If you find that the aerial is too long at the working frequency, you could apply the design of fig. 6 where a coil is inserted at the base of the aerial. The coil is made with 17 turns of 2mm electric wire on a 17mm form. These data are valid for the 27Mhz band and must be properly scaled at other frequencies. In all cases the vertical coax must be shorted at the far end because it works also as an impedance matcher for the rest of the aerial.
Installation can be vertical, as shown in the drawings, or horizontal.
Gain was measured between 6 and 9 dB but the instrument available was not very accurate and I cannot bet on the number reported. A reverse biased transistor does the job in a nice way. If a BC337 is used, then the resistor is 390K for a 220V mains and 180K for a 110V mains. The capacitor could be damaged as well. A permanent ground, or no signal, at the trigger input starts the timer.