Enigma Amplifier for 70MHz

 

My Greek 70MHz amplifier died a few months back and after a rebuild and a new transistor, I was unable to keep it stable so it got consigned to the junk/scrap box.  Around Xmas, someone mentioned that Enigma Components in London were producing medium power amplifiers for 144MHz and when I looked on their website, they had also produced one for 70MHz.  It uses the MRFE6VP6300 device, so 300W output, no problem with bad VSWR and a reasonable price.  The amp is supplied in class C so suitable for FM but Nick at Enigma confirmed it could be put into class A by adjusting the bias potentiometer.  The unit comes fully assembled and with detailed instructions on how to set up etc.

 

They supply the board mounted onto a heatsink so this turned into a boxing exercise, shown in the following pictures.  I used another Perancea tinplate case, same as my other amplifiers.  This is smaller than the heatsink so I used 3 pieces of ally angle to hold it in place, the holes are drilled and tapped into the heatsink.  A similar arrangement holds the fan on the top.

On the Enigma website there is a link to the 144MHz version built by Dave G4FRE and he recommended cooling of the output filter.  So I have put a couple of small fans on the end wall to blow a little bit of air around.

Bias and fan power are fed into the case via two (colour coded) phonos.  The main fan supply is then fed out via an SMA - complete overkill

I think the only negative with the Enigma is the output connection, it’s not easy to get a really short-lead RF connection onto the pcb.  At 70MHz it’s not too critical so I used a 25mm long piece of UR58 inner for the RF.  A short length of braid and a solder tag link the groundplane to the case and the outer of the output N-type.

The amp is designed for the bias to come off the 50V rail which I’m not too keen on so I cut the tracks and removed the associated Zener.  Now my externally switched 12V bias feeds straight to the on-board 5V regulator.  As supplied the bias was set around 0.6V but it was easy to readjust to the 2.5V required.

As usual, first switch on was using my current regulated power supply set to 30V and a limit of 1A.  I brought the bias volts up and set the current to 500mA – all ok.  Then I tried a sniff of RF and got a few watts at the output – more importantly the amp was nice and stable.  Then I slowly wound everything up, setting the bias current to 1A.  Eventually, the amp was peaking at around 300W out, the transistor hardly gets warm so I’m rather pleased!

Drive is 300mW, output is 300W, power input is 50V @ 8.45A = 71% efficiency

2021 Lockdown contesting

 For the 432MHz AFS contest on 7 February, I operated for the first time from home.  

Mast on the drive with 28 element M2

  

The joys of living in a three story house - this is the view looking NE from the bedroom window and with the mast retracted

Finally, the operating position next to the front door.  Feeder and rotator go out via the letterbox

And after all that, I won a certificate - leading fixed station running 25W or less

New mast arrives

 

After a few years of pushing up my antennas on two scaffold poles, I decided it was time to get a mast to make setting up easier - so an MM0CUG trailer mast was bought during the summer.  It's got a manual winch to take the mast to vertical and an electric winch for going up.  I spent some time sorting out a big storage box on the front, this contains my Honda generator, battery for the winch, mains cable and feeder.

                                      

That's Melody cat sat on the trailer!!                   First time out, the mast is vertical, honest!!                                               

I also changed my car at the same time.  After contemplating various cars that would also be suitable for contesting, Helen spotted that the Peugeot 3008 has a fold-flat front passenger seat.  So I sit in the back seat, the rig/transverters go on the front, with the logging laptop on a little table in front of me, the second laptop for KST on the other back seat and the amplifier in the boot

Picture gallery

73 de G4HGT

    

Last time out at IO93EV, 21 May UKAC - coming next - a new car and a trailer mast!!

                                                                                                                        

G4ZAP/p receiving the Surrey Trophy for winning VHF NFD 2018

Tony G4NBS, John G4HGT, RSGB president, Paul G0SPA, John G3UUT

                      

G4VVZ arriving with Versatower, JO01PU

                      

G3UUT attaching 12ele onto van mast

A view of G0VHF/G4ZAP/P, VHFNFD 2019

G0VHF/p:  4x  28 ele on 432MHz and 8 x 55ele on 1296MHz

                                                                                         

Low powers 2017, Sheffield IO93EH

G4HGT, G4VVZ, G4NBS, G3UUT (photo: M0GAV)

2 x 11ele DK7ZB, 144MHz built by John G0JLF

Site photos

Over the last couple of years, I've used three sites for portable operation: IO93BV, Ilkley Moor; IO93DV, Otley Chevin and IO93EV, near Bramhope.


