# cubical quad reflector tuning stub design

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.

• Please explain why you need a tuning stub. – Brian K1LI Mar 24 '19 at 13:30
• 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 – rodv92 Mar 24 '19 at 13:55
• Great - we love experimentation! And, please tell us the frequency range over which you want to experiment. – Brian K1LI Mar 24 '19 at 14:00
• 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 – rodv92 Mar 24 '19 at 16:10
• 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 – rodv92 Mar 24 '19 at 16:13

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.