Morden2004 makes a 'J' pole antenna!

Morden2004

Morden2004

Relaxin'
Morden2004 makes a 'J' pole antenna! And it works!

After reading the 'J' pole thread elsewhere in this forum, I decided that it was a good project for me. I recall years ago making one out of 12 ga.wire just for fun and it was quite the performer.

My first stop, Home Hardware for the pipe and fittings. I decided early on that I would beef-up the bottom (support) section to ¾” pipe so one of my parts was a 1/2x1/2x3/4 ‘T’.

With all the physical hardware in hand I made my next stop at Radio Shack (now called ‘TheSourceCC’ in Canada (the CC is Circuit City) and I parked myself in the RF/Cable isle. I always leave final decisions as to wiring until I can check out the bits in the bins on the shelves. I was lucky this time because I found a perfect component to interface the RG-6 co-ax to the antenna stub; see picture #1 below. :banana:

This is a 75 ohm grounding block RS# 1500909 for satellite cabling, but it will become my primary connection to the antenna and will allow me to run a ‘un-cut’ 50’ RG-6 cable end-to-end. I always like it better when I don’t have to jury-rig a new connection to factory-terminated co-ax. Factory connections are both stronger and are weather-proof. Mine often fail on both counts.

I cut the sections to 100 MHz as a good center frequency. Then I soldered together the basic antenna and cut two pieces of wood to tie together the ¼ wave stub to the ¾ wave rod. I fabricated these simple wooden tie bars (two) with one large hole and one tight hole. These slip down over the two sides with a little tapping with a rubber mallet. The large hole has a sharp-pointed screw to lock the bar in place.

Picture #2 shows the ‘working end’ of the ‘J’ antenna with the grounding block located on the lower tie-bar at about 5” above the cross-over. I think you can now visualize how the co-ax will connect and how I will extend the other connector with solid copper wire to become the split-T connection to the antenna. Picture #3 and picture #4 shows why I was so happy to find that grounding block! Picture #4 shows the completed wiring with the RG-6 connected. I will create a small loop in to co-ax below the antenna and wire wrap it to the support to be a strain relief for the co-ax.

Next, I’m going to stand this baby in my stair well (I have 7 foot ceilings in this old house, so can’t stand it up in my living room :no: ) and see how it performs.

Paul
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Looks good. It's hard to tell from the picture but you might have the ground and signal wires going to the wrong elements. I think the red center conductor should go to the shorted 'matching' stub and the ground to the longer element. Also if you ever plan to mount this outside you might consider adding some silicon RTV to where you have the wire going into the connector block, otherwise water will surely finds it's way into that connector block.

Lefty
 
Lefty said:
Looks good. It's hard to tell from the picture but you might have the ground and signal wires going to the wrong elements. I think the red center conductor should go to the shorted 'matching' stub and the ground to the longer element. Also if you ever plan to mount this outside you might consider adding some silicon RTV to where you have the wire going into the connector block, otherwise water will surely finds it's way into that connector block.

Lefty
Click to expand...

Mixed up wires? Interesting thought. Back to the books for another look I guess! I missed that possibility. Thanks for the heads up.

And, yes, before I mount this baby outdoors I will cover both connections with silicone grout to keep Mr. Moisture out.

Paul
 
It works!!

Yes, the 'J' pole antenna works! :banana: :banana: :banana:

It delivers about 1 full increment more on the signal level meter (4 vs. 3) on my Sansui G-8000 using my reference station - 103.1 FM Moncton N.B., which is about 80 Km across mostly open water. And that is with the 'J' pole indoors and propped up inside my stairwell about 1 foot above floor level. Awesome.

As mentioned in a previous message, I reversed the two wires on the antenna; the co-ax shield was connected to the 1/4 wave and the center conductor was connected to the 3/4 wave segment and --- worse by nearly a full increment! So, in my case, the above mentioned connections are correct. Interesting. Can someone explain this?

I would like to get a more meaningful measurement of the increase and am open to suggestions from some of our more 'RF' knowledgeable members. I have a scope and I know how to use it. Any suggestions?

