Portable Amateur Radio Club
and how I beat it. John GØUBE
Last year I posted a short video clip on our FB page of the noise level I had on the 80, 40, 30 and 20 mtr bands. Here is how I went about living with the noise in a built up area in Birmingham.
All of the pictures show 80 mtr band where the noise was highest. 40 and 20 mtrs were about 2 ‘S’ points less
The Antenna at this time was a long wire fed a the furthest point from the building. There was a commercial 9 to 1 transformed at ground level and the wire ran up a fiberglass pole to form an Inverted ‘L’ up to my roof. I did install a Common Mode Choke made from Coax on a 4 inch former that did help to bring the noise down to the level in Fig 1. The noise was 20 to 30dB over 9 before the choke was fitted.
So you can see my starting point was quite bad and I was only able to hear the strongest of signals and it made operating on HF impossible.
Before the passing of Chris Maxwell we were looking at Core Materials used in BalUn’s and UnUn’s and we were going to do some practical experiments in this are, having both just investing in a VNA analyzer. So I decided to continue some of the thing we spoke about while off work due to Covid.
First thing I found was that the use of the normal Red type ‘2 ‘core material used by most to make 4 to 1 and 9 to 1 transformers, gave totally unacceptable SWR across the HF band. I tested 2, 9 to 1 transformers constructed on type ‘2’ material and obtained the same results. 1.3 to 1 SWR at 13.5Mhz rapidly increasing each side of that point. At the center of the 20 mtr band 3.5 to 1 and 12 to 1 on 80 mtrs. So why is this core shown all over the Internet as the one to use??? See Link 1 at the bottom of the page.
So I wound a new 9 to 1 on a FT 240 43, Wow 1.2 to 1.4 from Top Band to 10 mtrs and would be possible to use at 6 mtrs at 1.8 to 1 SWR??
So I installed it onto the same long wire. This change made a massive difference to the SWR on the long wire so the ATU was doing much less work matching the antenna. But another advantage was I gained a little reduction in noise. About ‘2 S’ points less.
So back to the books and Internet about noise and how to beat it. I found a video by GM3SEK which is well worth a look See Link 2. The interesting thing to note is that Common Mode current works both ways. So RF in the shack on transmit and noise on receive!!!! So I started making a mains filter more on that latter, and found a better way to make a Common Mode Choke. The choke is wound on a FT 240 43 core using wire rather than Coax see link 1 below.
I took a measurement with RF meter (Home Made) to see how much RF is on the outside of the coax when I transmit. At 100 watts there was about 5 watts coming back into the shack as I found out when I touched the brass body of my Morse key.
I made a choke using this method and installed it next to the 9 to 1 in place of the Coax Choke. Now on transmit there was much less RF in the shack and the noise level was massively reduced. Fig 2.
These results were very encouraging so back to the advice in the video by GM3SEK you need a choke at both ends of your coax. So I constructed a second and installed it where the coax enters the bedroom. There is now no RF on the coax to the rig and yet more improvement in the noise level. Fig 3
In Fig 3 you can see that the noise level is now only flickering up to S2 and is much more like atmospheric crash type noise.
I have changed the antenna for a EFHW with transformer wound on the same core type and the noise is even better.
Conclusions and Observations.
Can’t find any reason why the Type 2 cores are used in so many articles on the Internet when they are measurably not the thing to use. See Link 1. TRX Lab has the test gear to demonstrate how the cores work or don’t work. He has a group of videos on this subject including 4 and 9 to 1 Unun’s.
The problem with Internet and social media is the same incorrect info just gets repeated over and over, good or bad and people believe it. You need to try it yourself.
The type 43 core material is far better and there are others of use to us but harder to get hold of and more expensive than Type 2. Type 52 and 31 have there uses.
Well worth watching all the TRX Lab videos even if he calls them balloons.