A disk-what? Ah, a Discone!

Well,

Today has been a day of antenna fiddling. The first thing I did was put up a Discone antenna on the front of the house:

This is supposed to be a general coverage receive antenna from 25MHz to over 1.3GHz plus it is resonant for transmit on the Amateur 6M, 2M, 70cm and 23cm bands.

Here's what it looks like connected to the return loss bridge and Spectrum Analyser:

The markers at the bottom of the image above show the resonant points we are interested in and it does indeed seem to be a reasonable match to 50R at those frequencies (as is a 50R resistor, of course). How good an antenna it is remains to be seen. I am hoping this will hear more of LEO satellites than the collinear I have.

We have had some more really bad weather over the last few days and very much by luck I found a broken jubilee clip (Read hose clamp) in the garden yesterday. Looking up I could see it was off the 4M and 6M beam antenna I installed back here:

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

So I have had the antennas down again today, it's surprising how big my "mini beam" is when it's on the ground:

You can see that I have to remove the outer parts of the one element to get it past the fence next to the raised border in the garden! But I have replaced the clip and also altered the resonant point of the 4M section to be exactly at the Meteor Scatter frequency of 70.110MHz.

After adjustment the "dip" is now ~70.12MHz with a return loss of > 32dB ; the SWR at 70.11 (my target) is 1.05:1 and across the band is no more than 1.7:1.

and at 50.23MHz the SWR is a respectable 1.2:1:

Looks pretty damn fine to me!

Cat's been next to useless:

Fun, egh?

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?

That'll Never Work - Will it?

Well,

The VHF antennas here have always been somewhat of a compromise. Recently I moved my 4M or 70MHz antenna from the side of the house and put it on top of the HF antenna on the main mast. I also took down and filed (in the hedge) a 6M or 50MHz antenna. I saw this a while ago and wondered... could this really work:

InnovAntennas 4 & 4 ele

So, you guessed it, I bought one.

Here's the antenna as it starts to go together:

As you can see, there are 4 elements for 6M and 4 elements for 4M on the same boom. The thing that I didn't (and still don't) understand is how the thing works electrically as the 4M elements are all parasitic (there's no wires connected to them from the RF supply). The antenna has a direct coax connection without any kind of connector:

There's plenty of self amalgamating tape under the white PVC tape you can see, the connections themselves were then coated with Copper grease. The antenna also needs a coax wound balun - this is preventing me from feeding the antenna directly with Westflex 103 - it's just too thick to wind tight enough. I've had to connect a length of RG213 to the antenna, use that for the balun then have a join to the 103 close to the boom.

I then brought the tower down and changed the antenna on the top for this one:

While the antenna was in this position I attached the analyser thingamabob; the 70MHz resonance looked really good, but the 50MHz resonant point was more like 55MHz.... hmm I thought, perhaps I should raise the antenna and see.... no change at all.

I scratched my head for a while and re-checked all the dimensions - they seemed correct to me, here's the diagram I was following:

Suddenly I realised that the element centres that are labeled as 1800mm on the diagram are only 1500mm on the antenna! A quick email to InnovAntennas confirmed that I had indeed got the wrong parts.

The antennas are back in the air:

But the 6M or 50MHz elements on the top antenna are currently exactly 300mm too short.

Useless, hugh?

Is this what I was waiting for?

Well,

I'm still in A71 land, but I have been checking my WSPR beacon that I left running on 4M from time to time. I mentioned about it here:

http://g0mgx.blogspot.co.uk/2013/05/movements-are-afoot-its-all-go-you-know.html

But here's what I found this time:

So my 20 watts on 4M has made it all the way to PA0O - clearly there has been some propagation while I am away (typical).

Good, egh?

Mixing Better than Before (70MHz)

Well, following on from my efforts at a 70MHz "transmit converter" here:

http://g0mgx.blogspot.co.uk/2012/06/even-more-4m-or-70mhz.html

I've made some of the changes I said I would do. The main two things I have done are re-tune the local oscillator for the 60MHz harmonic rather than the 80MHz harmonic that I was using previously and also re-made the board thinking about layout and symmetry where required:

So now I have a 60MHz oscillator mixed with my 10MHz frequency reference I made here:

http://g0mgx.blogspot.co.uk/2011/07/ocxo-oh-you-mean-ocxo.html

to make 60 + 10 = 70MHz signal. As you can see from the output on the spectrum analyser, the signal is now really much cleaner than before, even the 60MHz oscillator is at least 45dB down. I always like to remember here that 6dB is half voltage, so 45dB down from the 70MHz signal is a really good result.

So what I need to do now is swap the 10MHz frequency reference for the DDS from here:

http://g0mgx.blogspot.co.uk/2012/06/its-been-ages.html

and check that the bandpass filter at the output from here is wide enough for the parts of the 4M or 70MHz band that I am interested in.