Paul
 
Morden Glad your J pole works so well. The center conductor connects to the long arm. One thing I did notice is that you used very wide spacing between your elements. At the FM frequency the spacing is normally 2 inches. Also you can move the connection point up and down till you get the best signal.Moving it will get the best impedance point and the max in transfer of signal to the coax. The short arm acts as a matching transformer and the spacing will cause the match to the coax to be off.In the real world what you have is most likely as good as it will get.If it works leave it alone.Nice job.
Ed
 
Here is a page that will give you all the numbers.
http://www.hamuniverse.com/jpole.html
Interesting I found several articles where they made the connection the other way around and it worked beter that way so I guess you have found what works best for you.
Ed
 
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EddyR said:
Here is a page that will give you all the numbers.
http://www.hamuniverse.com/jpole.html
Interesting I found several articles where they made the connection the other way around and it worked beter that way so I guess you have found what works best for you.
Ed
Click to expand...

Ed: I wish I had seen that site before I cut tubes :yes: .

Here are their measurements for 100 MHz:
A (Long section) dimension is: 84.6 inches
B (Short section) dimension is: 28.08 inches
C (Feed point) dimension is: 2.76 inches
D (Spacing) dimension is: 2.64 inches

Here are my measurements for 100 MHz:
A (Long section) dimension is: 88.4 inches OK
B (Short section) dimension is: 29.5 inches OK
C (Feed point) dimension is: 4.75 inches 70% wider
D (Spacing) dimension is: 3.75 inches 45% wider

I wonder what impediance they were anticipating? If I recall correctly, a bigger gap will increase the impediance, right? I think that their dimension 'D' is likely calculated for 50 ohm co-ax whereas I'm using 75 ohm co-ax. Also, the feed point 'C' will certainly affect the SWR and the null point of the center frequency (digging back into my memory here - probably wrong) I think. Moving the feed point may yield more signal.

Paul
 
Morden2004 said:
Yes, the 'J' pole antenna works! :banana: :banana: :banana:

It delivers about 1 full increment more on the signal level meter (4 vs. 3) on my Sansui G-8000 using my reference station - 103.1 FM Moncton N.B., which is about 80 Km across mostly open water. And that is with the 'J' pole indoors and propped up inside my stairwell about 1 foot above floor level. Awesome.

As mentioned in a previous message, I reversed the two wires on the antenna; the co-ax shield was connected to the 1/4 wave and the center conductor was connected to the 3/4 wave segment and --- worse by nearly a full increment! So, in my case, the above mentioned connections are correct. Interesting. Can someone explain this?

I would like to get a more meaningful measurement of the increase and am open to suggestions from some of our more 'RF' knowledgeable members. I have a scope and I know how to use it. Any suggestions?

Paul
Click to expand...


Well I goggled a few J-pole articals and I see examples of wiring if both ways.
From my ham radio days long ago the one I build was wired like this example, with the center conductor wired to the shorter element:

http://www.vcars.org/tech/J_Pole2M.html

As far as tuning the thing for best match your kind of on your own using the signal strength meter as a measurement value. In ham radio they use their transmitter power through a SWR bridge meter, and they adjust the center wire attachment position for minimum standing wave reflection. This is where the match is most efficient. For receive only it's probably not as critical and won't make that much difference in received signal strengh. i'm sure you will find that the easiest improvement is in how high you can get that thing in the air. FM is VHF line of sight mostly (there can be weaker signal reception from reflection off hills and buildings) and the higher you are the further you see.

So keep on the experimental side, antenna building can be alot of fun and nothing returns better reception results then improving the antenna. Even a TOTL megbuck tuner can sound like crap if it doesn't get enough signal to work with.

Lefty
 
For the sealant, try the Radio Shack "connection sealing tape" or some such name. It is a small roll of a black gooey stuff with a paper backing strip. Mine came in a hanging package near the F-connectors.

You unroll what you need, cut, pull off the backing strip, then wrap around the connection. There, treat it like putty and mold to make a little encapulation of black goo. Easy to apply, cheap, and seems to work great.
 
Lefty said:
Well I goggled a few J-pole articals and I see examples of wiring if both ways.
From my ham radio days long ago the one I build was wired like this example, with the center conductor wired to the shorter element:

http://www.vcars.org/tech/J_Pole2M.html

As far as tuning the thing for best match your kind of on your own using the signal strength meter as a measurement value. In ham radio they use their transmitter power through a SWR bridge meter, and they adjust the center wire attachment position for minimum standing wave reflection. This is where the match is most efficient. For receive only it's probably not as critical and won't make that much difference in received signal strengh. i'm sure you will find that the easiest improvement is in how high you can get that thing in the air. FM is VHF line of sight mostly (there can be weaker signal reception from reflection off hills and buildings) and the higher you are the further you see.

