A practical antenna testing range

    If you have an interest in antennas it can be very illuminating to take real-world antenna measurements. This post describes a simple set-up to allow practical VHF and UHF antenna measurements on a budget.

    The first requirement is enough free space as to not cause interference or obstruction to your antenna. As much distance as possible in all directions, ideally many wavelengths. I used a field because I grew up on a farm, but you could use a public park, an empty beach, or common land.

    The second requirement is for some means of measuring RF field strength. There are specialist instruments to do this, but for cheapness and practicality in this case I used an RTL SDR, one of those USB digital TV sticks. I plugged it into my Android tablet, ran the SDRTouch app, turned off the automatic gain control, and turned the gain down to about 10% of its range. My receive antenna was the nasty little 15cm wire antenna that comes with the SDR, the only time I've ever used it in action.

    The third requirement is a transmitter. Any transmitter for the frequency of your antenna will allow you to take measurements, however for this practical test range you will need it to be capable of micropower output. I used a Raspberry Pi with my RF breakout board and an attenuator network. I calculated that my 47R-470R-47R Pi network resulted in an output around 0.03 mW, or 30μW. This resulted in a detectable RF field that was entirely within the boundaries of my range, in other words the transmitted signal was undetectable 100m from the antenna. For the purposes of legality I used a low-pass filter and a modified version of the freq_pi signal generator that inserted my callsign in CW into a continuous tone once every few minutes. 

    So, given my three prerequisites, I set to work. I set up my 2m HB9CV antenna in the middle of the field, chose an unoccupied frequency - a part of the repeater input segment not used in my part of the world - and fired up the Pi.

    The detection setup was a clipboard with a piece of paper, and the tablet and receive antenna secured with rubber bands. I walked away from the antenna at each point of the compass, noting down the received signal strength at each metre interval.

    My resulting figures could then be put into a spreadsheet and rendered as a radius plot to draw the radiation pattern for the antenna. And so I proved by measurement that an HB9CV is a directional antenna. Hardly unexpected, I'm sure you'll agree, but the real proof is that using a £20 single board computer and a £10 USB receiver it's possible to make real-world antenna performance measurements.

A Raspberry Pi gives me a lifetime DX record

    The other day I dug out and dusted off my amateur radio logbook. The last entry was in 1993, a 70cms FSTV test with 2m talkback with a friend a few miles away. I noted at the time that my power output on 70cms was 50mW, and that the test was a failure. The transmitter was homebrew and I still have it, though I think I may have scavenged the output transistor.
    It's weird, reactivating a callsign so long dormant. Long enough dormant in fact that what always felt like a rather new callsign is now something of an old-timer. A lot's happened since 1993 in amateur radio, there are some interesting new bands and modes, and I can now use all the HF bands if I want to. I bet that went down like a lead balloon with the sheds-and-allotments nets on 80m! :) But to be honest I'm no longer interested in sitting in a shack at a microphone. I was always in it to experiment with radio and though I spent a lot of happy hours on 2M FM back in the day I'm probably not returning.
    Of the new modes that have appeared since I buried my personal amateur radio time capsule, WSPR caught my eye. Extreme QRP and extreme narrow bandwidth, and best of all, possible using just the internal clock generator of a Raspberry Pi. So with a quickly assembled 70MHz low-pass filter and an equally hastily built dipole I put together a 4M WSPR beacon.
    It's a while since I had an operational transmitter of my own. This one's only producing about 100mW, but it's still important to ensure a minimum emission of unwanted radiation. When your transmitter is at heart a logic level square wave there is a lot of possibility for harmonics.
    I hope it's not a damning admission when I say I was never able to really ensure I wasn't transmitting harmonics back in the day. I had my wavemeter but it couldn't go up into the high harmonics so if I built anything I had to rely on overprovision of low pass filtering. I suspect I was not alone in this and I never had the DTI breathing down my neck so I am happy I got it right.
   Now I have something unavailable to me in the 1980s and '90s, an RTL-SDR. I can have a waterfall spectrum analyser view of any couple of MHz slice of spectrum between 25MHz and 2GHz, so I can take a look for harmonics. Of course it's not a calibrated device so it's difficult for me to check relative values even if I turn off the AGC, but I can still check for presence or absence of radiation.
    In the room next to the Raspberry Pi, there are the harmonic peaks. Much lower than the carrier, but still there. Half a mile away though, the carrier is just as strong but the harmonics are no longer present. The lower-level harmonics I detected in the same room are not reaching my antenna, the LPF is doing its job. Good, that's what I want to hear.
    So, turn on the Pi, run WsprryPi, and keep an eye on the WSPR map to see who spots it. 4M is neither an easy DX band nor a popular WSPR band, so it wan't exactly a surprise when none of the five or so stations active at the time in Western Europe spotted me. But a Raspberry Pi doesn't use much power, so just leave it running.
    After two days, a sporadic-E opening. And a single spot, from just north of Madrid, in Spain. Nearly 790 miles to this part of Oxfordshire, for 4M that counts as pretty extreme DX.
    So, EA4ETR. The first callsign in my logbook since 1993, the first non-G callsign, and far and away my furthest DX on a difficult band all in one go. And all with a £25 computer and a filter and antenna from junkbox parts. Not a fancy chequebook transceiver in sight.
    That for me is what amateur radio should be all about.

A transceiver for not a lot

    I've just built a transceiver for the 4M amateur band. QRP(low power) and AM, it's not going to be something I'll work DX with, or indeed given my rural location not something I'll work *anyone* with, but I've had a lot of fun making it.
    My starting point was a 2M QRP AM transceiver courtesy of G3XBM, the 'Fredbox'. The receiver is a FET super regenerative design with an RF amplifier to stop unwanted radiation, and the transmitter is a very simple series modulated PA giving <100mW. I used 2SJ310 FETs in the receiver instead of MPF102s, and 2N3904s throughout in the transmitter. Everything else came from my hoard of electronic bits. I can't claim any astounding electronic design innovations as I was following someone else's proven design with a few adaptions when it came to winding coils and a few minor tweaks to bias resistors to get the desired voltage.
    The only significant difference from the G3XBM design is the lack of a crystal. Instead I'm using a Raspberry Pi as my frequency generator, running Jan Panteltje's freq_pi software. This controls the Pi SoC's clock generator, and can produce anything from the low kHz to 250MHz. Quite happy working on 70.260MHz.
    So there we are. A home made transceiver costing not a lot, and for not a lot of construction time. Rather more satisfying than a twenty quid BaoFeng handheld, I think.