I wanted a robust, portable, compact antenna usable for horizontal and vertical polarisation to take along with my FT-817 when I go on vacation, camping etc. I also wanted the dipole to cover the VHF and UHF bands and if possible with some gain! As everybody knows a dipole has about 3dB gain over a quartre wave antenna, is why my preference went to a simple dipole. To make it portable I used two identical telescopic antennas of 80 cm slid out and only 14 cm when pushed in.
What you need to make this handy portable wide band dipole is:
Two chrome-plated telescopic antenna's (14cm/80cm)
A robust PVC box of 75x50x22 mm and 1,5mm thick
a silver plated stereo mini switch
a professional female BNC connector
some low loss RG174 50 Ohm coax
carbon/ferrite bead or toroid (to act as a shoke)
some 0,6mm enamelled copper wire to make the coils (like from a transformer)
a small ferrite core to place in L2 (approx 4 mm diameter)
glue gun and/or silicone
BNC --------------------------- silver plated switch
carbon based bead ------ RG174 thin low loss coax
chrome telescopic antenna ABS plastic anti-static box
I did not find a telecopic antenna small enough to reach 450 MHz and still long enough to reach 50 Mhz (1,5 meter), so the only sollution was to tune the dipole (made out of two 80 cm telecopic antennas) for 50Mhz using two coils (base loaded dipole). A double silver plated switch is used to shorten the coils when using it for frequencies above 50 Mhz. The gain when using the coils (loaded dipole to get a shortened dipole) on 50Mhz is approx. the same as a quartre wave, meaning no gain.
You can use any size of telecopic antenna, but just beware that the minimum length determines the highest frequency and the maximum length of the telescopic antenna determines the lowest usable frequency!
In my case, using a 14cm/80cm telescopic antenna, I calculated the working frequency range:
90MHz to 470MHz (switch closed)
48MHz to 90MHz (switch open)
You can tune the dipole to the desired frequency by sliding both telescopic antennas in or out.
This little antenna could also be used for satellite as the dipole tuned to 145 MHz works reasonable well on RX @ 435 MHz.
We need two pieces of emalled 0,6mm copper wire (stripped from a transformer will do). Wind the wire around a 5,5 mm drill till you get 8 turns. Both coils are exactly the same with one exception: in the coil mounted on the ground side (L2) we put a little ferite core inside the coil and seal it with some glue. When the switch S is open they will allow you to use the dipole on 50Mhz when both telescopic antenna's are in full length (2 x 80 cm).
Inside the dipole box
The insides of the finished dipole
After you tested the dipole you can finish it by using your glue gun tor silicone) to seal everything up.
The maximum TX power is approx. 10 watt.
Finished dipole mounted on the back side of the Yaesu FT-817