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A Simple QRP Transverter for 4m (70MHz)




Status Updated 14.12.11 

Added video links and emails from MI0BDZ (see bottom of page)

DONE!  It's finished for now, but definitely capable of being improved.

  • RX converter and LO worked at first attempt. Hearing GB3BAA at 89km.
  • TX mixer, drivers and MRF237 PA built (equivalent to the SD1127) plus LPF. About 1W pep out
  • Sorted out the RX/TX switching. In the end I did the FT817 mod to put TX 5V DC on the antenna pin - easy.
  • Boxed the whole unit in a diecast box
  • Tested on the bench with power meter and sig gen. and had on the air QSOs in SSB, AM and FM.
  • Simple wire dipole erected in the loft space aiming E-W.
  • Full schematic drawn (see below)
TO BE DONE
  • Make any improvements as a result of operational issues (add tuned circuit on SBL1 output to improve match and spectral purity).
  • Should consider TX/RX supply switching: currently only RF paths switched and DC supply to TX and RX on all the time.
  • Make some more QSOs and enjoy using it.

Best DX to date? G4RFR at 229km on CW

Introduction

  There is almost no commercial equipment for the 4m band and certainly almost nothing from the traditional Japanese manufacturers. So, to get on the band it's necessary for me to design and build a simple transverter. 4m is not a
difficult band to build for and I already did a transverter design some years ago for 6m, so a 4m one has been straightforward. However, I lacked test equipment for that part of the spectrum other than a
signal generator. My scope does not g o that high and I don't have a proper receiver for the band yet. So, some improvisation was needed to check the build as it progressed. I still need to check it on a spectrum analyser at a friends house.

This page talks through my experience of building the transverter and operating it on the band with QRP. Please check back in the coming weeks to see how the project and activity with it progresses.

The image left is the GB3BAA beacon 89km away being received on the transverter and loft mounted dipole viewing the beacon on Spectran.

Spec

  • All modes (i.e. a linear driver/PA)
  • 1W pep output
  • 0.1uV or better sensitivity on SSB
  • Minimum number of parts consistent with satisfactory operation
  • Reasonably easy to copy design using "dead bug" construction as long as people take the design as a starting point.

Transceiver Design

The transverter is 70MHz to 28MHz as on 10m I have various rigs that I can use. Also, it only requires a simple 3rd overtone crystal oscillator at 42MHz. Crystals are available from Spectrum Communications for £4 each plus post and packing. My aim with this design was simplicity: more complex designs may work better, but I hope this one will encourage you too to have a go at the 70MHz band. The OZ2M transverter design is, to my simple mind, a far too complex design to just get one going on the band, although I do not doubt this design works well.

Receiver down-converter

For the receiver the NE602 is suitable as an RF preamp and mixer. These are cheap and pack a lot in a small DIL8 package. A suitable design appeared in the latest edition of SPRAT down-converting to 20MHz. In terms of noise figure and large signal handling, I think they will be adequate, although a BF981 or similar may be better as an RF amp and mixer. The first parts assembled were this RX down-converter.  At switch-on it immediately worked copying the GB3BAA Tring beacon 89km away using my 10m halo as the (inefficient) antenna. Sensitivity on my generator looked fine. There is some out-of-band breakthrough as a result of the minimal selectivity in the front end and this may need to be addressed later with better input filtering, but has not been an operational issue yet.

Transverter (see picture at top of page for boxed transverter) with TX strip added

In the end I went for an SBL1 passive double balanced mixer as I've used these before. Any similar DBM would be suitable. This is followed by a BF199 pre-driver (probably a 2N3904 will do here), a ZTX327 driver (a small Ferranti/Zetex device ex PMR) and an MRF237 TO39 RF PA (also ex-PMR). The TX strip was aligned initially using a 70MHz drive signal from a signal generator with trimmers between each stage. When optimised, some of the trimmers were measured with an LC meter and replaced by fixed value caps.  This could also be done in the PA output PI filter too. Output is about 1W pep. More may be possible by optimising the PA load. Adding the TX mixer and driving from the 28MHz source the power was the same with just a minor change of position of the tuned circuit in the pre-driver output. The TX strip is both stable and linear although the bias resistor values may need optimising. keep the diodes used to bias the driver and PA in thermal contact with their respective transistors. A relay is used to switch RF paths between RX and TX and this is activated by a DC 5V available at the FT817 after a simple mod to the rig. This mod requires the addition of one wire from the TX 5V line, a 10nF capacitor and a 10k resistor. It is described visually on this site http://www.bergtag.de/download/ft817.pdf.

