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Made a J-pole and getting KQRS out in the sticks!

DENNYDOG

DENNYDOG

Addicted Member
I ran across a thread here on AK about building a J-pole antenna and decided to finally build one. I have thought about it before but never got around to it.

I went to the local Runnings and picked up a couple sticks of 3/4" copper pipe. You can build it out of 1/2" and up to 1 1/2" but thought the bigger pipe was more than I wanted to spend. I want to mount it on the roof and thought the 3/4" would be a good trade off between strength and cost. I figured the 1/2" would be way too flimsy. Total cost with pipe and fittings was about $35. I'm sure it would be cheaper yet if the parts were bought at a larger store like Menards or Home Depot.

Plans are to mount this on the roof but I thought I would see how well it worked first. It is currently sitting at the top of my stairwell with the cable running down to the tuner.

Before and after results are stunning. I am using a Pioneer Elite F-91 digital tuner. I was using a wire that originally came with one of my newer receivers plugged in to the coax in. I always thought the reception should be better as this is supposed to be a decent tuner according to the tuner information site. There was always static and never could pick up many stations.

Most of the stations now show a much stronger and cleaner signal. I couldn't even get KQRS which is a Twin Cities station. Now it comes in very well with no static. I am hoping it well be even better once I mount the antenna on the roof.

I followed the plans from the ham universe site. I used 98 MGhz as the center for the tuning. Just follow the measurements that the calculator gives you. I had good luck using the feed points it gives you as the reception didn't change a whole lot moving them up or down the J-pole.

http://www.hamuniverse.com/jpole.html

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Hope it's not just the time of day that it is coming in decent. Everything is coming in better than it did before though so I am hoping once I get it mounted on the roof it will do better yet.

I had it sitting vertical on the ground right outside the window when I first tried it and it was better than the other antenna but still not great. It made a huge difference just getting it up a little higher in the stairwell.
 
I always wanted to build one of those....be careful positioning on the roof as getting higher might cause you to get higher than you want to be.
 
Wow Congratulations on the DIY J-Pole. Thanks for the pictures.

Good work on using an RF choke at the antenna feed point.:thmbsp:

Great soldering job.:thmbsp:

You may already be planing on doing this...

But you will need to weather proof the coax at the antenna...You do not want moisture to get into the end of the coax.

You may still be able to buy a product called Coax-Seal at Radio Shack.

And make sure that you have good secure connections for the shield and the center conductor...Something more than just the wires twisted around the copper pipe.

And you may want to use more than just electrical tape to support the RF choke...Since you just may get some ice and snow where you are...

And be sure to use a good quality grounding block for the coax with a good ground rod. An advantage of the J-Pole antenna is that all elements are at " "DC Ground Potential". That is a good thing...But you do not want the "Ground" to be through your tuner.

Antennae can build up large static voltages from near by lighting strikes and actually from the wind in low humidity conditions...The grounding block will "Drain" away those static charges and help protect your tuner.

And remember that with antennae just like real estate it is "Location..Location...Location"...If you have the option...You may want to experiment with the location on your roof...Some times a few feet even a few inches one way or another can make a big difference.

And again you have a very nice looking J-Pole antenna and if you are already planning on doing these things...Just disregard...But I thought it might be of some help to others planing on building a J-Pole antenna. And for those of you that might be considering building a J-Pole antenna...If they are built and located correctly they are great performing antennae.

Hope it works good for you and you get to listen to
some great FM :music::music::music:
 
A j-pole is omnidirectional; it's relatively simple and not very difficult to make and reasonably rugged. Mounted on the roof (and solidly grounded!!), it should work well for many people. The disadvantage of an omnidirectional antenna is that you can't null out multipath distortion with analog FM, if it's a problem in your area. HD Radio seems to be more resistant to multipath.

If I can get the loan of an antenna analyzer, I'm going to try to retune mine for 75 ohm impedance (it's more like 40 ohms at the moment) and see if a larger diameter will help make it more broadband. Mine was based on the design shown in figures 1a and 1b here:

http://www.buxcomm.com/jpoles4ever.htm

I liked it because of the built-in connector; easy to wrap it in electrical tape for waterproofing.
 
DennyDog:

Doing some RnR (Readin' n' Research): try sliding the coax feedpoints up the rods a bit; this should help tune the antenna for a specific frequency by changing the impedance closer to 75 ohms (0 ohms is theoretically the bottom of the rod, highest at the top of the quarter wave rod (the short one). Shouldn't move it more than 2-3 inches, I'd think. It might "peak" the antenna for a weaker station and you'd see it on a signal strength meter. Final connection does need to be soldered.

Without a proper impedance measuring instrument, I'm shooting in the dark.
 
Nice work. I built one a while back and like it. I "tuned" up and down with the wires and when I found the sweet spot I drilled a hole and used self tapping screws for the final attachment. Soldering would probably be even more weather proof.
 
Maybe you could find a ham radio guy and borrow his antenna analyzer (don't all hams have one of these?) to tune your J-pole and optimize your attachment points precisely to 98 mHz. and then lock down the adjustments with some small hose clamps.


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Sorry to dash water on your endeavor but I bet I'm farthur away than you and I get KQRS from my indoor upstairs dipole, all day every day.
The thing to understand is that you have a vertical dipole with that setup and all dipoles will give the same gain. Your j-pole require a really tricky matching system and if you didn't get it right, you will have losses in the transmission line. And, you're only guessing that you got it right.
As you can tell from my post, I don't think much of j-poles. It's just another dipole to me.
All half wave dipoles give the same gain before any transmission line losses.
 
