Double directional coupler for 0.1-100 MHz - Page 1
Forum section: electronics / rf-microwave

author: RoGeorge
author_rank: Super Contributor
timestamp: July 10, 2019, 11:00:52 am
content: Questions at the end of this post.Many pics in between, yet very small (1 MB in total).The TL;DR:Was trying to learn more about RF, and made a directional coupler as in this W2AEW's video:In the YouTube comments,https://www.qsl.net/kl7jef/Directional%20Wattmeter.pdfis mentioned as reference.The build:(please note that because the reflected wave travels backwards through the circuit, and thus through the transformers, the reflected waveform seen by the oscilloscope has an inverted sign - is 180 degree shifted - from what is expected from theory, meaning the oscilloscope will show the 'Reflected' signal in phase with the 'Forward' signal when the 'RF out' is shorted, and 'Reflected' in anti phase with 'Forward' when 'RF out' is left open, which is quite the opposite):The setup:The results:Upper half of the screen:- Dark blue trace (CH4) - RF out voltage (on a 50 ohm resistor load as antenna)Lower half of the screen:- Yellow trace (CH1) - 1/50 of Forward voltage (into antenna)- Light blue (CH2) - 1/50 of Reflected voltage (from antenna)RF out open at 0.1 MHz, 1 MHz, 10 MHz and 100 MHz:RF out shorted at 0.1 MHz, 1 MHz, 10 MHz and 100 MHz:RF out 50 ohm at 0.1 MHz, 1 MHz, 10 MHz and 100 MHz:The following results are for a linear sweep between 0.1-100 MHz (10 MHz/div), 1.25Vpp connected to the RF in port of the directional coupler. RF out port is left open at first, then shorted, then connected to a 50 ohms BNC cable terminated in 50 ohms at the oscilloscope side.RF out open, linear sweep 0.1-100 MHz (10 MHz/div):RF out short, linear sweep 0.1-100 MHz (10 MHz/div):RF out 50 ohm, linear sweep 0.1-100 MHz (10 MHz/div):For the RF out 50 ohm, an ideal directional coupler would have had zero reflected voltage on a 50 ohm load, yet this is not the case here except for very low frequencies. Also, in theory the yellow trace (Forward) would have been always constant, but here it is acting strange, especially when the RF out is shorted.The questions:1. Why is it working properly only at very low frequencies (around 1 MHz works great, at 10 MHz is already lousy)?2. Any chance to make it work up to 100MHz, and how?3. For a low power directional coupler (max 2.5Vpp on a 50 ohm load, +12 dBm), is the range 0.1-100 MHz unrealistic for this type of design?

author: awallin
author_rank: Frequent Contributor
timestamp: July 10, 2019, 07:37:59 pm
content: I guess you don't have a spectrum analyzer? if you make or find software to scan the input frequency and capture the waveforms on the scope and from that calculate S-parameters then it would be much easier to say something about how your dir-cpl design works.

author: RoGeorge
author_rank: Super Contributor
timestamp: July 12, 2019, 06:02:40 pm
content: After replacing the ferrite cores, it works much better, but still far from ideal.This is the response with the new ferrite cores, and only 25 turns instead of 50:- BLUE trace is the voltage at the 'RF out' port with a 50 ohm cable terminated with a 50 ohm load- YELLOW trace is the voltage at the 'Forward' port- LIGHT BLUE-GREEN trace is the voltage at the 'Reflected' port- the 'RF in' port was fed at 1.25 Vpp and a 10MHz/div linear sweep in frequency (between 0.1-100 MHz for the whole screen)Compared with the previous picture and previous build, this one looks much better, but I still don't get it.Why does the reflected voltage (light blue trace) increases with the frequency almost linearly? Is this just an artifact caused by the imperfections in the build or by the RF losses in the ferrite cores , or is it that indeed the reflections increases with frequency?

author: ogden
author_rank: Super Contributor
timestamp: July 12, 2019, 06:36:44 pm
content: Quote from: RoGeorge on July 12, 2019, 06:02:40 pmIt's because this is "exploded view coupler" aimed at educational purposes. Example of DIY coupler designed for practical use:https://www.changpuak.ch/electronics/DIY-DualDirectionalCoupler.php. More about DIY couplers:http://michaelgellis.tripod.com/direct.html Yes, at high frequencies usually directivity gets worse.