Adding Additional Elevated Radials To An Antenna

[azprospector]

I have a question for the group here. I’m considering putting up a Sigma Venom 5/8 Wave High Gain Silver Rod CB Base Station Antenna antenna at our place up north of us. Winds of 65 or more mph are not uncommon up there so an antenna like this one appears to be a good fit. The antenna would be mounted on a 2” X 25’ heavy steel tilt mast with 8 guy wires. This antenna has its own radials. These short radials seem extremely short in my opinion to be a really effective elevated ground plane. My question is this. Would it be beneficial to add 4 additional radials about a foot under the antenna itself? These radials wouldn’t be connected to the antenna itself in any way but would be mounted directly on the mast itself.

 

[ko6jw_2]

The radials function to decouple the antenna from the mast. They are not a ground plane as such and do not need to be 1/4 wave length. The people who make the antenna may actually know what they're doing.

 

[prcguy]

Without modeling the antenna its hard to say if you can improve it and a dozen 1/8 wavelength radials can be as good as three or four 1/4 wave radials in some cases. If you were to do anything I believe lengthening the existing radials to about 1/4 wavelength would be the most beneficial. That might lower the impedance changing the VSWR and you might have to retune the antenna. The second best method would be to place three or four 1/4 wave elements one 1/4 wavelength below the original radials. This used to be a common way to increase gain and improve the pattern at the horizon by adding the equivalent of another 1/4 wave element below the original ground radials.

Edit: I just looked at this antenna and it only has three short radials, I thought it was like some of the Sirio antennas with a dozen or more short radials. I believe these radials are just for a little decoupling and probably not very effective. You might experiment lengthening them to 9ft and see what happens. I would take very accurate field strength measurements from a distance then lengthen them and take field strength measurements again. I'll be the field strength goes up a little with longer radials.

 

[KEWB-N1EXA]

I have the SIgma 5/8 wave and it works great as it is.
Its mounted 20 feet in the air on a giant Conduit Pipe.

The biggest problem is to replace the the steel Jubelee clips with SS pipe clamps
and make aluminum shims so the Vertical has no wiggle room when assembling
also replacing the Metric Steel screw connecting the coil to the antenna with
a good SS steel one.

 

[KEWB-N1EXA]

I have a question for the group here. I’m considering putting up a Sigma Venom 5/8 Wave High Gain Silver Rod CB Base Station Antenna antenna at our place up north of us. Winds of 65 or more mph are not uncommon up there so an antenna like this one appears to be a good fit. The antenna would be mounted on a 2” X 25’ heavy steel tilt mast with 8 guy wires. This antenna has its own radials. These short radials seem extremely short in my opinion to be a really effective elevated ground plane. My question is this. Would it be beneficial to add 4 additional radials about a foot under the antenna itself? These radials wouldn’t be connected to the antenna itself in any way but would be mounted directly on the mast itself.

I think you looking at this antenna wrong - you are thinking of a ground plane 1/4 wave setup.

There is a coil in the base ( tapped to ground) of this antenna and along with the short aluminum pieces above it form a resonate trap between 26.5 and 28mHz so the antenna functions n the 10-11 meter band and is a dc coupled short on anything else. The small aluminum pieces are for Capacitance they are not radials to hold the antenna at 50 ohm impedance. Basically adding radial would be pointless because the trap would kick in on 10-11 meters.

Many hams buy these and change the coil value to make these work on 12 - 15 and 20 meters with no problems.

Hope this helps explain how the silver rod works. This also applies to the Sirio series and other 1/2 wave cb antennas like the old Hi Gain's.
Dont confuse this with the Macom 5/8 wave that has a different circuit and has 9 foot radials or the Avanti 4000 that is a J pole...Different Animals at the zoo !

 

[prcguy]

The coil in the base is not a trap, its for matching and its common to most 5/8 verticals including VHF and UHF mobile types. A 5/8 radiator can be easily matched stand alone with no radials similar to the IMAX2000. Placing a 5/8 over a good ground plane will improve the pattern and gain at the horizon. I don't consider the radials on the Sigma 5/8 here as a ground plane and I think the antenna will benefit from an actual ground plane.

I think you looking at this antenna wrong - you are thinking of a ground plane 1/4 wave setup.

