A ROTARY DIPOLE FOR 17 AND 20
After assembled a two element Quad, Mario (ik7zcq) needed an antenna for 17 and 20, so he asked me to build a 17/20 dipole; I hote rotary dipoles and traps..but why don't try it!
I hote projects because I never
find all I need: so always I do the best I can with what I found.
At the beggining my project was:
1) Grounded-driven element;
2) Gamma-match feeding;
3) Traps with coax-cable;
After first test I decided to use a direct-feeding with no Gamma-match: the Gamma will give best results with single-band feeding. Dipole will be no more grounded.
THE DRIVEN ELEMENT
Driven element is tapered and I use 8 aluminum pipes with the following diameters:
|30 mm||162 cm||2 mm||
|25 mm||164 cm||2 mm||2|
|20 mm||131 cm *||2 mm||2|
|16 mm||28 cm||1 mm||2|
* here will be the trap (108 cm + insulator + 23 cm)
I suggest you to reduce thickness for tips (2 - 1,75 - 1,5 - 1 mm).
First part (diam.
30 mm) is center-insulated with a nylon bar (diam. 25 mm); outside I used PVC (diam. 40mm,
5 mm Thickness); there are two screw bolt (3mm diam.) for feeding the dipole (See Picture 1).
The mast to boom attachment is an home-made steel plate (15x30x0,35 cm) (See Picture 2).
Second part (diam. 25 mm) is easy to assemble;
Third one is a little bit difficult: cut a 20
mm aluminum pipe (lenght 131 cm) at 108 cm so to obtain two pipes (108 e 23 cm).
Put a PVC bar (5cm) into and between them, then use fibreglass to block them and put over them an insulated support (See Picture 3) (TNX Antonio,ik7ytx).
Than make two 3 mm-holes (for traps-feeding).
Last part is a 28 cm length (diam. 16 mm) aluminum pipe, used for 20 mt tuning.
Radiation resistance for an half-wave dipole will be around 69-70 ohm (free space); if you put it 0,5/0,75 lambda high resistance will be lower so you don't need to match it; I only used 20-25 turns of coax-cable as balun.
At the beginning I believed traps was the more
difficult thing to build, so i bought two RF capacitor (See
Picture 4) but that was not true!
Using a dip-meter check the resonance: 8 turns of RG58 on a 27 mm support will resonate at 18.1 (See Picture 5).
When tuned, cover the traps with RF glue (See Picture 6) so that water can't enter in and then cover all with PVC tape (See Picture 7).
I tested the dipole on my garden, on a 10 mt
pole, with no objectes near it.
At first assemble first two partes (with no traps) and tune it on 17 mt; than add traps and 20 mt part and tune it on 20: S.W.R. will not change.
On my garden S.W.R was 1,2:1 on 17 and 1,5:1 on 14.200, 2:1 on 14.000 and 14.350; when assembled in the qth of Mario it was 2:1 on 17 and 1,1:1 on 14.200 (but there are some objects near it, see Picture 8)!
The length (half-dipole) is the following:
Tips (18 MHZ)
Half-Dipole (18 e 14 MHZ)
Dipole's weigth is less than 6 Kg but you can reduce it reducing the thickness for aluminum pipes.
73 de iz7ath, Talino