FARNHAM VHF GROUP

GB3FM - GB3FN - GB3FX

The GB3FNM Beacons

The group also hosts microwave beacons on behalf of the UK Microwave Group. The beacons operate on the following frequencies:

 2.320 920 GHz(ERP = +16dBW)
 5.760 920 GHz(ERP = +17dBW)
 24.048 920 GHz(ERP = +13dBW) (Currently off air - November 2020)

There is a plan to re-establish the non-functional 24 GHz beacon and a 47GHz beacon which was operational for a time has been taken off the air.
 
The beacons on 5.7 GHz and above were developed, built and maintained by G8ACE and G4NNS and project managed by G8KQW. The 2.3 GHz beacon was developed, built and maintained by G4SJH, G1EHF and G8GTZ. The antenna systems are located at approximately 15 to 20m above ground level.
 

Beacon descriptions

All the active beacons nominally provide omni-directional coverage subject to variations in the antenna pattern. Where possible the antennas are oriented to favour directions up-country and towards the continent.
 
The beacons provide a useful resource for propagation monitoring and enable microwave operators to check their receiving equipment.
 
The complex consists of indoor units (IDU) which include the power supplies, keyers, stable frequency references and intermediate or final frequency generation. The intermediate frequency for the 5.7 GHz beacon and power are supplied to the corresponding mast mounted Outdoor Unit (ODU) via coaxial cables. Since early 2010, the OCXOs have been locked to the 5MHz reference used for the GB3FN and GB3FX repeaters, to give improved frequency stability. For the 2.3 GHz beacon the final frequency and power are supplied to the ODU (pictured below) for final amplification at the mast head. The 2.3 GHz beacon uses GPS frequency stabilisation.
 
Beacon Image All the beacons use slotted waveguide antennas which produce a polar diagram in the form of a four leaf clover with approximately 10dB of gain in the main pair of lobes (180 degrees apart) and 6dB gain in the minor lobes (at right angles to the main lobes). The nulls are at approximately 0dB. The main 5.7 GHz beacon lobes are aimed ENE/WSW while the minor lobes are at 90 degrees to these, i.e. NNW/SSE. The 5.7GHz antenna is housed in a separate radome connected to the 5.7GHz ODU by a short length of low loss cable. The 24GHz antenna is housed in a radome which is an integral part of the 24GHz ODU. The 2.3 GHz antenna is aligned so that the main lobes are more NNW/SSE.
 
The 2.3 GHz beacon runs a short PI-4 sequence (so long as the GPS source is available) in addition to the usual CW tone and FSK identification sequence. The PI-4 sequence can be decoded with the appropriate software at low signal levels for automatic monitoring.

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Updated 23rd December 2020
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