SWR: the Good HF QRP SWR

@8/27  2015

An SWR meter is mandatory for any Ham, i think.  surfing  on WWW, there are many SWR builder and many SWR bridge presented.  classify  SWR might base on different aspect of the SWR.  

for working frequency range, some only work on HF, some only function well on V/UHF band, there are some type SWR working in a really wind band, but limited on other factor, ie, resistance bridge SWR working on a wide range band but it's not a In-line SWR, after tuning the Antenna, you need remove the SWR or lift the switch to switch out the SWR.

some SWR don't need to remove from transmission line, this is in-line type SWR, most of this SWR is base on some directional coupler method.

An popular classify is based on what kind of bridge it use,  ie, the resistance bridge, Tandem bridge SWR,  Bruene bridge SWR, etc. 

some directional bridge might be good performance but building them is challenge, like sloted line coupler(<<Radio handbook>>) or the left picture *(from http://ve6atv.sbszoo.com/ ).

Selecting an Bridge 

A transformer voltage sample Bruene from QRP article by W7EL 

 http://www.qsl.net/kl7jef/QRP%20Directional%20Wattmeter.pdf

this is actually design for a Power meter, i use it as a QRP SWR. 

D1, D2: 1N60.  

T1, T2, T3:  20T on FT-37-43 (no available FT-37-72), secondary of T2, T3, primary T1 is one turn (use the component lead). 

Detector and compensate

see the schematic below.  i don't use the U2A, just use one CA3140 as compensate. 

and not use the 1k output resistor and the 0.001 uF capacitor i see no unstable problem:

how the compensate works?  the 1N60 is almost a resistor changed depending output, before it's driven to fully conduct. here is a IN/OUT testing result, the Input read on oscilloscope(pp), output is DC voltage reading from Multimeter. 

Result, a good HF up to 6 meter  SWR

* it's works well from  at least 3Mhz to 30Mhz, get well Null while connect to dummy load.

* it's also works for 6 meter, ie, the 50 Mhz. still get well Null. 

* it's can not get null result at VHF 2 meter band. 

how to use it

FWD, adjust sensitive let it get full scale.

then use REV, you got the SWR reading.

how directional coupler works

basically it's how we different the power transfer direction. In a transmission line with a matched load, the current in the direction from source to load peaks 90° than the voltage. If power flow in the other direction, this relationship is reversed. so the meter get a +-90 degree current sample add to a same phase voltage separately, we get the 2 direction's power.

one confuse thing is while i calculate the Vi + Vv, do i need to consider another side of the T1? i always make concept there mixed together, because current flowing through both T2 and T3's secondary, so i wondering the FWD position's output will impact in some way by the T3's secondary. it does affect the total current,  but actually, it's nothing to worry about to get the correct Vi+Vv.  just considering the T1's each output terminal as a AC voltage source.

for further explain, maybe more intutive, the v and i in transmition line is represent by: 

That's very obviously, to get v, -v , eachone of them plus  i, ofcause voltage get from i , we get toltally fwd or reverse wave.  To fully understand why v and i related such way, you can read following article or any text book introduce transmition line.  In the very end, you needs toltally trust Maxwell and Mathmatics.

  https://athena.ecs.csus.edu/~milica/EEE161/lecturenotes/TransmissionLines.pdf