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70MHz Beacon

Description of the beacon hardware and DDS frequency source used for my personal beacon

Written by: G4RFR ( FRARS ) Friday, 25 April 2003

The G4JNT/P 70MHz Beacon on Bell Hill

My personal beacon on 70MHz located on Bell Hill that was installed last year has now been replaced with one having much higher frequency stability. The design is based on an Analog Devices AD9851 Direct Digital Synthesiser chip which is used in a modified version of my DDS Module as published in RadCom November 2000.

The AD9851 has a built in Phase Locked Loop X6 multiplier for the drive clock frequency, and can operate with a clock of up to 180MHz, so allowing frequencies of up to around 72MHz to be generated directly. However, I don't do it this way .....

Instead, the DDS Chip is driven from a high stability ovenned 10MHz oscillator (donated by Julian, G3YGF). The PLL multiplies this to give a 60MHz clock and the DDS is programmed to generate a frequency of 10.031MHz. A side effect of using DDS techniques to generate signals, is that a whole comb of frequencies are actually generated in the DDS chips itself. For a specific wanted frequency, Fo, when driven by a clock of frequency Fc, a DDS will generate all products equal to
N.Fc +/- Fo
Where N is 1, 2 , 3 etc. Normally all products higher than approximately 40% of the clock are filtered off. For our situation with F0 = 10.031 and Fc = 60MHz, the frequencies generated are therefore:
10.031, 49.969, 70.031, 109.969, 130.031 etc. generated with decreasing amplitude.

The 70 MHz term is extracted by a bandpass filter that rejects all the other products, and amplified to form the transmitted signal. The frequency shift for the CW is generated by reprogramming the DDS chip for a frequency 85Hz higher whenever the keying line is active. The reprogramming sequence only takes about 600us after the input keying line changes state, so is capable of working with quite fast data if needed. A modified PIC software routine from that appearing in the RadCom article was written to allow the frequency keying to be incorporated. A figure of 85Hz frequency shift was adopted to allow for RTTY telemetry at a later date; most RTTY decoding software includes this shift as an option.

Power output to the horizontal dipole antenna at 2.5m above ground is 0.6 Watts.

For anyone who wants to use this beacon to set up a 70MHz transverter, the frequency is maintained to within a few Hz of nominal, such that the lower tone, the one carrying the CW information, is 70.031MHz and the space tone is 70.031085MHz. The frequency was chosen so as not to interfere with reception of two (Yugoslavian) beacons listed as being on 70.029 and 70.03MHz

Current measurement puts the actual frequencies as being 4Hz high, ie Mark / Space = 70031004 / 70031089Hz, but this shift will change by a couple of Hz as temperature changes. I reckon it will always stay within 5 - 10Hz of nominal, and I can always measure it off air to 0.01Hz accuracy if given a few hours notice.

Telemetry from the site is transmitted as SLOWCW (also known as QRSS) with a one second dot period, designed for monitoring using Spectrogram or similar software. The data is repeated in fast CW, along with station ident, approximately every five minutes.

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