Using GB3UHF

Propagation monitoring

The beacon can be used for determining if propagation conditions are enhanced or have deteriorated over a period of time. Careful monitoring of the beacon can give a useful indication that conditions are likely to improve, and as such give rise to an early alert of an opening.

The beacon can be monitored using JT65B as shown on the Receiving GB3UHF page, and using the decoding software the beacons signal strength can be monitored over a period of time, even when inaudible in a conventional receiver bandwidth.

Another method is to use your receiver S meter reading to determine relative signal strength again over a period of time and the data can be logged using software. The software can then be used to display either in real time or historically, a graph showing any changes to conditions.

Together with GB3VHF, there is a great opportunity with GB3UHF to gain valuable information simultaneously on propagation characteristics at both VHF and UHF frequencies as shown below in a report from Alwyn G8DOH near Hook Norton, Oxfordshire (IO92FA) observed on the 15/6/2016.

The path profile is shown. Receive site height is 239m asl. The pink line shows the boundary of the first Fresnel zone. Obstructions in this region will weaken the signal by diffraction. Alwyn says “I calibrated both 2m and 70cm receivers using calibrated signal generators and the signal levels are referred to the aerial feed point. Accuracy should be 1dB”. “Results yesterday (receiving aerials shown bracketed) were: GB3UHF is at -96 dBm (2x18ele @33m agl) GB3VHF is at -90 dBm (2x8ele @34m agl) I made a path loss calculation, assuming each beacon is 10W to a 3ele pointed in the RX direction (RX site is about in the middle of the beam). This was based on 30W pa output, 1.7 dB coax loss, 3dB split between the 2 x 3ele yagis and 7.2 dBi gain for each. The 2m atten. is 30dB greater than free space, 70cm is 33dB greater than free space. A rather good result for the path length, helped by the good take-off at each end. Signals were very stable, slow fading (5s) of about 1dB on 2m, faster (1s) fading of about 2dB on 70cm and conditions very poor at time of measurement”.

John GI7UGV has an interesting waterfall display of three beacons received at his QTH in IO74DO on 17/6/2016 and he says “I've had a few decodes from the GB3UHF beacon this morning. Found this a bit of a surprise given I'm in a location few people would bother trying VHF/UHF in! Antenna wise 8 element x-quad RHCP on 70cm and 5 element horizontal on 2m both at about 5m AGL 10m ASL in the middle of Newtownards. Frequency movements are this side, been swapping between radios to get three decoding at once.

0718 -23 2.6 1542 # GB3UHF JO01EH

1028 -18 2.6 1572 # GB3UHF JO01EH

1202 -21 2.6 1356 # GB3UHF JO01EH

1214 -25 2.6 1362 # GB3UHF JO01EH

I'm receiving GB3UHF, GB3VHF and GB3NGI at the same time at the moment as shown in the image of the waterfalls, GB3VHF is stronger than normal and GB3NGI is off the back of beam with lot of aircraft scatter from the SE as usual”.

Alwyn G8DOH took an off-air measurement of the GB3UHF instantaneous frequency, compared rather indirectly against GPS, not using an SDR.

The method was as follows. A Halcyon professional Droitwich-locked OCXO is used as a reference to a frequency synthesizer set to 432.429000 MHz. Droitwich derives its frequency from a GPS-disciplined Rubidium source, so this should be accurate to about 1 in 10^11 over periods of seconds to minutes. This signal is coupled into the feeder from the receive antenna pointing to GB3UHF. The receiver is set to AM. Using this method, errors in the receiver frequency calibration therefore have no effect on the result.

The audio output goes to an Agilent 8903B Audio Analyzer that includes a reciprocal frequency counter, sampling the frequency 2.5 times per second, and the frequency is displayed.

If GB3UHF is on 432.430000 the 8903B should show 1000 Hz and as the photo shows it does indeed, although there is a little bit of fluctuation- at the 0.1 Hz level, but the old 200 Hz offset from the old driver frequency generator is no more.

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