Guides for installersUpdated: 13.11.2026

Antennas for CORTEX transmitters: how to choose and install them

Which antenna a GSM/LTE transmitter needs and which one suits a VHF/UHF radio system, when an indoor antenna is enough and when to fit an outdoor or directional one, and what to avoid when installing a radio transmitter.

CORTEX antennas at a glance

The antennas fall into two groups: GSM — for RT4-5GP/4, RT4-5GL/4 and other CORTEX GSM devices, and VHF/UHF radio — for radio transmitters, receivers and repeaters.

Antenna	Type	Use
ST1-1G	Indoor GSM	Compact antenna for RT4-5GP, RT4-5GL and other GSM devices, indoor mounting
GP1-G	Outdoor GSM, SMA connector	Improves reception in weak coverage areas
ST1-1	Indoor VHF/UHF	For CORTEX transmitters and RS-63 system components; supplied with the RR-63RT receiver
GR-3	Outdoor VHF/UHF	Higher gain for a wide coverage radius
UHF-M	Outdoor UHF	Long-range links: RS-4000 and base stations
Yagi	Directional UHF/VHF	Remote sites: high gain, narrow beam
Split 2/3/4	Antenna splitter	Connects 2, 3 or 4 RR4000d, RR4000h receiver modules to one antenna

GSM transmitter: indoor or outdoor antenna

Start with the ST1-1G indoor antenna and power the transmitter on.
Check the GSM LED: green — high signal, yellow — medium, red — low.
For an exact figure, send SMS 99.C3: a CSQ of 15 to 20 in the reply is acceptable, 20 to 30 is good.
If the signal is low or CSQ is below 15, fit the GP1-G outdoor antenna with an SMA connector and place it where reception is better.

The RT4-5GP/4 and RT4-5GL/4 have an SMA antenna connector. For more on checking the link, see “Checking a GSM transmitter”.

Radio systems: from an indoor antenna to a Yagi

The standard RS-63 kit includes an ST1-1 antenna for the RR-63RT receiver; several types of external antennas are available to order.
If a site is far from the receiver or the signal is weak, use an outdoor antenna: the higher-gain GR-3, or the UHF-M for long-range links in RS-4000 systems.
For individual remote sites, use a directional Yagi antenna. Because of its narrow beam it must be pointed at the receiver.
The RX_LEV LED on the RR-63RT receiver shows the signal level: during reception it lights red for a poor signal, yellow for medium and green for good.
To connect several RR4000d or RR4000h receiver modules to one antenna, use a Split splitter with 2, 3 or 4 outputs.

Installing a radio transmitter: rules from the manual

These requirements come from the installation manual of the RT4-5se radio transmitter (139–174 MHz, up to 5 W):

Connect the antenna before applying power — directly to the antenna connector (whip) or via coaxial cable.
Choose the transmitter location where the receiver picks up its signal best.
Keep the wires from the battery to the transmitter as short as possible.
Do not run the wires to the terminal block near the antenna, and never along it.
With a dipole antenna, lead the cable away at right angles to the antenna for at least 1 m.
Equipment should be placed in a metal box; with a whip antenna the transmitter housing must be in direct contact with the box.

⚠ Never power a radio transmitter without an antenna connected.

Outdoor installation

Outdoor antennas are often installed on roofs and masts. CORTEX carries out this work: design and manufacture of antennas and antenna masts, antenna installation at height, as well as lightning protection, surge protection and grounding.