Directional Coupler

 When I erected my 80m aerial I delberately made it longer so I could trim it later. The usual way to do this is to use a transmitter and SWR meter and, by trial and error, just chop off short lengths of wire until the performance is optimum for one's favourite frequency.

Having acquired a spectrum analyser I wondered if this could be used to look at the aerial's performance directly.

Looking on the Net it seems a directional coupler could be used to do this. This device is basically the main part of an SWR bridge. Add a couple of diodes and a sensitive meter and you can measure forward and reflected power. The greater the difference between these two values should provide the resonant frequency of the aerial.

The picture below shows a directional coupler I made in an hour or so from junk box bits and pieces.

The tracking generator feeds in at the lower left and the 80m aerial upper left. The spectrum analyser input is fed from the right.

 The main part is the box. I salvaged this from an old tone switch. This was once used to select an LNB and had three of those satellite connectors which I unscrewed and replaced with screw-in BNC sockets. A solder tag and a pair of 100 ohm resistors (to give me a 50 ohm load) plus two ferrite rings and some wire completed the parts list. I followed the circuit for a home-made directional coupler from here...

I suspect I connected one of the coupling coils in the wrong phase so I just switched the input and output cables. This did the trick and I was able to use my spectrum analyser and its tracking generator to provide the following traces. The ten turns used on the ferrite rings provide 20dB attenuation.

Circuit used

See the Elecraft dual directional coupler which has a similar circuit but can provide switched forward and reflected outputs.

Below: 80m Inverted Vee fed with URM67 coax

 The picture above reveals the resonant frequency of my inverted Vee aerial. On the right is a second resonance at the third harmonic of resonance. As you can see, subject to measurement inaccuracies the point of resonance is around 3.28MHz.

The RF baseline is about -21dBm and the peak -37.6dBm. The actual numbers reflect the power level of the tracking generator and system losses.

 Above you can see the second marker at what is ostensibly the third harmonic of resonance which is indicated as 10.38MHz, not a true third harmonic of 3.28MHz.

 Above you can see the third marker at what is ostensibly the fifth harmonic of resonance which is indicated as 17.26MHz, again not a true harmonic of 3.28MHz and below the 4th marker at 24.29MHz, representing roughly the seventh harmonic. Other resonances appear as the scanning frequency increases.

 The next day I decided to continue experimenting. First, I chopped 30cm off the end of one leg of the dipole. The resonance went from 3.23MHz to 3.28MHz then another 30cm chopped off the same leg raised the resonant frequency to 3.31MHz.

As it wasn't convenient to chop anything from the other end of the dipole I paused to consider the results. What's the impedance of the dipole? I noted the resonant point at -50dBm. This value of course is dependent on the tracking generator level and the loss in the directional coupler. Fitting a 50 ohm terminator in place of the aerial gave me a flat level of -62dBm.

The difference between -50dBm and -62dBm must indicate the impedance of my dipole?

I decided to fit resistors of various values in place of the 50 ohm terminator.

See the table below

 R ohms

RF level dBm













Plotting the figures produces a graph which roughly indicates my aerial is about 62 ohms. Fitting a potentiometer at the aerial socket would allow me to determine the dipole impedance.

I imagine that chopping lengths from both ends rather than one will shift the dipole impedance, not just its resonance? Another experiment perhaps?

The next experiment was to raise the height of one end which was around 3 feet above the ground. I'd noticed the resonance peak shifting around with the wind but when I raised to end to around 8 feet by adding a guying wire the variations lessened considerably. Resonance was also higher to around 3.4 MHz with third, fifth and seventh resonances at 10.69MHz, 17.89MHz and 25.09MHz respectively. I then raised the other end but as this is now close to a tree I think extra ground capacity has come into play. Impedance measured around 67 ohms.

I then experimented with the directional coupler. By adding 300pF across the aerial connection and ground the resonance peak became a lot sharper. I think this might be happening because the aerial impedance doesn't match 50 ohms and the additional capacity shunting the coax is bring this into line. The impedance of 300pF at 3.4MHz is about 156 ohms and this across the previously measured 67 ohms gives about 47 ohms.

Next I tried the aerial on 80m. Running the transmitter at 3.5MHz gave me an SWR of around 2:1 and higher frequencies were much worse, so the next job is to make sure the aerial is in a clear area and shorten it to give me resonance around 3.65MHz........


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