LF/MF Loop antennas on transmit

For the last few winters I've used a wire loop antenna on 136 and 500kHz transmit with moderately successful results. Well, considering the extremely low ERP it is better to say excellent results. My wire is not that thick: around 1mm PVC covered multistrand wire and the loop area around 80m square, with the lower part of the wire either very close to, or actually on, the ground.  Using the TX loop calculator at http://www.66pacific.com/calculators/small_tx_loop_calc.aspx the efficiency is around -30dB at 500kHz, so that 5W from the PA would result in around 5mW ERP radiated. My calculations and measurements, with less than ideal loop geometry, suggest the ERP is some 3dB lower at around 2-3mW. At 137kHz the loop is considerably less efficient at around -50dB.

For a small plot with nearby trees I am not sure how well this compares with a small Marconi with a short vertical section (around 6m max) and a horizontal section of around 6m. Anyone know the answer?

Updated MF transverter schematic

The current Rev D version of the 10W MF (472/500kHz) transverter is shown below. There are a few changes around the PA and output filter.

Revised 10W transverter schematic (27.7.12)

472/500kHz video

There is now a short video describing the 472/500kHz 10W transverter on my YouTube channel. This shows the transverter working on WSPR with the associated simple ATU and antenna current meter, all on the operating bench.

472/500kHz antenna improvements needed

For the last month whilst testing my 472/500kHz transverter I've been using a highly compromised antenna. Essentially it is a very short Marconi tuned against my central heating system ground. The first 6-7m is a coax cable running next to pipes in the loft horizontally. Eventually the coax runs outside and up 5-6m to my 6m/10m halos. The coax inner and outer are strapped together when used on 500kHz. In the last week we have had a new central heating boiler fitted, this time in the loft, so there are even more copper pipes and wires running close to the coax. Reports appear to be a few dB worse on 500kHz as a result.

The time has now come to design a "no compromise" 500kHz Marconi! Looking at the geometry of my garden I should be able to erect a 5-6m vertical away from the house with some 10m of horizontal top loading. I can add some inductance near the top of the vertical section and tune it all against a reasonably decent ground (4 x 1m long earth spikes, a couple of radials and the central heating copper).  Not sure how soon I can work on this but I would hope the ERP increase could be up to 6dB. At least with WSPR (and OPERA) one can do a series of tests over several days with each antenna and compare results. It will be easy to measure antenna current which gives a good indication of ERP when the antenna effective height is calculable.

New MF transverter ready for testing

472/500kHz transverter built more presentably

Being satisfied with the latest 10W version of my 472/500kHz transverter breadboard, I have now rebuilt it more tidily so it will fit in a small plastic or diecast box. This uses my normal "dead bug" construction using a piece of copper laminate as ground plane with small PCB pad islands to join some components. It will be tested tomorrow, but I am not expecting any issues.

UPDATE 26.7.12
Tested the transverter above on-air this evening and getting reports from several stations on WSPR.

WSPR reports so far this evening on 500kHz

Results with the new 500kHz transverter

WSPR reports received this evening (40mW ERP)

A decent evening testing the new 472/500kHz transverter with WSPR reports from 6 different stations (best DX 440km) and copying 3 different stations on OPERA Op4 mode. There is still a slight PA instability that needs to be fixed (probably decoupling changes), so there is a bit more optimisation to do.

472/500kHz transceive transverter

Rev C Transverter (one correction: o/p LPF cap 6n8, not 100n)

Armed with the sensitivity data in the last post I have now modified my MF transverter to add the receive parts, without a preamp. I used the same scheme adopted in my old transverter to automatically switch through the 500kHz (or 472kHz) signal on receive directly to the FT817. Only 4 additional parts were needed to make it into a TX/RX transverter.  As proof that the RX part is working I have just copied PA0WMR with a huge signal on OPERA Op4 with the signal passing through the transverter.

472/500kHz TX transverter - first schematic

10W 472/500kHz TX Transverter (CW, WSPR, OPERA, QRSS)

OK, here is the first schematic (subject to corrections, changes and improvements) of my simple TX transverter for 472 or 500kHz. This one needs no expensive parts and went together painlessly. The PA filter parts used the GW3UEP approach of winding thickish copper wire (1mm in my case) onto 22mm white PVC drain pipe pieces. The mixer, LO and squarer stage all use ubiquitous 2N3904 transistors (my favorite!) and the PA a cheap IRF510 FET that produces 10W RF without even getting warm.

Next stage is to add the RX preamp and filter and sort out RX-TX switching. I also want to try to go for a higher PA output power which will require changes to the output network.

500kHz tranverter (Mk 2) breadboarding

This weekend I've made considerable progress with the new transverter for 472 and 500kHz and fully breadboarded a 10W TX transverter.

My objectives are to make the design simpler than the original 2010 design  (no expensive SBL1 double balanced mixer!), produce more power and improve the RX performance. Also I want to reduce the need for special parts such as toroids.

The design evolving is, like all my designs, a mixture of my own ideas and the best from other people. When the design is ready I will put it here and on my main website. I've moved away from 28MHz IF (too high, too much start-up drift) and instead gone to 3.7MHz using a 3.2MHz crystal available from GQRP inexpensively. The TX mixer (none is needed on RX as it is uses the FT817 in split mode receiving directly on MF) is a simple bipolar stage followed by a low pass filter. For the PA I have largely copied GW3UEP's excellent designs for a 25 (and later 100W) amplifier. Initially I am using an IRF510 from 13.8V and getting around 10W RF out. The next stage is to look at the RX front end. I want a little gain ahead of the FT817 but with good large signal handling and decent filtering. The FT817 works OK at 500kHz but is a little deaf.

