Which one do you trust?

Well,

The linear that I started way back here:

http://g0mgx.blogspot.co.uk/2013/11/now-that-smoke-has-settled.html

is almost complete, I've done my best to calibrate the input power detection circuity and will certainly "trip" into an alarm state if I overdrive the amplifier brick. I have also calibrated a simple directional coupler on the output to give me both forward and reflected power indication.

I needed to understand the match that my antenna gives me on 4M, but the Antenna Analyser that I bought some months ago only goes as far as 50MHz and the other MFJ unit that I share with Vince, G0ORC seems to have a gap in the tuning at exactly the 70MHz band.

So, I used the return loss bridge from here:

http://g0mgx.blogspot.co.uk/2013/09/return-loss-bridge-where-does-it-lead.html

and explained how to use it for SWR measurement back here:

http://g0mgx.blogspot.co.uk/2013/10/so-lets-get-bit-smarter.html

So, hooking the antenna from here:

http://g0mgx.blogspot.co.uk/2013/05/thatll-never-work-will-it.html

we see this:

and focusing right on the 70MHz allocation only:

So, whilst it's not perfect, the antenna is a good match across the whole band. As the MS frequency it looks to be 1.3:1 - which is more than acceptable.

I need to introduce you to two new residents at G0MGX, they are Mother and Son, please meet Pepper and Freddie:

 

Fun, egh?

Honestly? It's a mess!

Well,

I've been boxing the components for the 70MHz linear amplifier that I have been making:

http://g0mgx.blogspot.co.uk/2013/12/testing-times-for-my-linear-project.html

today. The control software is still very much under development, but in my usual build a bit, test a bit fashion, I need everything to be wired to the Arduino processor board now so I can make progress.

All in all, here's the list of separate bits I've ended up with:

• Linear brick and heatsync
• 48V SMPS Power Supply
• 9-0-9 transformer for DC supplies
• Fan for cooling
• Veroboard containing the ancillary electronics for the Arduino relay and fan control plus the DC regulators for the +24, +12 and +9 rails
• Low pass output filter
• Arduino
• LCD display
• Directional coupler for output forward and reflected power reading
• **huge** coax relay for the output switching
• input attenuator
• small coax relay for input switching
• Current shunt

So all together there is a lot more than I bargained for!

Here's what it looks like today:

and I have to confess that this is one big mess that I hadn't set out to make. It should work OK, but the "junk box" style of part sourcing has resulted in some really bigger than needed bits....

Florrie cat's not been helping much today:

 

Progress, though, egh?

Attenuator - Take II

Well,

I popped down into the Big City this morning and called at R F Potts. A wonderful electronics outlet that seem to stock anything an everything. I came away with a pile of 2W carbon resistors of suitable values that I can parallel up and make my attenuator (again).

Here's the take II model:

And the attenuation I am seeing at 70MHz:

Which looks very much like 7.71dB - much closer to my target of 7dB than I managed last time!

Also the return loss of the attenuator shows a good input and output impedance at 50R:

So, all in all, this is looking much better!

I need to progress the metal work for the amplifier now, then I can start to fit relays and other such gubbins and get the software progressing in my usual, write a bit, test a bit way.

Good, egh?

More Mixing to 4M

Well,

Been fiddling some more in the shack today with the 70MHz linear amplifier that I am building. The first total screw up was that I need to make an input attenuator so I can drive with a 10W ish input signal and reduce it to about 2W to the amplifier module.

I used an on-line attenuator calculator to design the pad and purchased some suitably power rated components for the job. I soldered it together and then measured the attenuation with the Spectrum Analyser, here with a single fixed frequency output from the tracking generator:

Now, this looked very much like it was about a mile from my target value of attenuation of 7dB and I seen to have more like 20dB at 70MHz. This didn't make much sense.

So I stuck it on my return loss bridge I made back here:

http://g0mgx.blogspot.co.uk/2013/09/so-whats-happened-to-bridge-then.html

to see what the input match to 50R was. Here's what I saw:

Now, this is telling me that at 50R impedance the input SWR to my attenuator at 70MHz is 23:1 (ish) i.e. a complete load of dingos kidneys.

If I just connect a 20dB attenuator (much lower power rated) I made a while ago, I see this:

This one was only really built with HF In mind, but still the SWR at 70MHz is a respectable 1.2:1.

So - why is this attenuator I have made so pants? I now realise that the resistors I bought to make the attenuator are wire-wound rather than carbon. So we have buckets of inductance as well as resistance. That just won't do at all!

Whilst I was messing about with the attenuator I decided it would be a good idea to box the 70MHz TX converter I started to make here:

http://g0mgx.blogspot.co.uk/2012/07/mixing-better-than-before-70mhz.html

and then revisited recently here:

http://g0mgx.blogspot.co.uk/2013/12/testing-times-for-my-linear-project.html

Well, I made an output amplifier for it to raise the output level a bit and have boxed it with it's own PSU:

Clearly this should be built into a metal case but I only have this plastic one which will have to do. I must be aware of external RF when I am using it that may make the output wobble somewhat:

The output looks OK on the scope:

but if I were to use this as a TX converter and put the signal on-air I clearly need another low pass filter post the output amplifier I added as the harmonics are quite high:

So, another box for the shelf and a failed attenuator build.

You win some, you lose some.

Testing Times for my Linear Project!

Well,

You may recall the 50/70MHz linear that I last reported on back here:

http://g0mgx.blogspot.co.uk/2013/12/70mhz-linear-progress-is-steady.html

The first thing I need to do is to test the main amplifier block to confirm all working as planned. Now, to do this I need a fairly accurate 70MHz signal to amplify - the problem is that the signal generator I made back here:

http://g0mgx.blogspot.co.uk/2013/10/so-hows-sig-gen.html

doesn't go that high.

