136kHz RX activity

In the last few weeks I've received 136kHz signals from several different countries and stations:

DK7FC/P     CW/QRSS
DF6NM     QRSS
G8IMR      WSPR
M0BMU    WSPR
PA0A    WSPR
SM6BHZ     WSPR

Certainly my receiving system is now up with the best since adding the tuned preamp. My main drawback is the TX/RX loop's directionality which reduces signal levels in a N-S direction. We just need a lot more stations active on the band, especially on WSPR.

Further 8.76kHz VLF test transmissions today

8.76kHz earth mode tests - best DX 5.1km and 4.75km

Today being mild and dry (for the first time in almost 6 weeks) I decided to do a field test with my improved 8-9kHz tuned preamp and loop antenna. A 5W "earth mode" (through the ground) test signal was emitted on 8.760kHz from the Burwell QTH and I ventured out into the countryside armed with the 80cm loop and preamp to see where the signal could be copied. In addition to my usual test sites at 1.6 and 5.1km I tried other directions and sites today. The map attached shows where signals were received and where no copy was achieved. The signal does seem to travel along the metal water pipes, which must be absent to the east where I assume they must be plastic, at least in places.

A 4th UK station with VLF NoV

Just received this from Gary G4WGT:

"Today I received my SRP from Ofcom to carry out research and experiments on the 9kHz band. The NoV is effective immediately and expires on 10th January 2012. The allocated frequency, power and additional conditions are the same as for other UK 9kHz operators, ie. 8.700kHz to 9.100 kHz with a maximum of 100mW (-10dBW) e.i.r.p. I am not ready to transmit a signal yet but it will be quite soon. I will make an announcement through the usual channels.

73  Gary - G4WGT"

What next at G3XBM?

With my recent semi-successful WSPR transmissions on 136kHz and a few outings on 500kHz I feel I'm at a crossroads now: do I go on with the LF experiments or do I have a change and do something completely different? My original aim on the LF bands was to see what could be done "simply" i.e. without large antennas and with real 5W or less QRP power from the PA. Already I have the measure of this and I doubt I'll prove much more to myself by striving further.

Next challenge? Ideas please.

On-line CW training

If, like me, your CW is not as good as it could be, then you may want to visit http://lcwo.net/ where you can both train yourself and test yourself. On plain callsigns I could read 22-23wpm on average, but I still made mistakes at much slower speeds. 30 minutes a day on this site and anyone's speed would rapidly improve.

Short Marconi vertical on 136kHz - forget it!

ATU for 136kHz. Antenna connected to top of ferrite rod

Disappointing results yesterday with a very short Marconi vertical tuned using a ferrite rod loading coil and variable capacitor. ERP was calculated as around 200uW, but in reality the ERP was around 10dB down on the vertical loop (based on WSPR reports locally), so the ERP was around 5uW only. Most of this can be put down to the coil losses and to tiny current actually flowing in the antenna part actually radiating. A much more efficient antenna for the band will require a BIG loading coil outside and more wire in the air with some top loading. I could erect a 6m vertical antenna with a sloping top capacity hat horizontal section with an overall vertical rise of 10-12m. Antenna improvement could increase ERP by >6dB. TX power could be raised by 6dB. Taken together this would make a difference, but it would not meet my QRP ethos.

My conclusions are that an ultra-simple QRP approach to 136kHz TX is just not possible if one is to achieve useful results consistently.

ERP with the loop on 136kHz

Today I did a recalculation of my 136kHz ERP using the loop antenna with my WSPR transverter. I have a small RF ammeter in series with the loop wire and I measured the current as 1.3A.

For 136kHz the ERP of a loop is = (I^2 * A^2) / (205 * 10^6)
where I = loop current and A = loop area (see "LF Today" RSGB book p101)

Putting in the numbers ERP = 1.3^2 * 80^2 (205 * 10^6)

So my ERP = 52uW on 136kHz.

A further 3dB could be obtained by thickening up the wire used in the loop taking the ERP up to around 100uW. Other improvements could come from increasing the TX power output from the present 5W. Later today I will work on a small ATU to resonate my 5m feeder to the 28MHz halo as a Marconi vertical. First I will work out the antenna current needed to beat the ERP of the loop. At least a Marconi will not be directional, which is a drawback of the loop.

Reports on 137.5kHz WSPR and QRSS3

In the last couple of days I have managed to get a couple of reports on 136/137kHz. Yesterday G3UUT copied my QRSS3 beacon and today G3XIZ copied the WSPR beacon. Neither represent any great distance - G3XIZ is 46km and G3UUT about 18km - but these are my first reports since getting back on the band after a few months of absence. ERP was about 10-20uW. G3XIZ was getting me at about -22dB S/N suggesting that around 100km may be possible on WSPR with the current kit (I did get a few spots from G3YXM at 148km last autumn with the same set-up), so I hope a few more stations in range can manage to detect me. I really must increase the ERP by 10dB: I have been saying this for too long and not doing anything about it!

DK7FC/P on 136.8kHz normal CW

DK7FC/P very strong on 136.8kHz CW today

Stefan has been out /P with his 100m long kite antenna, this time on 136.8kHz CW. He was excellent copy here in the UK (JO02dg) with a signal some 20-30dB over the noise floor even when sending fast CW. Attached is a screen shot from Spectran showing him working DF6NM at 1715 this evening. Distance is around 649km.

Improved E-field probe for VLF

Today I made an improved version of my VLF E-field probe. The first hi-Z FET input stage now an amplifier with an 8-9kHz tuned circuit in the drain. Sensitivity is now much better than before with strong Alpha beacon reception. It works well with the 80cm loop too and with the large TX loop in the garden via a step-up transformer. I'm looking forward to further tests by DK7FC and G3XIZ on 8.97kHz as my sensitivity is now better.  I hope to test this out in the field with my 8.76kHz transmitter over the weekend.