So, a satisfying few hours building.

500kHz WSPR

I left my 500kHz WSPR station running overnight last evening to see what reports I got. People are now generally using 500kHz USB dial for WSPR so that simultaneous monitoring on RX of both OPERA and WSPR is possible. Personally I can't tell much difference.

500kHz WSPR reports earlier (20mW ERP)

Anyway, my best reports were from F59706 (560km) and F5NWK (559km) when I was running around 15-20mW ERP from my current transverter and antenna.

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.

First 500kHz OPERA reports

Having managed to match my 6m long coax feed line (as a Marconi vertical) up to the 10/6m halo antenna on 500kHz by adding some inductance wound on a ferrite rod in the shack, I've just received my first OPERA beacon reports on the band from ON6EO at a distance of 220km and G0KTN at 210km. Reports were -30dB and -29dB S/N which is marginal.

OPERA mode on 500kHz

500kHz OPERA RX this afternoon (on left are my spots)

Having now uploaded the latest version of OPERA weak signal mode software I have been looking on 500kHz using my, as yet, untuned short vertical antenna. CPU load without waterfall is running at around 40-50% on my Dell Inspiron 630m and the PC is running in a stable way. So far copied PA0A, PA3ABK and M0FMT. If I can get the antenna resonated crudely I may try OPERA TX on 500kHz later this weekend.

472kHz designs

GW3UEP's website has some useful new designs for 25 and 100W transmitters for the new 472-479kHz band which is expected to become available Jan 1st 2013. These designs use very standard and easily available parts and are very simple to duplicate. They would be an ideal starting point for anyone wanting to get going on the new band in 7 months time.The designs are based on earlier ones for 500kHz.

Part of GW3UEP's simple TX for 472kHz

New LF Yahoo Group

After some debate, the old RSGB LF-reflector is now migrating to a new Yahoo group at http://uk.groups.yahoo.com/group/rsgb_lf_group/ . If you have an interest in the VLF/LF/MF amateur bands such as 8.97kHz, 136kHz, 472kHz and 500kHz then you may want to join. It is sure to become one of the most useful resources on the web for up-to-date information as well as a valuable archive of schematics, photos and ideas.

472-479kHz available Jan 1st 2013 in UK

G3XBM's 500kHz transverter. Ideal for 472-479kHz.

OFCOM are now going to renew NoVs for the 501-504kHz band for existing permit holders and this will be possible via an on-line webform on the RSGB website.

"Following the agreements concluded at World Radiocommunication Conference 2012, Ofcom has started discussions with the RSGB and others about the timescales for implementation of the new secondary allocation to the amateur service between 472 and 479 kHz which, it was agreed at the Conference, would be effective from 1 January 2013.
Ofcom has therefore determined that, to allow these discussions to be completed, the current NoVs to allow experimentation between 501 and 504 kHz should have their validity extended to 31 December 2012. This notice confirms this fact."

See http://www.rsgb.org/operating/novapp/500.php. So plenty of time to get an antenna up and a TX or transverter ready. You might like to start by looking at http://sites.google.com/site/g3xbmqrp/Home/500k where I describe my 5W QRP transverter which will work on 472-479kHz. I've plans to make a Mk2 version of this during the summer with higher power and other improvements. It would be nice to make a PCB this time so that others could build it.

472-479kHz secondary allocation

From the RSGB's website today comes the latest news from WRC-2012 about an amateur secondary allocation to replace 500kHz with something more permanent. It needs ratifying but it does now look very likely indeed to happen.

Agenda Item 1.23 – 500kHz
Progress through Committee 4 (COM4) was a little easier than expected and the frequency band 472-479kHz will be allocated to the amateur service, on a Secondary basis. This is subject to no further objections being received during the two final readings through the plenary meetings, of which the first ‘blue’ reading is expected to be during the plenary this coming Friday, late afternoon.

A 472-479kHz band?

Latest reports from the WRC2012 conference suggest the possibility of a secondary amateur allocation between 472-479kHz look "promising" although this hinges on a crucial meeting on Tuesday Feb 7th. Some countries remain against an amateur allocation, but the majority do support one, with caveats. A 7kHz wide worldwide allocation would be a good outcome, so keep fingers crossed for a few more days.

Slight progress towards a new 500kHz band

The latest reports from WRC2012 about a possible new allocation just below 500kHz sound slightly more hopeful than a few days ago with China and Russia softening their positions, but unfortunately the Arab group seems dead set against an allocation at all.  We'll have to wait until Working Group 4C (next level up) discusses it.  In the meantime let's keep our fingers crossed.

WRC-12 Radio Conference (Jan 23 - Feb 17)

This most important international frequency revision conference takes place Jan 23th - Feb 17th. As always, the amateur service can never be certain of the outcome with a possibility of precious spectrum being lost to other services. However, this time there is a good chance that we will GAIN a new amateur MF band somewhere between  415 and 526.5kHz. The most likely outcome is a band near 480kHz around 8kHz wide. 8kHz of spectrum would allow much more experimentation, especially if this allocation was world-wide. This would supersede the special permits around 500kHz allocated in some countries.