Search This Blog

Thursday, 20 July 2017

First On-Air Test Complete

Well,

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

Well,

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

Well,

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.

Sunday, 11 June 2017

Its brick time!

Well,

To compliment the DATV transmitter I made here, I've been building a PA rated at 60W RF out - it will be used at way less than this, but for any kind of TV transmission we need loads of overhead in the PA to avoid nastyness in the output.

The PA is this design here, the PCB from G4DDK.

The module itself is a RA60H1317M1A and I got mine from Anglia Live.

The heatsink feels like a great find, I saw it listed on eBay by JPG Electronics in Chesterfield; as it's just up the road I paid a visit - what a find! Loads of goodies!

Anyhow, here the PA under test:



The TX RF from the Portsdown will come in through the LPF we tested last time; then through the PA and out through the SMA relay. The RX Signal will pass through the BPF also from last time, and to the Receiver we made here.

The relay was one of a number I found some time ago; they are Ducommun latching 12V SMA relays. These need a driver circuit which I made like this:


and that's built on the veroboard you can see at the front of the PA block. The output lines do this when the PTT is grounded and then disconnected:



All I need to decide now is what to set the Bias voltage to on the PA - not sure about that!

Throughout Miss Luna Cat has been supervising from a distance:


Good, egh?

Saturday, 3 June 2017

Filters Filters Filters

Well,

Following on from the success of last time; it was time to make some filters around the 146.5 MHz DATV frequency on the NoV allocated bit of spectrum we have above the 2M band.

I've also built up a kit I have had here for a while, it's a PGA144 from G4DDK.


So at the top we have the PGA144, middle is the LPF and bottom is the BPF. The designs are really quite simple - just ask if you need the details. Here's the spectrum from all three:


The yellow is the PGA144 - it has a 20dB attenuator at the input so the signals are actually 20dB higher than shown - the gain at 145MHz is exactly 20dB.

The purple is the LPF being swept and looks just fine.

The Blue is my BPF which I am very pleased with - it looks great.

So next will be a 60W "brick" amplifier for 146.5 MHz - waiting for the bits but I have to go work in foreign parts for a week or two so will pick this up on my return.

Local conditions.

Wednesday, 31 May 2017

I think its working

Well,

Following from my musings last time on the BATC Portsdown project; I think mine is now up and running.

I've been working on a box for the project and the various bits and bobs are now inside:





So, following the suggested test setup I've configured the transmitter to TX on 1255 MHz using 2000KS (thats the symbol rate) and my newly invented DATV receiver from here sees this:


So I conclude it's working. Now to try and stream some video and then think about external amplifiers and filters!

I've decided to initially aim at 146.5Mhz in the NoV only allocation above 2M as my first target frequency.

I've set the Portsdown to tx on 146.5MHz, 7/8FEC with a symbol rate of 333KS. The output close up looks like this:


Checking on the harmonic content we see this:


So I made a LPF (needed!) and now the output looks like this:


In reality the LPF looks like this:


It is a standard 3 inductor design with 22pf at each "end" and 43pf in the middle two locations. The inductors are 3 turns open wound on a 6mm drill bit.

I've hooked up the BATC supplied EasyCap USB device to the Portsdown and I have coupled up my AntennaCam and we can see this on the MiniTiouner receiver we made here:


So, the next thing I need is a TestCard for TX; enter another great use of a Rasperry Pi. I've installed the software called TCANIM from here. I've followed the instructions to the letter but I cant seem to get a video signal out of the Pi AV socket....


Local conditions.

Tuesday, 30 May 2017

Portsdown where?

Well,

As part of the project I mentioned last time, I've started to construct the hardware for the BATC Portsdown project.

The fist board I have tackled has been the LO filter. This goes post the AD4135 LO which uses the same development board as we used on the 4.4 GHz signal generator.

This is extreme, extreme soldering! I've invested in a flux pen of decent quality from Farnell and that's made my life much easier. Previously I was using some cheap eBay sourced flux which was a load of dingos kidneys.

Here's the results of my days soldering:




There's basically a 2 bit input thats status determines which of the three on board filters are in line (or bypassed on 23cm). I've tested this and can see three filters, not too sure about their shape though.

4M:


2M:


70cm:


23cm:


Looks a bit odd to me, but lets see.

Local conditions.