J-POLE FOR 145
If you need a simple, strong and efficient atenna for 145 mhz, you need a J-pole; I think it's the best antenna for newcomers because it has better performance than a GP and is easy to bulid; moreover it's electrically gronded.
It's an half-wave radiator fed
with a quarter-wave stub; for the mine (that i built in less than one hour) I use aluminum
coming from an old VHF-band TV beam;
Aluminum pipe (better with different diameter, I used 200cm/diam. 20mm, 80cm/diam. 16mm, 10cm/diam.10mm);
1 plastic box;
10cm/diam. 32mm PVC pipe;
1 screw bolt diam.6mm/8cm lenght + washers and nuts;
A sheet steel (or zinc, or aluminum ecc) (few cm);
Cut the 20mm pipe in two part:
150cm and 50cm; in the same way cut the 16mm pipe: 60cm and 20cm; join the pipes at the
following way: 20mm/150cm + 16mm/60cm and 20mm/50cm + 16mm/20cm so to obtain two
poles: 180cm and 53cm.
Now hole the base of the short pole and pass trough a 6mm diam. screw bolt; make the same at 53cm from the base to the long one; space the two poles 29mm.
Mount the SO239 using a sheet steel (or aluminum, zinc, ecc.) on the 180cm pole, just some cm up the 6mm screw bolt.
Now you need to feed it: ground goes to long poles (180cm) and central to the short one (See Picture 1);
make two 20mm diam. holes on the 32mm PVC pipe (space it 50mm) and pass trough the poles; put this insulated spacer at 30cm to SO239 (See Picture 2).
You can keep the base of the long pole (below the 6mm crew bolt) of any length; you'll use it to support the antenna.
If you use big diameter for pipe,
tuning will be very easy and you'll have a wide-band antenna; on the mine I obtained 1:1,1
S.W.R on all the 2 mt band.
Anyway you can tune the antenna making the tips shorter or longer: will be better if you use only one tips keeping (i.e. the 180cm one) fixed.
After tuning you only need to cover holes with Rf paint or glue (to avoid water comes in).
How I said before, the J-pole
antenna is an half-wave radiator fed with a quarter-wave stub; because I'm obstinate I
tried to feed the antenna using the long pole (180cm) as driven ( that is half-wave!) and
grounding the other one...well: no changes in S.W.R. and performances!
Anyway I suggest you to use standard construction just because I never saw one like the mine!
If you need, you can build a BI-bander J-pole or TRI-bander: download the article on how to do that (cactus-antanna) (See Picture 3).
73 de iz7ath, Talino