10m and 20m Low Pass Filters

After a bad experience with a purchased low pass filter, I wanted to make my own for my WSPR transmitter for 10m and 20m bands. The ARRL have some good documents on line for filters. A low pass filter should prevent harmonics of a higher frequency from entering the transmitted signal. After looking into other people's designs on line, I would put the values into some modelling software to see the results. I understand that computer models rarely reflect real life, but they are a good starting point. I wanted to use achievable homebrew inductors and preferred values for other components that are close to the ideal model values. The filter I decided to make was a 3rd Order Butterworth design. I started with the inductors. I had some miniature 545nH inductors that I liberated from a scrap radio circuit. Then I started to add preferred value capacitors into the model. I decided to move the cut-off frequency forward a bit so that my working frequency would be in the very low loss section before the 'knee' at -3dB. I'm only working at 10dB so losses have to be kept to a minimum. Some purists would say you must use silver mica capacitors which are very expensive. If you look at commercial offerings though, they don't seem to use anything special in their circuits for low power use. I am going to use this filter QRP, about 10mW, for my WSPR transmitter. I will be using normal multi-layer ceramics for my project. Below you can see the outputs from the modelling software:

10m Low Pass Filter

20m Low Pass Filter

I built both filters onto an old audio switch board that I found in my bits drawer. It's ideal because there is a large surrounding ground plane around the connections for shielding. I achieved the 273nH inductor by using two 545nH in parallel, and similarly with the capacitors to get the desired values.

 

I can switch between 3 bands if required. Next stage was to test the circuits to see if I had come close to the modelling software results. Running the filters through my VNA set for S21 Gain, I got the following results. The markers indicate target frequency, 2nd and 3rd harmonics:

I'm impressed how similar the graph results are between virtual and real readings. The insertion loss figures were higher than the modelling suggested. This could be due to using lower quality components. So, does it work? Well I set my WSPR rig running for a while and got these results after only 1 hour:

I think that is not bad for only 10mW. A couple of spots in Germany, one in Denmark, Italy and Switzerland.

And then, rather surprisingly, a spot in USA the day after.

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.

The 17-5 HF Vertical Antenna

This is the next project to be undertaken. Named the 17-5 because the top vertical element measures 17.5 feet in length. A short counterpoise wire of 3 feet completes the antenna when connected to the feed point via a 9:1 unun. The installation will be terminated with a 1:1 balun to deal with any common-mode current issues. I am reliably informed that the antenna will match up on 40m down to 10m, maybe even more.

I decided to model the antenna first to give me an idea of the radiation patterns. As you can see below, the take-off angle looks quite impressive - very similar to a ground plane antenna.

The far field plots show good gain figures at low angles with the antenna mounted at a height of 3m above ground. The 3D plot shows the highest energy radiation in pink / purple which is perfect for DX.

I have some fishing pole sections which I keep using and re-using for my antenna projects that can be utilised once again here. I slotted some sections together to give me a total of about 22 feet. I intend to feed the wires along the inside of the pole. The unun will be fixed at just over 3 feet from the bottom of the pole.

Completed Antenna

To test the antenna before erecting, I put it together and then stood it up in the washing line receptacle in the lawn. This is the temporary fixing of the 9:1 unun on the pole.

The antenna is 20.5 feet in total, which is still quite long.  A quick scan with the Nano-VNA showed reasonable SWR readings across the bands. So up it goes!

I decided to fit a waterproof box onto the pole to house the unun and partially cover the PL239 connection. Final position is above the garage on the gable end of the house, not ideal, but the most practical for my situation.

When permanently installed, I found that I could not get a decent match on 7 MHz and below. Also 18 and 28 MHz were a no-go. Proximity to buildings, metal brackets or other antennas? Who knows, probably all of the above. However, it did match up on 50 MHz which was a bonus, however, this may be outside of the efficient operating frequency of the unun, and consequently, I haven't made any contacts on this frequency yet with it.

