What issues need to be considered in the early stage of the relay system construction?

  1. Frequency selection
  2. Equipment selection
  3. Antenna position selection
  4. Repeater and antenna distance
  5. Estimated system call distance

How to choose the frequency?

First the repeater is not a personal device.Before use, you need to report to the local radio committee.When reporting,The radio committee will give you a small range of frequencies,Let you choose one or several frequency points,Use these frequencies to use the repeater legally.
note,The frequency range selected by the Radio Commission,Generally at both ends of UHF,
Is not commonly used frequency.This is to avoid affecting the use of popular frequency points.For example, in my area,The radio frequency range offered by the Radio Commission is between 400 and 430 mHz.So before customizing the repeater,Be sure to consult and report to the local radio committee first.If you want to build a GMRS repeater.The frequency of the GMRS repeater is fixed, as shown in the figure:

GMRS Repeater Channels

GMRS Repeater Channels

The center frequency of the repeater should be:
Lower frequency(simplex / repeater output, MHz): 462.625 MHz
Upper frequency (repeater input, MHz): 467.625 MHz
This frequency point is the center value of the 8 channels of the GMRS.Each repeater is allowed to float 1 mHz above and below its center frequency.Using 462.625 MHz and 467.625 MHz as the center value of the repeater,Any of the 8 channels can be used.

What equipment is the best? (The right one is the best)

There are many brands of radio repeaters: Motorola, Hainengda, retevis.Repeater can be divided into: digital trunk, analog trunk
There are also many latitude classification methods.Which is the best?People here often enter a misunderstanding.Just at the time of purchase,To pay too much attention to performance parameters,Inspirational to get a best repeater.But buying a repeater doesn’t just need to consider the performance of the repeater.But what should be considered is what effect do I want to achieve?What are the effects of these features?As long as the purchased repeater has these functions, it can be used as an alternative target.There are a lot of walkie-talkies,Many functions,But not every function you need,But buy him,Need to pay for all the features.The purchase price is also an important indicator we have chosen.Within the acceptable range of prices,Choosing electrical appliances,This way you can purchase enough features to use,The quality is very good,The price is still low,Good radio repeater.

Such as Retevis RT97 radio repeater.It is a portable repeater.Center frequency can be customized.You can also purchase the frequency that retevis writes.UHF or VHF is optional,The frequency difference is 5~10mhz optional.Support sub-audio,CTCSS: 50, DCS: 104 pairs,Control panel adjustable channel,Small size: 310 × 283 × 170mm,Weight 2700g,Support 220v mains, DC12V ~24V mobile power, car power,It is suitable for: concerts, outdoor walks, small factories, farms and other scenes.These scenarios are not very demanding on relays.Mainly need a temporary erection,Or in the open area, the requirements for the relay station are not high.Using the Retevis RT97 is fully capable of doing the job.And the price of the Retevis RT97 is very affordable.

RT97 Portable Repeater Power Amplifier UHF

RT97 Portable Repeater Power Amplifier UHF



How to choose the antenna position?

Everyone knows that the higher the erection, the better.The antenna should be installed in the highest building or on the hill in the area of ​​use.The closer this location is to the center of the area of ​​use, the better.The specific height of the erection is also a little tricky.High energy for antenna mounting,Can be seen with the eyes,The edge of the repeater signal coverage area.This height is the most appropriate.
And also,In the specific environment of erection,The antenna remains the highest,Don’t let objects such as walls, billboards, barbed wire, etc.Block the signal transmission of the antenna.The antenna is preferably 2 to 3 meters higher than these objects.

Antenna on top of high building

Mounting antenna on the top of the mountain

What is the distance between the repeater and the antenna?

The repeater should not be too far from the antenna.The radio repeater and the antenna are connected by a feeder.There is a certain loss in the transmission of radio signals in the feeder.The longer the antenna, the greater the loss.Therefore, the radio repeater is installed to be convenient for maintenance.Position as close as possible to the antenna.

How to estimate the system call distance?

After the radio repeater erection plan is formulated,You can calculate the communication distance of the system radio.Practical formula for radio wave transmission loss engineering:


  • F—Communication operating frequency (MHz)
  • h1—A communication antenna A point antenna height (m)
  • H2—communication object B point antenna height (m)
  • d-A and B communication distance (km)

The above practical formula is limited to the VHF 150MHz and UHF 400-470MHz frequency bands, and the terrain relief height is about 15m, and the communication distance is within 65km.

System wireless device communication distance calculation

  1. Assume known conditions
    (1) System operating frequency:
    TX 465MHz RX 455MHz
    (2) Repeater parameters and erection data:
    Transmitting power: 10W (40dBm)
    The formula for converting power P(mW) to x(dBm) is as follows:
    Receive sensitivity: -116dBm
    Coaxial cable loss: 2dB
    Omnidirectional antenna gain: 9.8dbi
    Antenna erection height: 30m
    (3) walkie-talkie parameters
    Transmitting power: 4W (36dBm)
    Receive sensitivity: -116dBm
    Walkie-talkie antenna gain: 0dBi
    Walkie-talkie height: 1.8m
  2. System gain of repeater and walkie-talkie
    The system gain is the maximum value of the attenuation allowed by the intercom to transmit the signal to the repeater receiver.
    If cable loss and antenna gain are not considered:
    System gain (dB) = transmit power (dBm) – receive sensitivity (dBm)
    If the cable loss and antenna gain are considered, the gain of this example system is:
    SG(dB)=Pt+PA-(RA+CL+RR)=36+0-(9.8-2-116)=144.2 (dB).
    Pt——transmitter transmit power
    PA – walkie-talkie antenna gain
    RA – repeater antenna gain
    CL – coaxial cable loss
    RR——radio repeater receiving sensitivity
  3. Judgment of whether communication can be established
    If the system gain is equal to the loss of radio transmission,
    It means that the radio wave energy of the communication distance has reached the limit.
    If the system gain is less than the transmission loss, the communication may not be established.
    Substituting the system gain into the electrical wave transmission loss engineering formula:

    Retevis RT97-relay station theoretical communication distance

    Relay station theoretical communication distance

  4. the system’s conservative call distance
    The above formula only calculates the conservative distance that the uplink signal (the intercom is sent to the repeater) for communication.The distance that the downlink signal (the repeater sends to the intercom) can be covered,Usually because the radio repeater has a large transmit power,The downlink signal is often better than the communication distance of the uplink signal.Since the system call is two-way,Therefore, the conservative call distance of the system is often calculated based on the above signal.
The transmission distance of the walkie-talkie is smaller than the coverage distance of the repeater

The transmission distance of the walkie-talkie is smaller than the coverage distance of the repeater