Re: QCX for 2m VHF? #chat #qcx #vhf #mods

Jim Mcilroy


When I got my ticket back in the 70s everybody I knew at my local club knew how to build and use stuff, and was very interested in doing so. The exam was a written one, and as far as I can recall there were few multiple choice questions, and various questions about valves.

I do recognise these days at my local club that there are a significant number of hams who are only interested in operating gear and their eyes glaze over a bit when backs get taken off radios and soldering irons get fired up. One hopes they will grow out of their fears and get more out of their respective ham worlds.

Nevertheless everyone had to do an exam and surely some of that knowledge must get taken on board. I can see you have reservations about that.

I am generally in agreement with your last paragraph and it is much easier to build at higher frequencies these days. A good quality BNC connector will work up to about 3.5 GHz. I used to work at a microwave components factory and would measure anything I could get my hands on. Prior to that I cut my teeth on GAT-5 and GAT-6 transistors working at 8 GHz for satellite comms and they were tricky to match and easier to blow up. Had steadier hands then.

So, yeah, we agree on some things.

I noticed your other post about a simple 2m CW rig. Good idea. Not so simple for many to do a prototype but double sided copper sheets would be your friend.

I also have a Datong PC1 in my junk box that I bought in a lot as scrap. I intended just to use the box but I notice that it has a parametric mixer and a 144MHz filter. I'm sure I could have some fun with that.

Also have a couple of Avala PCBs which were the forerunners for the Genesis SDRs. They are intended for HF but they are easy to work on and could possibly be integrated with 2m VHF components. That would have to be a downconverter design as the mixer chips are 74HC4066s. Not as good as the QCX design but probably pretty good at 40m. Later on we'll see.


On 10/02/2021 08:41, Jim Allyn - N7JA wrote:
On Mon, Feb 8, 2021 at 09:51 AM, Jim Mcilroy wrote:
I must confess that I assumed that most hams would know the distinct differences between HF and VHF design and construction.
I don't know why you would assume that.  Clearly the original poster didn't know the differences between HF and VHF design and construction, else he wouldn't have asked, "Would the QCX design not be able to make an VHF version?"  Maybe things are different where you live, but here in the United States, it seems to me that a significant number of hams, perhaps a majority, don't know the difference between HF and VHF.  I have heard lots of US hams admit that they know basically nothing: in their own words, they just memorized enough questions and answers to pass the test, then promptly forgot everything.  An example of how much some hams here (don't) know:  within the last year or so, I heard a discussion on one of the local repeaters about baluns.  One of the guys wanted to know what kind of balun he needed: "I see there are one to one baluns, and four to one baluns, and nine to one baluns.  How do I know which one I need?"  The answer he received was, "If you want to operate on one band, you need a one to one balun.  If you want to operate on four bands, you need a four to one balun.  If you want to operate on nine bands, you need a nine to one balun."  I looked up the gentleman who gave that answer on, and he holds an Extra Class license.  I'll leave it to you to consider whether that gentleman is likely to know much about the differences between HF and VHF design and construction.

On the other hand, one could argue that there is no difference between HF and VHF design and construction.  The principles are the same: keep all component leads and signal paths as short as possible, minimize the loop area circumscribed by the signal paths and their return paths, keep different types of signals (analog and digital, for example) away from each other, ensure that bypass capacitors have a low impedance at the design frequency, and so on.  The differences are in the details.  Practically speaking, an inch of wire doesn't exist in a circuit designed for 80 meters, at 70 centimeters that same piece of wire is a fairly large inductor.

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