VHF Sleeve Antenna Suggestions

[JustinWHT]

Back in the '70s SWBell installed mobile phones on oil well drilling rigs with a sleeved vertical antenna. To the best of my memory, it had a 3 or 4 ft stainless steel 1" diameter mast that terminated in a collar at the top. The collar had an 18" long and quarter inch diameter rod attached to coax center conductor. The coax braid attached to a 1-1/2" or 2" stainless steel sleeve extending down. My understanding was the outside diameter of mast to inside diameter of sleeve served to match impedance and a decoupling balun.

I am looking for suggestions on duplicating the sleeve using coax braid. The assembled antenna will strap to top rod of a 20 ft length of fiberglass banner flag mast where lower three sections are tubes and top section a thin rod - so my antenna needs to be lightweight.

Thinking of taking length of coax and cut through insulation and braid 26" from end and pull off the braid, then cut center conductor to 22.5" for 144 MHz. Slide a 1" diameter clear tubing over the top of the length of coax and slide the 26" braid over that. 26" was estimated length as the brad length will shorten as you expand it over the clear tubing.

Ideas and suggestions welcome.

 

[mmckenna]

Pretty much how all the lower tier consumer VHF marine antennas are built now. Just a coax sleeve antenna stuffed in a fiberglass tube.

 

[JustinWHT]

Commercial colinear antennas use alternating sections in series.
Where the first and lowest bay cable braid (or sleeve) connects to coax feed line via an RF choke, the top of the braid connects center conductor of next bay and its braid connects to center conductor of lower bay. That's not what I'm looking for.

Dang... That sounds like Abbot and Costello's "Whos on first".

My idea is a basic center-fed vertical dipole. I could build several using different diameters of clear tubing and check the VSWR. I'll cover the whole antenna with heat shrink. I could even attach metal ring at top and toss a weighted string over tree branch.

 

[mmckenna]

Commercial colinear antennas use alternating sections in series.

Right, the colinear antennas do, and there's lots of plans on how to build those on line.

But, crack open an old low end VHF marine antenna, and it's just a piece of coax inside. Outer shield pulled down around the coax jacket, and the center conductor with insulator sticking up. I had an old broken one I took photos of, unfortunately I lost the photos.

I had an old Motorola base dipole like you mentioned. Unfortunately it broke, but was a really nice antenna. Porcelain insulator and all. Shouldn't be hard to copy using some copper tubing.

You could copy one of these designs:

156 MHz (Marine Band) VHF 6dB Antenna

Heavy-duty style marine VHF antenna. 6dB gain at 156 MHz. Otherwise known as the "Hot Rod". One of our most popular VHF antennas! Morad Antenna Company, the best marine VHF antennas, made entirely in the USA. Our marine VHF antennas are trusted globally.

www.morad.com

 

[JustinWHT]

I don't want to build a multiple bay antenna only a single bay one. I've built couple if 18' tall 6-bay antennas using half inch coax from left over AT&T cell site grades.

The VHF antenna in your link exactly what I'm thinking of but lighter weight. Though I'm suspicious of their 6 dB gain. Attached is their diagram using 1" sleeve and no impedance matching or choke shown

 

[prcguy]

A problem you will have peeling back the braid on coax is the outer covering on coax is fine for protecting the braid but it’s a lousy dielectric as the RF insulator between the folded back braid and the braid under it. If you use Teflon coax then the outer covering is ok as a dielectric.

 

[JustinWHT]

I wouldn't be folding back per se, I would slide the 26" or so braid over the outside PVC sheath.

You have a good suggestion. I could cut the sheath and braid at 26" roughly to get me a length of braid, pull off the sheath and braid, trim the inner dielectric and conductor to 22.5" for antenna whip. Then instead of sliding the braid back down over the PVC sheath, cut 22.5" of the PVC sheath and replace it with a Teflon sleeve and slide the braid over that.

 

[prcguy]

I wouldn't be folding back per se, I would slide the 26" or so braid over the outside PVC sheath.

You have a good suggestion. I could cut the sheath and braid at 26" roughly to get me a length of braid, pull off the sheath and braid, trim the inner dielectric and conductor to 22.5" for antenna whip. Then instead of sliding the braid back down over the PVC sheath, cut 22.5" of the PVC sheath and replace it with a Teflon sleeve and slide the braid over that.

Any material that is between the coax outer braid and whatever is used for the sleeve needs to be a good RF dielectric if the sleeve hugs the coax and the PVC sheath over coax is not appropriate. Even though the braid sleeve is grounded it’s a hot radiating element and must be treated as such, otherwise you loose efficiency and will have tuning problems.

