Yet Another QRP Portable T-Tuner and SWR Bridge for Cheap

As part of project LOAP (Lipstick On A Pig ===>, I wanted a small cheap matching and SWR indication unit for the Pixie-based transceiver system.  The T-Tuner and SWR bridge unit on the right, in the picture below, is not vastly different from others, but here it is for your consideration as a good performer at a low cost.  Note however, that it has been characterized into real-world impedances with a worst-case loss of less than 0.9 dB.

BMU and T-Tuner Top View QRP SystemThis unit has been run with 5 Watts CW for long periods into the load impedances presented by a BMU matched 43 to 48 foot vertical, sloper, or Inverted-L.  All bands are easily and efficiently matched on 80 M to 10 M ham bands..  The corresponding QRP Base Matching Unit (BMU) is described here ===>

XCVR - T-Tuner - BMU - Antenna - QRP

Above is the block diagram of the QRP system.  Note that the BMU is placed at the Base of the antenna.

Here is the schematic of the QRP Portable T-Tuner and SWR Bridge ===>

FIGURE 23 - T-Tuner SWR Bridge QRP Schematic - 2

Note that the standard 50 Ohm SWR bridge shown above uses a 270 ohm resistor at “R Led” for operation at the 1 Watt level.  This resistor must be increased in value for higher power operation.

The “Tapped Powdered Iron Toroid” used in this design is a T68-6.    It has 40 turns total, and is tapped as follows:

5 T   –   0.23 uH  –  connected to S8
6 T   –   0.30 uH  –  connected to S7
8 T   –   0.47 uH  –  connected to S6
11 T  –  0.79 uH  –  connected to S5
15 T  –  1.36 uH  –  connected to S4
20 T  –  2.27 uH  –  connected to S3
25 T  –  3.70 uH  –  connected to S2
33 T  –  5.70 uH  –  connected to S1
40 T  –  8.05 uH  –  connected to variable capacitor T-junction

Note that the dirt-cheap DIP switch used in the unit pictured is a surplus unit and was more or less tried as an experiment to see if it would reliably work – which it has thus far to at least 5 watts.  However the voltage spec for the unit pictured is lower than voltages actually encountered.  It would be a good idea to use a DIP switch or rotary switch with proper specs in the neighborhood of 50 volts or more.

T-Tuner SWR Bridge Bottom View QRPThe miniature variable capacitors are available in various capacitance values on eBay, generally for less than $3 apiece.  Cores are available from standard parts suppliers like DigiKey or direct from Amidon at a few $ each.

T-Tuner SWR Bridge Top View QRP


Efficiency of the unit into actual impedances presented by a BMU connected to actual antenna imnpedances was measured using the first step in the procedure shown in the  article “Z-Substitution” Method for Measuring UNUN Loss Into Real-World Load Impedances”.  Loss for both of the unis above was virtually identical.  Since the impedance seen by the T-Tuner is a function of the length of the transmission line between the BMU and the tuner, actual loss values will vary.  Losses shown below are for a short coax line of 4 feet:

80 M / 75 M:  -0.32 dB
60 M:  -0.90 dB
40 M:  -0.85 dB
30 M:  -0.22 dB
20 M:  -0.85 dB
17 M:  -0.65 dB
15 M:  -0.73 dB
12 M:  -0.30 dB
10 M:  -0.30 dB


Copyright Dave Benzel- KD6RF – 2017 Mar 2


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