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.

A new reception report on 500kHz

This morning I was received on 500kHz WSPR by G7NKS near Biggleswade. This is his screen shot showing my signals. Notice my drift upwards when left on 100% TX - must do something to slow this down. It's not a problem when not on continuous TX, it's a thermal issue in the up-converter oscillator.

So far I've received reports now from 4 different stations on 500kHz, which is encouraging.

500kHz - antenna "improvements"?

This evening I am trying a different approach to the 500kHz antenna. My 15m longwire is strapped to my ground wire and now is acting as a radial. My main antenna is now the vertical coax feeder up to my 28MHz wire halo antenna. The coax has inner and outer strapped together so it is acting as just a thickish wire. The 28MHz halo will act as a top capacity hat.

The WSPR beacon will run overnight, so I will see whether the changes make my signal any different. Measured antenna current is quite a bit higher, so I am moderately hopeful a few more stations might receive my WSPR beacon if the ERP has increased a few dB. It will still be <1mW though.

After the first hour, I think my reports from M0BMU seem slightly better than before, maybe 2-3dB stronger? This is a hopeful sign, although it may just be conditions and QRN levels at the far end.

The 500kHz drifter....

G6ALB sent me a screenshot tonight of my 500kHz WSPR beacon as it was warming up. For the first 30 minutes it moved some 59Hz, not a lot really, but it looks dramatic on the WSPR waterfall. The reason is the transverter crystal oscillator settling down after first switch-on. At the moment the whole thing is a rats nest in the open on the desk: it needs putting in a box. The WSPR beacon was again heard by G3XVL at 61kms today.

Even lower power success on 500kHz

This afternoon I fired up my 700mW 500kHz WSPR beacon and wet string antenna (ERP microwatts) and got 3 reports from G3XVL at 61kms. So, I tried putting a 6dB then a 10dB attenuator between the PA and the ATU. Even with a power from the PA of just 70mW (never mind what that would be as ERP from the antenna!) he still gave me a -25dB S/N report! These were the three reports in reverse order:

2009-09-14 14:42	G3XBM	0.503829	-25	1	JO02dg	0.001	G3XVL	JO02nb	61	112
2009-09-14 14:40	G3XBM	0.503827	-23	1	JO02dg	0.001	G3XVL	JO02nb	61	112
2009-09-14 14:36	G3XBM	0.503823	-18	1	JO02dg	0.001	G3XVL	JO02nb	61	112

This suggests that just 35mW from the PA would have been enough to span this distance, even with my tiny antenna and poor ground.