Performance

It does work well, outperforming the 58ft wire sometimes. The long wire does work better on the lower frequencies though. I find antenna comparisons really difficult. Any tests you might perform are always against a backdrop of constantly changing conditions. I test using WSPR and comparing spot reports and also FT8 reports, but there are many variables to factor in. A good way I find to test the reception performance is to tune into a morse beacon and switch between your antennas to compare the S Meter readings. My wire antenna is horizontal while this is one is vertical. The long wire is in free space, while this one is quite enclosed between buildings. Despite all this I have made contacts in the USA and Brazil and across Europe, which is quite impressive. I will continue to try to improve the band coverage by firstly doubling the counterpoise to 6 feet. I have modelled this and it shows an improvement in SWR so let's see what happens. More to follow........

 

Simple Remote Antenna Switch

 I'm getting a bit bored with having to go to the airing cupboard to switch my antenna over. That's where the switch is located. My plan is to switch the three antennas I have installed by remote control. There are many ways to do this - just take a look online.  I intend to build a relay box to house the connections to the SO239s and Schrack miniature relays with contacts rated at 250VAC 8A. (see images below) 

Each antenna will have its own relay that will connect or disconnect as required. The coils of the relays will be operated by a 12 volt DC signal from a rotary switch, located in the shack. This arrangement also allows me to move all of the cables and switch box into the loft out of the way. Protection diodes were included along with some indication LEDs on the control box. The circuit diagram can be seen below:

Some designs use the normally closed contacts on the relays to ground the antennas not in use, while others just leave them open. I decided to go with non-grounded. I don't want to create a lightning magnet! I don't think there is any down-side to this. After all, my manual switch doesn't ground any outputs / inputs so it should be fine.

(Update 02/08/2025. I decided to ground the antenna not in use connections on the relays. After weighing up the pros and cons, I did it mostly to get better isolation between antennas and not as static or lightning protection.)

Testing:

I connected the Nano-VNA to the common input to the box and a dummy load to each of the outputs in turn. A sweep at different frequencies didn't show any adverse SWR readings compared to the manual switch. (in fact there is a slight increase in SWR with the manual switch and an identical increase with the relay box, so I'm guessing we're all good to go!) This is probably due to the extra connections being used. I think using coax to wire to the relays is a good idea here. I previously made a box, expecting the metal enclosure to provide adequate screening. Using coax makes for a reduced loss in the circuit.

I tested the unit live and made several contacts with no apparent problems. This is a project that has really been worth doing.

Why does anyone bother with Linux?

 The title says it all! I'm not on my own it would seem with my thoughts on this - just Google it. I installed it, gave it a fair crack of the whip, but in the end it had to go. I would have loved to stick with it. I get the idea that its an OS for developers and programmers, but so is any other OS. The difference is that other systems cater for both end-users and developers equally. Linux can be used like this because you can find some good complete package installers, but they are few and far between. Its like being given a luxury vehicle for free and then finding out that you can't get fuel for it. Trying to install and get something working is usually a nightmare! Pages and pages of code to type into the terminal and execute - this is after you have so called installed it in the first place. Most of the time it doesn't even work after this. You don't even know where the program has gone. But oh, the dedicated Linuxites are quick to tell you that you are doing it all wrong and you should try this or that. They don't use desktops or user interfaces, they run everything from the terminal, like this is a good thing. We are not all programmers or hackers. We just want an alternative OS to Windows so we can find and run applications on it. Linux can stay on the Raspberry Pi, best place for it! I capitulated in the end and this man went back to Windows. You need an application, you find it, you install it, and guess what? It works straight out the box! Don't get me wrong, Windows is not perfect by any stretch, but it is reliable for every day users. Ah well - rant over.

DXing In The 80's

During a bit of spring cleaning the other day, my wife found these old QSL cards dating back to the early 1980's. The one above was mine, AW26. I was immediately sent back to a time when CB and particularly DX was outlawed and it was a very clandestine operation to set up a radio station then. The QSL cards had to be sent by post because there was no internet. All very exciting! So here are a few more of them:-

This was from 1EP in Italy, handle "Hawkeye".

Kilo Sierra in Germany, handle "Meteor"

My wife also got in on the act using the callsign ESW26 and made this contact with "Anarchist" in Germany.

There were also a few from USA as you can see here:-

And also good old 'Blighty'

A snapshot in time. I really enjoyed finding these again. What would be very nice would be to get in contact with some of these people if they are still around.