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 Antenna tuning and matching impedance 
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Joined: Sun Aug 26, 2012 2:08 pm
Posts: 1048
Location: Lacey, WA USA
There is, of course, the use of an SWR meter with a transmitter to measure the SWR (standing wave ratio) of an antenna at the frequency of interest. Or a dip meter may be used to find the point at which the antenna is resonant.

But a return loss bridge with either a swept frequency generator or noise source and panoramic receiver can give you a graphical reading on the resonant frequency and bandwidth of an antenna with one look.

One way is using a wideband noise source or swept signal source, a directional coupler or return loss bridge, and a panoramic receiver (ie, spectrum analyzer) like an RTL-SDR.

50kHz to 1.5GHz return loss bridge:
http://www.wetterlin.org/sam/SA/Operation/Bridge_BalunPlusBeads.pdf

Three bead balun return loss bridge, 250kHz to 1.5GHz with 3dB pads on the input and return loss output:
http://www.wetterlin.org/sam/SA/Operation/3BeadBalunBridge.pdf

How it works: A return loss bridge sits between the wideband noise source and the antenna. With no load or a short, all of the signal is reflected back. That is your reference level from the return loss bridge. The better a match the antenna is, the more of the noise signal is radiated at that frequency, and so less of the noise is bounced back. This gives you a measure in dB between the fully reflected signal and the reflected signal at any particular frequency, which can be converted to SWR with a chart or lookup table.

A video that explains it pretty well:
https://youtu.be/EDbS-zlLPxw?list=PL1295F604A4A2D331

A page that goes along with the video:
https://drive.google.com/file/d/0B7Bk3JifFLbSbTVNUWduLU9lR2s/view

A good return loss bridge is easier to homebrew than a good directional coupler. If you aren't trying to get insane bandwidth, a ferrite core with bifilar windings and three 1% 50 ohm resistors (or six 100 ohm 1%) can give you very good isolation. Use metal film or carbon composition resistors as they have flatter frequency characteristics. You will lose 6dB of signal, but even so, 10mW is generally enough to drive it.

http://www.qsl.net/kl7jef/Build%20a%20Return%20Loss%20Bridge.pdf

Image

That one is $20 from Aliexpress. Although it can be better if you put a 3dB pad at the input and at the reflection output to isolate any impedance mismatch problems from those ports, it does require more power at the input.

Of course, you can have a few return loss bridges for different frequency ranges. It'd be nice to be able to tune 2.4GHz WiFi antennas, for instance.

Here is one that goes up to 3.5GHz using two sections of RG174 coax in ferrite beads, one "turn":
http://www.studioadriana.com/vk5fe/?page_id=684

Lots more interesting construction articles there.

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Steve Greenfield AE7HD


Wed Mar 08, 2017 6:20 pm
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Joined: Sun Aug 26, 2012 2:08 pm
Posts: 1048
Location: Lacey, WA USA
Whoa. From the Ukraine, 500kHz to 3GHz return loss bridge. It requires a reference load, which will affect its performance. Read the caveats: accurate measurement to 2.5GHz, indicative measurements to 3GHz. $10 plus $5 shipping.

http://www.ebay.com/itm/RF-bridge-0-5-3000-mHz-VNA-Return-Loss-VSWR-SWR-reflection-bridge-antenna-/332052527822?hash=item4d4fdf7ace:g:dK8AAOSw5cNYQH0A

Noise source with output up to 3.5GHz:

http://www.ebay.com/itm/2016-New-DC-12V-0-3A-SMA-noise-source-Simple-spectrum-external-tracking-source/171709817697?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D2%26asc%3D41375%26meid%3Dff9221e30a64488e999ba09075a11477%26pid%3D100005%26rk%3D2%26rkt%3D6%26mehot%3Dag%26sd%3D332052527822

