is stub matching really required for helical antennas?

[josephpangilinan]

can anyone help me with this one..

i want to know the importance of stub matching in helical antenna.. is it really required?

 

[flatulent]

The RSGB amateur radio handbook of some years back had a design where the wire from the helix to the feed coax was routed a special way and the feed coax connector was 50+j0 ohms. No stub was required.

 

[dozer]

Yes, and the same design has been in ARRL books for decades.

Essentially, the last turn of the helix closest to the ground-screen is used as a tapered-line impedance-transformer.

It's brought close to the screen -before- it gets around the circle to where it will connect to the coax-connector pin. You adjust the length that's 'close' to the screen, and the rate of decrease of spacing, to match the helix to 50+j0.

hope this helps.

 

[VSWR]

Here is the description of the 50 ohm feed: