Using HF/VHF DX Indicators

[VK2SWL1]

During active ionospheric or tropospheric propagation conditions on VHF, it is useful to determine which geographic locations are being received. The search needs to be assisted by monitoring various types of transmitters primarily in the 26-144 MHz region on a wideband communications receiver (e.g. Icom R7000, R7100, R8500, R9000; AOR 5000, or transceivers featuring 30-60 MHz general receiver coverage). HF/VHF indicators include the following categories:

25-26 MHz Globe Wireless HF base stations.
28 MHz 10 metre beacons.
30-55 MHz atmospheric radar stations.
50-52 MHz 6 metre beacons.
112-118 MHz ATIS Airport Navigation Beacons.
87.6-107.9 MHz FM broadcast stations.
45-138 MHz TV video/audio carriers.

Since the Icom R-8500 communications receiver features 1000 memory channels, it is often practical to program all of the above indicators within say a ~ 7,000 km range of the receiving station QTH. A DXer should not rely on real-time discretional judgements when monitoring DX indicators. The memory channels need to be programmed into the receiver in advance - no matter how unlikely they may be received. The writer even has selected South American HF broadcast channels programmed into the R-8500 memory, which by default, along with the more frequent DX indicators, are checked every day. One has to endure the inconvenience of switching in memory channels which may at best only come in one or twice a year on average - often less. For example, 138.26 MHz RTS5A Loxton is one example whose video carrier can be detected in Sydney every day via meteor scatter, but is generally too close for single-hop Es.

Fortunately the probability distribution for Es and tropospheric DX events are not uniform throughout the year. Someone tuning the VHF bands day and night throughout the year corresponds to a blind unassisted search. This approach is a waste of time and effort and is almost certain to fail. Searching for small targets (one single FM channel) in large spaces (geographic area / frequency range) over very small time windows (5-15 minutes out of 525600 minutes over 1 year - roughly 1 in 52560) is obviously inadequate for such searches; hence it needs to be supplemented with additional information, thereby transforming a blind search into a higher-level assisted search. There are several different methods which all contribute to a higher-level search.

Based on past records, Es peaks around the 21/22 December summer solstice. In practice, multi-hop Es often peaks within Dec 20 to Jan 16, although this can marginally vary from season to season. The seasonal peak days alone considerably narrows the time window. We further narrow the search by noting the diurnal distribution within the seasonal peak. Nothing exotic is usually noted outside of 0900-2100 LT. In practice, the Pacific path multi-Es can start as early as 0900 LT, while Asian / PNG area multi-Es tend to peak in the afternoon (1400-1800). This further reduces the reception time window. However, we can reduce the time window even further.

Past experience indicates that the best multi-Es events occur on days when single-hop Es is already well under way by 0800-0900 LT. This suggests that above average levels and distribution of Es ionisation has already formed by early morning. Hence we expect that the ionisation will increase to even greater intensity later in the day. For a 1 kW FM signal to traverse over 2,000 miles, Es intensity needs to be both widespread and intense. For this reason certain DXers will often defer work/study/shopping commitments assuming these conditions are in place by early morning. The inference is that multi-Es events occurring later in the day may be missed if we leave home.
We further increase the probability of detection by using VHF communication receivers to track 28-138 MHz precursor indicators from identical and/or nearby areas. These increased odds mean that a DXer is often prepared before the MUF hits 88 MHz and above. Because lower VHF Es signals propagate for much longer periods relative to the 88-108 MHz FM band, it is not unusual to be tuned to a vacant 88-108 MHz FM channel where an Es signal suddenly fades up out of the noise.

The low VHF indicators alert us to action. The response is automatic because we have already memorised the logical Es MUF progression. Some examples are:

Darwin 33.2 -> Darwin VK8 50.31 -> Philippines 55.25 -> 88-95 MHz
Indonesian/Irian Jaya FM.

50.11 VK9 Norfolk Island -> 93.9 Norfolk Island -> KVZK-2 AS -> 92.1
KSBS FM KH8.

50.11 MHz A35RK Tonga -> 90.0 MHz Radio Tonga FM.

Accurate pre-prepared station and frequency lists also assist in detection. The most likely channels are then monitored for Es exotics along the appropriate great circle path (i.e. north / north-west), while coincident single-hop Es signals are largely ignored.

The final step in refining the search is instant (not delayed) notification/warning from other DXers. When several DXers are searching for the same exotic Es signals over a similar period the probability of detection is considerably increased.

Direct observation tells us that lower frequencies are received more frequently over restricted distance ranges. The most common Es signal into Sydney is 28.27 MHz VK4RTL/b Townsville @ 1,000 miles (1,600 km). Somewhat less common is 28.26 MHz VK5WI/b Adelaide @ 700 miles (1,120 km). The Townsville beacon is more frequently received because the skip distance is near optimum; hence reduced E layer-ionisation is necessary to reach the MUF.

By contrast, rare sporadic E is when you need two or more Es clouds to propagate a high MUF signal over 2,000 miles (3,200 km). The
required ionospheric conditions to propagate 100 watt 90 MHz FM signals over 2,400 miles via 2Es is so provisional that the event may only occur once ever 3 years at best. The same can be said for eastern mainland USA to Ireland at 90 MHz via 3Es.

One further refinement to the search process is measurement of TV video carriers to 1 Hz resolution (see impending separate forum post dedicated to this topic). For example, two TV carriers are present on 45.25 MHz (nominal). When viewed on Spectrum Lab, the TV carriers are separated by 20 Hz. After accurately measuring the carriers by referencing to a precision frequency reference generator, the results are:

Wellington, New Zealand: 45.249985 MHz.
Hedgehope, New Zealand: 45.250005 MHz.

Once a TV carrier has been measured and a location assigned, this is added to an existing database for further reference. See:

http://home.iprimus.com.au/toddemslie/Tevhftvlist.html

Todd Emslie

http://home.iprimus.com.au/toddemslie/dx.html

[VK5PJ]

Todd
a correction for your list on the web site, the location is incorrect for this entry:

64.239 978 . ABAW2 Albany VK6 200kw

it should read "Mt Barker" which is 50km inland from Albany. There are some translators at Albany but no high power CH2, the high power stuff is all at Mt Barker.

Peter, vk5pj
(I grew up in Albany - vk6zsp)