NanoVNA: initial measurements

I paid $44 including shipping from Chicago, not from China. It arrived in several days. Started measuring my antennas like everyone else and then looked for more things around.

It can also be used as a signal generator. Showing 21.500MHz CW. The counter is fed with a GPS disciplined signal, so it is quite accurate. NanoVNA is off about 10Hz here.

CH1 can work as a spectrum analyzer, but wide band analysis is not possible since it take only 100 samples. Some peaks will be missed. Narrow band analysis will be fine.

1. My trap dipole (Diamond W8010) on 40m. Resonant around 7162kHz.

2. High-pass filter for BC band rejection

I made this in a EF Johnson LPF housing. It can probably handle upto 100W, but only using it for the ANAN-10 to prevent it from ADC overload when using certain antennas.

There is a strong AM station on 1400 kHz in my QTH. It even registers on NanoVNA CH1 with a resonant HF dipole, which attenuates non-resonant signals. The CH1 is calibrated against the output level of CH0 at each frequency. On 1400kHz, CH0 output is measured to be +4.5dBm, so the picture below is showing a -45dBm signal. With a long wire antenna, it is over -10dBm. In my case, just several dB of attenuation was enough to prevent the overload.

The filter attenuates the 1400 kHz signal by 6.75dB. This is more than enough to avoid overload.

At the same time, 160m is usable with this filter. At 1.8MHz, attenuation is -1.38dB. Considering typical noise floor, this is nothing.

The filter performance is acceptable up to 30MHz.

3. IF Filters

No effort was made to match the filter impedance. So ripples are pronounced and the results are not super accurate.

- 252kHz Collins mechanical filter. This filter is useful for the radios with 262kc or 256kc IF. The bandwidth is about 4kc.

- 455kHz Murata ceramic SSB filter. 3kc at 6dB

- Drake TR7/TR5 5645kHz IF filter. This one is a 50ohm filter, so had a good matching. 2.3kHz nominal bandwidth.

- Gonset 9MHz filter (1960s) supplied with a 8998 kHz xtal, wich is at the steeper side. The filter provides extra 30dB attenuation of the carrier. 3kc at 6dB.

- Inrad 4-pole homebrewer SSB filter kit.

3. Homebrew 40dB sampler.

The terrible cables were used here. The cables connected directly without the filter had about 1dB loss and their impedance was not stable. More on this later. The is based on a QST article using two toroid transformers. This is mainly used for spectrum analyzer input and PureSignal sampling for my ANAN SDR.

- 2MHz to 200MHz

- 2MHz to 30MHz

4. 1960s P&H AT-1 9dB attunation pad for the VHF transmitting converter input.

The resistors inside have increased values, so they are not precise. The converter input is at 14MHz.

5. Drake TV-3300-LP, HF low pass filter.
It says "attenuation better than 80dB above 41MHz. It shows -86dB at 41.5MHz.

6. Cables

RG-213, LMR-400 and LMR-240 cables are all being used, so not shown here.

- Monoprice 6ft RG-58 with molded BNC. It says RG-58, but the impedance is close to 75 ohm. 1dB loss at 27MHz.

- A 30ft Belden 9907 with BNC salvaged from DCL, UIUC at around year 2000. The cable was probably used for a 10 Base-2 network. They were of course long gone when I was there. Good quality.

- Cheap 6ft RG-8X from eBay. Very lossy. The cable warms up nicely when transmitting. My only complaint (you know I am kidding, right?) is it doesn't heat up uniformly.

- Cheap 10ft RG-8X from eBay. Even worse. I have a lot of these, but need to throw them away!

- 7ft Tram-Browning RG-8X, home crimped BNC. When I cut and strip it to crimp, I could see it is of good quality. I am going to replace the bad RG-8X cables with this one.

Copyright (C) 2020 Kihwal Lee