Antenna testing is hard (II)

This article follows up on this one. The title could also be: doing measurements on hand held radios is hard. Actually, some measurements I did in the past might not be so accurate after all.

Measuring in dBm
If you measure in dBm, what I always did, you assume that the device at the other end is 50 Ohms, just like the manufacturer promises you in the specs. Slowly but surely we are finding out that this is not always the case. As a result I might have to switch to microvolt (µV), a method which doesn’t require a perfect 50 Ohms at the other end.

Can’t we just convert it? 0.5 µV @ 50 ohms = -113 dBm, a piece of cake, right? No. You must know the actual impedance of the receiver if you want to convert from dBm to µV. Erik PE1RQF did some measurements on a few hand held radios. The outcome was, well, a bit scary.

Impedance

If a device is 50 Ohms, the SWR should be 1:1. Only a perfect dummy load and the Marconi RF generator are.

What this means for us
What this outcome basically means is that generating cold numbers on sensitivity and antenna performance are nice, but don’t tell the whole story, or could be misleading.

  • Antennas which prove to be the best performers (RX/TX, VSWR), tested under optimal conditions, are not necessarily the best performers on a specific brand/model of hand held radio.
  • There are antennas which aren’t 50 Ohms at all, but could very well outperform everything else on the market because your radio isn’t 50 Ohms either.

Do the test yourself
The best example I can give you is this one: take a Baofeng BF-666S, 777S or 888S, use the stock antenna, and make notes of the performance in the field. Try to hit repeaters which are barely in range. If you have a field strength meter, measure field strength when transmitting.

Remove the stock antenna and replace it by a Nagoya NA-701, NA-771, the $3.79 antenna, or the Baofeng UV-B5 antenna. I tried all of these; just pick whatever 3rd party antenna you have. Repeat the tests.

What happened here is that the short stock antenna was the best of the bunch, all 3rd party antennas had a negative influence on the performance of this specific Baofeng model. The same 3rd party antennas mentioned above did improve the performance of the Baofeng UV-5R, often by a wide margin.

Exception to the rules
The funny thing is that reception suffered greatly too. This is quite uncommon and a sign that hand held radios don’t follow the rules. For example, if you just want to receive on 20 meters and your dipole is 2×6 meters instead of 2×5 meters, you would never notice the difference. With your hand held radio however you will.

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Baofeng squelch: measurements

Erik PE1RQF offered to do measurements on his Baofeng BF-F9, before and after changing default squelch levels with Chirp. As expected after field tests, the differences are substantial. Here they are:

VHF (default settings, thresholds in dBm)

SQ Set BF-F9 setting Threshold
1 40 -124,6
2 41 -123,8
3 42 -122,2
4 43 -120,8
5 44 -120,6
6 45 -119,9
7 46 -119,4
8 47 -118,8
9 48 -118,4
VHF-Default

VHF, Default thresholds

VHF (new settings, thresholds in dBm)

SQ Set BF-F9 setting Threshold
1 24 -130,4
2 29 -131,4
3 34 -130,9
4 39 -129,6
5 44 -125,5
6 49 -120,1
7 54 -115,3
8 59 -110,2
9 64 -105,1
VHF-New

VHF, new thresholds

UHF (default settings, thresholds in dBm)

SQ Set BF-F9 setting Threshold
1 29 -140,1
2 30 -130,9
3 31 -129,0
4 32 -127,4
5 33 -126,6
6 34 -125,4
7 35 -123,8
8 36 -123,0
9 37 -121,6
UHF-Default

UHF, default thresholds

UHF (new settings, thresholds in dBm)

SQ Set BF-F9 setting Threshold
1 24 -133,1
2 29 -133,1
3 34 -125,6
4 39 -120,7
5 44 -114,1
6 49 -108,6
7 54 -103,6
8 59 -97,6
9 64 -93,7
UHF-New

UHF, new thresholds

Understanding FFT (Fast Fourier Transform)

If your old oscilloscope broke down and deemed beyond repair, you probably want to replace it. You will find out that modern DSO’s (Digital Storage Oscilloscopes) can do much more than classic, cathode-ray models. One of the many new and exciting functions is FFT, a feature I immediately checked out when I purchased my Atten ADS-1102CAL. A review of my model can be found here.

