Without a filter on the front end of the RF, the receiving effect will be greatly reduced. How big is this discount? In general, the distance will be at least 2 times worse when the antenna is good. Moreover, the higher the antenna's terrain, the worse the receiving effect! why? Because today's sky is flooded with a large variety of signals, these signals block the front-end receiver.
However, for the 435MHz band front-end filter is not so good. Start the analysis below.
This is a set of Chebyshev bandpass filters with a top-end capacitance and a center frequency of 435MHz. Due to the use of commercially available SMD inductors (this type of inductor has a Q value of up to 70), the insertion loss is so great that it reaches -11dB, and the other curve is reflection (which can be converted to standing wave). Therefore, after joining, the sensitivity is greatly affected because the sensitivity of the receiver is directly related to the noise coefficient of the first-stage high-level emission. Even if the technology is better, for example, the noise factor of the high-level emission can be controlled to 0.5, but the insertion loss of the front-end filter can be realized. Really worsening the noise figure by 11db.
So it is rare to see such use.
Look at this picture again:
Keeping other parameters and replacing the inductance with a good air-core coil, although the volume is large, the insertion loss becomes about -5, which is basically usable, but it is still very difficult to make. because:
The coupling capacitance at the top is only 0.2P. The capacitance of this capacity is not very good to buy, so you can only draw the capacitance on the PCB, which will make it difficult to successfully achieve 1 time.
Even a 12nH inductor is not a very good winding, and it must be hollow and winding. If you have insufficient experience, you cannot master it.
The inductance is still a bit large, and the parameters of those capacitors are sensitive, and a slight change will affect the performance.
So if you can continue to increase the Q value of the inductor, and there are ways to continue to reduce the coupling capacitor? Then shrink the bandwidth.
The situation will be the following:
In this picture, the Q value of the inductor suddenly becomes 1600, and the inductance also becomes larger. The figure becomes very beautiful. This filter can guarantee the receiver's many options and sensitivity, if there is no energy consumption. The considerations were immediately followed by an IC, which immediately pulled the distance up.
Better performance, but too large microstrip filter
Practical spiral filter design
For this kind of spiral filter, there are fewer people who will really design at home, and the software can already be well integrated.
Let's start with a picture of the actual spiral filter used in a 435MHz mobile device.
In fact, the better filters must be machined more strictly. We will design high-quality 2-chamber and 4-chamber filters for this tester.
The first is the four-cavity, the operation is very simple.
ShenZhen Antenk Electronics Co,Ltd , https://www.antenkcon.com