Homemade external antenna for CDMA2000 / EVDO standard (Intertelecom, PeopleNet operators)

Published on December 03, 2012

Homemade external antenna for CDMA2000 / EVDO standard (Intertelecom, PeopleNet operators)

Introduction and theory under the cut. Read carefully before asking questions and / or accusing me of incompetence.
A lot of information was shoveled on the Internet about self-made external antennas for 3g modems, but I did not find anything directional, that's why I am writing these lines. Very touched by people who believe that 3G is such a communication standard such as GSM, but in fact it is just a generation. These same people are looking for antenna drawings for a 3g modem ... So these drawings are not, or rather, they are, but it's the same as coming to the car market and stubbornly demanding a carburetor for a passenger car without even specifying its model. So, we will design the antenna for the CDMA2000 standard, in which the operating frequencies are in the range of 821-894 MHz (and not 800 MHz as many people think). The antenna considered here will not work.for MTS Connect, Utel (Kyivstar) operators. Of course, I came across offers to catch a signal on a “carnation” (it’s also a quarter-wave vibrator), make a can antenna (that's just the catch, according to calculations, we need no longer a can, but a whole bucket), the notorious Kharchenko antennas (a good option when the signal still there, but the gain is poor), etc.

I settled on the Wave Channel antenna, which is also Uda-Yagi. Advantages are a high gain, low windage, narrowly targeted DND, but the drawback is extremely significant - very high manufacturing accuracy is required. An out-of-size director will become a reflector, and an active vibrator will not resonate at the frequency we need. The more accurately you do everything, the better the result.

The base station is only 3 km from my house, but the windows face the other side of the tower and the signal leaves much to be desired. At first I wanted to make an antenna with 8 directors, but it turned out that ultra-precision is needed here because leaving by 1 mm instead of amplification will give attenuation. The 3-antenna antenna does not require such manufacturing accuracy, but has insufficient amplification. Therefore, I settled on the 5-channel directional channel, considering it a "golden mean." The receiving and transmitting channels are quite separated from each other, because the antenna was calculated in the middle of the receiving channel, that is, at 881 MHz. At first I wanted to design the antenna in the middle of the whole range (859 MHz), but since Yagi has a narrow-band antenna, we get a gain peak in the non-working range and a lower gain at the operating frequencies.

For design, the Yagi calculator program was used .

What do we need:
- an aluminum square profile with a cross section of 10 mm (bought by me at the epicenter), not aluminum is suitable, but still it is lighter, but does not affect the antenna characteristics;
- an aluminum rod with a diameter of 5 mm and a length of 1 meter (other material, including copper, is even better, which is even better, but aluminum is the best price / quality ratio);
- a copper tube with a diameter of 6 mm long half a meter (the outer diameter is indicated, the wall thickness does not matter);
- bolts with a diameter of 3 mm; 7 pcs.;
- cable with a wave impedance of 50 Ohms;
- Adapters, connectors - all individually for each modem, as they say "Google to help."

Separately about the cable. A television cable is not suitable for you due to its resistance of 75 ohms. More precisely, it can be attached, but due to inconsistency, the cable losses are more likely to be greater than the antenna gain. I took 10 meters of the RG58 cable, it is quite cheap, but the loss is 0.6 dB per 1 meter of cable, i.e. I personally lost 6 dB despite the fact that the difference in signal with and without an antenna is 20 dB. Therefore, saving on cable is not worth it.

Of the tools:
- saw for metal;
- drill;
- tap on the top three;
- drill 2.5; five; 6;
- the file is flat;
- caliper (in extreme cases, the ruler will come down);
- arms.

First drawings:

Reflector is marked in red, active vibrator in blue, and directors in green.

Drawing of an active vibrator (dipole):

All dimensions in the drawings are indicated in millimeters. The distance between the elements is indicated in the centers.

We begin to manufacture. We take an aluminum profile, we retreat an arbitrary distance from its beginning (this distance is necessary for fasteners, I took about 10 cm) and make a through hole with a 5 mm drill. I recommend immediately making a hole with a drill of the smallest possible diameter, and then drill a 5 mm drill. This is necessary so as not to deviate from the center axis of the profile. Next, we retreat 68 mm (according to the drawing) from the center of the hole made earlier and make a through hole with a 6 mm drill (this is the diameter of the active antenna vibrator). Next, we make all the holes with a 5 mm drill to place the directors.

We begin to make a reflector and directors. Actually all sizes are indicated on the drawing, I just want to give some tips on cutting. Cut the aluminum rod according to the drawing 2-3 mm more, after which we expose and fix on the vernier caliper the required length of the element. We file the rods with a flat file to the desired length periodically controlling the size with a vernier caliper. If the element is tight between the jaws for internal measurements, then you can proceed with the manufacture of the next element.

A rather complicated manufacturing of a loop vibrator. It is better to fill the tube cavity with fine dry sand to avoid tube fractures (I did without it, but it's better not to risk it). To make a circle, you need to find a pipe close in diameter and bend a copper tube through it. The rest is according to the drawing.

To fix the elements in the cavity of the profile, I propose this option. After inserting the element into the cavity of the profile perpendicular to it from the top of the profile, drill a hole with a 2.5 mm drill and cut a thread with a M3 tap and clamp the element on top with a small bolt onto the top three (the main thing is not to overdo it because aluminum is very soft metal). Maybe someone will come up with a simpler or more reliable option, but it seemed to me with my set of tools this is the most successful way of fixing.

All elements must be centered and perpendicular to the traverse (boomers, as the bourgeoisie likes to call it).

We proceed to solder the drop cable and the coordination loop. Cut off a piece of RG58 cable with a length of 132 mm. We remove 10 mm of external insulation on each side of the cable piece, taking care not to damage the braid. Then we expose the internal insulation and twist the foil and braid into one bundle, turn the piece into a loop, connect the braids on each side and solder well. We clean the internal insulation by 8 mm. I think the rest is clear from the figure:

Solder the central veins to the ends of the active vibrator in the place of its rupture (15 mm in the drawing).

Some explanations. Before changing or throwing something out of a design, better ask in the comments so that later there are no reviews “but mnu doesn’t work”. I made everything very precisely according to the calculations, but still the minimum SWR was not at the frequency of 881, but at 885 MHz, which was quite acceptable for such frequencies. If it is made inaccurately, then the effect will still be, but not the maximum. At the transmission frequency (average frequency 824 MHz), the antenna showed itself very poorly, therefore I recommend that you place the modem in the best reception zone anyway, because it is used to transmit, according to the sensations, an internal rather than an external antenna.

I almost forgot about the tests. AxesstelPst EvDO BSNL was used to evaluate the result.
The modem is simply plugged into the USB port:

Connect the antenna:

What do we have. The signal is -62 dB, for comparison, if you are standing 20 meters from the BS, the signal will be about -40 dB, -105 dB is almost complete absence of a signal. Also interesting is the DRC Requested parameter. 3.072 Mbps means that the modem asks for the highest possible speed and the BS station will give us speed depending on the network load. The specific speed depends on the database load, i.e. a further increase in signal level will not improve speed. Speed ​​in the morning, in the evening will be naturally worse:

Good luck in manufacturing. Waiting for questions in the comments.