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Steve Greenfield AE7HD


Thu Mar 09, 2017 10:36 am
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Joined: Sun Aug 26, 2012 2:08 pm
Posts: 1048
Location: Lacey, WA USA
The AD8362 looks very useful! True RMS measurement from 50Hz to 3.8GHz. The output is a DV voltage that is a linear 50mV per dB change in input. Dynamic range of 65dB.

http://www.analog.com/en/products/rf-microwave/rf-power-detectors/rms-responding-detector/ad8362.html

Quote:
Complete fully calibrated measurement/control system
Accurate rms-to-dc conversion from 50 Hz to 3.8 GHz
Input dynamic range of >65 dB: −52 dBm to +8 dBm in 50 Ω
Waveform and modulation independent, such as GSM/CDMA/TDMA
Linear-in-decibels output, scaled 50 mV/dB
Law conformance error of 0.5 dB
All functions temperature and supply stable
Operates from 4.5 V to 5.5 V at 24 mA
Power-down capability to 1.3 mW

Used as a power measurement device, VOUT is strapped to VSET. The output is then proportional to the logarithm of the rms value of the input. In other words, the reading is presented directly in decibels and is conveniently scaled 1 V per decade, or 50 mV/dB; other slopes are easily arranged. In controller modes, the voltage applied to VSET determines the power level required at the input to null the deviation from the setpoint. The output buffer can provide high load currents.



The AD8302 accepts two signals from very low frequency to 2.6GHz. It outputs the difference as a linear function of the dB ratio (log to lin) and a voltage proportional to the phase difference. This can be used for all kinds of things including measuring complex impedance. The dynamic range is 60dB, from -60dBm to 0dBm.

http://www.analog.com/en/products/rf-microwave/rf-power-detectors/non-rms-responding-detector/ad8302.html
Quote:
Measures Independently both Gain/Loss and Phase
Operation from Low frequencies up to 2.7GHz
Dual Matched Logarithmic Amplifiers with Phase Detector
Gain Measurement range -30 to +30dB
Phase Measurement range from 0 to
180 degree
Integrated Voltage Reference and Output Voltage Circuits
Single 2.7-5.5 V Supply

The AD8302 is a fully integrated RF IC for measuring amplitude and the phase between two independent input signals. The device can be used from low frequencies up to 2.7 GHz.

The AD8302 integrates two closely matched wideband logarithmic amplifiers, a wideband linear multiplier / phase detector, precision 1.8 V reference, and analog output scaling circuits.

The applied input signal can range from -60 dBm to 0 dBm (ref 50 Ω), which corresponds to a 60 dB dynamic range.

The AD8302 output provides an accurate amplitude measurement over ±30 dB range scaled to 30 mV/dB and the phase measurement over a 0 to 180° range scaled to 10 mV/°.

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Steve Greenfield AE7HD


Tue May 30, 2017 2:21 pm
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Joined: Sun Aug 26, 2012 2:08 pm
Posts: 1048
Location: Lacey, WA USA
Linear Technology has some similar ICs that have the ADC built-in. Unfortunately, the low end is 10MHz.

http://www.linear.com/product/LTC5587

LTC5587 - 6 GHz RMS Power Detector with Digital Output about $11 from Digikey.

Quote:
Features

Frequency Range: 10MHz to 6GHz
Accurate Power Measurement of High Crest Factor
(Up to 12dB) Waveforms
40dB Log Linear Dynamic Range
Exceptional Accuracy Over Temperature
Single-Ended RF Input
0.014dB/Bit (12-Bit) ADC Resolution (VREF = 1.8V)
ADC Sample Rate Up to 500ksps
SPI/MICROWIRE Serial I/O
Compatible with 1V to 3.6V Digital Logic
Fast Response Time: 1μs Rise, 8μs Fall
Low Power: 3mA at 3.3V and 500ksps
Small 3mm × 3mm 12-pin DFN Package

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Steve Greenfield AE7HD


Mon Jun 05, 2017 9:13 am
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