home brew base antenna questions

[BOOTY MONSTER]

i asked some question quite a while back about building a base antenna and i cant find those Q's and A's now . so i guess ill ask them again , thanks for putting up with me .

does a half wave or a 3/4 wave cb base antenna have to have a coil ? i know a 1/4 wave doesnt . seems like i recall reading somewhere that 1/4 wave multiples (1/4 1/2 3/4 and full length) dont need a coil . IF im remembering correctly is this true ? seems like i also recall that having 3 or 4 ground plane radials makes no difference in antenna performance . for cb channels on a base antenna should the radials always be 9 foot to be optimal regardless of wheather the main radiator is 1/4 1/2 or even 3/4 wave ? if i recall doesnt angeling the ground radials down also lower the angle of signal radiation from the antenna ?

 

[W5LZ]

'B.Monster',
The input to the average 1/4 wave groundplane type antenna is something around 35 ohms. If you 'droop' the radials that impedance starts going up. When it gets sort of close to 50 ohms (+/-), quit. There's no need of an impedance matching coil. (Vertical element + the radials are 'approximately' the same as a 1/2 wave antenna fed in the center. A 1/2 wave center fed antenna's input impedance is something like 73 ohms (+/-). If you bend it in the center a bit, that's like 'drooping' the radial on a 1/4 wave, it changes the input impedance. Imagine a one-legged 1/4 wave groundplane and you are very close to having a vertical 1/2 wave antenna. See the similarity?
If the 1/2 wave antenna is center fed the input impedance is close to 50 ohms, sort of. If that same 1/2 wave antenna is end-fed, the impedance is closer to something like 1000 ohms. That would require a matching coil to drop it to close to 50 ohms (reason for that coil). So, if your are thinking in terms of 1/4 wave sections, even multiples means higher impedance, odd multiples can mean lower impedances. Not 'low' as in a good match to 50 ohms, just lower than an even number of end-fed 1/4 wave sections. (Confused yet? Think you might just 'settle' for a 1/4 wave groundplane, or a 5/8 wave 'already made' antenna?) That thinking in 1/4 wave length multiples has some benefits, but it's still not a very simple thingy, there are 'catches'.

(If you have some preference for brand, or 'size' of antenna, do it. Don't let that 'low spot' influence you much.)

- 'Doc