The theory here is that a 10.0 to 10.5 MHz input signal should give me a 70.0 to 70.5 MHz output signal - if (and it's quite a big if) the bandpass filter will let that range of frequencies through.

Once thats all sorted I need to think about an RF amplifier for the output and an attenuator for the 10MHz input which eventually will come from one of my HF radios. The RF amplifier will be designed around an MRF151 power FET (because I have some), so that's something to start the next round of fiddling with, I also have a neat 48V switched mode PSU that should do nicely, now maybe I can make something for 6M also....
I even managed to be here and work someone on 4M the other day using the modified FT-847; it was SP6GWB, Stainslaw. He's got a really neat site here:

www.sp6gwb.pl

but thanks for the QSO! My first on 4M....

More madness to follow I'm sure, but it's good though, egh?

Even More 4M or 70MHz

I've been fiddling in the shack today with an oscilator I started back here:

http://g0mgx.blogspot.co.uk/2012/06/4m-70mhz-and-counting.html

It's basically an 20MHz crystal oscilator that's multiplied up to 80MHz. I've then fed that into a mixer just like the one used in the BITX here:

http://g0mgx.blogspot.co.uk/2010/12/bitxing.html

And mixed it with the 10MHz reference oscilator I made here:

http://g0mgx.blogspot.co.uk/2011/07/ocxo-oh-you-mean-ocxo.html

The idea being that 80MHz - 10MHz = 70MHz.

There are a couple of amplifier stages in there followed by a (my attempt at) band pass fiter. The iportant thing with mixers is that you get the sum, the difference and all sorts of other harmonic products out, so you have to really filter out the one you want else you just get a real mess!

It all looks rather badly built and like this:

But the output on the spectrum analyser looks like this:

So you can see that the 70MHz signal is the main "feature" in the ouput (It's 10MHz per cm from left to right where the far LHS peak is 0MHz and it's also 10db per cm vertically).

I think I will re-make this with consideration for all the bits and where they need to be; then I'll think about seeing if this will make a transmit converter for 4M.

Good fun, egh?

FT-847 PA Mod - there's a better way!

Well,

Following the mod of the PA filter in my FT-847 here:

http://g0mgx.blogspot.co.uk/2012/06/ft-847-that-i-have-here-includes-70mhz.html

I've been thinking some more about this; given that the PA is capable of delivering 100W on 6M or 50MHz, it wasn't clear to me why I Was struggling to get more than about 60W out on 4M or 70MHz. There had to be some very large inefficiencies in there; then I discovered this link:

http://www.70mhz.org/847filters2.pdf

And all became very clear, very quickly! I don't know what package Marc used, but he has done a fine job. Once again the FT-847 came apart and I kept removing bits until I had discovered the PA board:

Now, to make the mod described by PA1O, Marc, I need to lift the PA board out. This was a bit scary as I had to remove all the screws holding the MOSFETs onto the heatsync and then very gently flip the board over. Once upside down, I could clearly see the two caps described in the article:

Then with rather a lot of brute force and ignorance I managed to get the one cap removed; it's been stuck down rather firmly so it kind of came off in bits with no hope of ever putting it back again!

So, after carefully putting it all back together (including a dab or two of heatsync compound), the radio is back together. I've removed the ferrite I added when I did the previous mod:

http://g0mgx.blogspot.co.uk/2012/06/ft-847-that-i-have-here-includes-70mhz.html

And I now have a full 85 - 90 Watts out on 70MHz with about 16A current draw and still have 100W output n 50MHz with a reduced current draw of 17A (it was about 21A); so I think we can conclude that efficiency is improved.

I can't see any negative side effects of this modification to the PA, the output seems stable on all bands but I'll be sure to let you know if I find out differently.

I'm going to look at the output spectra a some point in the near future to make sure the signal is nice and clean and all harmonics are suitable suppressed.

Cat's not impressed:

Good though, egh?

FT-847 Pre-Amp Installation

So, now that I have built and done the basic DC test on the new FT-847 pre-amp last time:

http://g0mgx.blogspot.co.uk/2012/06/ft-847-4m-70mhz-pre-amp-mod.html

It's time to install the board in the Radio. This looks to be a fairly straightforward operation, replacing one board for another.

Here is the old "daughter" board inside the radio just prior to removal:

And this is the gap - this picture was taken to ensure I could remember where the old board wired into the main board - re-fitting being the reverse of removal!

So here is the new daughter board which includes the 70MHz pre-amp just being tacked into place:

And here with the wiring complete:

All I need to do now is find a suitable signal and peak the variable inductor for maximum smoke; that should be a doddle. I can hear the relays operating as the tuning range enters and leaves the operating frequencies, so something is working as planned.

Not bad, egh?