So keep on the experimental side, antenna building can be alot of fun and nothing returns better reception results then improving the antenna. Even a TOTL megbuck tuner can sound like crap if it doesn't get enough signal to work with.

Lefty
Click to expand...

Yes, height is a big contributor to improved signal strength. I added the 3 foot bottom section thus raising the antenna 3 feet and the signal strength went from 4 to about 4.25 - that is significant as we're still indoors!!

Paul
 
Whitehall said:
For the sealant, try the Radio Shack "connection sealing tape" or some such name. It is a small roll of a black gooey stuff with a paper backing strip. Mine came in a hanging package near the F-connectors.

You unroll what you need, cut, pull off the backing strip, then wrap around the connection. There, treat it like putty and mold to make a little encapulation of black goo. Easy to apply, cheap, and seems to work great.
Click to expand...

Great idea. This must be something like the self-vulcanizing tape we used to use eons ago. It looked like regular black electrical tape but in a few hours it 'healed' the seams into a continuous skin.

Paul
 
Very interesting DIY project....

Paul.. when you get a chance can you post a pic of the whole thing so we can get a better sense of the overall dimensions and appearance? Thanks, I'd appreciate it.
 
hpsenicka said:
Very interesting DIY project....

Paul.. when you get a chance can you post a pic of the whole thing so we can get a better sense of the overall dimensions and appearance? Thanks, I'd appreciate it.
Click to expand...

Holy smokes - do you know what you're asking? :nono: I can't even get it upright in my room - it just a few inches shy of 11 feet high!

Yes, I will take it outside tomorrow and shoot a few pics of it for you.

It is awesome in both appearance and operation. I've been thinking of ways to make it less 'plumbing project gone awry' looking and I think that a wrapped covering of vinyl tape may give it a more manufactured appearance. Being copper, it will eventually have a green patina (verdegris) especially since I routinely get salt spray from the ocean right over the house!

Paul
 
Morden2004 said:
Holy smokes - do you know what you're asking? :nono: I can't even get it upright in my room - it just a few inches shy of 11 feet high!

Yes, I will take it outside tomorrow and shoot a few pics of it for you.

It is awesome in both appearance and operation. I've been thinking of ways to make it less 'plumbing project gone awry' looking and I think that a wrapped covering of vinyl tape may give it a more manufactured appearance. Being copper, it will eventually have a green patina (verdegris) especially since I routinely get salt spray from the ocean right over the house!

Paul
Click to expand...

Yikes.. 11ft high!

I was imagining something around 7 or 8ft tall... guess I wasn't paying close enough attention!

Anyway, no rush.. but would love to see pics of it actually in operation outdoors when you get a chance.
 
Great job there Morden, it looks great.

If I might make some suggestions, since you went with a wood insulating block, you'll want to give it a hefty coat of spar varnish or the like. This will keep the water and salt out of the wood and so keep it as an insulator rather than reduce your antenna's performance. Also, painting it will go a long was of making it last, hence the zinc chromate and Rustoleum suggestions in the other thread. It sounds like you're already planning on sealing up the connections, but if that tape doesn't work too well, I've had excellent results with liquid tape -- a red goo (the cheap stuff is black, but doesn't work as well) that comes in a jar with a brush lid that you just brush onto the connections and seals 'em right up.

Get that sucker up in the air (and put a drip loop in the coax before you run it in through your wall or the like -- I used a telephone junction box with the rubber grommets and a chunk of PVC pipe glued into a hole in the back [and a blank outlet cover with a matching hole drilled in it on the inside] as wall feedthrough if you want to get fancy) and you will be impressed at the performance.

Sooo, are you going to start making these for other members too? ;)
- JP
 
Suggestion

Pictured you have it wired correctly. The center of the coax goes to the 3/4 wave pole, however that connection needs to be moveable.

You know what your weakest FM station in your area is, so tune it into that one and adjust the center coax up and down on the 3/4 wave pole until it comes in as loud as possible.

Fini

Oh, ground it before you walk away. I use HD braided wire and attach one end to the antenna using a clamp and the other to a piece of rebar I have driven into the ground. I drive it all the way in and then put a rock on top of it just so no one catches themselves and I know exactly where it is. I bury the path of the wire to the ground. Of course if you are doing this right next to the foundation of the house, you can leave it exposed.
 
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