vhf antenna made from coax, like a J pole marine antenna in fiberglass?

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wa5ngp

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I had a fiberglass marine antenna I think made by HyGain. Inside the fiberglass was a long piece of magnet wire with a matching coax stub below it. I've googled for details on this design but I cannot find them. As I remember the main radiator was simply a piece of magnet wire embedded in wax and then below it was a coax matching stub. I want to make another version so I can keep it rolled up when not used and then just strap it to a pole when I want to deploy it. Unfortunately, I did not document the design when I took it apart. Perhaps the design is more like a 5/8w vertical with the coax matching stub below it.
tks for any pointers
don
 
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mmckenna

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I believe what you are looking for is a coaxial sleeve antenna.
You can also make a roll up J-pole antenna out of old twin lead TV antenna cable if you can find some.
 

wa5ngp

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that sounds more like it. I'll google for it. tks. If I had some twinlead squirelled away somewhere I'd use that. I try to use the junk I have to justify keeping junk around. :giggle:
 

prcguy

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If the magnet wire was about 16" long with about 16" of coax below it with the braid folded back it would have been a coaxial dipole. If the magnet wire was closer to 3ft long it was probably a version of a J-pole with a 1/2 wavelength radiator and a 16" long matching thingee at the base.
 

wa5ngp

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for the J pole case for the matching thing at the base, I assume that would be a quarter wave shorted on the end? could it be parallel to the upper section or does it have to extend out the bottom or side of the antenna? seems like it could not go up next to the vertical part. again assuming the J pole design the coax could just come up the pole connect to magnet wire going 3 ft up and then the same coax continues down, I need to draw it out and post here.
 

prcguy

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A J-pole is a 1/2 wavelength radiator with a 1/4 wave shorted stub for matching. You could take a 3/4 wavelength of wire attached to the center conductor of the coax, then a 1/4 wavelength of wire running parallel to that connected to the braid of the coax, then short the two together at different points along the 1/4 wave section until it matches.

When your done you will have in my opinion, an antenna that is 3/4 wavelength long for only 1/2 wave dipole type performance at best and it will have common mode RF current problems along the coax, which can be reduced with some ferrite.

for the J pole case for the matching thing at the base, I assume that would be a quarter wave shorted on the end? could it be parallel to the upper section or does it have to extend out the bottom or side of the antenna? seems like it could not go up next to the vertical part. again assuming the J pole design the coax could just come up the pole connect to magnet wire going 3 ft up and then the same coax continues down, I need to draw it out and post here.
 

wa5ngp

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ok, here's my crude sketch, I think that this was a marine J pole, what threw me was the coax tapped at the center connected to magnet wire going up then the quarter wave section of the same coax going back down.
 

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krokus

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ok, here's my crude sketch, I think that this was a marine J pole, what threw me was the coax tapped at the center connected to magnet wire going up then the quarter wave section of the same coax going back down.

Any dimensions to go with the drawing? I realize you're doing this by memory, but approximation could help.
 

nd5y

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Dimensions would depend on the design frequency, the velocity factor of the coax, and since the coax stub is used for impedance matching it probably won't be exactly an electrical 1/4 wavelength.
 

wa5ngp

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I think I found it here
It says its 1.5M long so sounds like a J pole.

I modified it for 2M. I have not been good about documenting my various tinkerings. that and my memory are combining to make working with that old stuff more difficult.
 

majoco

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It says its 1.5M long so sounds like a J pole.

How can it be a J-Pole without that matching stub at the bottom? 1.5m long makes it a half-wavelength at 150MHz so I suspect there's a matching coil in that piece at the bottom.
 

wa5ngp

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according to Slim Jim / J Pole Antenna Calculator which is the first J pole calculator I stumbled onto the overall length of a marine antenna at 156.8mhz is about 1.4M including the matching 1/4 stub length so I think the dimensions validate that it is a a J pole. and its long enought to make a 2M antenna which requires a total length of 1.49m, so mystery solved.
 

majoco

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Here's my 'Slim Jim' that I made waaay back in the 80's - has always worked well. It's ali rods on the outside of a 1-3/4 PVC waste pipe with a braid-breaker choke inside the bottom of the pipe.

Slim Jim sml.jpg
 

wa5ngp

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Now I'm curious, the choke at the bottom, that's in lieu of the matching stub right, or is it to to keep common mode currents off the coax? Do you happen to have the turns and dimensions?
 

majoco

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Ths 'slim jim' is made exactly as per the original article here...... Original Slim Jim Antenna Article by F.C. Judd - The 2BCX Slim Jim Antenna - G2BCX

Here's a very good article too.... Notes On the Original Slim Jim Antenna by Dave Coomber, M0UXB

This also show a Pawsey Stub and a 4:1 coaxial balun which I suspect is what the WA5NGP was wondering about.

The choke disconnects the coax cable from the antenna. This prevents all sorts of problems that you read in this forum about not being able to 'tune' the antenna using an swr meter because the length of the coax affects the tuning. It's 5 or 6 turns of RG58 tied together with cable ties inserted up into the PVC tube with a female SO239 on the end - I guess the diameter is about 1-1/2 inches.
 

wowologist

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Um, why would you want OR need a j-pole for marine use? (it;s a compromise dual band antenna with a rather large blind spot) More then likely that's a 5/8 wave vertical without a ground plane (since presumably you would be over water while using it and really in no need of a high vertical take off.

Here's a 2m one that is easily adaptable to the higher marine band. http://www.arrl.org/files/file/Technology/tis/info/pdf/8009022.pdf
 
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krokus

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Um, why would you want OR need a j-pole for marine use? (it;s a compromise dual band antenna with a rather large blind spot) More then likely that's a 5/8 wave vertical without a ground plane (since presumably you would be over water while using it and really in no need of a high vertical take off.

J-poles are not inherently dual-band. Any of them that are a compromise design. (Like any multi-band antenna.)

As to why one might be wanted: the J-pole is a decent performer, which is fairly easy the make, and can be physically robust. There are plenty of design plans, that show how to calculate the dimensions for the desired band.
 

wowologist

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J-poles are not inherently dual-band. Any of them that are a compromise design. (Like any multi-band antenna.)

As to why one might be wanted: the J-pole is a decent performer, which is fairly easy the make, and can be physically robust. There are plenty of design plans, that show how to calculate the dimensions for the desired band.

No actually the entire methodology of the j-pole is its acceptable use as a dual band antenna without having to decouple, create and match stubs and its inherently ease of creation...you would never waste the loss of using a single banded (tuned) antenna that has been designed for a single band IE: ....the marine band is just that...a single band, 18mhz wide there is no need for a dual band antenna and the losses and compromises associated. Most, not all marine antennae are based on single pole 5/8 design for a reason...your over water and dont need a ground plane as your over a natural one...and 5/8 allows for a more line of sight take off of the signal for local area operations >75km with a 25w radio is not unheard of and no reason for over the horizon <180km on that band....you would use HF for those communications.
 

krokus

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Most, not all marine antennae are based on single pole 5/8 design for a reason...your over water and dont need a ground plane as your over a natural one...

The no ground plane antenna is so they can be used on fiberglass hull vessels, or somewhere up on the mast or yardarm.
 
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