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Tips on installing mobile signal repeaters for professionals. Cable length?

03 Oct 2019
cable length max

How do we find out precisely the maximum length of cable you can install between the outdoor antenna and the repeater?

In all the Stelladoradus repeater kits we provide a standard 15m length of cable to be installed between the outdoor antenna and the repeater. This is satisfactory for most customers.

However, sometimes it would be useful to install a longer length of cable instead. Perhaps the roof is far away from the area where you would like mobile coverage, and you need to install a long cable. In many professional installations, this is the case.

Now, we have a method with which you can use to calculate this maximum length more accurately. In many cases, you will find this length to be much greater then 15m.

The key tool you will need is an i-Repeater. The i-Repeater has an LCD screen that will tell you the “total loss” figure in dB’s for each frequency. Once you know this, it is easy to calculate maximum cable length.

Begin:

Go to the roof with a yagi antenna, an i-repeater, and a power source for the i-repeater.
Point the Yagi at the nearest suitable tower.
Power on the i-repeater and attach the Yagi to the outdoor antenna port.
On the LCD, press “menu” and Click on “dB page”. Take a note of the “Total loss” values for each frequency.

That’s it!

Analyzing the data:

These “total loss ” values are the amount of attenuation the repeater is applying to the signal due to the signal being too strong.
You should focus on the highest frequency that you intend to provide in this installation

If you are installing an LG repeater (800MHz, 900MHz ), then you only need to focus on 900MHz.
If you are installing a GDW system (900,1800,2100mhz), then you will need to focus on 2100MHz.

RULE: it is more difficult to get the higher frequencies to travel through air and cable.

2100MHz will be more attenuated in cable than 900MHz, so designing a system that incorporates 2100MHz (or 2600MHz) is going to be more challenging, and you will find your maximum cable length will need to be shorter.

total loss closeup

Looking at the example in the image above, we see a total loss dB of 16dB at 2100MHz.

This is a large value for 2100MHz and would be typically be the case in urban areas.

As we intend to install a GDW system (W=2100MHz), we need to focus on 2100MHz.

Calculation:

RULE: There is a loss of 0.2dB per meter at 2100MHz in SD400 cable(standard cable we supply).

So we divide our Total loss dB figure by 0.2 to get our maximum cable length: 16 / 0.2 = 80

So, we can use an 80m length of cable between the outdoor antenna and the repeater.

80m is far better then the specified 15m. This is very useful if you need to get the signal deep into a building. (You will probably not need a pre-amp)

 

Questions:

What if the db loss value is zero?

Theoretically, in this case, we should have a zero length piece of cable between the Yagi and the repeater. However, this is not practical, so we have to accept some loss.
This is why we specify a 15m cable in our repeater kits, so as to ensure we do not have too much loss in the case of a weak mobile signal outside on the roof of the building.

We have a new online repeater design tool. To use this online tool, email [email protected] and we set you up with an account.

design tool

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