Friday, 15 December 2017

Analysing my baluns - and an opening to VK on 12m!

Last week, I was reminded by a radio friend about the existence of SARKPlots - the PC-based software that takes input from the fantastic SARK-110 anatenna analyser.

Somehow, I had completely forgotten that SARKPlots was available, so immediately set about looking at my 4:1 baluns - one commercial, the other home brewed - with the analyser.

The first test was of my G-Whip 4:1 voltage balun into a 200 Ohm load.  It's spent several years outside, but is made of high-quality materials and fully potted.  Here's the output:

Commercial 4:1 balun by G-Whip (made ca. 2012)
Certainly, there's nothing much to worry about there, although the impedance transformation to 50 Ohms is, surprisingly, more imperfect than one might expect, more especially towards the higher end of HF.  It ranges from about 56 + 4j Ohms to over 60 Ohms.

Now I tested my 4:1 current balun, which has also spent much the same amount of time outdoors, and also uses high quality components, including marine-grade stainless steel for the contacts (it is very wet and salty here, by the sea!):

Home brew 4:1 current balun, made ca. 2013.
I have to say I'm rather pleased with my own creation!  It has a very good impedance transformation (about 53  and less than -1 j Ohms, and resultant low SWR.  The small dip near 18MHz is beyond my ability to explain, but it is very small, and is a change in the better direction to 48 - 0.6 j Ohms.

So that was baluns tested.

Next, a bit of real operating!

This morning, the propagation forecasts, which I never take seriously, didn't look too good to Europe, let alone anywhere else.

So, I left the radio running in the background.  Sure enough, at 09:31UT, I heard a pretty stable -12dB S/N FT8 mode signal from VK2EW.  After a couple of weaker CQs from him, I ventured to respond.

I was heard the first time, but at a pretty weak -22dB S/N (remember these S/N numbers are relative to an unrealistic, 2500Hz bandwidth, where FT8 only occupies 47Hz).

Unfortunately, the only direction I am rather blind to very low angle signals is in the short path to VK, where I run at a tangent and slightly into the copper mine hill to the NE, and then into another hill about 2km away.  HFTA modelling (blue line (red is to the central US, and much, much better)) clearly shows the low ground gain available at 066 beam heading.

Terrain profile to VK short path (066 degrees)


Ground gain VK short path (blue).  Red is to central USA.

Overall, taking antenna gain (+8.5dBi) and ground gain at the expected low arrival angles into account, I was putting out just over 400W ERP from an approximate input of 60W to generate that weak received signal at a well-equipped station in VK.

Oddly, I've never tried to work VK on long path, and I don't even know if long path is applicable at 24MHz, though it certainly is at 21MHz.

It must be worth pointing the antenna to the south west one morning, to see if anything can be done on the long path.  The ground-plus-antenna gain figures rise, depending on propagation angle, to anything between 500W and 4.1kW ERP!  Sadly, HFTA doesn't compute for the important angles down to and very slightly below the horizontal (elevated QTH).

Terrain profile, VK long path (220 degrees)

Ground gain, VK long path.

Whilst I was busy doing other things, a couple more VK stations called at about -15dB S/N.  They will have to wait for another day.  Not bad for 12m at this point of the solar cycle!

No comments:

Post a Comment