AmateurRadio.com

A few weeks ago I made a rubber duck helical antenna for my TH-F7E. I didn’t get around to writing about it at the time, plus it took a while to get some reports of its performance.

Well, it works extremely well, and I have just added a description of how to make it to the main G4ILO’s Shack website. See A DIY Ducky for 2m.

I’m preparing to build my 40m version of Roger G3XBM’s XBM80-2 QRPP transceiver into a little box but before I start I have one thing more to do. I have to design an output filter for the transmitter.

Roger didn’t bother. Comments from him suggest that he was aiming for a low parts count and assuming that any filtering would be provided by an external ATU. However Alan VK2ZAY comments that the output was more like a square wave. The second harmonic of my 7.030MHz signal would be on 14.060MHz, also a QRP frequency and I can’t be sure that my antenna won’t radiate this too. So I think an output filter is a good idea.

Alan used a two-stage pi network, each stage consisting of two 820pF capacitors and a 2.2uF inductor. I built this up on my breadboard, terminated the end with a 50 ohm load (actually 47 ohms, the nearest resistor value I had) and checked it with my SWR analyzer. The picture above shows the result.

Out of interest I also checked the response of a single stage of filtering using the same values, shown in the next picture. You can see that the cut-off frequency is a lot shallower than when two stages of filtering are used, so clearly attenuation of harmonics would not be as effective.

My problem is that I need a filter for 40m not 80m. I tried searching the web to see if I could find some online design tools, and I did here, here and here. However when I fed the design parameters for 80m into the calculators the results they came up with were a lot different from each other and also from the values VK2ZAY used.

In the end I decided to use trial and error. I figured that since the frequency is higher the values I would need to use would be lower. The next lower value inductor I had in my parts box was 1uH, so I don’t have a lot of choice in the matter. I substituted that for the 2.2uH. The result shown by the SWR analyzer was promising, but there was a big hump below the cut-off frequency and the SWR at 7.030MHz was a bit on the high side. So I then tried lower values of capacitors. The lower I went, the shallower the cut-off but the lower the hump as well.

In the end, I settled on 680pF capacitors with the 1uH inductor. There is still quite a steep hump below the cut-off frequency but I’m only concerned with the performance at 7.030MHz where the SWR is 1.2:1, which I don’t think is too bad. The cut-off curve of this one stage filter seems as steep as the two-stage one VK2ZAY used on 80m so I think one stage of filtering will do.

After I’d written the above, the thought occurred to me to try the low pass filter in the circuit while it was on the breadboard. I’m glad I did, because I’d have been disappointed to find after soldering everything into place that the output power had fallen from 100mW to 25mW. After a bit of experimentation, it appears that connecting the low pass filter directly to the collector of the first transistor as VK2ZAY did in his version of the transceiver loads the circuit and reduces both transmit power and receive sensitivity. The solution seems to be to couple the transistor to the filter using a small value capacitor – I found 180pF gave the best results. This removes the need for the DC blocking capacitor on the output. The power is still down quite a lot, but part of this may be due to all the harmonic energy which is now not making it through to the power meter.

My QRP crystals finally arrived. Murphy worked overtime on this eBay purchase: the first batch apparently got lost in the post and the replacements were sent by mistake to another customer with the same surname. But I have at last been able to try out the little QRP CW transceiver built to G3XBM’s design on the QRP working frequency.

The transceiver is still on the breadboard as I was waiting to try it on both 40m and 80m before deciding which band to make it up for. Certain components need to be optimized for the band in use, and even the crystal in use, since to save space I’ve hand picked a fixed capacitor to set the transmit frequency instead of using a trimmer. I need also to decide how much power output I want because that is governed by other components. I have had over 200mW out of it but I’m not sure if a 2N3904 without a heat sink could really handle that, so at the moment it is set up to give 100mW output when powered by a 9V rechargeable PP3 battery.

I’m leaning towards 40m as this would make a nice little fun portable rig and the antenna being half the size of 80m would make things much easier. 40m generally has more activity during the daytime, too. I have never found 80m an easy band to work with limited antennas and low power, so it has always surprised me that the lower band is so popular with QRPers.

Of course, it being the weekend there is a contest on today, so not much chance of being heard with 100mW. Even if someone did hear me and reply, it’s doubtful if I could pick them out using a receiver with no selectivity whatever! This tiny, simple transceiver is amazingly sensitive, as you can hear in this 1 minute audio clip recorded on 40m this afternoon using my MFJ magnetic loop for an antenna. How many different stations can you copy?

This evening I may put the 80m crystal in and see what it sounds like on that band. Unfortunately the little receiver has no tools for fighting the terrible QRN that comes on during the evening when all the neighbourhood plasma TVs are switched on, and sometimes 80m is all but unusable.