CB Base Antenna Questions

[Zack08]

I was recently given a CB base antenna, and I was thinking about converting it for 46 MHz Rx, but I have a couple questions about it's construction.

It has a 102" driven element, but there are three short (roughly 10") radials mounted about a third of the way up from the bottom. What purpose do these serve? Should I bother trying to move them and scale them when I convert the antenna, or just remove them?

It also has three radials, about 108" each. I remember reading somewhere that ground plane radials need to be at a 45 degree angle to create a 50 ohm load. These radials are angled much more downward. What is the reasoning behind this, and will this cause any problems when trying to retune the antenna for a higher frequency?

 

[ramal121]

You're right. By lowering the radials angle the radiation resistance at the feedpoint is changed. Guess mfg was trying to lower radiation angle. The doo dad on the driven element is a capacitance hat and is used as a tuning device to bring antenna back closer to 50 Ohm. I would leave it in however I don't know if scaling by frequency would be correct for that. The best way is to use an antenna analyzer to dial it in.

Or you could leave it out and try to raise the ground plane angle. The ground plane radial length should be 5% longer than the driven element.

 

[Zack08]

Thanks for the info! I'll have to google it and see if I can scale the capacitance hat, now that I finally know what its called. =)

 

[prcguy]

If the antenna is fairly old you probably have an Antenna Specialists "Starduster" which is about the equivalent of a center fed coaxial 1/2 wave dipole. There are some newer and poor quality clones around. It could easily be scaled down to your target frequency if you don't cut past the point of no return.
prcguy

 

[Zack08]

prcguy,
It looks real close to a starduster, but the little radials on the capacitance hat are bent down instead of sticking out at 90 degrees. It looks well made, but I have no idea what model it actually is.

From what I calculated, the length of the driven element would be about 61" at 46 MHz. What are your thoughts on the capacitance hat? Should I scale it down or take it off? Wouldn't the impedance be quite high if I removed it and left the ground radials at their current angles?

 

[prcguy]

A true capacity hat belongs at the top of a whip but lower will have some effect on shortening the overall length. I believe the original Starduster and probably the clones have telescoping elements held together with a sheet metal screw. You could split an inch at the end of the larger diameter element with a hack saw, telescope them shorter and use a small hose clamp to hold it together instead of a screw. I would make the radials a calculated 1/4 wavelength long at your desired freq then telescope the top until it resonates.
prcguy

prcguy,
It looks real close to a starduster, but the little radials on the capacitance hat are bent down instead of sticking out at 90 degrees. It looks well made, but I have no idea what model it actually is.

From what I calculated, the length of the driven element would be about 61" at 46 MHz. What are your thoughts on the capacitance hat? Should I scale it down or take it off? Wouldn't the impedance be quite high if I removed it and left the ground radials at their current angles?

 

[Zack08]

The elements are exactly as you describe. I was planning on cutting them off, but I think I'll go with your idea. That leaves plenty of room for adjustments.

I guess the only thing I'm still curious about is impedance. I guess I need to leave the hat on the antenna to get it closer 50 ohms, right?