Contents :

A collection of thoughts, and my notes about experiments and ideas, technical or otherwise, connected to Amateur Radio, Satellite working and monitoring and other electronics.

Saturday, January 30, 2010

Some experiments on the (V)XO of the RM80

After Joachim's experiments with the RM80 frequencies and MEPT, using the varicap, giving some problems with chirp I decided to try another approach, and we will see what the results are.

In place of the varicap I mounted a trimmer capacitor, in parallel with the R10 (feeding the varicap through I mount a 2.2nF capacitor, so that Q2 creates a RF-short of the trimmer capacitor, giving rise to the lower frequency RX LO of about 3579.350.

Just trying, and as expected the oscillator does not oscillate on the stray capacitance of the circuit without any capacitor in stead of the varicap.

First test was with a 2-10pF trimmer, giving rise to a frequency above 3580 (even with full capacitance), and stopping oscillation at low cap. values. Not good enough.

A 2-18 trimmer with a parallel fixed capacitor of 10pF is better : the (high) frequency range is now about 3579.75 - 3580.10. I noticed that the oscillation on the higher frequency is weaker on the higher frequency by about 10 dB, still the oscillation holds. A lower value coupling capacitor to the buffer, as Joachim indicates, is probably in order.

This makes me think that it should be possible to replace the trimmer with another varicap with lower capacitance than the one supplied., and with a zener stabilized voltage make the tx frequency tunable. A range of about 250 Hz should be attainable with one or two BB105 diodes - and I should have some of those somewhere.

On with the experiments, I will leave the RM80 with the trimmer cap for a while and see if the TX frequency remains stable when transmitting.

Update :

The lower value coupling capacitor to the buffer increases the oscillator frequency by 50 - 100 Hz - the oscillator now tunes down to .830 .
Parallel capacitor changed to 22pF, trimmer still 2-18pF, tunes (upper frequency) about .760 - just above 3580.000. nice range.

Monday, January 25, 2010

Rock-Mite 80 update

A new crystal in the oscillator brought the lower frequency down to 3579.350. That is quite acceptable for a grabber, the audio range for the 3579.500 - 3580 will end up at 150 - 650Hz. Not ideal, but workable.

Here is what it looks like with a few minutes of low - high - low frequency :

Still, with the 5V shift the TX frequency only went up to 3579.500, the shift is just too low at 150 Hz.

Enter the Q2 switch transistor and the 12V pull-up resistor. Voila, the upper frequency is now 3579.850, a 500 Hz difference, bringing the TX frequency just inside the 3579.800 - 3580.000 band.

It looks like it will be a good idea to have more than 2 crystals on 3579 for this construction, so a selection of the most suitable frequency can be made, alternatively find a better suitable varactor diode with more capacitance variation at 0 - 5 V.

The other point, with the current (original) "firmware", the RM starts up at the higher frequency rx local oscillator and will have to be switched manually to the lower lo frequency.

Later the test of the receiver as such, enough done for today.

I am optimistic that this will work as a MEPT tx and Grabber rx, but the rx test will show.

Sunday, January 24, 2010

Rock-Mite 80 Initial Test

The RM-80 is essentially assembled, first test has been done.

As opposed to Joachim I have only 100 Hz shift between the two steps, that is not enough to be useful. This has to be a fault, and I will seek it out later this week.

Saturday, January 23, 2010

Rock-Mite Kits Have Arrived

In the mailbox I found a nice little package today :

Two Rock-Mite kits , one for 80m and one for 30m.

This week end I see soldering in my "crystal ball"

Now which one to begin with ? .... well I started unpacking the 80 kit, so 80m it will be, most likely the color burst frequency around 3579.5 kHz.

Wednesday, January 20, 2010

DI2AM reception on 505 kHz

The 500 kHz grabber is now set to two narrow frequency ranges, the "WSPR segment" and 100Hz around the frequency of DI2AM.

DI2AM reception seems to be there essentially all the time after sunset, albeit with some (expected) QSB, and WSPR signals coming and going.

Nice result with a "cheapo" frame antenna wound on a cardboard box 35x40 cm small.

A new somewhat bigger frame antenna (more than 70x70cm) will be made some time, but that will have to wait.

In the week end the construction of the 500 kHz TX should begin

Tuesday, January 19, 2010

500 kHz transmit permit

Today I received a permit for experimental transmissions on the 500 kHz band, 501.000 - 504.000 kHz.

Now I will need to build a transmitter, I will most probably go the way Joachim proposed, using a 27 MHz crystal on the fundamental frequency (9 MHz), then dividing by 9 with a 4017, then by 2 with a flip-flop, followed by a "buffer amplifier" , and later an IRF510 as power amplifier.

The hard part will be the antenna, it needs to be compact, so a "magnetic loop" is probably the only way to go here.

Rock-mite kits

A discussion with Joachim today made me go buy some more kit stuff.

The Rock-mite kits look very suitable for a simple QRSS modification , as seen on Joachim's blog.

I ordered the 80 and 30m versions, one of each.

The best thing is that it is useful as both a grabber receiver and a QRSS MEPT - and the price of $32 a piece to have it sent here should not discourage anyone from doing QRSS.

The Rock-mite is a simple crystal controlled CW transceiver and can be used as such. It is a single frequency device with a crystal as the input filter and another one as (VXO) local oscillator/TX oscillator.

Monday, January 18, 2010

160m - 80m Experimental Loop

Yet another receive loop is born.

The construction is similar to the first 500 kHz loop, i.e. cardboard box loop, but made with Litz wire and with only 6 turns.

