I'd bet that the guys who design AM antennas for cars dont think in terms of making them resonant. They are just too small (short.). There is one interesting design consideration that sometimes gets abused. It is the coax lead that connects the "antenna' to the receiver. That feed line is seen by the receiver as a capacitance that shunts the input terminals. And, since the "antenna' looks like a highly capacitive load, the feed line capacitance is shunting alot of signal from the receiver input. Some radio designers used to provided a way to tune the combination of all the reactances for each individual installations. I dont think thats done much any more.
I dont actually have any data on the polarization of AM (broadcast band) radio waves. I do doubt that the horizontal component of any AM radio wave can exist so close to the ground as where a car is.
I was the one who first brought the term 'ground plane' into the discussion. Of course a better term is counterpoise.
As you say, it's a two terminal antenna with the whip being one terminal, and a steel plate of some indeterminate size, more or less perpendicular to the whip, as the other terminal.
Counterpoise, reflector, etc. are better terms.
The impedance between the vehicle chasssis and the ground is obviously pretty large.
Come to think of it, the antenna in question was an 80 meter dipole. It would have required two streches of wire (three poles) and I think the formula is 246/f(mhz) to give something like 82 feet. So the poles were a
Don't doubt you, Bob. This was not your mainline railroad, who knows whose poles they were. There were grade crossings where this RR crossed a couple of roads, but definitely no gates and I don't even recall there being lights. We called it the "tryweekly" RR. An engine and a caboose would crawl down to Hillsdale one week, try to get back the next. I never did see a real train on it.
Whoa. Check out the ARRL Antenna Handbook (if they still publish it; many libraries have it). Or an old high school physics book.
AM waves, because of their frequency and not because of their modulation, propagate mostly by ground wave, which follows the earth's surface, during the daytime. Power = distance, because it's a nearly a straight distance-squared (not distance-cubed) relationship. There is no skip. Unlike VHF and above, the waves follow the curvature of the earth, "clinging" close to the ground.
At night, the ground wave shrinks to very short distances, and, given the right conditions, classical ionospheric skip takes over. This is low-ionosphere skip; it requires multiple hops to go long distances. But the high-power, clear-channel stations can make multiple skips.
I didn't read the earlier posts to this thread so I apologize if I'm repeating something that's been said.
"Practical Receiving Antennas -- The main considerations involved in receiving antennas are the amount of energy that can be be delivered to the receiver, the directivity, the cost and the freedom from extraneous disturbances.
A receiving antenna should abstract sufficient energy from passing waves so that even the energy abstracted from the weak radio waves representing static and other noises will under normal conditions be at least comparable with the thermal-agitation energy existing in the input of the receiver. The signal-to-noise ratio cannot then be improved by further increase in received energy, and no further improvement is possible in the antenna system as far as energy pick-up is concerned. The amount of energy that can be abstracted by an antenna depends upon its physical size, the frequency, and the loss resistance. The abstracted energy tends to decrease with increased frequency, particulary when the antenna dimensions do not exceed a quarter of a wave length. As a result, the problem of obtaining adequate energy pick-up is most imprortant at high and ultra-high frequencies.
When it is desired to avoid marked directional effects at broadcast and lower requenmcies, it is customary to employ a single wire running to a height of 15 to 50 feet. With such an antenna the horizontal portions are relatively unimportant because waves of broadcast and lower frequencies are vertically polarized when near the earth.
Automobile Antennas -- A great variety of antenna arrangements are used in automobile receivers. Perhaps the most effective is a vertical whip, connected to the receiver by means of a transmission line. Other arrangements that are employed include plates mounted below the running board and insulated from the body of the car, and a plate connected in the top. This latter arrangenment is suitable only for cars in which the top is not all metal."
Terman's comments reflect the general premise of most of the foregoing thread. That is: Broadcast antennas are best when they are composed of a wire vertically mounted. Horizontal sections of the wire do not appreciably add to the amount of energy abstracted because broadcast signals are vertically polarized. Terman's comments re. placement of antennas in different locations within the automobile do, in some measure, substantiate other posters' claims concerning diversity. I can't say for certain that diversity antennas aren't used for broadcast reception, although I doubt it. In recent practical experience, "diversity" is only seen in UHF practice, particularly in frequencies above 1000 mHz. An antenna system can employ both space diversity and frequency diversity. Both have long been in use at microwave frequencies. Sectorization and tilt as commonly employed in cellular practice are not included in the definition of diversity. As I see it, presently evolving "smart" antennas utilize the diversity concept whereby individual elements of the same antenna are diversely managed. Smart antenna design is a cut-throat business. A company I worked for had a PHD vice president heading up a large department engaged in smart antenna design. He squandered vast amounts of company funds, never got an antenna design past the FCC as far as I know, and was eventually fired.
The question (or statement) as I understood it from the OP, was that if you didn't consider the body of the car a "ground plane" (presumably at MF frequencies) you don't know how radio signals function. I guess I have to be among the unknowing, which is a shame since I've been spec'ing and measuring antennas for ground vehicle and aircraft applications for too many years.
I can't say for certain that
Space diversity FM broadcast antenna systems are fairly common in autos to reduce the effects of multipath. These are generally pretty simple systems with two antennas, with the receiver being switched to whichever antenna has the strongest signal. The first factory supplied system that I heard of was the Saab, about 8 or 10 years ago.
