Good Enough

Designing anything new rarely goes as expected.  Of course you try to design as much as possible using technology and designs you are already familiar with.
I was working with a monopole wire antenna this week.  I needed to tune and impedance match the antenna to be as efficient as possible between 905 and 930 MHz.

The tool I had available to do this tuning with is a Spectrum analyzer with a built in Tracking Generator and an external return loss bridge.

Note: For optimal transfer of energy from a transmitter to an antenna, they have to have the same impedance.  Most rf transmitters are designed with an output impedance of 50 Ohms. This means that the most efficient antenna will have a characteristic impedance of 50 Ohms. The characteristic impedance of an antenna depends on it’s length and it’s surroundings.

When the impedance doesn’t match, a matching network is needed to fill in the gap.  If an antenna looks more like a capacitor than an inductor, then a series inductor can be used to make up the difference.  If the antenna looks more like an inductor, then a series capacitor can bring it back to close to 50 Ohms.  Of course the series inductor or capacitor will burn a little energy but most will get transfered to the antenna.

The antenna I worked on this week tuned fairly easily around 860 MHz.  When I cut the antenna shorter for 915 MHz, it became very hard to tune.  My job would have been a lot easier if I had a network analyzer that operated in the 900 MHz Range.  An analyzer would have told me the characteristic impedance of the antenna as well as whether it was more inductive or capacitive.

I didn’t have the a network analyzer and was under a time and budget constraint. My solution was to put different inductors in series and see if that would work. It worked great at 860 MHz. It didn’t work at 915 MHz.  My next idea was to put a capacitor in series, the only capacitor I had that would fit on the PCB was a 0.1uF.  This would be too high of a value of reactance to be useful.

My next idea was to put an inductor to ground to reduce the inductive reactance of the antenna.  This produced mediocre results.  I could only get about 85% efficiency from the antenna in the 900 MHz Band.  I tried successively larger values looking for the best return loss.(most emitted energy).

I had to settle for “Good Enough”. If I get the chance, I would like to go back and buy a capacitor kit that will let me try the series capacitors.  The antenna will work well, but the overall range of the radio will a little bit smaller.

Have you had to settle for good enough?  What are your thoughts about how “Perfect is the enemy of good”?

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