The tall onces: 5 windings and a 271 pF 2 kV capacitor in parallel, they resonate at 14.125 MHz, the small onces are similar, but the resonate in the center of the 40m band. You design them with the dipmeter program on the VNA. The Q of the LC circuit is not really large, the bandwidth of the trap is of the order of 1 MHz. The capacitor has to withstand high voltages, so get the capacitors that survive 2000 volt or more. Not sure to what power you can use them, but we will soon find out I guess.

A trap is stopper for a designed frequency, so you can cut a 40m antenna short with a 20m trap and a 80 meter resonant antenna short with a 40m trap. An antenna with a few traps is then in resonance at a couple of frequenties. On hindsight I like the large traps for the 20 meter band better than the smaller traps which are wound around PVC tubing. The large onces are made from an antenna to spanner cable separator, a piece of plastic 12 cm in length and 2 cm diameter, including two mounting holes suitable for a M4 or M5 bolt.

Last update: 14-oct-2017


What is the noise floor of my SDR?

The SDR that I use next to my transceiver is the airspy/spyverter combination. It should, like any other receiver, have a certain noise floor (NF), but this figure was hard to find on the internet so that I decided to measure it myself. In this blog I will describe how I measured the NF, the end result is 132 dBm for a bandwidth of 500 Hz. That makes the combination equivalent to what you find for several other amateur radios. For this experiment I used: a VNA, a directional coupler, a dummy load and two fixed attenuators of 20dB and 30dB. The used VNA from metropwr has all required options that you need. The idea is that we insert a known signal (S) into the receiver, and that we measure with the SDR# software the signal to noise ratio (SNR) of this signal at a various frequencies in the HF domain. I simply picked them in the middle of each amateur band.

The metrovna has a rfgenerator program and the output level can be measured with the DET port. The rfgenerator signal level is approximately -8,5 dBm which is a bit too high for the SDR. The dynamic range of the SDR is determined by the number of bits of the ADC which is 12, you can count 6dB per bit, so the dynamic range is approximately 72 dB. The inserted signal S should be within the dynamic range relative to the noise floor of the receiver, with the 30 dB attenuation from the directional coupler and 20 dB or 30 dB from the fixed attenuators we can get down to more or less acceptable signals that can be inserted into the SDR as a reference signal. The AGC should be off, and the tracking filter should also be off in the SDR# software.

The inserted signal level is then known in dBm, you measure the peak dBFS (dB full spectrum) and the floor dBFS and the difference results in the measured signal to noise ratio (SNR) by the SDR. Subtract the measured SNR (in db) from the inserted signal and you get the noise floor of the receiver which holds for the bandwidth of the ADC. Actually, this is rather complicated with the airspy/spyverter combination, the full bandwidth is 8 MHz, but you normally decimate the full 8 MHz  64 times so that the effective bandwidth becomes 125kHz. In order to go to a noise floor spectral density you should therefore subtract 10 log10(125000) from Sdb – SNRdb, the units become in the end dBm/Hz.

Next you need to correct the spectral density to a dBm value characteristic for a 500 Hz bandwidth, this is how they are shown in the Sherwood table. To summarize the mathematics:

NFdb = Sdb – SNRdb – 10 log10(125000) + 10 log10(500)

where Sdb is derived from the rfgenerator of the VNA inserted into a 30dB directional coupler connected to a dummy load, the 30 dB directional coupler has a higher loss at low HF frequencies, but this is what you can measure with the DET port on the VNA. The output of the directional coupler is brought down with 20dB and 30dB fixed attenuators. In the end Sdb in the equation is what we insert as a known signal in the SDR, and we represent the noise floor for a 500 Hz wide signal. The outcome is:


The noise floor NFdb is displayed against the frequency in MHz, its average value is 130,5 dBm if we include all measured values, and 131,8 dBm when we leave out the frequencies below 5 MHz. So this means that the noise floor becomes to 132 dBm for a bandwidth of 500 Hz. The excess noise density relative to the room temperature thermal noise of 174dBm/Hz is 16,5 dB/Hz.

Two meter 3-element Yagi

You get everything from the local hardware store, 20mm square pipe, 8 mm tube, 5mm stick, all aluminum and 1 meter in length. Some polycarbonate, butterfly nuts, bolts, wire straps and hot glue do the rest. The balun is a 1/4 and 3/4 lambda RG58u coax balun, all dimensions and positions of the reflector, radiator and director are listed here. Here are some photos.

Yellow is the reflector, blue is the radiator, red is director
Backside of the reflector
backside of the radiator
backside of the director
Frontside of the radiator
Radiator and coax balun
Frontside of the director

I’ve tested it once in the garden, the 857D said the SWR was fine, the radiation pattern of this yagi suggests a 16 dB front to back ratio. The 3dB beam angle is approximately from -35 to +35 degree, it could be nice for fox hunting on two meter. Also, it is lightweight: 700 grams.

