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Monday, 30 May 2011

Am I biased?

My conundrum regarding the linear standing current may be coming to a close; with some great help from the GQRP Yahoo! group; especially G8SEQ, G3OOU and G8BEQ, it's clear that the minimum bias voltage that I have available, and hence the collector current flow is way too high.

Here is the bias circuit as published:

So I've tinkered about with different values in the circuit and now have the minimum voltage down to 0.71V and the maximum is 0.91V.

This is allowing me to set the standing current to just about any value I like; there's about a 20mA draw without the collector power connected so I've set the bias to 220mA total with both collector power circuits connected.

Now for the tests.....

Sunday, 29 May 2011

How much current?????

I've been building the linear amplifier as part of the ongoing HF tranciever project in RadCom. This specific article was in March 2011. Here's the schematic of the amp:

Now, the actual build in the article uses a TPV3100 dual transistor (it's 2 devices in one package - I think). Being somewhat clueless about transistor characteristics and equivalents; I've ended up using a pair of 2SC1971's.

I've built the amp here:

And the bias is currently set to minimum which is 0.748 volts. With just the bias connected the current draw is about 70mA. If I then add power to either transistor (not both) via T2 the current draw increases to 1.85A. This seems rather excessive.

What I don't know is a) if the transistors I have used are suitable for this application and b) what sort of standing current I should expect.

Nothing is getting excessively hot, and no smoke has come forth, so it's not a total disaster (yet).

All this has been possible with the assistance of my second in command:

Can anyone help?

Sunday, 15 May 2011

IMD Test Oscillator - Finished!

The thing that I was fiddling with here:

Well it's now complete and in a box:

The output of the mixed signal is also looking neat and tidy:
I made a quick video of the unit under test, which is here:
Not bad - wouldn't you say?

Are we receiving???

Well, here we go again.... I was very interested in the Collpits test oscillator that featured in May 2011 RadCom Homebrew. After some deliberation I decided I would have a go at making one of them. This process has been rather traumatic!

Here is the schematic that I'm following:

It all looks rather simple - right? Well I built the basic oscillator based round the MPSH10, and then tried to find something similar to the listed 2SC495. I managed to destroy several transistors whilst doing this and eventually ended up using an MRF476 - which is total overkill but as I have them I figured this would be OK.  Whilst destroying transistors (smoke, bad smell et cetera) I was unable to get the suggested 4V p-to-p output and could only really get about 2V p-to-p. I instantly blamed the amplifier section and just continued to blow up more alternative transistors.

Eventually I noticed a slight construction error in the basic oscillator - this corrected everything is working as expected. Here's the first oscillator and it's Cousin just beginning to take shape....

The output from the oscillator is very clean and the LPF in the design should have removed any harmonic content hence we see a very clean sinewave:

Just for some entertainment, here is my bench after constructing this simple little project:

What a mess!

And now, because I can, here is a closer shot of the construction of this little baby:

You'd have to guess that there's more to follow here... what you recon?

Friday, 6 May 2011

More Bedini Madness

I've been fiddling more with a Bedini monopole and have created a quick video to capture my latest round of insanity:

Rather nuts, hugh?

Monday, 2 May 2011

Bank Holidays - Don't you just love them?

Well, it's a beautiful bank holiday Monday here in the UK (this basically means I and the rest of the country have the day off).

The grand plan for today is simply to play radio! I have no antennas to put up, no desperate projects to get on with, so, just because I can, here is a picture of the shack as it stands today:

What a mess!

Sunday, 1 May 2011

Well, what's it like then?

I've had a bit of time to play with the new HF antenna that I put up earlier in the week. It's a Dual Beam Pro:

I've only had a Windom here up to now, so I can only compare it with that antenna. So far I can report that I am very impressed with the Dual Beam Pro. Some signals are significantly stronger on the new antenna, and none so far have been weaker. The other evening on 17M USA stations were 5 S-points up on the Dual Beam Pro. This will have been aided by the Dual Beam Pro pointing E-W at the time and the Windom being permanently N-S, but still a significant improvment in my working conditions!

Anyhow, on all 5 bands except 20M the FT-950 internal tuner will tune the antenna, and on 20M it easily tunes with my external ATU. I was expecting the rig to be unable to tune the antenna at all on any bands! So the expected miss-match can't be all that bad. I haven't actually looked at the SWR of the antenna on the bands, but it was expected to really need an external tuner.

I have to confess that I was rather sceptical about such a simple and small antenna, but so far its performance has been far grater than my expectations. Well done Pro Antennas!

6m and counting

I've added yet another antenna to the house (or as my wife seems to like to call it that damn mess). This one is for 6m SSB activity.

Rather than adding another rotator, I've just stuck this on the pole that holds up the centre of the HF Windom. It's permanently fixed in the South East direction - that's kind of where Europe is from here. It's a HB9CV 2 element from Moonraker; I'm rather worried about the elements as they are rather loosely held together with nuts and bolts and rattle rather a lot when you shake it! The SWR is perfectly acceptable though, and I've already witnessed an opening into southern Spain. I was being hoovered at the time so was unable to call back.

We'll just have to see what happens!