Davemade groundplane / birdcage antenna

[Marconi]

Thanks Homer.

Bob I got a link to a #5 deal from Photobucket. The slideshow was running, but no images.

I opened the link again and clicked the X to shut the SS down and the images popped up and working.

I remember this from back in the days from you.

Is the copper pipe tapped into the radiator or does it look like a J-pole feeder that is open to the air?

 

[HomerBB]

 

[bob85]

The guy who took the pics for me said it was connected at the gamma strap dc short circuit,

Radioworld's advertising for sirio cx series agrees

"Grounding protection: All metal part are DC-grounded, the inner conductor shows a DC-short"

 

[Marconi]

Thanks Homer, I see the tap point now in image #2 with the close up and the zoom clicked on.

 

[Shockwave]

The Dave made uses a folded aluminum strap with more inductance than the Sirio CX series. It extends up from the base of the center radiator and back down to the plate with the cone base attached to it. The person that took the pictures made effort to hide this but some magnification made it apparent. Notice the base of the center radiator is insulated from the base of the cone too and still DC grounded.

 

[Marconi]

The Dave made uses a folded aluminum strap with more inductance than the Sirio CX series. It extends up from the base of the center radiator and back down to the plate with the cone base attached to it. The person that took the pictures made effort to hide this but some magnification made it apparent. Notice the base of the center radiator is insulated from the base of the cone too and still DC grounded.

Donald, that is the way I see it too and I agree with the intentions you noted.

I posted a screen shot of the image and tried to mark the spot. If I can find the link I will add it to this thread. If you check close look for my marks and you might be able to make out what Donald is talking about.

I just finished my model for this antenna and will post later.

 

[bob85]

I see it now Eddie, the angle makes it look like the gamma is tapped to the cone until you blow the image up.

if it is connected as Donald describes with the monopole isolated id call it a stub matched antenna.

 

[Marconi]

Donald I see what looks like the radiator insulator too and I marked up the antenna as I saw it. IMO that insulator area looks similar to the Maco V58 radiator setup,

 

[Marconi]

Here is my Bird Cage model

1. antenna notes
2. Real Earth model - 3 pages
3. Control Center image showing the Free Space Average Gain results.

 

[bob85]

Eddie,
if its using a stub across the feed-point the antenna needs to be lower than 50ohms & electrically short or capacitive before you connect the stub,

hairpin matched yagis, & the dollar match for hf mobile antennas work like that.

 

[HomerBB]

@Marconi
You'll notice an L shaped connector that apparently connects the stub matcher to the center radiator and the wire from the center of the coax connector attached to it.

 

[Marconi]

Very good Homer, I also see what the 2 bolts that I marked up earlier below the radial base plate are doing. They are securing the stub L-bracket that attaches the other side of the stub to the top of the 12" inch radial bracket. You can see these 2 bolts in the PDF file below. They are close to the coax connector below the radial bracket.

I also think this stub has a purpose that Bob can describe.

I'm still thinking the radiator is floating, because I see it setting inside what looks like a white insulator. I marked the insulator earlier with a big ? in one of my images. It looks like both the radiator and the white insulator are inside the antenna base mount. Always more questions.

I wish I had some measurements for this 3 wire stub.

I removed the 9.6 pF capacitor from the model's feed point and the model then shows R = 169, X = +324, SWR = 16.036, at the antenna Feed Point showing to be very inductive...without the little capacitor added. Right now I don't see how adding this stub will work, the model's feed point seems to me to be looking for capacitance. HELP!

 

[HomerBB]

@Marconi, look at the diameter of the main vertical radiator above the bottom cone plate. Now look at the diameter of the vertical below that plate. The bottom is larger than the top because the upper vertical slides down into the lower and with the insulator between the two. This forms a capacitor.

The Ringo antennas have that setup. There is a tapped inductor, and a coaxial capacitor to complete the L/C network, however, there is additional capacitance created by the upper vertical inserted within the bottom section with an insulator between them. When I homebrewed a 1/2 wave Ringo I could not get it matched until I realized I needed to recreate that capacitance on the vertical.

I think you will find the capacitor you're looking for there.

BTW, because of this capacitor sleeve at the base of the vertical I do agree the basket/cone appears to be floating except for the stub connection.
I am thinking this antenna is more like the 5/8 waves I asked you with the radials turned upward I asked you to model for me, but longer.

 

[Handy Andy]

Forgive the intrustion, but I had to go to that Sirio and look again... Hoep you don't mind but I cleaned up those shots so I can see it better myself - Ok to leave them here?

Thanks in advance for letting me put it here...​

 

[Marconi]

BTW, because of this capacitor sleeve at the base of the vertical I do agree the basket/cone appears to be floating except for the stub connection.

Homer, I don't think I intended to say the radial basket was floating.

Instead, if I'm seeing the BG antenna correctly in this area, with the radiator inside a PVC type insulator and that insulator inside the base mount...similar to what we see on the Maco V5/8, then I think we are seeing the radiator floating. I also agree this creates a capacitor effect.

I am thinking this antenna is more like the 5/8 waves I asked you with the radials turned upward I asked you to model for me, but longer.

I vaguely remember such a model and I found it in my models folder. However, I don't remember what we learned about that idea. You are correct, the radiator for that model was raised up 1" above the radials and was floating above the groundplane. Did you ever build that idea?