How to connect two antennas?
If you use two stacked antennas instead of one, we get double the signal strength at the input. However, we should not forget that in “decibel” terms this doubling means a gain of only +3dB. Therefore, in the case of a weak signal from the base, instead of using the stack, you may want to think about choosing a more efficient antenna, such as a mirror antenna. Also, many people come to this page looking for an answer to the question, “how to sum the signal from two TV antennas to different channels?” You don’t need to do that, you need to choose an effective broadband antenna for DVB-T2, which we have a whole section devoted to on our site. However, antenna arrays of two or four antennas are quite common, so let’s not escape the question: how to connect multiple antennas together after all? Let’s take a closer look.
If such a question has arisen, then of course it is clear that simply paralleling two antennas will not work. In the best case we will get half of 50 Ohm at the feeder input, and in the worst case – unpredictable complex resistance with reactive component. In any case, the matching will be broken and the VSW will increase prohibitively. Therefore, a special addition device called a splitter must be used. How does it work?
- First, each antenna must “see” on its feeder a matched load, such as 50 ohms.
- Second, the signals from the antennas must add up in phase.
- Third, the modem or your other device to which the antenna array is connected must “see” that it is loaded with 50 ohms.
The simplest splitter that satisfies all three conditions is a star of N+1 resistors, where N is the number of taps. In the case of a 1-input to 2-output splitter, the resistance value is 16.7 ohms. An online calculator for such a combiner can be found here . Such a splitter is very broadband, but it has one, and very significant drawback: it introduces attenuation of 3dB. That is why it makes no sense to use it, because we lose all the signal gain from the second antenna in the splitter. If you decided not to bother with the problem and buy at the radio market or order a ready wideband splitter like the one shown at the beginning of the article, most likely you will get a resistive adder. If we look inside such a combiner, we can see it. In this case, the star is replaced by an equivalent resistance triangle.
Another resonant splitter circuit, which is called Y-bridge or Wilkinson bridge (online calculator with the circuit) allows to avoid losses. Such a combiner has a bandwidth of 10-15% of the center frequency and losses less than 1dB. In centimeter waves such a splitter looks like this: Can we build such a splitter with our own hands? Of course we can. If we replace the helical line with equivalent quarter-wave sections of coaxial cable. Then the schematic of such a combiner will look like this: P1 – downlink cable, P2, P3 – to antennas. The 71-ohm cable can be replaced with 75-ohm cable without significant degradation of the combiner parameters. An online calculator for calculating the coaxial Y-summer is here. The practical design looks like this:
At frequencies up to 800 MHz you can use splitters based on ferrite broadband transformers.
In addition to the bridge matching schemes that we have considered, there are schemes on quarter-wave impedance transformers on transmission lines from segments of coaxial cables. This approach has long been used in television reception. It is based on the principle that to match the two resistors Z1 and Z2 A quarter-wave section of cable with a wave impedance of Z = √ Z1·Z2 Here are examples of connecting two and four antennas using this method:The length of the quarter-wave cable segments can be calculated using the online calculator on our website. Don’t forget that the wavelength in cable is shorter than in air. The schemes are fairly uncomplicated, but on centimeter-wave quarter-wave sections are very short and difficult to install. The protruding central cores should not be more than 1-2 mm, otherwise the parasitic inductances of the assembly will negate the matching and the combiner will not work. Note that lengths ¼λ ¾λ, etc. (odd number of quarters of wavelength) – are equivalent and transform the resistance in the same way. However, increasing the length in this way sharply narrows the bandwidth of the adder. Considering the above mentioned, such adders on coaxial lines are better to use at frequencies lower than 800 MHz, and for 3G-4G and Wi-Fi the best design is the splitter on the overhead line, which we will consider below.
