Easy and PA friendly QRP SWR meter

In most cases I measure SWR only in QRP mode. Using low power level is safe and does not disturb other stations I can not hear at the time of testing.
I don't like to leave my SWR meter in the daisy-chain of the feed-line. The reason is obvious: more cables, connectors and measuring devices the bigger loss both ways. All my transceivers are able to show (let's say indicate) SWR, hence I can be aware of any changes around my antenna system simply observing the meter on the display of the transceiver. 

I used to build tiny antenna tuners for QRP field operation with built in SWR meter. The idea came from an old SPRAT. Basically it was a resistive bridge. If the output is getting nearer to 50 Ohm, which is the feed-line impedance, the SWR is lower, the needle will go back to zero. The scale of the meter is the same logarithmic one as the traditional and well known strip line type meter. Value "3" is around the centre of the scale, "2" is somewhere at the first third, and 1.5 is between "2" and the very left end. If you try to calibrate with a factory built device you certainly will find it correct. Several images of scales can be found in Google search depending on the layout of your meter.

The circuit is simple. I am using only 100-500mW to test my antennas, but if you need to use bigger power, you will need to use bigger resistors.

The detailed description and schematic can be found here:
http://www.interalia.plus.com/q_tech15.htm

To avoid stray inductance and capacitance one should keep leads as short as possible. As underestimated it is I am able to measure SWR up to 70cm UHF band.


There are some disadvantages of course: 

-You can not use it with 5kW. (I do not want to burn my hair with such a big blast on a field day, or at home, where my mast is only 9 feet (3mtrs) away from my room:) )

-You can not leave it in the daisy chain while operating your station. (I always use cables in mint condition, always make connectors waterproof, and the guys from next door are friends, hence they never want to cut my coax lead.)

-The device does not look as good as a brand new high-tech factory built item. (Who the @ cares?)


On the other side:

-It is so cheap. You can not buy a simple CB SWR meter on this price.

-It is as accurate as strip-line type meters.

-It protects your PA even in worst cases.

-It works up to UHF.

-Building something instead of spending money makes me happy:)

-Size matters on field days... it can be so small it fits in to my backpack.

It is a split version of the meter. The sensor unit (see later) is in a different box with BNC connectors on each ends. 
The meter is in a tiny plastic box with a variable resistor on the side and an RCA connector on the back. This way it can be used with other devices as well such as field strength meter, RF indicator and so on...

 The sensor circuit (actually the bridge) is in a tiny aluminium capsule. Previously it was an attenuator, but I am not sure. 
There is RF choke on the cable to the meter. It is very important to isolate the meter from the transmitter. If the ground is not perfect (it happens sometimes:) ) there is current on the braid which appears on this cable as well.

Finally a quick look inside. Does not look nice, but works reliable. Hot glue is only to keep the cable tight. All the parts are holding each other in "free space". 
Testing an antenna with an FT817 is so easy connecting the sensor to the front antenna connector instead of struggling with another big box of an SWR meter. 
The device can be built together with the meter as well of course.

How to use:

There is no switch, only the variable resistor on the meter box. All you need to do is to disconnect your antenna, set the needle to the right end of the scale and connect the antenna back, then measure. 

If you really want to leave the device in a the feed line, you should use a bypass switch to let the power go straight to the antenna.

Thanks to GQRP club for the nice doc!
:)