Application Practice of Low Carrier Frequency FM Television

The low carrier frequency FM TV modulator has been introduced for some time. Its circuit structure has been improved and the circuit parameters have become more reasonable. However, some problems have been found in practical use, which are detailed below.

Under the same frequency, power and other conditions, the transmission of TV signals by FM can reach much farther than that by AM. This is because the carrier frequency level of AM must be 30-40 decibels higher than the noise level to obtain good image indicators, while FM only needs to be a few decibels higher than the noise level. Practice has also verified this point.

Using the same transmitting tube, the FM mode can reach much farther than the banner mode. This is because the transmitting tube of the AM mode can only work in Class A state, the power efficiency is low, and the output power is much smaller. Take the MRF581 commonly used in our products as an example. In the range of 300-400MHz, the output is only more than 100 milliwatts, while in the FM mode it is more than 1 watt (the standard given in the manual is a maximum of 2.5W).

According to the above reasons, at the same frequency and using the same transmitting tube, the transmission distance of the FM method should be much greater than that of the AM method. In fact, this is not the case. There are many factors that restrict the transmission distance of the FM method, and the most important factor is the interference signal in space.

Recently, I wanted to use the signals in this frequency band for long-distance transmission, and did some experiments, which are detailed below for reference.

First, a pair of Yagi antennas were made using the structure shown below.

The center frequency of the antenna was measured to be around 400 MHz, and the performance was ideal. The test was then carried out in the following manner.

At a distance of 8 km, the image is very bad. The transmitted image can be seen in the black and white bars rolling up and down. When the transmission signal is turned off, there is still a strong interference signal on the screen. It can be felt that the transmitted signal is very strong, but it cannot form a good image.

We have tested multiple frequencies from 340MHz to 400MHz, and all of them are worse than 400MHz. We have not tried anything above 400MHz, and there is no need to try it. The degree of congestion above 400MHz to 450MHz is well known to everyone.

Then I tried several frequencies above 800MHz, such as 800, 855, and 922MHz (which can be directly received by satellite receivers), but all ended in failure. No wonder, satellite receivers are more sensitive, and the frequency band occupied by FM is relatively wide. How could there be more than 20 megahertz of relatively clean spectrum space in the frequency band below 960MHz?

According to practice, we can draw the following conclusion: Low-carrier frequency FM TV signal wireless transmission has some advantages when used in more complex environments, but is inferior to AM for long-distance transmission. Of course, AM cannot be used in this frequency band because ordinary TV sets can receive it.

The modulator is equipped with a pair of folded oscillators as transmitting antennas. Most places within one kilometer can receive reliably, but only some places can receive at two kilometers. There are many reasons for this, but there are two main factors: one is interference, and the other is obstacles.

The folded dipole antenna is shown in the figure below. Installing the folded dipole vertically can obtain better omnidirectional transmission characteristics.

When a larger transmission range is required, an additional amplifier can be added after the modulator. The simplest and cheapest method is to use the power multiplier module of the cable TV system, with a maximum output power of 7-8 W. For ease of use, we integrate the amplifier module and the vertically placed folded oscillator into a transmitting unit, as shown in the figure below.