The calculation of resonance frequency found here indicates a frequency range covering the whole of the 160m and 80m band, and this looks like what I have.

The coupling is made with a further full loop, and it looks like that creates a too hard coupling to the antenna (not really unexpected), making the antenna rather broadband.

Later this week some experimentation on the coupling loop size is in the planning.

Sunday, January 17, 2010

Better antennas for LF/MF

In order to make loop/frame antennas with higher Q for the low bands (160m, 600m, 2.2km) I have ordered some Litz Wire from across the "pond".

It will be interesting to see how much improvement I can get from this.

"eBay" , of course. It can be a dangerous place, but it is our last, best hope for strange raw materials.

Saturday, January 16, 2010

TVEPG up and running from The Netherlands

I got the TVEPG set up with the HF-3 and the 500 kHz antenna, you can find it at here

The grabber is running on an Acer Aspire One 110, the old model with 8 GB SSD, under Linux, running SpectrumLab under Wine and an upload shell script originally written by Allan OZ5AR.

The frequency range is set to 503.5 - 505.5 at the moment, but I do have another setting for the 500 kHz WSPR band.

Update :
New setting for the 500 kHz Grabber : split screen with WSPR segment and a wider segment shown together.

Friday, January 15, 2010

500kHz antenna connected to the HF-3

The antenna is now connected to the HF-3 and receiving there. DI2AM is visible, if not too strong. Some WSPR signals are easily visible, with the WSPR program they should decode nicely.

For now I will leave it running, and in the week end the 500 kHz system could come on the TVEPG grabber page. no time now, and tomorrow some non-radio activity.

Still the signals are not all too strong, so a better antenna is required here.

The noise levels, however seem very low here.

"Progress" report 500kHz RX antenna

Well - let us say - the antenna tunes down to 500 kHz, but the output looks lower than I get from the ferrite antenna of the ATS-909. The S/N, however sounds like it is better.

The source of the lower output ? I am not sure, but suspect the small size of the loop contributes, and perhaps the coupling winding is not enough with a single turn. further tests are needed.

OK - this antenna is rather small and uses standard hookup wire, so the highest efficiency is not to be expected. a somewhat larger antenna, say 1x1 m made from Litz wire should provide substantially better results.

The first test was with a "dual gang" variable capacitor, but with only one plate set used. of course the hand capacity detunes the antenna so it is essentially untuneable.

SO the second test was with both plate sets, and now we are getting there. because the hand is at the center of the capacitor, no detuning or hand capacity effect is noticeable.

With the 25 windings, the antenna tunes down to just under 500 kHz, so the antenna is a qualified success.

The ATS 909 seems to have a rather good sensitivity on MW, so it is a bit difficult to fully judge the efficiency and usability of the antenna.

The tuning is not *very* sharp, but with a decent maximum at resonance.

The next test should be connecting the antenna to the HF-3 RX which should have a very high sensitivity on 500 kHz. Oh, now for soldering another connector to the cable and see what it will do.

Another solution to the low output could be to build an amplifier, but that sort of defeats the purpose of a 'simple antenna'.

More room for experimentation.

Tuesday, January 12, 2010

500 kHz Receiving Antenna in the Make.

I have been looking into the options for receiving amateur signals on the 600m band (500 kHz). Since I live in an apartment size does matter, so looking for a small sized antenna I looked around and found this article from VK2ZAY , showing a simple 'frame antenna' made with a cardboard box.

Being close to all kinds of noise sources I think the loop is probably the best option for a receiving antenna on such a low frequency.

Since I intend to use the antenna indoors the support material does not matter much.

The box measures are on the small side - 35x40 cm (14x16 inches if you like) - but as a proof of concept I have put 25 windings with taps at 15 and 20 windings, as well as a single winding coupling loop.

Now I will be looking in my junk box to find a suitable variable capacitor to complete the project (this may take some time).

If this works out to my satisfaction I will try out frame antennas for 80m, 160m and 2.2km as well, at some time.

Since there seems to be an experimental license available here in The Netherlands I may even try to put out a 600m signal, yes, I am considering joining the experimental group. The decision, however, has to be made soon before the opportunity expires.

Saturday, January 9, 2010

Idea Box : Direct Conversion RX for Color Burst Frequency Grabber

Using 3.579 crystals as crystal oscillator and filter for a PSK31 receiver/transceiver is well known. Getting a decent sideband rejection with a simple circuit is not easy with this design.

Here is a thought : One of the reasons for the lower sideband rejection is the proximity of the 'BFO' frequency to the signal frequency. Possibly oscillator phase noise is contributing to the problems. Here is a trick for a simple QRSS receiver for 3579 :

N4ESS has crystals on 3575.6kHz this is quite a bit further from the signal frequency than is easily done with 3579 crystals. OK, the beat (AF/'IF')then is at about 4kHz, but using SpectrumLab that is not really a problem, since it can 'convert' and filter signals. Of course, the input filter as in Joachim's 30 QRSS Grabber RX is necessary to get some unwanted sideband suppression.

This should make for a simple design for a DC RX Grabber for the color burst frequency on 80m.

Edit : I looked in the 'stock' and I have a 3575.6 crystal (and of course 3579.5 as well, so building this rx for color burst rx is possible. Other projects come first, such as finishing the setup of the 80m SDR from Box73.

Note :

The same concept should be usable with a 160 RX at 1843kHz with a 1856kHz Xtal, also available at N4ESS, albeit with a somewhat higher 'IF' of 13 kHz.