I do not like buying antennas if I can build my own that works just as well as a purchased antenna. Throughout my amateur radio "career" I only purchased two Diamond X50 antennas. One needed refurbishing after two month. See article HERE. Living by the sea takes its toll on any antenna if it is not very well constructed. This is why I prefer to build my own antennas. Be as it may this is my choice. Others might prefer to purchase commercial antennas which is also fine.
In this article we will be looking at building two low budget antennas - The DIY Stacked J-Pole Antenna(s) for a 70 cm DMR Repeater. Currently only a two stacked antenna as I will later add the other two that will result in a 4 element stacked J-Pole antenna. The idea for this antenna was derived when I saw a product pamphlet in the shack of Nico ZS4N of Hy-Gain's 4 Element 2 Meters Stacked J-Pole Antenna. I searched the Internet and only found copies of the advertising pamphlet on this 2 meter antenna and none on a 70 cm version of this antenna. So I decided to roll my own.
Now why a J-Pole Antenna? Glynn E. "Buck" Rogers Sr (68 years as K4ABT) tells us more:
The J-POLE has been around since the early days of HAM Radio, and is a direct descendant of the "Windom" Like the Windom or ZEPP, the J-POLE is a spin-off, or a modified WINDOM for VHF and UHF. One of the first articles I wrote about the J-Pole was in HRC magazine in 1958. Since 1958, I've written several j-pole articles in other HAM Radio publications.
Here, my references are to the early, 1923 (version) Windom (Article by Loren G. Windom September 1929, QST magazine) . If you look at the feed of the early Windom that was fed with a single wire, you may soon see the similarity between the Windom, ZEPP, and the J-Pole.
Look close at the configuration of the Jpole and the Windom, and you will understand why in many of my articles in CQ Magazine and other publications, that I often refer to the Jpole as a Windom, with the short section folded back on itself to form the parasitic element. It is for this reason that I feel these are two of the best antennas ever designed.
Having said this, you will also note that the Windom (and the Jpole) are powerful antennas that provide outstanding performance on all bands above the band for which they are cut or designed for. The reason these two antennas perform so well (as Multi-Band antennas; Windom for HF & lo VHF, Jpole VHF & UHF), is because they operate at harmonics of the fundamental or lowest frequency for which they are cut/designed. To add additional feeders (ladder-line), other than 50 ohm coax or UNUNs is a waste of RF energy. Only 50 ohm coaxial cable and a BALUN at the feed-point is all that is necessary. Anything more, adds losses into the equation that cannot be overcome after-the-fact.
Remember the axiom:
"When you have reached perfection, anything more becomes a point of diminishing returns." Enough said!
Trust me on the above paragraph, as I have experimented with every Windom and J-pole concept or design that can be imagined. Having built and sold thousands of these two antennas, I've found that
It's difficult to improve on perfection.
- For now, let's look at some of the features of our J-Pole, whether for; 140-150 mHz, or 430-450 mHz
- the J-Pole is easy to erect
- the J-Pole needs no radials
- the J-Pole has low angle radiation
- the J-POLE has greater bandwidth.
- the J-Pole has greater immunity to terrestrial noise
- the J-Pole is great for local nets or distant repeaters
- the J-Pole has more gain than most Ground Planes
- the J-Pole is more durable than most Ground Planes
- the J-Pole meets most "stealth" antenna restriction agreements
- the J-Pole has less static-charge noise, and static-charge build-up
- the J-Pole is a end-fed half-wave dipole
The J pole antenna uses the stub to provide a good match to 50 Ohm cable. The feed point is moved up or down the stub to provide the best match, and adjustment can be made once the antenna is in position if required.
The main disadvantage is that it can be a little more difficult to adjust than some other forms. The reason for this is that impedance matching has to be accomplished by altering the trimming length of the stub.
Thanks, Buck K4ABT!
Now I needed a 70cm Stacked J-Pole Antenna and the only information that I have was the two images above and the info from Buck K4ABT, No measurements or images for a 70cm version! I decided to design my own 70cm version from scratch and sorry Buck I might have transgressed the axiom here?