There is a coil in the base ( tapped to ground) of this antenna and along with the short aluminum pieces above it form a resonate trap between 26.5 and 28mHz so the antenna functions n the 10-11 meter band and is a dc coupled short on anything else. The small aluminum pieces are for Capacitance they are not radials to hold the antenna at 50 ohm impedance. Basically adding radial would be pointless because the trap would kick in on 10-11 meters.

Many hams buy these and change the coil value to make these work on 12 - 15 and 20 meters with no problems.

Hope this helps explain how the silver rod works. This also applies to the Sirio series and other 1/2 wave cb antennas like the old Hi Gain's.
Dont confuse this with the Macom 5/8 wave that has a different circuit and has 9 foot radials or the Avanti 4000 that is a J pole...Different Animals at the zoo !

 

[KEWB-N1EXA]

The coil in the base is not a trap, its for matching and its common to most 5/8 verticals including VHF and UHF mobile types. A 5/8 radiator can be easily matched stand alone with no radials similar to the IMAX2000. Placing a 5/8 over a good ground plane will improve the pattern and gain at the horizon. I don't consider the radials on the Sigma 5/8 here as a ground plane and I think the antenna will benefit from an actual ground plane.

I know I’m not climbing up there to experiment!
Let’s see if this guy builds it and gives us a report.
Mine as is shoots 1,000 mile skip pretty easy and
One crazy day hit wales and Northern Ireland a few weeks ago.
The radials will not mount to the plastic base where the Small capacitance elements go I’ll say that!

 

[KEWB-N1EXA]

The coil in the base is not a trap, its for matching and its common to most 5/8 verticals including VHF and UHF mobile types. A 5/8 radiator can be easily matched stand alone with no radials similar to the IMAX2000. Placing a 5/8 over a good ground plane will improve the pattern and gain at the horizon. I don't consider the radials on the Sigma 5/8 here as a ground plane and I think the antenna will benefit from an actual ground plane.

So if you want to decouple the antenna from the mast that would not be A resonate trap?

Honestly asking a question not arguing the case!
id think the small elements act as a capacitance
Hat with the tuned coil to make the separation at the resonate frequency say 27.385 MHz as an example and sat 20 MHz would be at a ground potential?

 

[prcguy]

A "trap" is a parallel resonant inductance and capacitance that resonates at some prescribed frequency. Its most commonly used to break up an antenna by frequency in a multiband antenna. For example if you wanted to make a vertical antenna that covered CB and 40m amateur you could do that by having a 9ft radiating element, then a trap tuned to 27MHz at the end of the 9ft, then whatever length of wire is needed for the entire thing to resonate on 40m amateur. The trap ends the antenna for 27MHz use and makes the additional wire for 40m disappear until you fire up 40m, then the entire length resonates on 40m. A trap would have no real use in matching a 5/8 wave CB antenna.

Instead you will have a tapped coil as a matching device because the feed point impedance of a 5/8 is higher than 50 ohms. Its kind of like an auto transformer where the bottom of the coil would be at ground potential, a tap point is found that provides a good match to 50 ohm coax then the remaining coil turns connect to the vertical radiator. A 5/8 should really work against a good ground plane and lack of a ground plane will loose some gain at the horizon. Adding a ground plane to a 5/8 without one should improve its performance at the horizon.

The short ground radials on the Sigma 5/8 do provide some capacitance between ground and the vertical element and probably help with matching and a little feedline decoupling, but at a very minimal level. My opinion is add more ground plane and make it more like a Hy-Gain Penetrator or Avanti Sigma 5/8.

A capacitance hat is not the same as ground radials and they are always at the top or upper end of an antenna. It effectively lengthens the antenna and helps pull the maximum current point of the antenna further up the vertical element making better use of the vertical element. If you add a capacity hat you will have to shorten the vertical element to compensate or the resonant frequency will go lower. You can take a resonant 1/4 wave ground plane as an example and shorten the 9ft vertical element to maybe 6ft, the add a capacity hat to get the resonance back to 27MHz and you will not loss any performance to speak of. They are very handy as a tool to shorten an antenna and better than using a lossy loading coil.

So if you want to decouple the antenna from the mast that would not be A resonate trap?

Honestly asking a question not arguing the case!
id think the small elements act as a capacitance
Hat with the tuned coil to make the separation at the resonate frequency say 27.385 MHz as an example and sat 20 MHz would be at a ground potential?