You probably got a mixture of messages while reading these posts. But, I would put myself amoungst thoes who wouldnt find value in using the term "ground plane" when working with a conductor thats shorter than a few electrical degrees at the frequency of interest. I would not (hopefully ever) write that "someone doesnt understand how radio waves work". I expect to benefit from any experience others might have for getting the maximum peerformance from short AM and FM antennas.
My news group provider shows thr OP to be Andrew V, who wants to provide his home with adequet RF into his radios on both AM and FM. I jumped in with some advice that doesnt seem to be acceptable to all readers. It is my assumption that all the information submitted so far by me is absolutely correct. But, I have no quarrel with anyone who wants to theorize that a
1/100th wave long conductor is a "ground plane". That terminology just doesnt fit with my knowledge.
What would you consider the minimum length of a conductor that fits the definition of "ground plane"?
Jerry, unless the terminology has changed in recent years, the term ground plane refers to an artificial, highly conducting ground, not to the radiator. Typical ground planes are crossed or radially displaced conductors, wire meshes, or metal sheets that lie in a horizontal plane and are grounded as effectively as conditions permit. If the grounding is very effective, the ground plane doesn't radiate at all. That's the condition that's usually desired.
As for the radiator, it can be any length. But the usual purpose is to have effective radiation perpendicular to the radiator (vertical radiator; horizontal radiation, with a single lobe pointing just slightly upwards, shaped like a donut cut off just below its greatest periphery). At frequencies up to the edge of UHF, that usually dictates a quarter-wave radiator.
Is this what you're talking about, or am I on the wrong page here?
I believe the idea was that the size of a car top was not nearly large enough to serve as a good ground plane, esp. for AM broadcast freqs.
Sorry, as for my comment re. broadcast space diversity, I meant AM. I'm not surprised that space diversity is used for FM broadcast, although, I doubt very much if the improvement in reception is commensurate with the increased cost.
Yes, I should should identify the OP by other than "OP". As far as I can tell from the thread path the original statement was by a "Steve W." I am agreeing with you, although I am doing a lousy job of it.
But, I have no quarrel with anyone who wants to theorize that a
Or mine.
To me, a "ground plane" is a mythical beast. I usually don't run into structures performing as a perfect counterpoise. ;)
And I may have screwed up further by asking Bob if he missed the antenna factory; Bob has worked for Andrew, and his adaptive antenna story is one I've seen duplicated in other places. I'm guessing from your other posts that you may have worked for a manufacturer too. If you've designed anything painted green, I have probably had them bolted on to something.
My apologies if I somehow gave the impression that you didn't know the subject at hand.
Ed, You're on it like a cheap suit! A ground plane must have an area at least
1/4 lambda ^2 to be effective. The top of a car doesn't have sufficient area to work as ground plane at broadcast freqs. Essentially, ground plane radials provide an artificial ground that is elevated to the effective height of the antenna wherever it is above earth. It is generally understood when we speak of "ground plane" re. communications antennas we are referring to 50 ohm antennas.
Yeah, it would take one hell of a car. And an effective radiator would be tall enough to wipe out the power lines wherever you drove, while the ground plane would wipe out the utility poles on both sides of the road.
Thanks, Bob. It's good to hear that antenna theory didn't invert itself since I studied for my 1st Class Phone license.
I'm not following this thread very closely but it sounds to me that some people are mixing up transmitting-antenna theory with receiving-antenna theory. A car antenna is just a conductor stuck up there to suck up as much electromagnetic radiation as possible. Some of them are loaded at the base, but I always assumed that was for FM. 'Don't know for sure.
If I understand it correctly, (which I doubt ), since the car doesn't act as the ground plane, the ground must. If this so, how come a car radio that worked very well at recieving faint stations, hardly picks up anything when I tried to use on the bench with a regular car antenna? The antenna was connected properly, with the cable plugged into the radio and the antenna mounting screw connected to the ground screw on the radio. Thanks, Eric
I'd submit that your experience with transferring the car's "antenna" to the bench (without the car) gives credence to my theory that the car is the antenna, and what we call a car antenna is a probe that senses the currents induced in the car by the radio wave as it passes (at AM frequencies).
It's a semanitic issue, I know. For the sake of argument, one could say that the body of a car is the 'other part of a non-resonant dipole antenna.'
Because it's a mostly flat part and mostly planar in shape, the temptation to call it a ground plane is obvious.
If it were a 2 meter whip antanna stuck on the roof of the car, it would be a very good description. For 1 MHz radiation the description does leave a bit to be desired as you say.
Circuit board designers call the continuous conductor on their board a "ground plane" even though that could be a few inches square.
Obviously it would look silly stuck at the end of even a 2-meter quarter wave vertical.
For me the term "ground plane" does not have to have any particular wavelength of interest to be applied. I like the term 'ground plane' even for a car body, for 1 MHz am because it's descriptive of the *shape* of the conductor more than anything else. That, and the input coil of the receiver is stuck across that coax feedline, and the shield of the coax is bonded in most cases to the chassis of the radio, and to the car body at the other end.
The idea being that there is some rf voltage developed between the bottom of the vertical and the body of the car, by virtue of it (the vertical) being immersed in the local rf field.
This voltage is larger than the voltage that would be there, if the car body were absent; that is if the coax shield simply stopped and the whip were tagged on the end, out in the middle of nowhere.
Granted not a lot bigger, about a factor of two or three probably. Remember that while the car is sitting on top of rubber tires, there's a large capacitence between the car body, and the actual ground. Consider that if there is only 0.1 mfd between the car body and ground, that is one ohm at a MHz.
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