I am currently building a two element portative cubical quad with one reflector and one driven element. the conductors of the driven element and reflector are encased in pvc tubing.

I was wondering what tuning stub design I could use for the reflector. I didn't find a lot in the literature.

Currently, I am thinking of a conductive bar bolted across two long screws. it would be quite precise for tuning.

Now for the choice of metal: it has to be a compromise between corrosion resistance, price and conductivity. I was thinking of corrosion resistant brass. for a better and more expensive design a silver or gold plated stub would be better, I suppose ?

Anyway, I have no idea of the best geometry for the stub. I suppose it has to have a decent spacing or it will behave like a transmission line.

Any ideas or references welcome.

  • $\begingroup$ Please explain why you need a tuning stub. $\endgroup$ – Brian K1LI Mar 24 '19 at 13:30
  • $\begingroup$ Basically to alter the resonant frequency. and since i have an antenna analyzer, I would be able to see what's going on when i tune the reflector. So basically : experimentation $\endgroup$ – rodv92 Mar 24 '19 at 13:55
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    $\begingroup$ Great - we love experimentation! And, please tell us the frequency range over which you want to experiment. $\endgroup$ – Brian K1LI Mar 24 '19 at 14:00
  • $\begingroup$ well I am building a prototype that I will use as a low power max 10W FM broadcast antenna and/or RX antenna. currently the driven element is 3.22 m long single strand insulated copper wire encased in a 16mm pvc electrical conduit. I am trying to make the resonant frequency a bit below the center of the broadcast FM band so it is 95 Mhz. I used qsl.net/yt1vp/CUBICAL%20QUAD%20ANTENNA%20CALCULATOR.htm $\endgroup$ – rodv92 Mar 24 '19 at 16:10
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    $\begingroup$ the pvc and wire insulation will probably change the velocity factor and the electrical wavelength ? I will post the pics too of the driven element $\endgroup$ – rodv92 Mar 24 '19 at 16:13

Using this Cubical Quad Antenna Calculator, it appears that the length of your driven element targets 95MHz operation. As you surmise in your comment, wire insulation and PVC housing will reduce the velocity factor of the elements, which in turn will make the elements electrically longer and lower the antenna's operating frequency.

Tuning stubs are made from transmission lines; the word stub refers colloquially to a short section of transmission line. The tuning stub acts as a reactance in series with the antenna conductor: series capacitive reactance will shorten the electrical length, while inductive reactance will lengthen it. Applying stubs to the driven element and to the reflector will both affect driving-point impedance; applying a stub to the reflector will also change the antenna's directional characteristics.

The stub's reactance depends on its length, the characteristic impedance of the transmission line comprising the stub and whether the end of the stub is a short- or open-circuit. For short-circuited stubs: $$Z_{sc} = jZ_0 tan(\beta l)$$ while for open-circuited stubs: $$Z_{oc} = -jZ_0 tan(\beta l)$$ where $l$ is the length of the stub and $\beta=\frac{2\pi}{\lambda}$. Since $\lambda$ is the wavelength in the transmission line, the line's velocity factor must be figured into the calculation. Assuming you want the shortest practical stub, $\beta l$ should be kept to less than $\frac{\pi}{2}$, the length at which the sign of the reactance changes, so $l$ should be less than $\frac{\lambda}{4}$.

Since you are operating in the vicinity of 100MHz and using only 10 watts of power, it will probably be most convenient to make your stub from an inexpensive RG-58 or RG-59 type of coax rather than making your own transmission line. 50-$\Omega$ RG-58 coax is popular in amateur radio, while 75-$\Omega$ RG-59 coax is commonly used in CATV installations. Using RG-58/U as an example,

$$\lambda = \frac{v c}{f} = \frac{0.66 \times 3\times10^8 \left.\mathrm{m}\middle/\mathrm{s}\right.}{95\times10^6\mathrm{Hz}}= 2.08\mathrm{m}$$ and $\frac{\lambda}{4}\approx0.5\mathrm{m}$ Changing the length of an open-circuited line is simply a matter of cutting the line to different lengths. The length of a short-circuited line can be trialed by inserting a pin in the coaxial cable to short the braid to the center conductor.

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    $\begingroup$ thanks for all the information. I will try a parallel line made with some durable material. it will be easier to retune multiple times. making a puncture to short the coax at various points is a nice trick that i will try too. the advantage of a screw and bolts stub IMO is that I can short the stub at various points with a bar made of single side PCB. I could even push the experimentation with a placeholder for a variable capacitor or an inductor on the PCB to stitch the stub so I can test both inductive and capacitative setups. $\endgroup$ – rodv92 Mar 26 '19 at 11:38

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