CB base station antenna question

[M0GVZ]

i wonder if something like flexseal spray would stop that sprayed all around the base and coax conector. maybe yours was getting into the coax not the antenna.dont seem right theyd build an antenna to go up in swrs when it gets wet

No. It is because of the skin effect of the rain. RF flows over the surface of a conductor, not through it, so anything on that surface will impede that flow. Like insulated wire, rain covered wire/metal tubing appears electrically longer to RF, lowering the resonant point of the antenna.

 

[br549]

if you have 9 feet going up and 9 feet going out as radials then you got a 1/4 wave but when the radials bend down isnt that a 1/2 wave?

a horizontal dipole is a 1/2 wave . make it for CB'ers by dividing the ground elements a bit , make it vertical and it's a 1/4 wave ground plane . move the feedpoint/tuning to the bottom and it's a 1/2 wave vertical , add length to the top it can be a 5/8 or .64 . don't forget your ground elements

 

[M0GVZ]

move the feedpoint/tuning to the bottom and it's a 1/2 wave vertical , add length to the top it can be a 5/8 or .64 . don't forget your ground elements

Won't work without a matching section as the feedpoint impedance will be thousands of ohms once you get to a 1/2 wave or longer.

 

[br549]

Won't work without a matching section as the feedpoint impedance will be thousands of ohms once you get to a 1/2 wave or longer.

when i said "feedpoint/tuning" ..... tuning meant a matching network . BTW a starduster can be tuned a bit by adjusting the length of it's vertical and ground elements just like clipping the wire length on a dipole .

 

[Meglotron]

a horizontal dipole is a 1/2 wave . make it for CB'ers by dividing the ground elements a bit , make it vertical and it's a 1/4 wave ground plane . move the feedpoint/tuning to the bottom and it's a 1/2 wave vertical , add length to the top it can be a 5/8 or .64 . don't forget your ground elements

OK it's been a while, I know, but isn't 9' up plus 9' down 18 feet? and isn't 18' a 1/2 wave? Isn't splitting it from the coax in the middle a 1/2 wave dipole in the vertical?
So I still believe when you bend the 1/4 wave horizontal ground planes down you have made a 1/2 wave antenna from a 1/4 wave ground plain and it don't need a match because it's going to be real close to 1:1 swr.

 

[Meglotron]

No. It is because of the skin effect of the rain. RF flows over the surface of a conductor, not through it, so anything on that surface will impede that flow. Like insulated wire, rain covered wire/metal tubing appears electrically longer to RF, lowering the resonant point of the antenna.

but how much? is the tuning going to go sky hi? I doubt it will change more than maybe 5 or 10 channels and you can always tune the matching rings to fine tune it.

 

[whiteastro]

Nothing wrong with either antenna but please note that the P 500 will go up in SWRS when it rains or is damp out..

I experience that with my SPT500 but there is a fix for that on the internet I'm looking for. If anyone knows it please pot it. Thanks

 

[Ranch55]

I experience that with my SPT500 but there is a fix for that on the internet I'm looking for. If anyone knows it please pot it. Thanks

Yes, there is a "fix" ......
Search this forum.
I posted photos of my "fix" that works very well ......

 

[Ranch55]

Yes, there is a "fix" ......
Search this forum.
I posted photos of my "fix" that works very well ......

Penetrator 500 upper insulator upgraded with PTFE (teflon) rated for RF and electrical insulating properties @ 500v per mil thickness ......

#21Ranch55, Jul 6, 2018

 

[whiteastro]

Thanks looks good but I have the newer version that has the flat sheet metal matcher piece.

 

[Ranch55]

Doesn't matter, the insulator is the issue ......

 

[whiteastro]

thanks, is the insulator readily available somewhere or is it home made, and if so where is a good place to get the material.

 

[Ranch55]

I fabricated the improved insulator from PTFE tube stock.
I had to use a Dremel tool grinder and enlarge the hole in the bracket slightly.
I had to hand ream the ID of the tubing to size in order to slip over the vertical radial.
You will need to determine the correct ID and OD, order as close to that size as possible, and then do the final sizing yourself for a good fit.
It was not an issue for me, as I am a machinist by trade.

I got the material from here:

http://catalog.fluoropolymerproduct...fe/extruded-tube-teflon-and-ptfe-2?&forward=1

"Electrical applications: PTFE is one of the best insulators known. In thin sections, it will insulate to 500 volts per mil. There are grades of PTFE which have even greater dielectric strength. It is frequently used in wire and cable wrap, and to separate conductive surfaces in capacitors. Thick walled close-tolerance extruded tubing is the PTFE shape of choice where machining or drilling long lengths to close tolerances is impossible. Multi-hole tubing can be extruded. PTFE can be machined into standoff insulators, and many different types of high voltage encapsulation devices for electrical components."

 

[whiteastro]

Thanks for the help, it is a start but I'll have to wait for warmer weather to dig out my SPT 500 to get some measurements and get some ideas. I'll let you know when I fix it. Thanks again.

 

[whiteastro]

I like the looks of your new insulator and I'm a old fabricator so I'm sure I could make it, but what tool did you use to open up the inside diameter. a file or sandpaper? Did anyone ever try just turning the factory insulator upside down so the long part was on top. And is any sealant used at the top of the insulator?