So, Why would you want one?

Well,

It's a great question - we're talking Spectrum Analysers here, by the way - my first offering would be, why not? Second would come, because I can.... but really, let me try and show you.

None of this has any real importance, but you can say that about anything, can't you?

I've just been aligning (read fiddling with) an SSB transmitter that I made a while ago. Now, part of the circuit mixes the BFO frequency (in my case it's about 12.9MHz) with the audio that's being injected. Under normal operation this would be from a mic, but for this exercise I'm using an audio signal generator and injecting a 1KHz tone.

The signals are mixed together in a simple diode based doubly balanced mixer, and the output contains a whole pile of stuff. There's the BFO frequency, the audio frequency, the sum, the difference and all sorts of other harmonics in this signal. On a scope, when the balanced modulator is un-balanced it looks like this:

As you can see the signal is a complete mess, and very hard to see what's going on. This was a single shot take on the signal as there is so much that the 'scope cant really decide what to trigger on.

Here's the same signal on the Spectrum Analyser:

Now, this makes a whole pile of sense. The left hand peak is the LSB, it's exactly 1KHz (the audio frequency) below the next peak which is the carrier or BFO (remember I said the one of the things in the output was the difference between the carrier and Audio frequency). The right hand peak is the USB - this is the sum of the carrier and the Audio frequency. What we actually have here is an AM transmission - a carrier and two sidebands.

So it's very clear here - this part of the circuit contains some components to suppress the carrier (remove it) but the circuit needs to be balanced before this will happen correctly. Using the adjustment available and looking at the Spectrum Analyser I can adjust to this:

So, I can clearly see that the carrier is now well down into the noise. The next part of the transmitter chain is the crystal filter, that will suppress the unwanted LSB to create SSB as what we have here is DSB (double side band). Once again, using a Spectrum Analyser, I can watch the RF output and alter the BFO to place the USB inside the filter passband and the LSB well down the skirt of the filter. Only then will we have what we are trying to create; Single Sideband (SSB).

Simple, egh?

Why is this just so insane?

Well, in a recent post I mentioned that the 2m/70cm beam antenna here had decided that once horizontal it didn't want to work so well. It was OK when I used it for satellite operation at 28 degrees, but when we put it back to horizontal the SWR went through the roof. I managed to take the head of one of the grub screws that allow you to adjust the gamma match - so that wasn't too handy either. Anyhow, as an antenna for conventional SSB working it's not going to be much cop, as it's a real compromise for both bands.... so......

I bought a 7 element ZL special from moonraker for 144MHz:

http://www.moonraker.eu/

and it duly arrived a few days later.

Now I am no real antenna expert; my understanding of antennas is that you attach a piece of wire to the inside of a length of coax, place the wire on something nearby that's handy - maybe a hedge or something - then you work the world on HF.... not so on VHF!

There were two confusing aspects to this antenna for me; firstly there are two driven elements (which are actually folded dipoles) and the directors (elements) are not ordered smallest to biggest from the front of the antenna to the dipoles. Hmmmm.....

The two dipoles are fed at 180 degrees out of phase with each other, here you can see the main feedpoint which contains the coax from the radio, a piece of 300 ohm ribbon cable which feeds the other folded dipole and a tuning "stub" - this is in effect an open length of RG58:

These two folded dipoles are sightly different sizes, presumably this offers a broader bandwidth to the antenna tuning point.

Having struggled the pole back down, me and the ever patient Vince G0ORC, we had the new antenna mounted ready to go back up in the air:

You can see the 6m beam and also the new 2m ZL special:

We tuned the antenna on the ground; you chop small bits of the end of the open "stub" for best SWR. It seems to be quite straightforward, once up in the air the SWR was perfect in the SSB portion of the 2m band:

So, the antenna is back in the air, the SWR is near perfect and we could hear PI7CIS all the way from JO22, the directivity of the antenna seems quite sharp, so something is working OK!

Fun, egh?

BITXing Some More

I've just about finished the other bits and bobs for the BITX I was aligning yesterday. I've made a digital dial for the frequency readout and an s-meter; the readout is a kit I have had here for ages, it came from QRP Kits:

http://www.qrpkits.com

The build using surface mount components is really tricky, but it works OK second try!


1st time through I'd got some pins on the main microprocessor chip shorted out which was screwing up the multiplexing of the counter display. You really need to use a magnifying glass to see this stuff properly. Soldering it is another game again! I can't get the camera to take a picture any closer, so you'll just have to believe me when I say the components are small!

I've got all the bits of the BITX V3 wired together on the bench here (the mouse is not an essential part of the BITX):



Now, in a nutshell, what you should be able to see below is exactly what I love about ham radio, you can see the mess above which is all my own work, I turned it on, made a slight circuit modification (connected the speaker) and this is what I soon came across. There's only a piece of wire rammed in the antenna connector right now:

Not bad, egh????

RadCom Dec 2010 SSB Exciter

I saw the SSB exciter in the Hombrew column of RadCom Dec 2010 and thought I'd have a go at building it. I had a rummage in my components and found a 9MHz Crystal Filter and two suitable oscilator crystals; 8.9985 and 9.0015, so this seemed like a good starting point.

The first item I built was the Mic amplifier:

I built this using vero board; the output seems quite clean and tidy, and I make the gain about 18dB:


I've built the main part of the exciter and I'm seeing OK carrier suppression through the balanced modulator, however I think this can be better. I've used some BAT85 diodes in the modulator, but I didn't match them first, so I think I need to match some more and see if I can suppress the carrier some more. This image is taken from the spectrum analyser with a 1.5KHz tone injected into the mic amplifier. We should expect to see the carrier plus a signal at +1.5KHz and -1.5KHz (they are the upper and lower sidebands). The balanced modulator should be adjusted for minimum carrier at the output:



The Upper and Lower sidebands aren't quite the same amplitude for some reason, but the carrier is some 30dB down in the noise, so that will do for now. The output of the Crystal filter looks like this with USB selected:



And like this with LSB selected:



So, I need to try some different diodes in the balanced modulator, then I need to try a range of audio frequencies to try and figure out where the skirt of the crystal filter is to ensure that the carrier and the unwanted sideband are set in the correct place for maximum rejection.

Here's the board as-is:



So once I'm happy with the SSB exciter I need to mix this RF output with a VFO and try and get a signal on the Air!

I'll post more when I've figured out the best band to aim at.

Mark.