Simple J Pole FM Antenna Plans

[theodoric]

My J-Pole project

So I decided not to miss out on the J-Pole bandwagon.

I picked up a copy of Joseph Carr's Practical Antenna Handbook (4th edition) and the American Radio Relay League's ARRL Antenna Book, and found they both recommended the J-Pole as well. Carr's math was a little different than the schematic linked in another thread (that I can't currently locate the link to), so I went with his, as the source is known.

Tuning the length to a wavelength of 98MHz (halfway through the 88-108MHz band), I got the following dimensions:
3/4 wave element: 90.18"
1/4 wave element: 30.12"
coax attachment point: 2.95"
spacing between elements: 2.82"

While it would be optimum to put the thing on top of my two-story roof, the line length of over 100' would result in a huge line loss, and besides, I wasn't in the mood to get up on the roof in December, and I certainly wasn't in the mood to run cable through the attic, down through the walls, under the house, and up into the living room. Seeing as there was a perfectly good wall nearby, with a suitable area conveniently obscured by blinds, and only a dozen feet away, I decided to give it a try.

Components:
1 - 10' length 1/2" copper pipe
1 - 5' length 1/2" copper pipe
2 - 1/2" 90 degree copper elbows
2 - 1/2" pipe caps
2 - 1/2 copper pipe straps
2 - 1/4" copper pipe straps
4 - #10 - 32tpi x 1/2" brass machine screws
4 - #10 - 32tpi brass machine nuts
8 - #10 brass washers
6 - 1/2" Oatey nylon Pipe Support Clamps (part #33916)
12 - #6 x 1" Philips head coarse thread drywall screws

Parts and tax were about $12.

Using a pipe cutter, I cut the pipes to length, fit them with the caps, elbows, and support clamps. The nylon support clamps act as both mounting hardware and insulators, and keep the J-Pole approximately 1/8" from the wall.

I put the 1/4" pipe straps in the vise and flattened them out. The mounting holes matched perfectly with the 1/2" pipe straps. I bolted the copper pipe straps together with the brass bolts, nuts, and washers.

Then I mounted the whole assembly to the wall with the drywall screws, making sure that the spacing between elements remained 2.82" when mounted.

The whole thing came out very clean, and is completely hidden from view (except the top 8") by the window blinds.

Connection from the J-Pole is via 15' Belden RG-6 75 Ohm shielded coax cable.

 

[drspiff]

I think we need a sign that says

"This thread is worthless without results!"

 

[Whitehall]

So how in heck do they calculate "C" and "D"?

"A" and "B" are half and quarter wavelengths corrected for velocity for an "electrical" length.

C+2D could be the impedance match of the quarter and half wave poles plus the line impedance of the feed line. One source claimed that the poles could and should be as close as possible. That's one reason why hamsters like the idea of cutting up twin lead to make their portable J-pols. There might be capacitance problems somewhere down that path. That's as deep I I understand it so far.

If that's the case, then I can fold over my dipoles tape around a sheet or cardboard and "D" would be found by experiment and would vary with 75 ohm and 300 ohm feed lines. I'll make the dipoles for the longest FM wavelength (lowest frequency) for plus some for safety since C+2D will eat into A and B.

See cross-sectional (edgewise) sketch.

 

[Lefty]

"While it would be optimum to put the thing on top of my two-story roof, the line length of over 100' would result in a huge line loss, and besides..."

Well I certainly support your desision to try the lower wall for the trials, I would say you should consider next spring to try the roof top location. The added signal strength from the higher elevation will more then offset the coax losses. Now if the wall location receives all your desired stations at full stereo with no noise then maybe no further hight is needed.

From my ham radios days, there was a reason most hams if they could afford it would always for the the 60ft tower over the 20ft tower. The increased gain for the same antenna was always noted....

They had a saying: If your antenna didn't fall down last winter then it isn't big or high enough :yes:

Lefty

PS: Nice job on the J, I think copper pipe is THE material of choice. You can always paint that last 8" white BYW....

 

[drspiff]

How 'bout this as an idea: Take a piece of 1 1/4" PVC pipe cut to the "A" length. Get some of that aluminum tape the HVAC guys use to tape ductwork together. Run the tape down one side of the PVC and up the other side for distance "B". If the "B" tape overlaps the "A" tape, trim it back with your utility knife, leaving at least a 1/8" gap between the tape traces. You now have a J pole antenna which you can stand in the corner or build into a hat stand, or hang from the ceiling. Since the tape is aluminum, you could secure the coax to the antenna using a couple of ty-wraps.

 

[theodoric]

I haven't had a chance to do a serious A/B comparison so far.

Carr's book says that changing the spacing between the 1/4 wave and the 3/4 wave elements will yield different impedances, specifically,

Taken together, the matching section and the radiator form a parallel transmission line with a characteristic impedance that is 4 times the coaxial cable impedance. If 50 Ohm coax is used, and the elements are made from 0.5 in OD pipe, then a spacing of 1.5 in will yield an impedance of about 200 Ohms. Impedance matching is accomplished by a gamma match consisting of a 25 microfarad variable capacitor, connected by a clamp to the radiator, about 6 in (experiment with placement) above the base.

I didn't solder any of it together, so it will be easy to tune (except for the overall length), so I'm taking a 'wait-and-see' approach to see how effective those dimensions are. They seem to simply be a starting point.

 

[Whitehall]

Once you understand how aluminum tape can be made to work and what's needed in separation, then the physical implementation becomes wide open.

I got a 1.5 inch wide roll, 48 feet long for $6 or so. I can split it since half inch should be plenty wide and make 3 X 5 = 15 antennas from it at 40 cents each, plus leads.

That's the hope.....

 

[theodoric]

Whitehall, I love your proprietary/confidential nuke-yoo-lur drawing.

Lefty, I think you seriously underestimate my disdain for getting up on the roof and pulling cable this time of year. I'll attach it to the upright on the roof in the springtime, provided I don't just go whole hog and put a big APS Yagi up there.

 

[Whitehall]

Sorta makes it look I know what I'm doing, doesn't it?

HA!