Last update: 17-sep-2017.

Bandpass filters

During the last lighthouse weekend in Ouddorp Zeeland we found out the hard way that there is a lot of radio and radar equipment in lighthouses; this resulted in high QRM levels on the HF bands, the noise floor on my FT-857D was S7 to S9 on the 80m band near the lighthouse. Also, radio operators next to one another on different bands interfere with each other. The advise I got was to invest some time in set of a bandpass filters (BPFs). I got a DIY filter kit from DG0SA. His BPFs are rated at 200W, they are traditional 3-kreis filters and you can get them for all amateur bands. I have built the 80m, 40m and 20m BPFs and this is what you get:

The documentation of the filters can be found on Wolfgang Wipperman’s website
In the end this is what they look like, it took me 12 hours to build three filters. Tricky is the L2 torroid which comes with a trap. You can adjust C2 with tuning capacitors.
Performance of the 20m bandpass filter.
Performance of the 40m bandpass filter.
Performance of the 80m filter.

The insertion losses of the BPFs within the amateur bands is less than 1dB, and the crossband suppression of each filter is approximately 30dB, if you want something better than this then you need to build higher-order filters. I thought it was too much trouble for what I wanted. Wolfgang provides tuning capacitors to adjust the C2 capacitor, but in the end I did not use them. I did check the resonance frequencies and the filter characteristics with my VNA, without a VNA you need to find a friend to help you out.

Receiver test with the Yaesu FT-991

I tested the 40m filter on a weak station, an S1 station on the 40m band with the FT-991 directly connected to the g5rvj antenna (DSP filtering was off, no pre-amplifiers: IPO and no noise blanker). The BPF was inserted in the 50Ohm segment before the tuner that I use for the g5rvj antenna. Once you insert the BPF the S1 station becomes somewhat stronger, possibly up to 2S points. Reason is that the FT-991 is challenged by strong out of band signals that enter the receiver. With a BPF you make it easier for the receiver to select only that signal you are interested in, reason is that the BPF suppresses everything outside the 40m band. This may be an explanation why the signal to noise ratio is somewhat improved with the BPF.

Receiver test with the SDR: airspy / spyverter combination

The test is similar, but also more interesting because the SDR comes mostly with digital filtering, nothing else reduces out of band signals and it is expected that the SDR will be more affected by large out-of-band signals. The SDR combination is connected to my favorite active antenna that I describe here. The consequence of inserting a 40m BPF is illustrated in the waterfall plot below, we are centered on the high end of the 40 meter band.


The black bar in the center of the waterfall plot is the BPF switchover point, the lower part shows the waterfall where the BPF is inserted, the upper part sis the waterfall where the SDR is directly connected to the active antenna. All other settings in the SDR# software (gains for HF, mixer, IF and the AGC are the most important parameters) are the same. So here we also see a gain of a couple of dBs, I measured 5 dB, moreover you can see that the intermods caused by strong broadcast stations is significantly reduced. This test works best in the evening when the 40m propagation is enhanced compared to the daylight hours.

Intermodulation products are likely to be reduced by inserting a BPF before the receiver, the plot below is the same as others here, the lower part is without the filter, and upper part is with. The intermodulation product of a broadcast station is indicated by the yellow arrow, and it disappears when you insert the filter.


Last update: 6-sep-2017

Fieldday 31-jul-2017

On 31-jul I was in the field, a CG3000 tuner and a wire round around the mast up to the tip. The ground connector of the CG3000 goes to three 10 meter radials. Benefit of this set-up is that you are QRV on all bands from 160 to 10m. The CG3000 can handle up to 200W, the FT-857D produces no more than 100W. Use 10W AM to activate the tuner.

12,5 meter glass fiber mast
use the chair for the radio
CG3000 tuner and three radials
this is what you need to carry

Last update: 13-aug-2017.

800 Watt 1:4 Balun

The site of DG0SA Wolfgang Wippermann has a large collection of baluns. I replaced the 1:4 guanella balun with the 800 Watt 1:4 balun designed by Wolfgang, goal is to be able use it for higher power. Two ferrite cores are used, the first is a common mode filter to go from unbalanced to balanced for undefined impedance, the second is a 1:2 voltage transformer. I used volcanic wrap tape to isolate the leads between the transformers. As you can see in the images below, PVC tubing of 75 mm is well suited as housing, so this is what you eventually get:

And this is what is inside, the wiring is PTFE AWG18, so teflon coated rugged litze which is somewhat difficult to wind. This was the last view before I glued the cap on the side of the SO239 connector.

And this is a similar view at the other rise, I used plastic and metal rings to keep any water and debris out of the housing. Prefer to leave the cores loose in the tubing, the may generate some heat, once installed it does not move, could be done better but I ran out of ideas here.

And this is what the SWR looks like when I measure it with a 200 Ohm resistor terminated at the end of the balun.

Last update: 2-Jul-2017