I started out on DV, it's the local pub car park, good for beer but too busy for Thursday evenings and Sundays.  






































Early low power operations - no rotator, power from car battery, manual rotation, old car



Most of my operation has taken place on Ilkley Moor, BV, but this is rather exposed and permanently breezy or windy -  getting the mast vertical can be a struggle.  
                                                       

IO93BV looking west

Looking east from IO93BV, the ground rises about 10m to the east. The dip to the left of the fence is ESE

For some strange reason, I don't have any pictures on Ilkley Moor looking south or south east

So last summer I found a site close to home in EV - nowhere near as high but usable in all weathers and two minutes drive home.  I'll never win anything from here but it's useful for testing:

Looking southeast from IO93EV

A little bit of history

Most readers will be aware of the VHF/UHF DX Book, put together by Ian GM3SEK and recently made available in PDF format:

http://www.ifwtech.co.uk/g3sek/dx-book/dxbk-new.htm

I contributed the 432MHz transverter section, which worked fine for me but this was probably because I had access to signal generators and spectrum analysers, not readily available in most shacks at the time.  At the time, my qth was suburban Leeds and rather poor, output power was around 10W and noise figure was probably 3-4dB due to cable loss and no mast head preamp.  However, I worked around 60 squares, all tropo and I think we used it a few times with G4ZAP/P during the low powers from Sheffield.  After publication, I swapped a set of boards with Conrad PA5Y, receiving a bright yellow drum kit in return!!  No-one else contacted me about copying the design.

Last year, I decided on an upgrade, pulled out the HGT unit, replacing it with a DB6NT module and a new Mitsubishi PA module 

50V PSU for SSPA

I decided that a bit of overkill / future proofing was needed for the PSU required for my SSPA.   There are many different PSU manufacturers and specs but I discovered that Kuhne sell the Meanwell units.  If they are good enough for DB6NT, it’s good enough for me!

So I have used the RSP-1500-48V supply.  There’s nothing much to say about this part of the design; it just squeezes into a 2U 19” rack case, the lid needs a push to get the fixing screws in place and the psu is positioned under the case vent holes.  The 50V output is fed via a 50A fsd panel meter – I couldn’t find a 30A off-the-shelf meter and sorting out a beefy current shunt was going to take time so I just went for the 50A unit from Farnell.

The 50V terminals on the back panel are just 25A rated 4mm terminals and I use two red and two black in parallel.  The internal wiring is **mm **A rated cable used for house wiring, rather overkill but it won’t fail if the output was to be shorted together.

On the front panel are a couple of LEDs in red holders.  These are wired in series and in series with a pair of 20V Zener diodes then connected to the 50V supply.

The regulator / control circuitry Is built onto the Veroboard.  I need a 12V 500mA supply for the amplifier fan and a switchable 12V or so for the bias output.  These outputs are produced off the 50V supply.  The usual 7812 or LM317 type regulators are not designed for 50V inputs, so I used a series pass transistor to do a first reduce to 28V which might be a useful supply for 28V RF relays.  I used a *** transistor here and it needs to be on a heatsink (it is continuously dissipating 22V and 300mA).  If I ever use this supply for a 28V relay, it will probably need a bigger heatsink.  Then there are two regulators for fan and bias, again these are both on small heatsinks.  Finally, the bias is switched by a simple NPN controlled by the PTT input.

Noise from the 50V line

  

A few weeks ago, I had everything set up to do a few checks on the amplifier.  I discovered that my receiver noise floor rose by a few dB when I turned on the 50V supply.  This was with no antenna connected, the SSPA was to be checked into a dummy load.  When I’m out portable, everything is plugged together and then switched on so I simply hadn’t noticed this.  Although both my 50V psu and 12V psu were connected via earth terminals, I found the noise would appear when I connected the outer of the coax between my transverter and the SSPA.  This noise happened when the SSPA is in standby i.e. no current being drawn.  So this looked like a conducted emissions problem on the output side.  I found a basic generic filter design on an old Meanwell data sheet and a quick filter was constructed.

I had no idea about the spec of the toroid I found in my junk box so I wound 10 turns round it and measured the inductance – it turned out to be an Amidon FT-140-43.  Using fairly thick wire (2 x 1.5mm²) I could get 5+5 turns on the core, giving roughly 20uH.  For the capacitors, I thought about using low ESR units but didn’t have any to hand (and I don’t know if you can get them with 63V or 100V rating) so I just used what was to hand.  I also installed a small earth plate which is connected to mains as it enters the case; it is also connected to the PSU metalwork with a wide strap (UR43 braid).

Once the re-building was completed, I put my station back together, connected up the 50V supply and switched on – no noise!  Hopefully that should help pulling weak signals out, I will find out in a few weeks’ time.