The Enfield Repeaters

By Don G3JNJ

Situated on top of the Enfield Civic Centre in Enfield Town, there are at present two 'voice' repeaters, GB3NLon 2m channel RV62 (R7) and GB3LV on 70cms channel RU244 (RB2). This was first installed at the site in December 1979.

GB3NL. Prior to this, the only 2m repeater serving London was GB3LO (R7) situated at Crystal Palace in South London. 'LO' was first operational in 1974 from the QTH of G8AAI on Epsom Downs,Surrey, moving to Crystal Palace in 1975. The equipment in use on 'LO' at the time was a Pye 'Westminster' based unit with a rather complicated logic system, including 'time out' tone window and various modes of operation.

In 1977 a standby repeater was constructed using a Yaesu FT2F for receive and 'Westminster' modules for the transmitter with a very simple logic system.This simple logic system was a success so the complicated version used in the original repeater was converted and reinstalled. Unfortunately, the repeater suffered from 'considerable abuse' and it was decided to set up four individual repeaters to cover essentially North, East, South and West London in an attempt to de-focus the attention that one repeater attracted. These being respectively, GB3NL (R7), GB3EL (R0), GB3SL(was GB3LO) (R2) and GB3WL (R1).

It was agreed that all four repeaters would become operational on the same day, with similar, simple logic systems. The existing, standby GB3LO repeater was pressed into service at Enfield as GB3NL and took over the old 'LO' channel (R7).

Over the years there have been a few modifications to the repeater. In November 1981 a 'Storno' was installed. One significant event was a lightning strike in July 1987. This vaporised the antennas and damaged the cavities and mains cable. Remarkably, only one transistor succumbed to the strike. The present repeater is still the Storno and the antenna is a 'J Beam' C5 fed through a 'hexalator'. This is essentially a device to enable transmit and receive to take place on a single antenna. This, together with cavity resonators prevents interaction between Tx and Rx. The height of the antenna is some 57m above ground level.

GB3LV. The first experimental VHF repeater was sited at the QTH of the late G3KSW in Cheshunt and commenced operation in April 1977. This was a much modified Pye VHF unit followed by a varactor and tripler. In 1981 'LV' was relocated to it's present position at the Civic Centre.

A 'New' repeater based on a Pye Westminster, was installed two years later and continued in service until 1988 when it was replaced by another Storno 600. The UHF antennas consist of a 12dB gain Colinear for receive and a dipole for the transmit side. Again, cavity resonators are used to prevent interaction between Tx and Rx. The antenna heights being the same as for 'NL'.

The 'Repeater Keeper' for both 'NL and 'LV' is John Jackson, G3TZZ. They were built and are maintained by Roger Glover, G8IUC. Thanks must go to both parties for their considerable input in providing us with a service which we tend to take for granted.

ATV REPEATER

Several years ago the Cheshunt and District Amateur Radio Club investigated the feasibility of installing an ATV (Amateur Television) repeater at Enfield. A considerable amount of good work was done but unfortunately the project got somewhat 'bogged down'. Following an increase in activity by 'Simplex' ATV'ers, The North London Television Group (NLTG) was formed in April 1998 with the intention of building upon the work already done by CADARC, this time driven by a small team with the ability to react quickly to the problems that occur in such a project.

It took some four months to perform tests and complete the proposal which was submitted to the RSGB at the end of September. With some surprise, the proposal was approved the following February. So, with some panic, on March 13th 1999, GB3EN was installed. At the time of writing, it was very much in it's initial operational stages and minor problems were being addressed.

Good signals are being received around the Lea Valley and North London. It has also been 'seen' in S.E.Essex, Croydon, Bromley (Kent) and Hemel Hempstead. Early on, some unidentified interference was also experienced (this has since been resolved).

GB3EN operates on 1312 MHz (TX) and 1249 MHz (RX). The antenna system uses an Alford 'slot' for Rx anda pair of double-8 flat plate type, beamed 30 and 150 degrees for Tx. The reason for the seemingly strange radiation pattern is to avoid any potential interference to the Heathrow radar system. The height of the antennas is the same as the other repeater antennas.

The 'Keeper' for GB3EN is John Douglas, G4DVG and the repeater was built and is maintained by Adrian Hurt, G0OJY. Again thanks must go to all those involved in the project and we wish it every success. Also to Roger, G8IUC for his help with the 'background' information for this article.


GB3EN Receiver & Logic.	GB3EN Transmitter.