You can get around the complexities of installing coaxial quarter-wave lines by making such a line overhead. The advantage here is that we can make the impedance of the matching quarter-wave section any way we want, rather than choosing from a standard set of 50.75 ohms. Here is one of the popular and proven in practice schemes of the splitter on the overhead line, which can be used on 3G and Wi-Fi: The outer square tube can be aluminum, of galvanized tin, inner – round, preferably brass or copper, as aluminum is difficult to solder. The dimensions of the coaxial line are calculated with the free English program AppCAD , which can be downloaded from the following link. Instead of the program you can use the online calculator on our website. In contrast to the program, in the calculator you can calculate the size of the square screen using the known Z and the diameter of the inner tube. This is how the connection point of the connectors and the coaxial line looks like:The main difficulty in making such a microwave combiner with your own hands is the selection of the right diameters of the square and round tubes. You can simplify the task by replacing the round tube with a metal strip, as suggested by W3RJW (see link below). Such a strip can always be trimmed or sharpened to the right size. Another advantage of this design is that the outer tube can be made rectangular, not necessarily square. Online calculation of a rectangular coaxial line with a central conductor in the form of a strip is also present on our website. As you can see, making an adder for two or more antennas at high frequencies with your own hands is not an easy task, but it is quite solvable. It is only necessary to observe the manufacturing accuracy and the condition of in-phase addition from two antennas. To do this, the antennas must be placed in the same plane perpendicular to the incident wave, and the lengths of the feeder segments from the splitter to each of the antennas must coincide with the accuracy of a millimeter.
All of the above applies to the case of summing the signal from two or more antennas received from one source. In the extreme case it is possible to combine two antennas on one feeder, receiving from different sources, operating in different channels, from different directions, but in the same frequency range. If you have the need to combine two antennas operating in different bands into one feeder, you need to use another device called a diplexer. This is a kind of filter, cutting off one antenna from the other at the operating frequency. And if you can try to make a similar diplexer / summator MV / DMV with your own hands, then to collect a similar device on the microwave (for example WiFi diplexer 2.4/5 GHz), without tuning by expensive devices – an unrealistic task. Therefore, such devices are quite expensive.
We make Splitter with our own hands
A bad dream will not give you peace. In my case – to my hands :)) So, in my head I had the idea that you can make a splitter yourself. Why not? Especially recently tigra-KRaero appeared in the “Cars Tuning” magazine and told me about how his magical splitter is made Armed with a tape measure.
Armed with a tape measure, measured and estimated the size of my bumper. I took it with a reserve – 600×2000 mm. Measured the length of the threshold – 2100 mm.
Yesterday, taking time, went to a store selling materials for outdoor advertising and bought a sheet of PVC foam with a thickness of 8 mm. In the trunk of my huge sheet (2440×1220), of course, did not fit. I had to saw it right there in the store into three pieces, which have no problem to fit into the car.
First of all, I took off my rubber EZ-lip lip, which was not that easy, but it was a bit of a pain. Then I took out a bunch of old magazines from the garage – Men’s Health and GQ – I did not keep them so carefully for nothing. Used them as a stand for a sheet of PVC =)
Bottom view. Adapting a simple pencil to the polishing paste bottle, I did this simple method to make a rough sketch of the bumper outline.
Already something is being drawn…
My device :))) Brought the sheet to the garage, took a knife and methodically cut off the excess parts from the sheet.
Minus the first piece.
Minus the second piece. Took the sheet out again and checked it against the bumper. Something was beginning to come out. But there was still a lot of unnecessary stuff. Originally I made everything with big margin, not to make a mistake….
So far the splitter is huge. Once again corrected the sketch of the splitter, but this time using a small spray can. Took the splitter into the garage and removed a few more inches. Then I treated it with a large file to give it the right shape.
Here it is, my helper is in the lower right corner. Big – you can nail it with this one So, I got the size and shape of the splitter you need – but there is a lot of material around the edges.
I decided for the first time not to bend it and leave the edges straight. Maybe in the future, if all goes well, I will make a splitter with bent up edges. For now, I bent some material off the edges of the splitter and made them straight.
Side view 1. Looking at the straight edges.
Side view 2. Looking at the straight edges. The shape, of course, is not yet complete. But I think the general view is already clear. This is where I stopped today – it was raining, and I had to still have time to clean up after myself and run errands.