating toward the heavens. Fiberglass wrap antennae combat this problem." This is just not true. A quarter wave antenna will have the lowest angle of radiation of any whip antenna. The lower portions of the antenna do have the highest current flowing in it. This means this part of the antenna does the majority of the radiating. This is why it is so bad to mount such an antenna on a bumper and have metal parallel to it. Wound antennas have "group delay" which causes weird vertical radiation patterns, not straight metal radiators. The quarterwave antenna radiates in all directions, including at its base, unless it has some other element in the near field, like a tail gate.

Wound antennas, or loaded antennas are a compromise. They are intended to mimmic a 108 inch antenna, but the reactive portion of the match goes inductive. (Resulting in something other than +/-j0.) This can be tuned out with a capacitor, but this increases the "Q" of the antenna, making it match poorly at the edges of the band. Generally this capacitor is included in the antenna, and is not adjustable. Some companies do make an external box with a capacitor in it, but be aware this needs to be located at the antenna, not at the radio to works best.

On the other hand, an antenna that does not hit trees and bridges is an advantage.

There are also "base loaded" antennas which are only about three feet tall. The best of the lot is the Larsen NMO-27. It is a base loaded quarter-wave antenna and both the coil and the capacitor are located in the base. In theory this will not work as well as an antenna where the coil is distributed along the entire length of the radiator, but this does not actually seem to be the case. See www.sinctech.com/sinclabs/excel.htm as an example of how this is done.

Finally, there are an odd case of "marine" antennas. These antennas are electrically one-half wave antennas. The radiation resistance of these antennas is VERY high (in theory infinite), but the manufacturers make some compromises to make it all work out. The great thing about these antennas is that they are ground plane independent, making them ideal for mounting or a corner of the truck, or on an open top. They tend to be only about four feet long.

All in all, the best antenna would be the 108 inch whip. All others are a compromise of some kind.

I am not sure about the "professionally tuned" comments. Sure, a technician is only as good as his tools. The Radio Shack VSWR meter won't be a great unit, to be sure. BUT, someone who can read, and has a Bird 43 with the right elements (a five watt HF element, and a one watt HF element) should be able to do a good job. Some "professionals" have done really criminal things to make their installation look good, including putting resistors in the base. Sure, the antenna makes a good match, but the power, both on transmit and receive, is turned into heat in the resistor.

One last idea for off road. There are special antennas made for railroad use. They are vertically polarized like normal mobile antennas, yet they are uniquely shaped. Look at www.sinctech.com/products/tetrapics/special_ant.html to see some pictures. I have no idea why they don't have more information on the web site.

The antenna that covers 25-50 MHz (Excalibur 121T for top tuned, and 121B for bottom tuned) (it only covers 0.03 to 0.06 MHz at 1.5:1 VSWR without retuning) is shaped like a 2x4 sitting on edge. It is 6.8" tall x 5.7" wide (at the base) x 57.3" long. It requires a ground plane of 48x72 inches when tuned to 30 MHz, so it will have to be a bit larger at 27 MHz. In 1993 the list price was $291. They also sell a VHF unit, which does cover 2-Meters, and is MUCH smaller (of course).

--chip



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