At present, the TX DC supply voltage remains on during RX and the RX voltage during TX. In due course I may address this with some DC switching too at a later date.

NEW Several people have asked me about L1.  L1 was a small 2 pin 4mm diameter coil with around 10turns of 0.2mm wire with an F29 core. It came from a PMR portable made by Philips. Its purpose is to ensure the crystal oscillates at 3rd harmonic on the right frequency. A technique that can be used is to remove the crystal (short it out) and make an LC tuned circuit (L1 and C1) that makes the "free running" oscillator oscillate at 42MHz. Then reconnect the crystal back in circuit and adjust either the L or C to bring back to 42MHz. It should be very close.

Click schematic to get full sized image

Antennas

For initial tests I put a wire dipole in the loft. A wire halo built in a similar way to my Homebase10 halo for 10m may be erected later.  In fact I may nest a 6m and 4m halo within the existing 10m halo. These antennas work well and are ideal for summer sporadic-E openings. Sporadic-E openings can be strong. With stations on 4m widely spaced across Europe an omni-directional antenna at a reasonable height should be a good choice. A small yagi and more power would help inter-UK QSOs but I have no plans for this and will be content with local QSOs when the band is dead.  Commercial yagis, HB9CV 2el beams and halos are available for the 4m band from several sources including Vine, Moonraker and Wimo to mention just 3. A 4m band halo is available from Waters and Stanton for under £30 when I last looked. In the short term, I am erecting a horizontal wire dipole in the loft.

Ideas from others

Dom M1KTA pointed me to http://www.mydarc.de/dk7zb/Transverter/4m_Transverter.pdf which in turn links to http://www.qsl.net/dl5dbm/50MHzTr/50MHz1.htm but that has broken links on it. The web archive pages still has this information at http://web.archive.org/web/20100901080709/http://www.qsl.net/dl5dbm/50MHzTr/50MHz1.htm

There is also a Yahoo group for a project called the Eden9, a club project design for a 4m 1W pep SSB transceiver from Ron Taylor G4GXO. See http://groups.yahoo.com/group/eden9/ .

4m (70MHz) Information

The best source of information about the band is at the Four Metres Website. Here there are tips on operating on the band and an up-to-date list of countries with access to the band. This list is growing rapidly with many more countries now having access to the 4m band.  See also the Wikipedia article on the 4m band.

4m WSPR

One thing that would be fun to try on the band is WSPR, a mode that I have used successfully on lower bands. I suspect that aircraft Doppler may make this less ideal on 4m though.

Some Builder Feedback

This was feedback from Mark MI0BDZ who has built a 4W and 45W derivative version of this transverter.



Hi Roger,

My name is Mark (mi0bdz) and I was browsing the net for 70mhz projects when I came upon your site. I could hardly believe the simplicity of the little 70mhz transverter you designed and built. I decided to copy it. I started on Friday afternoon after gathering the bits together and completed it by Saturday evening. I could not have done this without thanking you for sharing the project, which I class as the best usable project I have come across in many years.

I have found that by careful selection of the TX transistors and bias resistors that 3 watts is possible. I did have to alter the values of the bias resistors quite a bit, but that is all. Everything else is as your schematic. Thankyou again for a wonderful little project. This is a gem. The sensitivity is also excellent. You were right about packing a lot into a chip indeed. Thanks again Roger.

Regards

Mark
MI0BDZ


Hi Roger,

I have eventually been able to make two videos just to show the potential of your great little design. One with the 4 watt version and the other showing a 45 watt version. They give a little authority to your design, as I described earlier but with the alteration of the three pole filter section on your TX section now with 5 turns @ 6mm dia. and not 6 turns @ 6mm dia. as I stated previously. The bias values are good for those transistors and I was able to replicate the transverter twice as you can see with identical results of 4 watts plus with both before the addition of the 45 watt PA on one.

Regards

Mark
MI0BDZ

MI0BDZ's video

MI0BDZ's 2nd video








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