Immediate plans

For a very long time, I have owned an SDR dongle. One of the things I really must do this next few weeks is to get this set-up. 

My birthday is just before Christmas and my XYL (well actually me!!) got me a manual QRP ATU. I shall use this to try WSPR or FT8 on some different bands.

472kHz ATU

When running a long wire (actually a short wire, HI) on 472kHz TX with my 12W output transverter, I used a simple ferrite rod ATU on the shack table.

As long as ferrites don't saturate, they are perfectly fine in this application. With 10mW ERP, I have been spotted in Finland  with this arrangement many times on 472kHz WSPR.

472kHz ATU

There is a common misconception that 472kHz ATUs have to be big. With low power this is not the case. With my 5W I have had no problems at all with a tapped ferrite rod as the photo shows.  With the outer of a coax this makes a very simple 472kHz station. The 50 ohm connection is very low down and the wire will be much further up the rod.

Auto-ATUs for FT817ND

Until it started to misbehave recently, I have used the Elecraft T1 auto ATU with my 12yr old FT817 with good results. Its main use has been to resonate end-fed wire antennas on HF bands.  I always struggled on 160 and 6m.

Does anyone know how the LDG  Z817 auto-ATU compares? It is larger  than the T1 but less expensive.

What would  YOU recommend? I intend to get a new FT1817ND (as a second radio) and auto-ATU, then to fix the old ATU.

MF bands and local comms

This morning with my 20mW ERP on 500kHz I worked G3XIZ around 45km away on CW. Chris gave me 569 (on his meter) when using his loop antenna on RX. He was a very good signal here. Earlier I copied the other stations in his net M0JXM and G3DXZ. Yesterday G3KEV was a good CW signal and PA0A was copied later today.

500kHz QRP transverter with ferrite rod ATU on bench

The new 472-479kHz will be an ideal band for local area CW nets: as my far less than ideal set-up here proves, one can work up to around 45-50km without any problem even with very low ERPs. Running 6dB more power from the PA (I currently have just 5W out) will help further.

This afternoon my 500kHz OPERA has reached yet another new station: G8ALS in Coventry 126km away. My maximum daytime range on OPERA OP4 mode seems to be around 220km, although a few more stations reporting may increase this.

QRP Auto-ATUs

For some years now I have owned an Elecraft T1 auto-ATU which works very well with the FT817, especially with the FT817 interface cable which automatically retunes the ATU when changing bands on the rig if the settings have previously been used. Just recently I started to have problems with this ATU when sometimes it simply refused to work. I contacted Elecraft for help and they replied to my email overnight, which was pretty good customer service. In the end it looks like the problem is just battery contact pressure: I managed to snap the plastic that ensures good pressure on the battery contacts and, by chance this evening, I found that pressing the battery hard brought the unit back to life. I may replace the 9v battery contacts with a snap connector. 

On the subject of QRP auto-ATUs, has anyone experience of the LDG Z-817? This is a bit bigger than the diminutive T1 but in some ways is a better arrangement for home use. This has to be retuned every time bands are changed but the method of interfacing to the FT817 control (to change mode and power during tuning and then switching back automatically when done) is good. The memories also store previous tunes so retuning when changing bands takes just a second or two I believe. I also like the way the Z-817 sits on top of the FT817. The T1 is fine for portable use but I never quite know where to put it when used on the desk at home!

Now, of course, if the FT817 replacement ever happens (Dayton in a few weeks maybe?) we will expect to see an ATU inside the rig, as for the Elecraft KX3.

137/500kHz ERP improvements

Today I made a start on improving my ERP on 137kHz and 500kHz.

Firstly, I've bought a pile of Wima 1000V polypropylene capacitors from Farnell. With these I have now made a new ATU to match my TX loop (breadboard only ATU so far) which will allow me to run some 6-10dB more power into the loop without the ATU components overheating and failing. My old switched capacitor array was an old school capacitance decade box and this was marginal with tuning changing as caps heated up!

Secondly, I am going to change the MOSFET PA design to increase the 137kHz power from around 7W RF to about 30W. This should be worth around 6-8dB. I plan to do something similar on 500kHz too.

Finally, I've got some more wire to allow me to double the thickness of my TX loop. This will be done later and will be worth 2-3dB as long as I alter the 3C90 ATU (step down) matching transformer.

In all, these changes will give me up to 10-13dB more ERP and, with some luck will allow me to get more reports on QRSS and WSPR. It will take some weeks to implement these, but by late October my range should improve on both bands.

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.

Change to 500kHz ATU

This evening, as an experiment, I rewound the tapped coil on on ferrite rod based table-top ATU for 500kHz using 0.56mm enamelled wire rather than 0.2mm. The antenna current was up by (at most) 20%,  but on-air it made no difference with reports on 500kHz from M0BMU being the same as before. Before I put away my 500kHz kit for the summer I may try an L vertical with a 15m long horizontal top as I have access to a nice tree in my neighbour's garden which would provide a convenient support for the far end.

The 500kHz ATU

This is a close up picture of my 500kHz ATU which consists of a ferrite rod with thin copper wire tapped at various points to which the antenna and TX are connected. The TX connection is about 5t from the ground end. Also in view are the current transformer (the small toroid with 50 turns secondary) and the small meter used to measure the antenna current.

Antenna measurements at 500kHz

Matching my antenna at 500kHz has been a bit hit and miss: my technique, if you can call it that, has been to match the antenna "by ear" on RX, adjusting taps on the ferrite rod ATU inductor, to give me the best S-meter reading on the intermod product on 504kHz. Then, with a 3dB pad between the PA and ATU, carefully tweek the tap settings on the ATU for maximum antenna current measured using a toroid current transformer. The 3dB pad is to prevent damage to the PA during the adjustment process. Up to now this has been OK, but I could really do with a better way of measuring the antenna parameters. I'd really like to get an antenna analyser, but cannot justify the expense. I need to check out some other simple, low cost, techniques.