FFT adds an extra dimension to the standard time domain: the frequency domain. All oscilloscope users are familiar with the time domain; just feed a waveform into your scope and the display will show shape and voltage over time:

ADS00003The frequency domain on the other hand shows what voltage is present at each frequency and will produce a screen we know from the spectrum analyzer:

GT-3-UHF2Some measurements which are very hard in the time domain are very easy in the frequency domain. Consider the measurement of harmonic distortion. It’s hard to quantify the distortion of a sine wave by looking at the signal on an oscilloscope. When the same signal is displayed on a spectrum analyzer, the harmonic frequencies and amplitudes are displayed with pinpoint precision.

Another example is noise analysis. Looking at an amplifier’s output noise on an oscilloscope basically measures just the total noise amplitude. On a spectrum analyzer, the noise as a function of frequency is displayed. It may be that the amplifier has a problem only over certain frequency ranges. In the time domain it would be impossible to tell.

Most modern DSO’s, even the affordable models made by Atten and Rigol, are capable displaying both the time- and the frequency domain, and can do so at the same time. The time domain will be displayed at the upper half of the screen, the frequency domain at the lower half of the screen.

Atten-FFTWhile FFT on a DSO is not as accurate as a real spectrum analyzer (especially when reaching the upper limit of the scope’s bandwidth), it will give you a reasonably good idea of what’s happening.

How FFT works
If you’re new to FFT, your head might explode while trying to get a grip on it. I still struggle with it.

Fourier’s theorem: any waveform in the time domain can be represented by the weighted sum of sines and cosines. The FFT software samples the input signal, computes the magnitude of its sine and cosine components, and displays the spectrum of these measured frequency components.

Got it in one go? I didn’t when I tried it for the first time. Yesterday I ran into this website, which will be of some help.

fourierYou can see this in a simulation when you click here: http://hascanvas.com/fftvisualize2

The guys from Tektronix made a nice YouTube video which explains a lot too.


Note
: do not connect your FFT capable DSO to whatever transceiver without proper external attenuation. Check the manual for maximum input level (often referred to as ‘damage level’) before measuring.

Feed the DSO with as little power as you can get away with. This will prevent overloading the DSO’s circuits and will result in more accurate measurements.

Have fun!

Google+ community

While I’m finishing up the Puxing/Wouxun review I started a Google+ communityGoogle+ to discuss Chinese ham-related products. These could be (but not limited to) radios, test equipment, accessories, kits, spare parts or components.

I also plan to invite manufacturers and sellers to join, which will give members the opportunity to give feedback and ask questions.

There’s a fine line between information and SPAM; this will defined in more detail later in time.

 

What? Yellow multimeters? Destroy them!

$30K worth of multimeters destroyed – – because they’re yellow

Sparkfun, a hobbyist electronics retailer, recently received a letter from U.S. Customs saying a shipment of 2,000 multimeters was being barred from entry into the country.

The reason? Trademark law. A company named Fluke holds a trademark on multimeters that have a ‘contrasting yellow border.’ Sparkfun’s multimeters are a yellowish5-in1-multimeter orange, but it was enough for Customs to stop the shipment. Returning the shipment is not an option because of import taxes in China, so the multimeters must now be destroyed.

At $15 per item, it’ll cost Sparkfun $30,000, plus the $150/hr fee for destroying them. Sparkfun had no idea about the trademark, and doesn’t mind changing the color, but they say restrictions like these are a flaw in the trademark system. “Small business does not have the resources to stay abreast of all trademarks for all the products they don’t carry. If you’re going to put the onus on the little guy to avoid infringing IP then you shouldn’t need an army of consultants or attorneys to find this information.”

(Thanks Brick for the pointer, source Slashdot.org.)