AntennaSmith Tech Tips

TTTZ-01  Transmission Line Transformer

Description:  

Saving Six Meters on Field Day

The main tower with a six meter squalo just went up with a half hour to spare before the contest started.  The squalo won’t load with a built-in transceiver tuner or an external tuner.  There is no time to lower the tower.  What’s wrong and how do we fix it?  (hint: we were on the air on six meters in 10 minutes!)

Procedure: 

How we did it in 10 Minutes!

Brand M analyzer wouldn’t give a useful reading.  Fortunately the TZ-900 arrived on site at that moment and in about one minute, the problem was apparent – a two second sweep of the entire six meter band showed the transmission line was ok but the squalo was tuned about 1 MHz high.  The load impedance at the desired operating frequency was too high for the antenna tuners.  There was no time or components to make a L-C matching network to bring the load at the transceiver antenna jack within the range of an antenna tuner.  

Fortunately with the TZ-900 we could quickly evaluate another option – a transmission line transformer.   There also wasn’t time to make careful measurements with the TZ-900 and construct a precisely designed transmission line transformer, but two coax jumpers were located in the Field Day antenna kit.  One was four feet long and the other six feet.  Each was inserted in series with the transmission line to the squalo.  A quick check with the TZ-900 showed a significantly lower impedance at the operating frequency with the four foot jumper.  The transmission line was connected to the transceiver.  Keying the rig brought a quick thumbs-up!  The tuner in the rig had enough range to load the antenna.  Elapsed time - about ten minutes. 

Analysis: 

OK---why did this work?---clearly we didn’t fix the SWR by adding a piece of transmission line.  (For a lossless transmission line the SWR is the same one each end and everywhere in between.)

For a given SWR (let’s use 3:1 for this example),   the complex impedance (resistive plus capacitive or inductive (reactance)) presented to the transceiver can have an infinite number of possibilities.  These possibilities are described by a constant circle around the center of the Smith chart.   The actual value will be a function of the transmission line length and for every half wave of length the impedance will go around the circle one time.    It is easy to see that there are two cases where there is no imaginary (capacitive or inductive) part of the impedance but only resistive.   These points lie on the center horizontal line of the Smith Chart.   In our example of a 3:1 SWR the corresponding resistances are 50/3 = 16.7 ohms and 50*3 = 150 ohms.   We can easily go from one to the other by simply adding a quarter wavelength of transmission line at the desired frequency.   If the tuner likes 16.7 ohms better than 150 ohms,   well then this method will work.   Other easily identifiable points on the 3:1 SWR circle are where the resistance is actually 50 ohms and the impedance presented is purely capacitive or resistive.   If it’s purely capacitive,   the point on the Smith Chart will be at the bottom of the 3: 1 SWR circle and if inductive,   it will be at the top.    As the impedance can lie anywhere on the 3:1 SWR circle,   there is an infinite number of possibilities of impedances that can result.   One can easily pick where they want to be on the SWR circle by the length of transmission line.  One could then easily add the appropriate impedance to do the match-----which is what the antenna tuner did in the case of the Field Day Rescue.

Obviously, the best way to avoid the problem would have been to tune the squalo with the TZ-900 long before the tower was raised.   Since that didn’t happen, an emergency solution in the true spirit of Field Day kept us on 6 meters.

References:

The ARRL Antenna Book


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