QSO - VK1BL/P and VK2FZ on 13cm on 25/11/2007

This article documents a successful attempt on the current VK1 distance record on the 13cm band, including an approach to predicting likely path performance.

Ted (VK1BL) and Adrian (VK2FZ) planned an attempted QSO on 13cm from Ted's field station at Mt Ginini, 43km south west of Canberra to Adrian's home station at Lilyfield in inner suburban Sydney. Mt Ginini is 1760m ASL and provides a good vantage for a north east path to Sydney, though growing trees are becoming a significant obstruction in that direction, and the 90km road trip is a problem at some times of the year. Andrew (VK1DA) postulates that this is evidence of global warming, not enough snow falls on Mt Ginini in recent years to keep the vegetation and trees in check.

VK1BL/P

Location: Mt Ginini, ACT. The spot chosen for setup was just east of the summit and south of the summit road at a place where the clearest path to Sydney (51°T) was available to a low antenna. The weather was clear and still on Mt Ginini, but Canberra was covered in a layer of low cloud, probably at about 1200m altitude. From Mt Ginini towards Sydney, there was some cloud at around 2500m.

Fig 1 is a G/T model of VK1BL/P. The model is based on an assumed an ambient noise temperature of 290K. Characterisation of ambient noise in built up and rural locations remains a project. The article Effective use of a Low Noise Amplifier on VHF/UHF explains the relevance of G/T, how it is determined and used.

Fig 2 shows the field station with capability on 70cm, 23cm and 13cm. The top antenna is a 40 element loop Yagi, the next is a 16 element DL6WU Yagi, and the next is a 13cm GridPack.

Fig 3 shows the inexpensive 870mm x 660mm 24dBi gain GridPack antenna, its feed system, LNA and T/R relay in more detail.

EMR MPE compliance is an issue with systems of this type at near ground level. The safe distance (ARPANSA General limits) in front of this antenna with 22W constant carrier is 7m, reducing to 0m under SSB telephony with compression and 50% duty cycle. Safe distance under JT65 with 50% duty cycle or a CW beacon with DutyCycle*ModulationFactor of 50% would around 5m.

VK2FZ

Fig 1 is a G/T model of VK2FZ. The model is based on an assumed an ambient noise temperature of 290K. Ambient noise at this location may well be higher, not only due to the urban locality, but the risk of local WLAN traffic.

Path prediction

Fig 5 is a map of the path from Mt Ginini to Sydney. The path is straight over Canberra which has noise implications.

Link predictions were made using RadioMobile using the station parameters from the G/T model. The receive threshold in Radiomobile was set to the total system noise in 2kHz bandwidth for each of the stations. For example, VK1BL/P's receive system noise is 81.3K and the total antenna noise is 290.2K, giving total system noise of 371.5K. Using the Receiver sensitivity metric converter this is converted to a total system noise floor of -137.4dBm.

Fig 6:

Receiver sensitivity metric converter - results

03 December 2007 15:30:58

Rx Rin	50.0 Ω
Rx ENB	2000.0 Hz
Rx ENB	33.0 dB
Signal/Noise	0.00 dB
(Signal+Noise)/Noise	3.01 dB
Sensitivity	-137.4 dBm
Sensitivity	0.030 μV
Noisefloor	-137.4 dBm
Noise Figure	3.58 dB
Noise Factor	2.28
Noise Temperature	371.5 °K
Noise Temperature	25.70 dB

The RxRelative field in Fig 7 shows the strength of a CW carrier above the threshold which has been set at the total system noise in 2kHz, so the figure can be taken as the expected S/N ratio.

In reality, VK2FZ's ambient noise might be worse than modelled, and margin may be a little less.

The link prediction shows this to be beyond line of sight and within tropo scatter distance. Note from the diagram that the tropo scatter common area is quite low, and that stations further north than Sydney might well have path loss not much worse that above and within range of SSB telephony contact.

Fig 8 is a link prediction from RadioMobile of the path VK1BL/P to VK2FZ. The RxRelative figure, the path margin, is better, principally due to the higher EIRP from VK2FZ.

Signal reports

Both Adrian and Ted reported SSB telephony signals that were perceived by ear to be consistent with the model's 10dB S/N. Signals were affected by a quite rapid and quite deep fade though, and disappeared into the noise during the troughs, characteristic of tropo scatter. Signals received at Mt Ginini often had a burble superimposed, much faster than expected from aircraft or mobile flutter, and perhaps a hint of the propagation mechanism. There was no sign of aircraft enhancement of the path.

Reports were exchanged at about 21:30UTC on 24/11/2007 for a record distance (286.6km) for the 13cm band from VK1.

Table : QSL details
Station	Readability	Strength	Notes
VK1BL/P	5	1-2
VK2FZ	5	1-2

The future

Modelled and actual link performance give encouragement to attempts at longer paths that should be achievable using tropo scatter propagation.

Plans are being developed for attempts over a much longer path into VK3, but the possibility of longer paths to the mid north coast of NSW is also promising, subject to interest from hams in those places. Contacts on higher bands, especially 3.3GHz and 10GHz are also being contemplated by an active group of microwave enthusiasts in the region of Canberra.

Links

Changes

Version	Date	Description
1.01	03/12/2007	Initial.
1.02	07/02/2008	Record status updated.
1.03
1.04
1.05