Your vehicle's contribution to your antenna

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KL7AJ, Jun 3, 2010.

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  1. KL7AJ

    KL7AJ XML Subscriber QRZ Page

    There's a lot of unnecessary confusion about what contribution the body of your car makes when it comes to H.F. Mobiling.

    Before we can clear away the haze, however, we need to completely eliminate the term "counterpoise" from our vocabulary. The concept of a counterpoise is a relic from before World War I, and has been soundly debunked by both modern antenna modeling and Maxwell's equations themselves. Also actual measurements in R.F. anechoic chambers have confirmed the modeling. Everyone needs to read L.B. Cebik's (W4RNL's) fine work on the myth of the counterpoise. His analysis is absolutely correct. I am greatly saddened that he is no longer with us. :(

    With that out of the way, let's start with a simple fact: Your car's body is a piece of wire. Period. The fact that it has "mass" or "bulk" is immaterial. There is a SINGLE vector which will describe the net R.F. current flow and direction in your car's body, no matter how big or odd shaped it is. Feel free to model everything I assert from here on out, if you're familiar with NEC, or even better, make some close-in current measurements, if you have that capability.

    For the first few cases, let's ignore the natural Earth ground entirely. As we will learn, this generally has more of a detrimental effect than an advantage in most practical vehicle installations.

    Let's take a thin, rectangular plate of aluminum, say a 4x8 foot sheet. Let's feed some R.F. into one corner of the sheet. A little application of the Pythagorean Theorem will tell us the farthest corner of that sheet will be 8.944 feet away. That sheet of aluminum can be replaced with a piece of wire 8.994 feet long. The radiation resistance of the two items, the sheet of aluminum and the wire is identical.

    Let's modify this a bit, and replace the sheet of aluminum with an aluminum box that's 4x4x8 feet...about the size of a really compact car. In this case, the "cube" (actually a 'rectangular prism') can be replaced by a single wire stretching between the farthest corner of the cube. (It's early in the morning, and the formula for the distance between opposite corners of a rectangular prism excapes me, but it will be slightly more than 8.944 feet. (Anyone feel free to supply the correct answer).

    We can REPLACE that cube with a single wire extending between the fathest corners of that box. Again, its radiation resistance and pattern will be identical to the rectangular prism fed at one corner.

    Once you have this in mind, can you see how easy it is to model and predict your car's antenna performance? Next installment, we'll talk about the ground.

  2. W0IS

    W0IS Ham Member QRZ Page

    Well, I'll be darned. I've now seen the term "rectangular prism" used twice in my life. The other time was a couple of weeks ago in some of my son's kindergarten homework. (Yes, they have homework in kindergarten.)

    I assumed that my son's kindergarten teacher just made it up. Everyone knows that the correct term is actually .... um, er, uh, ....
  3. KL7AJ

    KL7AJ XML Subscriber QRZ Page

    I believe it was my second grade teacher, Mrs. Anderson, who told us all to go home and tell our parents there was no such thing as a cube of butter. It's a rectangular prism of butter.

    Alas, my mom already knew that. :)
  4. KL7AJ

    KL7AJ XML Subscriber QRZ Page

    I guess the really scary part is that I still REMEMBER these things. I guess I didn't have much childhood trauma I needed to suppress. :)

  5. KL7AJ

    KL7AJ XML Subscriber QRZ Page's the magic formula:

    D= SQRT(a^2 + B^2 + C^2), where a, b, and c are the edges of the prism. Silly me; I knew it had to be simple. :)
  6. W8JI

    W8JI Ham Member QRZ Page

    I'm not sure what you are saying, or what Cebik actually said since you include no references.....but.....

    Anyone who thinks the current distribution in a wide flat sheet fed in the corner is the same as in a piece of wire the same corner length needs to reconsider the physics.

    Since radiation resistance is a function of current distribution and even phase relationships both spatially and along the conductor, the radiation resistance is not the same either.

    The surge impedance of the sheet is not the same as a thin wire conductor, and although we might in one case find one conductor shape and length that approximates the sheet on one frequency, good luck when the frequency is or antenna is changed.

    Here we go again. Over-simplification at work??? :)

    73 Tom
  7. W0IS

    W0IS Ham Member QRZ Page

    Off the top of my head, I'm guessing that the rectangular prism automobile, fed at one corner, at any given frequency, is equivalent to a cylinder of the same length, with a diameter larger than a piece of wire, but much smaller than the width of the car. In other words, a very thick wire. But that's just a hunch, and the actual mathematics are far beyond my capabilities, other than I know that it would involve integrals.
  8. KD0CAC

    KD0CAC Ham Member QRZ Page

    This looks like fun .
    I did a search on , w4rnl counterpoise , it was the 1st thing that came up .
    But I also like references link in a post , " bad Eric bad " .
    With help here I may get to learn / understand some of this Ham stuff ?
    Keep it up .
  9. KD0CAC

    KD0CAC Ham Member QRZ Page

    My guess is the frequency is the distance between the 2 farthest apart points , the sheet diagonal ?
    And width or area is the bandwidth ?
  10. W0IS

    W0IS Ham Member QRZ Page

    Actually, I was just thinking about why this sounded familiar, and I realize a related issue often comes up on a camper forum where I'm a member.

    There is a certain religious fervor among some that the "right" way to provide a connection to charge a trailer battery involves, among other things, a copper wire (IIRC, 10 gauge is the canonical size) run directly to the negative terminal of the tow vehicle's battery.

    I always point out, to no avail, that this is unnecessary for a few reasons. First and foremost, the DC resistance between the front of the vehicle and the back of the vehicle is equivalent to a copper wire of, IIRC, 6 gauge. So assuming that you make a good connection to the chasis of the vehicle (which isn't inherently any more problematic than making a good connection to the battery terminal), the chasis actually makes a better conductor that the copper wire.

    But also, the charging current doesn't originate at the battery. It's coming from the alternator. So if you really want a good connection as close to the source as possible, then you would hook it to the negative terminal of the alternator. But, of course, there is no such thing as the negative terminal of the alternator--it's just bolted to the chasis. And it would look kind of silly to run that long copper wire all the way to the front of the car, and then just bolt it to the chasis, which you could have just as easily done at the back of the car. Instead, you run the wire to the negative battery terminal--where it connects to another wire which runs to the chasis.

    Finally, no thought is ever given to fusing the negative wire. But if the ground strap comes loose, then the starter current is all going to run through the new wire which, it turns out, is already grounded surprisingly well through the hitch coupler.
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