As has been mentioned and advised in previous threads; the use of a 8' x 3/4" round copper pipe as antenna instead of a whip will improve your signal due the additional surface area provided by the pipe..
But why is 3/4" pipe always seem to be specified? Why not use a larger one for even more surface area? Why not for example a 3"pipe?
Structural reasons?
Hi Rich,
Performance increase as the diameter is increased, primarily because the capacitance of the antenna increases with he diameter, so the inductance of the resonant tuning coil drops, resulting in lower coil loss. The result is higher coil efficiency leading to a stronger radiated signal.
Copper pipe is a common option because it is readily available. Standard sizes (ID) are 3/8" 1/2", 3/4", 5/8", 1", 1 1/4", 1 1/2,", 2". At the low end, 3/8" is a little flimsy and may not hold up to constant wind conditions, even though it would be marginally better than a standard .2" stainless steel 102" CB antenna. 1/2" is sturdy enough. Larger diameters will be even sturdier, but will be heavier and more costly. 3/4" seems to have emerged as very popular.
One thing to consider is the wall-thickness of copper pipe. There are three wall thickness standards: type "K", "L" and type "M". "K" is thinnest and "M" is thickest. I have observed that common home stores like Home Depot carry the "L" and "M" thicknesses for certain diameters. Thinner is good for low-pressure radiant heating pipes and drain pipes, while thicker is good for water supply pipes. For our use, in diameters of 1/2" or greater, the thinner-wall pipes are better because they weigh less.
Here is one site that shows the standard dimensions: http://www.sizes.com/materls/pipeCopper.htm
When doing any sort of calculations that involve the capacitance of the antenna, the length of the pipe and the Outside Diameter are the dimensions to use.
Thanks for that overview Phil.. So then, the reason 3/4" is suggested id because it's a happy medium between efficiency and cost.
When doing any sort of calculations that involve the capacitance of the antenna, the length of the pipe and the Outside Diameter are the dimensions to use.
I was coasting right through your post fine till you said that.
Let's say I were to decide to use a pipe instead of the whip on the Rangemaster,- the only thing to be concerned about is re-tuning the transmitter - right?
Right. You are correct!
To be specific, your coil tuning would reduce from:
4127 ohms reactance, 410.4uH inductance
to
3198 ohms reactance, 318.1uH inductance.
The numbers are only relative because other factors are involved, but you get an idea of how increasing the outside diameter of the antenna from .2in to 0.785in reduces the required coil inductance.
A "fatenna" is a fat antenna. Not a girl I knew in grade school. "Offset Radials" will come up in a minute.
During one of these pipe size discussions last year I stopped at the big box hardware store and found the largest copper pipe was o.d. 4", cost $104 for 10-feet. At that time I had not heard about thickness of the pipe wall, as PhilB described this time.
A believer in the 4-element cage monopole antenna, which I use for FM, I hereby suggest that an AM 4-element cage monopole antenna could be used with small width pipes to achieve the same result as with a single wide pipe. Would there be a cost saving? Uh....
Now, a totally separate question. In a situation where an AM 10-foot antenna is right at the edge of either the property or a hill, can the ground radials be offset so that the circular radial pattern would be entirely in available yard-space with the antenna on an outer edge?
Two blogs in one.
"Now, a totally separate question. In a situation where an AM 10-foot antenna is right at the edge of either the property or a hill, can the ground radials be offset so that the circular radial pattern would be entirely in available yard-space with the antenna on an outer edge?"
Well ... Radials operate best equidistant from the center of a stick antenna. In fact, the first meter or two carry the most capacitive effect, especially at lower freqs.
Now, if you mean: Can you set radials from the center of the antenna out on one side then, yes it will work, but it will be directional. E.g., if you only have, say, 180 degrees of a circle to work with, then the signal will be strongest on that side of the antenna.
If at the bottom of a hill, I suppose you could run the radials on the far side uphill. I'm sure that would cause some effects, but would be tough to predict what and how much. If the antenna is at ground level, it might have some negative effect on a loading coil too, which Ermi alluded to in another thread.
Thank you Ken N.
My antenna is at the top of a hill with level ground on 3-sides. About 10-feet down the steep side of the hill area available, but radials there would be pointing almost straight down.
If 3 quadrants of the 360-degree radial system (270-degrees) are at the same elevation, the system impedance on those radials would be nearly equal. The 90-degrees or so of the radials on the downhill side would have a higher impedance due to the greater angle (>90-degrees) between the not-so horizontal radials and the vertical antenna. This impedance difference would cause voltage in the system on the down hill side to be higher (Ohm's Law) and the current to be lower. Since radiated signal is a product of current (amperes), the signal on the downhill side would be somewhat less. On extremely short antenna/radial systems however, the difference is much less noticeable because of large system exceeding 60% (Part 15 3-meter systems). Note: It is good to remember that a full 1/4 wave antenna length is 142 feet at 1700 KHz. where theoretical system efficiency is near 85-90%.
In vertical antenna design, if the ground side of a vertical dipole was 180-degrees out from the antenna radiator, the theoretical system impedance at the feed point would be 72 Ohms. As the angle decreases to 135-degrees, impedance would drop to approximately 50 Ohms, and 90-degrees in the neighborhood of 35 Ohms. As a point of interest, vhf and uhf verticals are tuned to 50 Ohms impedance by adjusting the angle between the antenna and the ground radials.
There are a lot of factors that determine signal strength; ground conductivity, radiation efficiency (Ohmic losses) and angle of departure, not covered in this post, that would also effect the signal in a particular direction of the antenna azimuth.
How would a spiral ground work? Like those twirling spirals used for hypnotism in old movies; like the grooves of a record from the center to an outer circumference.
Spiral Ground radial systems using up to four radials would have to be much longer (as much as 3 times longer) to cover the same area. As a result of the extra length, the inherent Ohmic resistance in the radial wire would be higher causing the antenna signal return circuit to have less amperage, therefore less signal. This is the main reason reason licensed stations don't use the spiral scheme for the ground radial system.