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Saturday, 19 May 2012

More WSPRing and QRSSing


After my last ramblings here:

I've been doing some travelling with work, so progress is not so good. However, I have entered into a dialogue with a number of folk over my observation that the amplitude of the DSS reduces as the frequency increases. Some suggestions to overcome this mainly include AGC type circuitry or utilising pin 12 of the AD9851 to feedback a voltage to set the output amplitude based on a sample of the output (recursion see recursion). The most interesting feedback that I got, however, was not that the amplitude was dropping with an increase in frequency, but that my scope wasn't reading the signals correctly. The theory being that as the frequency increases my scope sensitivity drops off....

I ended up plotting this graph:

This is implying that my scope starts to take a dive, accuracy wise, after about 1MHz, this is the blue line with the y-axis indicating how many dB my scope is "deaf" by. The yellow line is the same signal but through the on-board low pass filter from the DDS module. This seems to imply that the LPF is rather badly designed also. If this theory is correct I need to subtract my scope deafness from the drop in signal out of the LPF. If at this stage we remember that 6dB is half voltage, this is not insignificant!

All rather confusing? My scope is a 100MHz rated fluke which I thought was supposed to be a good quality instrument. So, is this behaviour typical or is my scope a pile of dingos kidneys? I wonder...

I've made a bit of progress boxing the WSPR and QRSS beacon project:

So far, I have tried a number of output amplifiers, the first based on the circuit in my original QRSS beacon, from back in December 2010:

This gives me a nice clean 4v P-to-P voltage out, but I wanted more than that, so either I add another stage or do something else.

I have also tried an output amp based on the Analogue Devices AD8008, this works OK also. I'm off on my travels again tomorrow, so I'll pick this up again when I get back.

All good fun though, egh?

Monday, 7 May 2012

Back to real homebrewing!

You may recall my discovery back here:

So, Ive managed to make some progress. I've modified the software from W3PM from here:

and have managed to get the GPS module working and also the DDS drive routines....

The code is written in C for the Arduino; and it's now interfaced to a four-line LCD, the AD9851 DDS module and the GPS receiver. The GPS module isn't in the picture as it's hanging out the window so it has visibility to the sky.

I've started to box the project, and have also built an RF amp to get me a staggering 100mW out...

Progress has been quite good today, I need to think about how I can get the RF levels the same on each band because as the frequency increases, so the amplitude out of the DDS and hence out of the RF amplifier decreases. I'm really not sure how to tackle that one, so I may have to ask for some help here.

Yesterday myself and Vince G0ORC went over to the "Dambusters Rally":

The rally was really good, albeit a bit small, but there was loads of really good radio junk to rummage about in. I picked up all sorts of stuff including this meter which I am rather proud of:

I've cleaned it up, mounted it on some wood, and stuck it in the corner of the shack with some amps flowing through it...

The next step will be to put some lighting inside...

Looks god though, agree?

Friday, 4 May 2012

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:

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?