Wednesday 18 February 2009

HF propagation prediction: February-April 2009

The equinox periods provide longer daytime periods than winter, but logically, shorter night-time periods too. These tend to be the best months for working North-South paths, such as UK to South Africa.

On 160m (1.8MHz or Top Band), look for short-skip and DX openings at night. Again, no daylight skip is possible due to absorption, but openings out to 1,300 miles and occasionally further afield can be expected at night with conditions peaking around midnight and again at sunrise (greyline).

80m (3.5MHz) will generally follow the characteristics of Top Band at night, but will also provide good openings out to around 250 miles during the day. These will lengthen to around 500-2,300 miles at night with fairly good DX opportunities at times. At this point in the cycle 80m should still provide good DX as absorption is still quite low. Local communciation should be good as long as the critical frequency stays above 3.8MHz - register for the digisonde data at

40m (7MHz) Forty metres should open to DX in an easterly direction at sunset. Openings to the west should be possible after midnight and should peak just before sunrise. Contacts should be possible during the day, although, again, lower critical frequencies may mean that it is difficult to work other UK stations while perfectly possible to talk to European stations. If the flux rises then 40m may open up to NVIS contacts around the UK.

20m (14MHz) is likely to be the best DX band between sunrise and sunset. The bands may occasionally open after dark, perhaps to the southern hemisphere. Good openings will be possible during daylight hours out to around 2,300 miles.

17m/15m (18MHz/21MHz) should provide fairly good DX openings during daylight hours, especially to Africa and South America, with 17m being open more often than 15m. Once again, 15m may struggle to open during times of low solar flux, but could provide good openings if it rises above about 90-100. Both bands are likely to close after sunset.

12m/10m (24MHz/28MHz) These could be disappointing bands if the solar flux remains low. If the solar flux heads towards the high 80s/90s then openings will occur on both bands, although 24MHz will open first. If it breaks the 100 mark then expect to see some good DX openings on 10m, especially in early spring/late autumn.

Friday 6 February 2009

The Rybakov 806 vertical

I have no idea why this is called a Rybakov 806, but essentially it is a 7.6m vertical fed with a 4:1 Un-Un (unbalanced to unbalanced transformer) as described by IV3SBE.

It works very well with a fibreglass fishing pole and can be put up in a few minutes. The pattern (right) is what you get on 20m.

You need to put down an earth stake, and it really, really needs radials – the more the merrier.

The idea is that the antenna represents a non 1:1 SWR match at all frequencies – 7.6m is chosen as it isn’t actually a half wave (high impedance) or quarter wave (low impedance) on any band. The Un-Un (see below right) transforms the impedance to something closer to 1:1 and therefore reduces coax losses.

Does it work? I have tried a 7.6m Rybakov at a few locations and yes it does. Is it as good as a dedicated resonant antenna – no. You get losses in the Un-Un and there will always be a residual SWR on the feedline, which adds to the losses. You will also need to use an ATU to get the SWR down to 1:1, although most internal ATUs can usually cope with the mismatch.

As a matter of interest, here is the SWR I found with the 7.6m vertical (with the 4:1 Un-Un) with a single earth stake and two 20ft radials:

7.50MHz - SWR 3.2:1
10.1MHz – SWR 3.6:1
14.2MHz – SWR 5.6:1
18.1MHz – SWR 5.4:1
21.2MHz – SWR 2.6:1
24.9MHz – SWR 2:1
28.5MHz – SWR 2.1:1

The radiation pattern on 10MHz and 14MHz is typical of a vertical and good for DX. It is not so good on the higher bands as the antenna is longer than a quarter wave and gets complex.

It is also rotten on 80m (SWR 330:10) as it is waaaaay too short – if you extend the wire to 8.6m it will be better on 40m but you may as well make it a proper quarter wave (about 10.4m).

In tests it could hear local CB stations that were inaudible on my Windom, W3EDP or horizontal dipole. I know most CBers use vertical polarisation, but it shows that it might be good for low-angle DX radiation on 10m too.

It is a bit short for 40m, but does work. It was ok on 20m - some stations louder than on a Windom at 30ft, some worse. Same on 17m.

For 20m–10m use I think you are better sticking to 7.6m. The modelling shows that the radials are critical. You may get different SWR readings depending on how many you have. A single earth stake might give you a low SWR, but will be lossy.

If you have little space and like experimenting it might be worth a go, but I never found it matched a proper half-wave dipole at 30ft. More radials might help –if laying on/under the ground they don’t have to be resonant, just make sure they are roughly the same length as the radiating element and aim for as many as possible (32+ is good). If you try this antenna with just a ground stake I think you will be disappointed.

Let me know how you get on.

Sunday 1 February 2009

The W3EDP 85-ft end-fed

This is one of my favourite, cheap antennas. I have been using one for years and they don’t get much simpler.

Mine was catapulted over the roof at this QTH. It goes out of the ground-floor shack window, straight up, over the roof and then down to the end of the garden where it is tied off with fishing line, leaving the end about 10 ft high. The counterpoise goes off at 90 degrees (see illustration).

At the last QTH it went 50ft up into an oak tree. It is very stealthy and a firm favourite of the QRP fraternity.

It consists of an 85ft wire fed against a 17ft counterpoise. Some books say that you don’t use the counterpoise at all on 10m. Others say that you use a 6.5ft counterpoise on 20m. You do need an ATU though.

Some even say that you should feed it as a balanced antenna through a balun, rather than as an end fed with the counterpoise connected to the earth terminal.

I have tried it both ways and it seems to work either way. The tuning is slightly twitchier using it as a balanced antenna.

The antenna works well on 80m and 40m and even gives a dipole a run for its money on 20m. I am writing this as I listen to US stations on 20m and Californian and Michigan stations are actually slightly louder on the W3EDP rather than the 20m dipole.

DX worked on this £5 wonder include 6W/DL4JS Senegal, YK9G Syria, VQ5XF Turks and Caicos, VP6DX Ducie Island and VQ9JC Diego Garcia.

The downside of the W3EDP is that it can be noisier than a dipole and watch for RF in the shack. Running it with an earth wire to a stake and a counterpoise my field strength meter shows that it isn’t too bad though. Try one.

G5RV on 80m - inverted V or flat

After I posted my comments about how long antennas like the G5RV and Windoms don't like being set up as inverted Vs when being used on the higher bands such as 20m and higher, a friend asked if it makes a difference on 80m.

After all, for something like the UK's RSGB 80m club championship you need high angle NVIS communications.

I have found that it doesn't really matter what I use here - I get pretty much the same results. A G5RV, 85ft end fed (W3EDP) and full size OCF Windom all give signals of around S9 - S9+20db around the UK.

After modelling the G5RV in both inverted and flat modes with MMANA, it looks to me that it doesn't really matter. The performance on 80m is the same. See the plots (right)