Grandchildren time

After a busy few days building LF kit (and blowing it up!) it is now time again for our grandchildren. Our younger grandson came to stay last weekend and this weekend we have our elder grandson with us. The picture here is with me in the lane close to our home on his way to feed Maggie the horse, who is a favourite with both grandchildren.

Class E PA designs

Today I made a first proper attempt at a QRP class E PA design for 136kHz. Although the values in the output circuit seemed to tie up well with theory I wasn't able to realise the expected high efficiencies, possibly because of the gate drive design. In fact I managed to blow up my last IRF510 because it got very hot and this meant aborting a WSPR test (at a sensible power level).  One resource recommended to me is http://www.alan.melia.btinternet.co.uk/classepa.htm which has a E class design spreadsheet. 

I also discovered today that 16mm diameter 3C90 toroids do NOT make good LF PA output bifilar transformers: they get very hot. The larger diameter 3C90s appear to be fine.

Overall, a frustrating day!

Encouraging QRPp results on 136kHz tonight

This evening I had a good report and screenshot from Chris G3XIZ 48km away. This is my best report in terms of S/N and distance yet, with very clear images on the Argo screen.  I was going to try WSPR on 137.5kHz but had problems with the PA so QRSS3 beaconed instead. ERP was less than 20uW.

Comparing 500kHz and 136kHz WSPR over a 3km path

This evening I've been running about 5W PA output (not ERP)  from IRF510 PAs on both 136 and 500kHz WSPR and getting reports from G6ALB 3km away. I'm using the same ATU (just different capacitor settings) into the same loop antenna. On 136kHz I get-20dB S/N whereas on 500kHz +10dB S/N. This is a difference of 30dB!! 136kHz seems a heck of a lot harder than 500kHz.

LATER: Just got this from Steve Olney - VK2XV:

G'day Roger,

I read with interest your observation of the behaviour of the "in the air" loop via WSPR.

"On 136kHz the best report was about -20dB S/N whereas on 500kHz it was +10dB. This is a difference of 30dB!!"

I am not surprised at the 30dB difference as the radiation resistance varies with the 4th power of frequency for the same dimensions giving 23dB difference (i.e. 10 log ((136/500)^4)). In addition WSPR reports a S/N against in-band noise.    Atmospheric noise is inversely proportional to frequency and there is about 36dB difference (500kHz being quieter) between 2200M and 600M. This difference is effectively reduced by man-made noise which is higher wrt to atmospheric noise at 500kHz compared to 136kHz. This reduces the difference in noise levels from 36dB to about 10dB to 15dB (depending on location). Therefore I would expect about (23 + 10) = 33dB to about (23 + 15) = 38db difference between WSPR reporting for 2200M and 600M.

Your measurements are reasonably close to this - given there would be other differential effects such as ground loss.

Keep the interesting stuff coming !!!

73 Steve Olney - VK2XV

136/500kHz loop ATU

This morning I tidied up the arrangement to match my "in the air" wire loop antenna currently being used on 136kHz and 500kHz. The loop looks like a few ohms resistive when tuned so I have a 4:1 turns ratio step-down transformer wound on a 3C90 core. The loop is tuned using an old decade capacitance box and this is fine tuned with the 365pF variable capacitor. The picture shows the values needed to resonate the loop on both bands. It is simple but works fine at the QRP power levels I'm using (5W max from PAs on both bands). My report from M0BMU at 69km on 500kHz WSPR was -8/-9dB S/N which is pretty good.

A poem that moved me

Today I went to a memorial service for a local villager who died of Parkinson's Disease after years of failing health. The church was full. This poem entitled "He Is Gone" was said at the end and I found it profoundly moving.