I always think of a J-pole as (usually) a 1/2 wave vertical with a 1/4 wave matching stub. There's only one "pole", the 1/2 wave section, so it can't be a dipole.
 
It is really a vertical 1/2 wave mono-pole antenna. And it is correct that the "gain" is similar to a full 1/2 wave dipole...

But it is slightly less ground dependent in terms of take off angle (angle of radiation)...

And it is at DC ground potential...Which a vertical dipole is not...And that is a very good thing when mounting an antenna outside.

I have built both J-Pole and vertical dipole antennae and both work very well...But for use outside the construction and mounting of the vertical dipole is a little more involved...It is not at ground potential and to make it so would require a heavy duty RF choke to ground...Again making it a more complicated install.

And if the vertical dipole is mounted on a conductive mast there will be some pattern degradation...(And that might be a good thing if it favored a certain station).

I have found that the measured VSWR (Return Loss) on J-pole antennae to be reasonably good when using the standard calculations...Although I believe that the calculations for the antenna the OP used are for a 50 ohm feed point and he would be well served to move the feed point up the antenna a small amount for the best match. And as was posted earlier...An inquiry to the local ham club for someone with an antenna analyzer would be a good idea.

If that is not possible he should just tune in a very weak station (To counteract any possible AGC (Automatic Gain Control) that might be present in the tuner and simply adjust the antenna for the strongest signal.

Having said all that...DENNYDOG...Do not worry to much about the location of the feed point...Since from 88Mhz to 108Mhz the feed point only moves about 0.5 inches...It is not going to make a huge difference...

And as I mentioned above...I believe this antenna design is for 50 coax and you are using 75 ohm coax...So just raise the feed point about 0.25 to 0.5 inches and you should be good to go and listen to some FM :music::music::music:

And one last tweak...A better RF choke would be for you to wind the coil of coax in a single layer on something like a 1 litre soda bottle...It would provide slightly better RF decoupling and slightly less signal loss than the scramble wound choke you have now.

And if it is possible I would recommend that you use brand new low loss RG-6 coax...I can not tell for sure but it looks like the coax in your picture is used and may even have a small slit in the insulation in the coax in the RF choke.

DENNYDOG...You did a good job in the construction of your antenna and it will work well for you.
 
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JBL GUY said:
Having said all that...DENNYDOG...Do not worry to much about the location of the feed point...Since from 88Mhz to 108Mhz the feed point only moves about 0.5 inches...It is not going to make a huge difference...

And as I mentioned above...I believe this antenna design is for 50 coax and you are using 75 ohm coax...So just raise the feed point about 0.25 to 0.5 inches and you should be good to go and listen to some FM :music::music::music:
Click to expand...

Ok. I didn't realize I was supposed to use 50 coax.

JBL GUY said:
And one last tweak...A better RF choke would be for you to wind the coil of coax in a single layer on something like a 1 litre soda bottle...It would provide slightly better RF decoupling and slightly less signal loss than the scramble wound choke you have now.
Click to expand...

It was just a quick coil I wound so I could try the antenna out. Looks pretty bad huh?

JBL GUY said:
And if it is possible I would recommend that you use brand new low loss RG-6 coax...I can not tell for sure but it looks like the coax in your picture is used and may even have a small slit in the insulation in the coax in the RF choke.
Click to expand...

Yeah it was some old coax I found in the basement. I didn't want to spend any money on new stuff until I was sure the antenna was going to work for me. Do they make a 50 ohm RG-6 cable? I thought all RG-6 was 75 ohm.

JBL GUY said:
DENNYDOG...You did a good job in the construction of your antenna and it will work well for you.
Click to expand...

Thanks!

Denny
 
Oh yeah and I do plan on weather proofing everything and soldering the connections before I mount it on the roof.

I do have a grounding lug that I plan on running the cable through. It was one the sat. provider installed when they put in my sat. tv. It has an extra coax connector I plan on using.
 
Hey Denny...

You are correct you DO want to use 75 ohm coax...


I am sorry if I was not clear...

The design of the J-Pole antenna that you used is for Ham radio frequencies...

And most ham radio antennae use 50 ohm coax when then are fed with coax.

I was just commenting that the calculations for the feed point location on the J-Pole antenna as calculated on that web site are for 50 ohm coax.

The actuall feed point impedance of the antenna is determined by the location of the feed point...The higher up the antenna the higher the feed point impedance and that would change the feed point impedance from the design point of 50 ohms to 75 ohms...Just by raising the point where you attach the coax.

Again you are correct...You should use 75 ohm coax.
 
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DENNYDOG said:
Oh yeah and I do plan on weather proofing everything and soldering the connections before I mount it on the roof.

I do have a grounding lug that I plan on running the cable through. It was one the sat. provider installed when they put in my sat. tv. It has an extra coax connector I plan on using.
Click to expand...

Oh and by the way....The shield on most RG-6 is aluminum and you will not be able to solder it...Although I have run across some copper shielded RG-6...Just not very often.

You could as mentioned before...Drill a hole in each pipe and use self tapping screws...And I would secure everything with BLACK cable ties (The black ones are much more UV resistant than the white ones).

Appears that you are on the road to having a very nice antenna...

It is great that you are taking the time to DIY an antenna.
 
Ok thanks! I had fun building it. I am glad to see it is at the very least giving me better reception than I had before.

Denny
 
Ed:

That MFJ-259B is exactly what I wish I had; I want a Sound Technology 1000 FM alignment generator too. Keep getting outbid on eBay for the latter, but maybe I'll try for the former.

--Damon
 
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