 

[KEWB-N1EXA]

A "trap" is a parallel resonant inductance and capacitance that resonates at some prescribed frequency. Its most commonly used to break up an antenna by frequency in a multiband antenna. For example if you wanted to make a vertical antenna that covered CB and 40m amateur you could do that by having a 9ft radiating element, then a trap tuned to 27MHz at the end of the 9ft, then whatever length of wire is needed for the entire thing to resonate on 40m amateur. The trap ends the antenna for 27MHz use and makes the additional wire for 40m disappear until you fire up 40m, then the entire length resonates on 40m. A trap would have no real use in matching a 5/8 wave CB antenna.

Instead you will have a tapped coil as a matching device because the feed point impedance of a 5/8 is higher than 50 ohms. Its kind of like an auto transformer where the bottom of the coil would be at ground potential, a tap point is found that provides a good match to 50 ohm coax then the remaining coil turns connect to the vertical radiator. A 5/8 should really work against a good ground plane and lack of a ground plane will loose some gain at the horizon. Adding a ground plane to a 5/8 without one should improve its performance at the horizon.

The short ground radials on the Sigma 5/8 do provide some capacitance between ground and the vertical element and probably help with matching and a little feedline decoupling, but at a very minimal level. My opinion is add more ground plane and make it more like a Hy-Gain Penetrator or Avanti Sigma 5/8.

A capacitance hat is not the same as ground radials and they are always at the top or upper end of an antenna. It effectively lengthens the antenna and helps pull the maximum current point of the antenna further up the vertical element making better use of the vertical element. If you add a capacity hat you will have to shorten the vertical element to compensate or the resonant frequency will go lower. You can take a resonant 1/4 wave ground plane as an example and shorten the 9ft vertical element to maybe 6ft, the add a capacity hat to get the resonance back to 27MHz and you will not loss any performance to speak of. They are very handy as a tool to shorten an antenna and better than using a lossy loading coil.

Thanks for the Coil explanation because I thought it wrong. I was more thinking the decoupling of the antenna from the mast was A trap not
feed point impedance.
Ill tell you that mentioning the Horizon gain brings a good point that the Sigma Vertical acts completely Different than my V dipole on Gain.
The locals VS DX has A big difference on Vertical vs Horizontal Gain. So adding Horizontal Elements To the sigma would get it to have more gain on the Horizontal plane. Good Experiment but I lack the space to try it!
This is the Sigma on the shed and the V dipole. The String across the top is to hold the shape and not part of the antenna.
There is an actual feed point transformer added but I left the inductor coil there anyway.

 

[prcguy]

There is no radiation in the horizontal plane from any of these 5/8 wave verticals, the horizontal radials do not radiate. If you have just one horizontal ground radial on a vertical antenna you have made an inverted V dipole and that horizontal radial will radiate. But it doesn't necessarily radiate in the horizontal plane and it radiates more in a 45 degree slant as a split between the horizontal radial and the vertical element. As soon as you add a second, third, fourth, radial, etc, the RF is now flowing equally in each ground radial but in opposite directions which cancels radiation in the radials.

This is why a 1/4 wave ground plane has less gain than a 1/2 wave dipole. A 1/2 wave dipole radiates equally in both sides of the dipole, although most of the radiation is near the feedpoint where the current is highest. In a 1/4 wave ground plane the ground radials do not radiate so the radial half the antenna does not radiate. Years ago the antenna books used to show a picture of a ground plane then a mirror image below the ground plane to try and explain its like a dipole and the missing half radiates, but we now know this is false and there is no radiation from the missing half.

Thanks for the Coil explanation because I thought it wrong. I was more thinking the decoupling of the antenna from the mast was A trap not
feed point impedance.
Ill tell you that mentioning the Horizon gain brings a good point that the Sigma Vertical acts completely Different than my V dipole on Gain.
The locals VS DX has A big difference on Vertical vs Horizontal Gain. So adding Horizontal Elements To the sigma would get it to have more gain on the Horizontal plane. Good Experiment but I lack the space to try it!
This is the Sigma on the shed and the V dipole. The String across the top is to hold the shape and not part of the antenna.
There is an actual feed point transformer added but I left the inductor coil there anyway.

View attachment 105991

 

[KEWB-N1EXA]

My little hamster brain gets larger every time we talk!
Thanks PRC📣