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Thursday, 20 July 2017

First On-Air Test Complete


Thanks to the help of Col, G4OHV I have tonight tested my Portsdown transmitter on air.

Here's the video Col captured of my TX - Thanks!

Good, egh?

Tuesday, 11 July 2017

More InnovAntennas Fun


You may remember my sharing an instruction sheet from an InovAntennas purchase back here and basically explaining how poor I though the instructions were. Well, here's another excellent example:

So, credit where credit is due - this time I didn't actually have any missing parts for the antenna itself; however I did have some spare bits for the antenna (some extra end caps and element clamps) and unfortunately the antenna to boom mounting plate and associated u-bolts are missing completely.

But lets take a look at the instructions:

  1. The title tells me it's a 1.4m antenna; I assume that's the boom length, but, oh no, the boom is 1.7m long.
  2. The bottom of the page tells me the boom is 1.7m long - so which is it? Let's get a tape measure and check.
  3. The bottom of the page also also tells me that "guy and supports are supplied" - I don't think so.
  4. So let's look at the shortest element - there are three numbers 1705mm, 1405mm and 903mm. So I think one of these (the 1705) is the distance from the boom end, the 903 is the element dimension - no idea what the 1405mm is - perhaps this is for the 1.4m antenna mentioned in the title that I haven't got? If that 1703 is the distance from the boom end then the first element is nearly a foot from the boom start - that can't be right either.
  5. Then we have the added information "X-POL SIZES"; you have to assume this is for a cross polarized variant perhaps?
But once again, no actual information on which bolts or other bits to use where. One of the driven element clamps is metal - now I assume that's not at the end the feedpoint is and it seems the feed is at the back. I assume I need a coax balun near the feedpoint but that's clearly guesswork as there is no information on that aspect at all.

Local conditions.

Saturday, 1 July 2017

EMC 'n' all that Jazz


I've been having some issues when I TX on 6M CW. Very strange in that my Radio PC (the one sending the CW) shuts down - it doesn't crash - it performs an orderly shutdown.

This is definitely an RF issue as it only happens when the TX power is above a certain value.

So, by using a process of elimination, i.e. removing cables from the back of the PC one at a time and seeing if the problem goes away, I concluded that it's probably the HDMI cable to the monitor (well, one of the monitors) that's causing the problem.

This has lead me to question the effectiveness of ferrite suppression and other such gubbins.

Now, all of us hams will have purchased a bunch of clip on ferrites at a rally; these are supposed to be made of type 31 material which is rated up to 500MHz.

I mean something like this:

Now, these are designed to clip on a cable, effectively providing one turn through the ferrite. How well does that work then? So here's the spectrum analyser showing 0 to 100 MHz and a simple loop back from the generator to the input - no ferrite here:

So, lets now add a single turn of the clip on ferrite and see what difference it makes:

So the answer is, quite expectedly, not a great deal. So, let's increase that to 5 turns:

So, thats much more like it.

We have to conclude that clipping these ferrites onto cables around the shack is next to useless at HF - we need at least 6 turns through the material before we see any significant attenuation.

There are a number of larger ferrites available, using the same material, but bigger:

These will allow you to get multiple turns of coax or mains cable, or in my case a HDMI lead through the core, and most importantly they are also clip on.

I've tried a few combinations of different cores on my HDMI cable to see what works best; there are all sorts of other issues creeping in now though, like the resonant frequency of the cable itself:

The bottom picture above effectively gives me 6 turns by using 6 cores; the image above uses much more expensive cores and passes the wire through multiple times. They both have much the same impact.

So, I plan to add the cable above as an extension to my existing HDMI cable and see if the problem is solved.

Here's our very beautiful Elmo enjoying the fact that summer has finally arrived:

Local conditions.