DIY Coax Line Isolators

The advantages of using line isolators is:

1. Negates the common mode current on the outside sheathing of the coax.
2. Reduces noise levels.
3. Gives a more accurate VSWR reading.
4. Reduces RFI.
5. Can help prevent RF burns.

This has got to be the easiest thing to build for any radio ham, and probably one of the most effective. You only need some ferrite torroids and that's it essentially. If you are using particularly large diameter coax, this project may have to be built separately in a box with sockets and a piece of RG58 coax. I use RG8 mini and had no problems making the turns, even with the PL259 attached.

This is one of them near the feed point of the antenna

I created two line isolators. One near the rig and the other at the feed point of the antenna. The ferrite rings I used were FT240-43. These things aren't cheap! I paid £21 for two on Ebay, but I think they are the real deal. Some ferrites boast compatibility but the iron powder used is of poorer quality and contains other metals in there which are not conducive to good performance. So how do I make one you say. Pass the feed coax through the rings completing 9 turns. Space the turns out equally (important), fix them in place with tie-wraps and plug the coax back in - easy. If your antenna system relies on CMC to balance then this will affect your SWR readings, maybe for better or worse, but only slightly.

I can vouch personally for point 4 above. We have a small radio on the kitchen window sill. Whenever I transmitted it totally blanked the radio out, even when using 5W. Since fitting the isolators, this has been reduced to a faint background hiss.

WSPR Node with Raspberry Pi

 I had played around with creating a 10m morse beacon once which worked a treat. The problem is there is no instant way to see if anyone can hear it. However with WSPR there are quite a few websites where you can see immediately where your signal is being heard such as WSPRnet.org. I use MSHV mostly for my FT4 and FT8 work but it has no WSPR function. WSJT-X on the other hand does and it takes care of everything for you.

I had tried lots of different solutions on the Raspberry Pi that I have. It is an old one (Rpi 2B) and I found that installing an old version of Raspbian Stretch was the way to go. I eventually got it to work properly using this link to GitHub:

GitHub - JamesP6000/WsprryPi: Raspberry Pi WSPR transmitter using NTP based frequency calibration

Follow the instructions and install it from the command line. It can take a long time so don't think the RPi has given up because it can take up to half an hour to install it. Once installed you can call it from the command line with the correct parameters (see author's comments) The output should provide only around 10dB, 10mW, which is low but people have reported thousands of km contacts with it.

Now then, apparently the output from the RPi is too rough and dirty to send out over the air so we have to do a bit of cleaning up of the signal. I wanted to try it ASAP so I looked for an easy makeshift solution for a low pass filter and discovered that it can be done with just a couple of capacitors. See circuit below:

The signal is derived from the CLK output of the RPi GPIO pin 7. To prevent any dangerous DC voltages getting in or out of the RPi the first capacitor at 100nF blocks any DC in or out. The second capacitor at 110pF acts as a filter by smoothing out the imperfections in the waveform. As you can see from my scope, it hasn't done a bad job at all.

My scope is an old CRT one and struggles when getting above 20MHz.

Zoomed in a little it looks like a pretty good sine wave.

I tuned my radio to the right frequency for WSPR to catch the stray transmission and there it was.

This was good for 28MHz but it was a different story on lower frequencies as I expected. I set the RPi program to output at 14MHz to see what the waveform was like on the scope.

As you can see here the waveform is getting a bit out of shape - 'saw-toothy'.

Another check on 10MHz confirmed my suspicions even further. The trace for this is below:

You can clearly see the waveform is even more out of shape and also the formation of harmonics which is very bad for transmission. I decided to stop and not go down to 7MHz because I knew what the outcome would be. To get the shape back, further filtering would be required. I think that I can safely use this WSPR beacon on 10m though at the moment. I may purchase a proper LPF so I can use other bands.

 

10m Band J-Pole Antenna

 Because the performance of my long wire antenna seemed to be lacking on the 10m band, I decided to build a J-Pole. I have built these in the past and used them on the free band frequencies to great effect. Quite often I had the opportunity to build and compare several antennas and this one always outperformed the others. Previously, I had erected one on the side of the house, but these antennas are very tall and are subject to being destroyed in a bad storm as I found out eventually.

So this time I have a plan. The antenna will be erected just above ground level with the bulk of the mast being fed up the trunk and through the inside of our palm tree. This will offer great protection from the elements and also hide most of the antenna from prying eyes.

(Practically invisible - except for the bit at the top poking out!)

I used an online calculator for the lengths and proceeded with the build. The bottom matching section was crafted from twin feeder since the gap between conductors is not critical. The top radiator section is just a piece of 1.5mm2 wire. I found that the feed point had to be moved considerably higher than the calculator suggested, but once the point was found everything was fine. The bandwidth for this type of antenna is usually quite narrow. The SWR was on average 1.5 between 27.5 and 29 MHz. With the ATU in the rig, this can be stretched considerably. The trusty Nano-VNA became invaluable once again. What a great little piece of kit this has been! The screenshots can be seen below:

(Scan from 20 to 30 Mhz)

(Scan from 100 to 150 Mhz. Surprisingly tuning into the 2m band)

Below you can see the plots produced by MMANA-GAL

When comparing the antenna to my long wire I found the reception to be a lot clearer without as much background noise. However the long wire takes some beating for all-round performance and multiband use. This antenna is excellent on its design frequency of 27 to 28 Mhz.