J-Pole

Status
Not open for further replies.

davenlr

Member
Premium Subscriber
Joined
Jan 31, 2014
Messages
637
Location
North Little Rock, Ar
I want to build a J-Pole for 1090 MHz, but the small size and only 1/4 inch separation from the element to the stub is creating a rather difficult problem, as I was planning on using brass compression tee, and 3/8 copper tubing.

As I understand it, the J-pole is resonant on its designed frequency as well as its harmonic frequency (f*3).

Would it work to design one for 363 Mhz, which would be larger, and use it on the harmonic for 1090 Mhz?

Also, all the coax coils used to prevent the feedline from radiating in the diagrams are for 2M (5 loops). What would the formula be for 1090 MHz?
 

KC8ESL

Member
Joined
Mar 8, 2008
Messages
535
Location
Painesville, Ohio
I cannot realistically see how it would be a physical possibility to build this. Is this a theory or you've seen it in practice?

The more common antenna I've seen built for this frequency is a colinear type setup.

They can be amazingly simple to construct.
 

davenlr

Member
Premium Subscriber
Joined
Jan 31, 2014
Messages
637
Location
North Little Rock, Ar
Problem with a collinear is its beam is right on the horizon. I am on a hilltop, but inside a bowl type valley, with 150' higher hills all around me at about 1 mile, so I need an antenna that radiates at about 20-30 degrees, not 0-5 degrees. Ive tried the collinear, tried a discone. So far the best results have been with a regular old 1/2 wave dipole since its pattern is up off the horizon. Since the J-pole has gain to overcome the feedline loss, but the same relative take off elevation as a dipole, I thought that would be a good choice to try.
 

mm

Member
Joined
Dec 19, 2002
Messages
616
With the proper phasing between elements and if your interested in either, 978 or mode S ES 1090, but not both, then a collinear works very well.

I recently made a 1/2 wave over a 1/4 wave with a .25 wavelength phasing stub between the 2 upper elements and 2 X 1/4 wave decoupling stubs tuned to UAT 978 for a customer in Arizona.

It was designed and modelled in 4nec and test ran using BOTH a RTL dongle(with lna) and a Garmin GDL39, it was Receiving out beyond 250 mile radius from my lab in Salem Ore. I was just dropping one of our planes North of Santa Rosa California and a 2nd one near Vancouver B.C..

The setup was as a base station with the antenna 15 feet above ground at 250 feet msl.

With a .75 db NF LNA the RTL DONGLE is almost as sensitive as the Garmin GDL39.

Fortunately the Az. customer didn't need 978 to 1090 in one antenna otherwise the collinear would not stretch that far and then I would have had to use another design such as a a loaded left handed loop style of antenna to get some gain with wide bandwidth, but still gain much lower than the collinear, along with some complicated capacitive switching to get the wide bandwidth.
 

KC8ESL

Member
Joined
Mar 8, 2008
Messages
535
Location
Painesville, Ohio
Dave, perhaps you're looking too deep into your problem. For what you want would perhaps a quarter wave antenna with a ground plane coupled with an LNA matched to your feed line loss so your coaxial net loss is 0dBi?

I'm finding success with a selectable 5/15dB LNA and a 1dB/step 10 step attenuator. I might be amplifying 15dB but over 100ft of rg8, let's say I lose 10 db of signal at 1100MHz so at the sdr I am +5 dB. I knock down an extra 5 dB of signal through the attenuator and I have a lossless coaxial systrm. Don't forget that you're not only amplifying signals present but you're also amplifying the noise floor as well.
 

KC8ESL

Member
Joined
Mar 8, 2008
Messages
535
Location
Painesville, Ohio
Another thing to consider (and someone PLEASE correct me if I am wrong on this topic) is that if you set this antenna on the ground, your theoretical takeoff angle would be much higher that if it was in free space.
 

davenlr

Member
Premium Subscriber
Joined
Jan 31, 2014
Messages
637
Location
North Little Rock, Ar
I just built a coaxial dipole, 2.5" of center conductor, with the sheild folded back over the jacket for 2.5", for 1090 mhz. Taped it in the north window about 10' above the ground, and am getting twice as many hits now, and the range in the direction the window faces, out to about 50 miles, or 20 miles further than it was before.

I think Im going to seal this one up inside a PVC tube, and mount it up on the roof in place of the other antenna, and see how it works through the 50' of coax. If its acceptable, Ill try the LNA on it. Can you provide a link to the LNA?
 

KC8ESL

Member
Joined
Mar 8, 2008
Messages
535
Location
Painesville, Ohio
Honestly, it was a Hamfest special about 8 years ago. The only thing on the circuit board was "PWS C2004". It had a custom case so the markings on the outside were gone.

BNC in BNC out and status LED.

The only reason I know the switch is selectable 5/15 dB is because I put it on a SpecAn with tracking generator and measured gain out of it. Good up to 1.4Ghz then there is considerable roll off.
 

davenlr

Member
Premium Subscriber
Joined
Jan 31, 2014
Messages
637
Location
North Little Rock, Ar
I saw a thread on here I think, about some LNA's. Ill see if I can find it. If I go that route, Id want to mount it in a box at the base of the antenna tripod.
 
Status
Not open for further replies.
Top