5/8 Wavelength Antenna Top Loading

[fuseless]

When I was younger, I had a CB radio with a 5/8 wave base station antenna. The antenna had a capacitance hat on the top and a tapped coil on the bottom. I Know what the tapped coil is for, impedance matching. I'm not so sure about the capacitance hat. I've been reading up on top loading with capacitance hats with antennas that are less than a 1/4 wavelength and depending on the frequency and height may not even require a loading coil to achieve resonance. I learned that top loading with a capacitance hat, with respect to an antenna less than a 1/4 wavelength anyway, electrically lengthens the antenna like a loading coil would.

So does top loading a 5/8 wave antenna with a capacitance hat work the same way?

Can a capacitance hat electrically lengthen a physical 5/8 wave antenna to an electrical 3/4 wavelength to resonate it, then use a tapped coil to impedance match the remaining pure resistance at the feed point on the bottom?

 

[Boxnut]

That capacitance hat has little or nothing to do with loading. Its asset is to broaden the bandwidth resonance a bit.
The resonance is a function of the radiator length and coil portion above the feed tap. The impedance is matched by the coil portion from the feed tap to ground.
You might look for the ARRL antenna book and chapter dealing with 5/8 wave base loaded verticals.
Hope this helps.

 

[unclejed613]

have a look here: **broken link removed** it's kind of like having a loading coil and capacitance hat all in one. since the low impedance (high current) portion of the antenna is a straight element you get the benefit of it being not quite as efficient as a full length element, but the difference in performance isn't really very much...

 

[Boxnut]

have a look here: **broken link removed** it's kind of like having a loading coil and capacitance hat all in one. since the low impedance (high current) portion of the antenna is a straight element you get the benefit of it being not quite as efficient as a full length element, but the difference in performance isn't really very much...

Yes, I have seen that. A decent design with a lower Q figure and broader bandpass at resonance. These CB 5/8 wavelength have a bit of gain like 3 or 3.5 Db over isotropic. A discone takes that to extremes. More like a 2 element quad covering 6 to 50 Mhz.

 

[unclejed613]

These CB 5/8 wavelength have a bit of gain like 3 or 3.5 Db over isotropic.

that's because there are two portions of the element with high current instead of one.

A discone takes that to extremes. More like a 2 element quad covering 6 to 50 Mhz.

a discone actually behaves more like a normal vertical with a reflector (which makes it act like a dipole) the reason for the wide bandwidth is the effective diameter of the radiating element being very large. i've built biconical antennas with a very wide bandwidth (with the SWR less than 3 across a 5:1 frequency range)

 

[fuseless]

Thanks Boxnut, you really got me thinking now. I'm curious however.
How exactly does a capacity hat increase the bandwidth of the 5/8 wavelength antenna?
Also, is there a specific edition of the ARRL Antenna Book that I should look for?

 

[unclejed613]

two ways.... first, the bandwidth of an antenna is related to the diameter of the elements, which is why discones and biconical antennas have such a wide bandwidth. the element surface doesn't "need" to be solid, a wireframe element will suffice, which is why, on large ships, their HF antennas are bird-cage like in construction. the other reason is the capacitance of the hat and the inductance of the "extra" 1/8 wavelength of the element work as a complex impedance which flattens the resonance curve of the antenna, similar to the action of the coax sections of a "double bazooka" antenna... if you want to see an interesting wideband dipole, look up "planar disk" antenna.

 

[fuseless]

I was totally unaware that those three spokes of about 10" in length sticking off the top of the 5/8 wave radiator actually increased its effective diameter. Ok so the 1/8 wavelength length of wire that makes up the coil above the tap creates the 3/4 wavelength antenna(electrically). But does this coil prevent the antenna from ever having a purely resistive impedance? I ask because you mentioned that there is a complex impedance involved. Or does the added capacity of the hat help to tune out the inductive reactance. Is that what you tried to say in your previous post?
Are the shorter, horizontal elements on the top of the discone antenna a capacity hat?