Solutions to implement a back-toback passive repeater or a refletor


I have been using PTP from cambium networks for some time, but i still have a question. 

From time to time there is a need to overcome the attenuation caused by an obstacle, plus there is no power grid  near the obstacle. My question is : Does Cambrium has solutions to implement a passive repeater (i.e. without the need of having power supply near the site).

Best Regards.

Are you able to use back-to-back antennas on a tower at a repeater point to bypass the obstruction? For example, a short tower on a mountain?



Hi, Mark. 

Yes I can, the tower would have 10 meters (maximum height), but enough to have LoS.

Best Regards

Hi, Mark.

I was really looking forward for you solution or suggestion.

Can you tell me if Cambium can provide any solution?

Best Regards.

Perhaps you could provide a LINKPlanner file for this project? I'll contact you directly.

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Sure. Please contact me.

Best Regards


What are the distances between these links with reference to your passive repeater. For long RF paths might as well design your system using on umanned power source (solar). You will be dealing large scale antennas for this case, and might not support your tower/structure.




Mark really helped me here and was always available for my questions.

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Maybe you can also share the solution. Probably we can adapt it as well.




Yes, I'll do that. A few interesting things come out of the discussion. drbrebelo kindly agree to let me provide some details of his project.

drbrebelo provided this link profile:

Clutter at A

LINKPlanner uses clutter data collected by satellite. It’s not always perfect and it’s worth checking by carrying out a site survey. At A, we have antenna height 6 m, but the clutter map contains trees at 15 m.

Clutter at A.PNG

Google Earth suggests that there is no clutter on the summit close to A, and we confirmed this with drbrebelo. We edited the clutter in LINKPlanner like this:

Clutter removed at A.PNG

Deleting the clutter reduces the predicted path loss by 16.0 dB.

Excess loss due to refraction

Having sorted out the trees, LINKPlanner predicts excess loss of 37.7 dB for this path. This is loss caused by completely obstructing the Fresnel zone by the terrain in the middle. If we configure this link in LINKPlanner with 27 dBm transmit power and 1.2 m antenna, the predicted receive power is -65 dBm +/- 16 dB. The wide range here shows the difficulty of predicting loss when the signal is refracted. A so-called “knife edge” has the lowest loss. Here the obstruction is somewhat dome shaped, and we would expect the loss to be towards the higher end of the range. There isn’t an easy way to find out for sure except to carry out a trial.


It might be sensible to carry out a trial and measure path loss. If the measured received signal is -65 dBm or greater, and capacity required is relatively low, it would be tempting to connect without line of sight. That would certainly simplify the project.

Increase antenna height

Additional antenna height would reduce excess loss. 48 m at each end would clear the obstruction completely. 40 m at each end would reduce excess loss to a much more manageable 11.7 dB. We're assuming that a 48 m tower is not a viable solution in this case.

Passive repeater

drbrebelo wanted to explore the idea of a passive repeater. The passive repeater works well if the antenna fills a large part of the Fresnel zone. It follows from this that it’s better to use a passive repeater close to one end of the link. In this link, the repeater can only be located near the mid-point, and the loss introduced by the repeater will be quite high.

We should avoid locating the passive repeater on the obstacle in the direct path. This is because the refracted signal could reasonably be a similar strength to the signal from the repeater, and we want to avoid multi-path fading caused by destructive interference. Instead, we should look for a repeater site that is not in a straight line to create a "dog leg".

Just as an example, we picked a point "T" here:

Repeater at T.PNG

We now have line of sight paths from A to T (2.3 km) and T to B (3.2 km). Suppose we install back-to-back 1.2 m antennas here interconnected by RF cable for H abd V polarizations.

The predicted signal level using the passive repeater with 1.2 m antennas is -66.8 dBm. Ideally we would like a bigger signal, but this should be enough for the lower modulation modes.

Here’s the calculation:

Frequency = 5800 MHz
Transmit Power = 27 dBm
Antenna Gain = 35 dBi
Range = 2300 m
Free Space Path Loss (2.3 km) = 114.9 dB
Antenna Gain = 35 dBi
Receive Power = -17.9 dBm
Cable loss = 1 dB
Transmit Power = -18.9 dBm
Excess Loss = 0.0 dB
Antenna Gain = 35 dBi
Range = 3200 m
Free Space Path Loss (3.2 km) = 117.8 dB
Antenna Gain = 35 dBi
Receive Power = -66.8 dBm

It might be possible to find a better repeater location with the benefit of local knowledge.

Powered repeater

Normally we would like to find a relay point that already has power. It looks from Google Earth as though this is unlikely in this case. The system integrator could explore solar and batteries. This adds the complexity of the power system at the relay point, but removes the need for the 1.2 m antennas.


I hope this provides some insight into the design process we went through. The passive repeater has a high loss, and here we have used four 1.2 m (4 foot) antennas for a 5 km link that won't even reach to top modulation mode. The solution might not suit every application, but it might just be the best option this time.