You can shed tears that he is gone

Or you can smile because he has lived 

You can close your eyes and pray that he will come back

Or you can open your eyes and see all that he has left

Your heart can be empty because you can't see him

Or you can be full of the love that you shared

You can turn your back on tomorrow and live yesterday

Or you can be happy for tomorrow because of yesterday

You can remember him and only that he is gone

Or you can cherish his memory and let it live on

You can cry and close your mind, be empty and turn your back

Or you can do what he would want: smile, open your eyes, love and go on.

It was written in 1981 by David Harkins. See http://www.poeticexpressions.co.uk/

AM Day Sept 19th (info via João Costa, CT1FBF)

The ‘Associação de Radioamadores do Litoral Alentejano’ invites everyone to participate in a special Amplitude Modulation (AM) transmission commemoration, using the 80, 40, 20, 10 and 2m bands, on Sunday, Sept 19, 2010.
Frequencies and periods:

3.685 MHz - from 0600 to 0800 UTC
7.1 MHz - from 0800 to 1100 UTC *
14.286 MHz - from 1300 to 1400 UTC
29.075 MHz - from 1400 to 1500 UTC
144.575.0 MHz - from 1500 to 1600 UTC

*alternative frequency is 7.143 MHz

More loop experiments at 500 and 136kHz

Today I converted my earth electrode antenna with elevated feed wires into a full "in the air" loop for both 500kHz and 136kHz. As might be expected, tuning is now very sharp as there is no longer a 50 ohm earth resistance in circuit dampening the Q. At first I found matching and tuning impossible until I decided to match the loop with a step-down transformer (3.5:1 on a 3C90 core) and separately do the tuning. This works a treat and it is now a simple matter to tune the loop either for 500kHz or 136/137kHz.

First reports on 136kHz with the QRSS3 beacon suggest the signal is about 6dB stronger, which is a great result. The loop is wound with quite thin wire (1-2mm diameter), so a higher efficiency is possible with thicker wire, but with even sharper tuning.

LATER: Attracting any attention is turning out to be a serious issue with real QRP on 136kHz. Even with alerts on my blog, the LF-reflector and the GQRP Yahoo groups it's hard to get that many people taking a listen/look. With uW ERPs I'm beginning to think that I'm never going to be able to get much further than my present best DX of 62km.

Last days on 56MHz (5m) - lost recording?

Many years ago Jack Hum G5UM attended the Cambridge club and talked about VHF. During the talk he mentioned a gramaphone disc being made recording the very last night of operation on 56MHz in the London area in the 1940s. Does anyone know what happened to this most historic recording?  This would be a wonderful item of ham history if it could be located.

More results on 137kHz and a better ERP calculation

Today I managed, I believe, for the first time to accurately measure the ERP of my QRPp system on 137kHz. This is the method used:

• Using the E-field probe, FT817 (AGC off, gain backed off as far as possible and a 10dB pad between the EFP and the FT817) and Spectran I went to my usual test site 1.5km away from the QTH, 45 degrees off the main lobe of the TX loop/earth electrode antenna.
• Measured the signal level of DCF39 on 138.83kHz
• Measured the signal level of G3XBM on 137.675kHz
• Repeated this three times to reduce errors.
• Noted the difference in FS level.

Difference in signal level = 44dB . I feel pretty confident this is an accurate figure now and not effected by AGC and overload. Assuming DCF39 is 1mV/m here (info from Alan Melia)  then my FS at the test site is 6.4uV/m.  Using the formula ERP = (E*d)^2/49 where E = 6.4*10E-6 and d=1.5*10E3 gives an ERP = 1.9uW giving an antenna efficiency of -63dB using the earth electrode antenna with the elevated feed and 4W from the PA. 

The test site is about 45 degrees off the main line of fire of the antenna, so in the best direction it could be 10dB (?) stronger, i.e. 20uW ERP giving an antenna efficiency of -53dB in the best directions. Frankly I'm amazed that anyone can copy this signal at any distance, so full marks to G3XIZ (48km) and G3XDV (62km).

Mike G3XDV sent me another screenshot of my signals this morning during the tests. if you look carefully you can just make out "G3X..." in QRSS3.