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When an antenna is shortened from it's naturally resonant length at the frequency of interest, the feedpoint becomes capacitive and it becomes necessary to add offsetting inductance in the form of a loading coil. This restores the resonance of the antenna. Unfortunately, the additional inductor also has resistance that adds loss to the antenna. The resistance in the coil eats up your power and you are not going to be as loud as you would be if the coil loss were not present. The more that you shorten the antenna, the larger the coil that you need to use. The larger coil has more resistance and eats up more of the power. Mobile antennas are usually pretty short, require large coils and hence have large losses. This loss resistance is also seen at the feedpoint and is partly responsible what makes it easy to match this shortened antenna to the 50-ohm feedline and radio. The impedance seen at the feedpoint is made up of the radiation resistance, coil loss and ground loss. The radiation resistance of a full size vertical antenna is about 35 ohms. As the antenna is shortened the radiation resistance goes down quickly but the other losses are still there or even increase. Here's a table that shows what the real Radiation Resistance of your shortened mobile whip is.
So, for instance, if you have a 72-inch, 20m-whip antenna and you are measuring an SWR of 1:1 that means that you have a feedpoint impedance of 50-ohms. The true radiation resistance of your antenna is somewhere near 3-ohms. That means that you have a loss in your system of 47-ohms (50-ohms minus 3-ohms) and a radiation efficiency of about 6 percent (3-ohms / 50-ohms). So, if you have a 100-watt transmitter in the car, 94-watts are being used to heat the air and only 6-watts are being used to communicate! The 94-watts are used up in the ground system of your vehicle and in the loading coil of the antenna. If we were able to suddenly remove all the loss from the loading coil that we were using then we would see the base impedance drop by the amount of the loss in the coil. Of course, if it dropped all the way to 3-ohms you would have a problem matching the antenna to the feedline but it is unlikely to drop that far since you will have some residual loss in the coil and you still have the ground losses. How do we reduce the loss in the Loading Coil? Basically, we use one with less inductance and resistance. One way to reduce the size of the required coil is to install what is known as a capacity hat. It's not really a capacitor but it does resemble a hat. Basically, it is an extension of the Marconi antenna design that adds horizontal elements to the design of an otherwise vertical antenna and still preserves the vertical polarization of the waves emanating from it.
The resonant frequencies of mobile antennas using the Hustler MO-2 vertical mast and the indicated resonator can be configured over a wide range by using the DX Engineering Hot Rodz™. The base impedance of the antenna will drop to about 16-22-ohms when you increase the efficiency of the antenna with the DX Engineering Hot Rodz™. Any matching problems will probably be taken care of by your rig's antenna matcher or a more efficient way would be to use a small matching transformer. How much did the efficiency increase? Well, if the base impedance was reduced from 50-ohms total to 20-ohms total when the actual radiation resistance of the antenna is really 3 ohms you will have gone from 6% efficiency to 15% efficiency. More than doubling the RF power coming out of your antenna. How to use the DX Engineering Hot Rodz™ Patented Let's look at an example of the use of the DX Engineering Hot Rodz™. Suppose you want to operate 40 meters. Normally, you would install a 40m coil and 32 inch stinger on the MO-2 which will result in an antenna of about 102 inches in height -- Fifty-Four inches for the MO-2 and 48 inches for the resonator and stinger. This combination will resonate at about 7.16 MHz. Looking at the chart you can see that as an alternative you can install a Hustler 20m coil(RM-20) and the DX Engineering Hot Rodz™ with the 24-inch rods (supplied) and the 12-inch stinger which resonates even lower at 7.1 MHz but which is only 78 inches tall! As a result you will see the base impedance drop to about 20 ohms which indicates that you have dropped about 30 ohms of resistance from your antenna system! This may or may not present a problem for you. I have used the automatic tuner in an FT-900 to match this in my mobile and still gotten great signal reports.  Picture of a 40m Resonant Antenna made with a 20m Resonator and 24-inch DXE Hot Rodz™. |
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