Although there have been tremendous changes in the mechanical realities of these antennas over the passage of time, but some characteristics still have a lot of significance with respect to the antenna’s performance. An average magnetic cb antenna mount achieves its grounding when the magnetic base attracts the metal of the vehicle’s chassis.
But it is not as sound as a thorough ground connection that is made with the same antenna. The elements considered in these calculations are the Magnetic Mount antenna mount itself, the paint placed on the vehicle, and the metal in the body of the vehicle that will aid the formation of a “capacitive ground connection”. This ground connection will be least effective if you choose an magnetic cb antenna mount with a very narrow base. Hence the total area covered by the magnetic mount should be a wider one.
This articles identifies the type of ground connection that is created because of the capacitor that comes into existence when the antenna’s bottom comes into contact with the metal of the vehicle. But it is not as effective as the direct connection between the two. To prove all these facts, following calculations will be done:
First some assumptions
- Generally the paint thickness over the vehicle’s surface ranges from 75-200 microns. It might increase to 3x or so when the clear coat is applied over it. Thicker paint layer reduces capacitance. Therefore, 600 microns or about 24 mils (1 mil = 1/1000 inch) are considered for the calculations.
- Typical small 2m ham antenna with 3 inch base.
- Braid of coax is seen attached to the foil on the bottom of the antenna.
- The capacitive reactance of the details given above will be calculated at 50MHz, 144MHz and 440MHz.
- Dielectric Constant, K, of clear coat and paint coat.
- The Radiation Resistance is the thing that the transmitter needs to handle.
- The Ohmic loss is the loss of the antenna and its internal connections due to resistance.
- The sum of the Shield Currents can be declared equal to the current on the inner conductor and flow back to the transmitter on the inside of the outer conductor. If the antenna’s ground and local ground are not properly connected, the current will move back to the outer surface of the outer coax conductor that can cause trouble for your radio and can be dangerous for you as well.
- The ground mounted HF vertical antennas are also affected by the similar conditions and that makes radials highly important.
- Xc is a part of the impedance from coax shield to ground and needs to be added to the overall impedance.
- When the resistance or impedance value is low with respect to the radiation resistance, it can badly affect the antenna’s efficiency.
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Calculating Capacitance between Magnetic CB Antenna Mount and Metal
The formula for the capacitance between two conducting plates is:
C = 0.2258 * K * A / d
- K is the dielectric constant of the material,
- A is the overlapping surface area of the plates in square inches,
- d is the distance between the plates in inches, and
- C is capacitance in pF (pico Farads – 1×10-12Farads)
Capacitor Area = pi * r^2 = 3.14159 * 1.5^2 ~ 7 sq. in.
Capacitance(pF) = 0.2258 * 3 * 7 / 0.024
Capacitance(pF) = 198pF
Xc = Capacitive Reactance = 1/(2*pi*f*C)
- When f = 50MHz =>Xc = 16 Ohms
- When f = 144MHz =>Xc = 6 Ohms
- When f = 440MHz =>Xc< 2 Ohms
This unfortunate situation occurs when the surface is covered with a thick paint coat. Now let us perform the same steps and calculations for the 8 mils:
Capacitance = 0.2258 * K * A / d
Capacitance(pF) = 0.2258 * 3 * 7 / 0.008
Capacitance(pF) = 593pF
Xc = Capacitive Reactance = 1/(2*pi*f*C)
- When f = 50MHz =>Xc = 5 Ohms
- When f = 144MHz =>Xc< 2 Ohms
- When f = 440MHz =>Xc< 1 Ohm
Through these calculations, we now come to know that magnetic cb antenna mount can make the AC coupling of the RF shield currents possible with the car’s metal by utilizing the capacitor current.
The impedance measured from the mag-mount/metal interface needs to be added to the value of radiation impedance of the Magnetic Mount Antenna that gives us the total impedance of the antenna equal to 50 ohms~36 ohms + Xc. This figure determines the antenna’s efficiency. Here we come across many new and unique concepts that are completely new for our understanding. Higher frequency usually brings better outcome with it. This fact has been proven through various straight-forward mathematical calculations. When the other factors are kept constant, capacitors give better conduction results at higher frequencies. Although you can find this fact evident from these calculations, but you can strengthen your concepts by making the calculations yourself as well.
The results prove that magnetic mount antenna can easily cling to the metal and connect well with it due to the foil underneath that gets connected with the coax’s shield. It has been proven that with the decrease in frequency, the efficiency of these antennas also decreases. There are such CB antennas available for 27MHz that have been equipped with wider bases that keep the value of Xc low and also firmly support the antenna whip above.
As compared to the trunk lip mounts that first establish a connection with a couple of set screws and then with the trunk lid through the paint coat, the large capacitors in magnetic mount cb antenna is better alternative to be considered. The conditions might even change if the trunk lid has been made of some other kind of conductor. This case is usually observed when the antenna is working under a high power. The magnetic cb antenna mount has got the unique quality of allowing the ground currents to get propagated over a large surface area across the locking screws.
When you are out for buying a magnetic cb antenna mount, choose the one that is able to manage and keep the capacitive reactance approximately below 10% of the antenna impedance. If you are opting for a quarter wave Magnetic mount antenna of about 36 ohms, then make sure its impedance is under 3 ohms. The rest of the efficiency of a magnetic mount depends on the frequency, paint thickness, dielectric constant and area.
The above mentioned calculations are sufficient to highlight the efficiency and reasonable performance of magnetic mount cb antenna that are quite versatile in their approach. You will also find here the best way to mount your antenna safely without any need of drilling that otherwise can destroy the shape of a vehicle or the place where you intend to place the mount.
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