Sound powered transmitters

A few people are currently experimenting with sound powered RF transmitters, generating the DC power to run a microwatt level oscillator from the human voice or morse key activation. There have been a few such ideas published. What appeals to me is a sound powered TX coupled with a crystal set RX. Such a system might get a mile or two as long as the station at the far end was running a few watts of AM or MCW. This would be the ultimate in QRPp!

There is a patent for this sort of sound powered transmitter - see United States Patent US2981833 dating from 1955 which has some schematics. This is available to view at http://www.freepatentsonline.com/2981833.pdf

DXTV - 1938 style!

There is a fascinating clip on YouTube of 405 line BBC TV being received in New York (from London) presumably by F2 or multi-hop sporadic-E propagation in 1938. This is the only example of a pre-WW2 live 405 line BBC TV transmission known to be recorded. The image shown left is of the female TV announcer. The signal comes and goes as the QSB takes the signal out of sync. Totally fascinating.

See http://www.youtube.com/watch?v=4SniB0JEDGs

Back from holiday - WSPRing soon

Just got back from 2 weeks in Greece visiting the ancient sites. Hope to get back WSPRing on 500kHz within a few days after I've caught up on things at home.

136kHz band WSPR reception

Jim M0BMU was firing up his transmitter on 137kHz today using WSPR. Managed very good reception of his 200mW ERP here at 69kms range. See screenshot - he is the yellow dashes. He was clearly audible by ear too, so a 12 wpm CW signal would have been copied well.

500kHz TX transverter schematic

This is the schematic of my 500kHz QRP TX transverter (from 28MHz). The next steps are (a) to add a couple of relays and RF sensing so that I can use it as a full RX/TX transverter and (b) changing the output stage to an FET such as an IRF510 to increase the output to 5-10W from the 700mW currently.

ERP estimates on 500kHz

Just got this letter from M0BMU who has estimated my ERP on 500kHz currently .

Dear Roger, LF Group,

It is interesting to make an estimate of what G3XBM's ERP might be...

The noise level at M0BMU during relatively quiet daytime conditions I estimate to be of the order of 3uV/m in the 2.5kHz WSPR reference bandwidth. The reported SNR gets up to about -25dB, making the signal level from G3XBM about 0.18uV/m.

ERP = (Ed)^2 / 49; with E = 0.18 x 10e-6, d = 69000m , ERP is about 3uW with only "geometric spreading" propagation losses. But there is some additional reduction in field strength due to ground wave propagation losses, perhaps 3dB with "good" ground, so the ERP would then become about 6uW. Diffraction losses due to the curvature of the earth are negligible at this distance.

Obviously, this could be +/- several dB due to various uncertainties, but should be of the right order of magnitude. Plenty of potential for improvement then!

Cheers, Jim Moritz
73 de M0BMU

500kHz WSPR screenshot from G3XVL

Chris, G3XVL, in Ipswich has just sent me a screenshot of my 500kHz WSPR signal this morning when I transmitted at 100% for about 30 minutes. It clearly shows the drift when I go on TX continuously. The drift is about 40-50Hz worst case. I now suspect this is drift in the FT817's reference oscillator as the box heats up and NOT drift in the transverter. I may try using the IC703 as this has a larger mass and should drift less.

The very strong signal is M0BMU and the one towards the top is G7NKS. No new reporters overnight, so it remains at 4 reporters with best DX 69kms when using less than 1mW ERP.

Breaking the 100kms barrier on 500kHz QRPp?

500kHz is proving quite a challenge at the moment: before I modify my set-up in a few weeks' time to increase my ERP ( less than 1mW currently) I want to get a few reports from greater than 100kms away. This target seems elusive so far, with the best DX remaining at 69kms, some 31kms short of my target.

The picture shows the longest paths my signals have spanned so far.

Sixbox 6m AM .....in Japanese

A few months ago I was asked by Junichi Nakajima-san, JL1KRA, if he could translate an entry on my blog about the Sixbox 6m AM transceiver to use in a Japanese magazine dedicated to AM and its use. Of course I said yes. A few days ago he kindly sent me a copy of the magazine. Now, I can't read Japanese, so I have no idea what the article says but it is nice to see the article in Japanese. Incidentally the magazine called 6mAM shows there is a great deal of interest in AM in Japan.

Since this blog entry was written, the design of the Sixbox has moved on and the revised schematic is on the Sixbox page of my website at http://www.g3xbm.co.uk

Junichi, I do not have your email address but thank you for sending me the magazine, the information and the 50MHz crystals.

A Brazilian Fredbox 2m AM transceiver

Hamilton, PU2XLB, has built a Fredbox 2m AM transceiver using the G4BYE adaptation of my original schematic. The G4BYE version has an LM386 audio rather than a simple stage driving a crystal earpiece. Hamilton, PU2XLB, has managed a 10km QSO already. He also has a Youtube video of it in action on RX.