If you’re going to use a sleeve like braid that hugs the coax you might as well remove all the PVC insulation under the sleeve and replace with Teflon sleeving or something similar. Or make the sleeve from tubing that is much larger than the coax and keep the coax centered inside the sleeve as it runs through it using styrofoam or similar as the coax spacer.

 

[JustinWHT]

If you’re going to use a sleeve like braid that hugs the coax you might as well remove all the PVC insulation under the sleeve and replace with Teflon sleeving or something similar. Or make the sleeve from tubing that is much larger than the coax and keep the coax centered inside the sleeve as it runs through it using styrofoam or similar as the coax spacer.

That's what I was rethinking - remove the outside black PVC and replace with a better dielectric material such as Teflon tubing.

You had a *much appreciated* brilliant idea using Styrofoam (polystyrene foam) which has a dielectric constant close to 1, the same as air.
Remove the PVC insulation. Glue 22" strips of pool noodle to exposed braid. Slide recovered 26" of braid over wood dowel to expand it (to 22" length) then slide over foam strips. Cover with heat shrink tubing.

While I was originally asking about impedance matching and RF decoupling, the thread evolved to construction suggestions that found extremely helpful. Thank you!!!

Moving forward - how am I going to mount it on my Explorer while parked. I have no qualms drilling holes in the side panels (no XYL to contend with, and XYM has his Technician lic).
Secure a vertical 3" PVC pipe to side panel between rear door and wheel well with EMT conduit "U" straps.
Mount another vertical 3" PVC pipe on cargo rail with 6" horizontal extension to mount vertically above lower pipe.

 

[JustinWHT]

I'm pretty sure this is the antenna SWBell used. Phelps Dodge. Dated 1971.

 

[prcguy]

I'm pretty sure this is the antenna SWBell used. Phelps Dodge. Dated 1971.

I have one of those old Phelps Dodge coaxial antennas. I also have a couple of Kreco versions and several gain type stacked coaxials but those are too much work taking out of the garage rafters for pictures. The grey painted version below is actually all brass and I need to strip the paint off and polish it back up


.

The Kreco stacked coaxial look like this.

 

[JustinWHT]

. View attachment 163814

That's it! The one on the left.Must be 50.years old.

 

[prcguy]

That's it! The one on the left.Must be 50.years old.

Yea probably. I have a lot of antennas, good chance whatever you can think of is buried in my garage somewhere.

 

[cbehr91]

They were used for railroads a bit too, either for interlocking towers or remote dispatcher bases, despite the low gain. Here is one with a folded whip element, but I have seen many more with a straight whip.

Looking for older Motorola TAD1000 series VHF loop dipoles

Looking for the old VHF loop dinosaurs, specifically the TAD1002 or 1003

forums.radioreference.com

 

[JustinWHT]

While I was originally asking about impedance matching and decoupling

The information sheet for the Phelps Dodge antenna answered my question.

Outside diameter outside diameter of inside support mast 1-5/15" and inside diameter of sleeve 2" for about 5/16" air gap.

 

[JustinWHT]

Looking for older Motorola TAD1000 series VHF loop dipoles

Looking for the old VHF loop dinosaurs, specifically the TAD1002 or 1003

forums.radioreference.com

I've seen them on 60' wood poles on RR's ROW.
A folder dipole where the folded over end attached to grounded sleeve - virtually lightning proof.


I'll look for my pic of a DB224 on a ONE HUNDRED FOOT wooden pole in Bridgeport, TX I snapped 20 years ago.

 

[JustinWHT]

The information sheet for the Phelps Dodge antenna answered my question.

Outside diameter outside diameter of inside support mast 1-5/15" and inside diameter of sleeve 2" for about 5/16" air gap.

 

[RFI-EMI-GUY]

You can find some nice hobby brass tubing at the Home Depot, ACE etc, stores that will work for the sleeve. Also some TEFLON stand offs in various diameters that would work for insulators and spacers.

 

[JustinWHT]

Might be to heavy attached to them then top most fiberglass rod. But I'll check it out.

 

[cbehr91]

I've seen them on 60' wood poles on RR's ROW.
A folder dipole where the folded over end attached to grounded sleeve - virtually lightning proof.


I'll look for my pic of a DB224 on a ONE HUNDRED FOOT wooden pole in Bridgeport, TX I snapped 20 years ago.

Yeah, every one I've ever seen was mounted on a wooden pole.