Some of you may recall when I first started meddling with 70MHz that I made an experimental 70MHz transmit converter to take a 10MHz signal and mix it up to 70MHz back here:

http://g0mgx.blogspot.co.uk/2012/07/mixing-better-than-before-70mhz.html

So I have dug that board out again, dusted it off and straightened a few bits that had got bent in the draw and stuck some power on it. I have the signal generator at 10MHz connected to the RF in port:

So in the picture above we have a 60MHz local oscillator and associated amp and filter along the top of the board, the connector on the RHS has the 10MHz signal from the signal generator then there is a diode doubly balanced mixer feeding another amp and output filter along the bottom. The output of this looks like this on the Spectrum Analyser:

And here on the 'scope:

It's important to understand that the Spectrum Analyser is a 50 ohm instrument - the 'scope isn't - hence the 'scope is terminated in a 50 ohm feed through you can see in the picture below:

So, using the data from the 'scope (which I suspect isn't too accurate at 70MHz) we can do some maths to calculate the power out of this concoction:

So, using the peak-to-peak voltage reading from the 'scope I get a power reading of about 1dBm. So, let's now connect the signal to the accurate power meter I built back here:

http://g0mgx.blogspot.co.uk/2012/11/the-art-of-calibration.html

And this is what I see (apart from a stupidly bright power light):

and if I set the reference level on the Spectrum Analyser to 10dBm this is what the signal looks like:

So it looks very much like a 0dBm signal at 70MHz to me.

All this test gear is really coming together- I trust the dBm meter I made very much - a great deal of effort went into calibrating it and there is no reason it should deviate from it's set up.

So now I need to hook up some power to the amplifier brick, terminate it through a power meter to a dummy load, throw the switch and see what happens.... watch this space.

Good, egh?

70MHz Linear - Progress is Steady!

Well,

You may recall the Linear Amplifier I mentioned I was going to make back here:

http://g0mgx.blogspot.co.uk/2013/11/now-that-smoke-has-settled.html

And I was testing the Low Pass Filter for the output stage here:

http://g0mgx.blogspot.co.uk/2013/12/low-pass-looks-more-wobbly-to-me.html

Well, I've completed the main amplifier module and it's mounted on a suitable aluminium pallet which in turn is mounted on a heatsync.

Now that I have this main part of the linear, I can begin to build the peripheral components in my favourite way; build a bit, test a bit.

Watch this space.

Low Pass - Looks More Wobbly to me!

Well,

You may recall the basic ideas for a 4 and 6M linear that I discussed back here:

http://g0mgx.blogspot.co.uk/2013/11/now-that-smoke-has-settled.html

I've started to fiddle with the software and have most of the basics working. The software will just about control everything in the amplifier including cooling and alarms. The most important alarm will be the over drive - I suspect even with input attenuation I am going to find it very easy to overdrive the amp.

I started a few days ago to make the low pass filter. My first attempt looked kind of OK and used air cored inductors, but when I hooked it up to the spectrum analyser and tracking generator I didn't like what I saw at all:

The overall response wasn't very flat and at the target frequency I had quite a bit of attenuation. As a consequence and thinking it was my inductors that were suspect I ordered some T50-10 torroids as these would be good at 70MHz and also would create the required inductances with a reasonable number of turns.

The Low Pass Filter design has evolved a little bit and now looks like this in theory:

So after the T50-10s duly arrived a re-wound all the inductors only to find exactly the same result. Hmmm.

So, then I tried:

• Winding Air Cores with a smaller inside diameter
• Reducing the capacitor values and adding trimmers so I could fiddle with the filter
• Going for a curry

None of the above helped much, the results were still rather rubbish at my target frequency i.e. I had buckets of attenuation where I didn't want any.

Then I tried using some different test leads:

Conclusion? The test leads I was using are spectacularly rubbish at 70MHz - Aaarrrghhhh!!!!

I do have a slight bit of attenuation at my target frequency - the slope is starting just a little too soon, and I need to decrease the capacitors either side of the center inductor a tad.

So here's the filter as built right now:

So I can only conclude that the first attempt at the filter was fine, I just didn't know! The moral of this story is to always connect the test leads from the Tracking Generator output to the Spectrum Analyser input without the circuit under test in the way to ensure you have a flat line - do this first!

Here's the ham cat:

Annoying, egh?

Now that the smoke has settled

Well,

You may remember the mishap I had with my FT-847 back here:

http://g0mgx.blogspot.co.uk/2013/11/my-ft847s-got-no-nose.html

It's been repaired, and as predicted I had melted some of the components in the 4/6M low pass filter. I had also burned some of the PCB tracks - I got lucky really as the PA itself survived.

To further my adventures on 4M Meteor Scatter I think I need to find a better way of generating some high power RF without running the 847 at maximum bananas. Hence a new linear project has been born.

I've scribbled down my thoughts over the design, I want to include some software based control and alarms so I can watch out for:

• To much drive
• High SWR
• PSU problems
• Over temperature

It would also be good to include a software based power measurement and SWR reading on the output.

Here's the rough starter design:

I've stuck the appropriate numbers into Elsie and have the Low Pass Filter design also:

I've ordered a 300W 50-75 MHz RF module (no PSU, heatsink or cooling) so I'll start to think about the software for the linear controller over the next week or so - it will be Arduino based for sure.

There was a lovely UK morning captured on Antenna Cam here today:

Good, egh?