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.

136kHz WSPR DX reception

Today there were several stations active on 136kHz WSPR with best DX heard the 50mW ERP beacon of SM6BHZ. Also copied PA0A and G8IMR. So far this evening no-one has heard my own modest (10-20uW ERP) WSPR beacon on 137.5kHz though. I'll keep trying.....

SAQ Jan 3rd

SAQ was audible well last night around 2330z tuning up for a transmission on 17.2kHz CW at midnight. The signal was about 20dB above noise using the 20m spaced earth electrodes as an antenna.

VLF success at last for G3XIZ (37km on 8.97kHz)

Chris G3XIZ achieved some VLF success today when his 50m high balloon supported vertical driven with around 50W was copied at up to 15dB S/N by M0BMU at a distance of 37km. Mode used was QRSS600. This is a new UK record for an amateur VLF transmission and is the first confirmed "far field" radiated signal. Well done Chris.

M0BMU estimated the ERP as follows (from a posting to the LF-reflector by Jim):

"I made an estimate of the field strength of your signal by injecting a test signal at a known EMF into the loop antenna, and comparing with the level of your signal. The received FS at the strongest point (around 1330utc) worked out to be 1.8uV/m. Taking the distance between our locations as 37km, your ERP would have been 87uW. The SNR at best was around 15dB, making the noise level 0.32uV/m in the FFT noise bandwidth of 2.1mHz, or a noise density of 7uV/m per sqrt(Hz)


Assuming a 50m vertical wire (heff ~ 25m),  Rrad of your antenna at 8.97kHz would be 880micro-ohms. Assuming 2.62dB directivity for an electrically short monopole compared to a dipole, and Iant of 200mA, the calculated value of ERP would then be 64uW. So there is reasonable agreement between these two calculations, the difference only being 1.3dB."

Night time SAQ VLF transmission - Jan 4th 2011

There will be an extra transmission with the Alexanderson alternator on 17.2 kHz primarily for listeners in North America, the times shown below. QSL cards will be issued for correct reports. These can be sent via post, email or via the SM QSL bureau.

16:00 PST (Pacific Standard Time) in USA. Monday, Jan 3rd, 2011 

19:00 EST (Eastern Standard Time) in USA. Monday, Jan 3rd, 2011

00:00 UTC (Coordinated Universal Time) Western Europe. Tuesday, Jan 4th, 2011

01:00 CET (Central European Time) and local time in Sweden. Tuesday, Jan 4th, 2011

Diurnal Variation in Dreamer's Band Noise

Over the next 24 hours I am doing a test to measure the change in VLF noise floor between 0-24kHz. DK7FC has let me have a suitable .usr file for Spectrum Lab to allow this to be recorded. You can see the results on the G3XBM VLF grabber which is updating the plot every 15 minutes.