I looked at the second image depicting the two meter version above. I decided to "invent" my own way of building the 70cm version of the Stacked J-Pole Antenna. I used an online calculator to calculate the 70cm dimensions. However the final dimensions for my version of this antenna differ significantly from the calculator which is fully understandable as the feeding arrangement and parts used is completely different to the traditional version of the antenna. Up to now I could not find any information about this Stacked J-Pole Antenna for 70cm. I however now have all the information at hand after constructing two of these antennas.
The dimensions for the antenna was calculated using a free antenna calculator program (available for download from here). I set my frequency to 438.260 MHz. The resulting calculation is shown in Figure 1 below.
The above dimensions did not work for the upside down J-Pole antenna for 70cm. Here is the dimensions that worked for me.
Herewith the small amount of info that I have about the antenna:
The antenna is a high performance all driven stacked array of vertically polarized dipoles (J-Poles) that presumably delivers 6-9 dBi (or 4-7 dBd) or more omnidirectional gain. Uniquely designed phasing and matching harness maintains a perfect parallel phase relationship and is center fed to minimize beam tilting for better low angle radiation. The driven element maintains an extremely broad band response and effective isolation from the supporting mast only if the correct distances is set. Open construction minimize failure due to moisture and condensation. The entire antenna is DC Ground for static removal and lightning protection. Feeds with a good 52 Ohm coaxial feed-line.
Electrical Specifications:
Gain: 6-9 dBi (or 4-7 dBd) (2 x Phased Stacked J-Pole Antennas)
Power Limit: 1 Kw PEP
VSWR (at resonance): 1,1:1 Note: Using the NanoVNA not connected to the computer I obtained a 1:1 SWR but using the NanoVNA Saver program the SWR was reflected as 1.4:1 and 1.2:1. Why is still a mystery.
Impedance: 50 Ohms
Mechanical Specification:
Mast Height: To be calculated
Isolators: Stauff Gr 2
Mast Diameter: 50 mm
Wind Survival: 100 km/h
With all the information now on paper, I was ready to move onto the next phase and that was to gather all the materials I was going to need.
Materials that I used for one antenna:
Bending the J-Pole Element:
(Click on images for larger view.)
Now how did I bent the 180 Degree bend of the antenna? I used a small pipe bender. The 10mm aluminum tubes are cut to 1 x 565mm length. I made a mark at 465mm on the long side of the antenna. On the short end I measured 120mm and made a mark. The first mark on the long side is placed on the 0 | 0 mark of the pipe bender. Now start bending until the 0 on the top lever reaches the 180 mark on the bottom lever. You now have a 180 Deg bend and your J-Pole antenna element.
I am not going to go into detail how I further constructed my version of the antenna. I will however describe my findings in constructing and testing the antenna. The images below provide good detail of how I constructed the 70cm Stacked J-Pole Antenna.
My build observations and findings:
I used the material that I had in my "Junk" Box. The aluminum was left overs from other antenna projects. Measurements is absolutely crucial and feed point connections must be short as possible for obvious reasons.
I used the outer braid of RG58CU Mil-Spec to connect the 3/4 wave element. The 1/4 wave radiator is connected using a short piece of 3mm Solid Copper Wire.
Tuning the J-Pole correctly is very important:
- SWR Meter or Antenna Analyzer
- Coaxial cable
- Tools for cutting/adjusting wire/tubing
- Initial Setup: Mount the J-pole at its final operating height, away from ground/obstructions if possible, and connect your SWR meter/analyzer between the radio and the antenna's feed point.
- Find Resonance (Length):
- Measure the SWR across your desired frequency range.
- If the lowest SWR dip is below your target frequency, the antenna is too long; trim a little from the ends (main radiator and stub).
- If the lowest SWR dip is above your target frequency, the antenna is too short; add length.
- Repeat trimming/adding in small increments until the resonant frequency (lowest SWR dip) is at the center of your band.
- Match Impedance (Feed point):
- With the antenna resonant at your frequency, find the point on the 3/4 wave element and the 1/4 wave radiator for the lowest SWR and 50-ohm match point.
- Once you find the right points for the lowest SWR and 50-ohm impedance your antenna is now tuned.
- Add Common Mode Choke: Install a choke (ferrite beads or a few turns of coax) at the feedpoint to prevent common-mode currents, which improves performance.
- Final Check: Re-test across the band to ensure good performance and bandwidth.
- Two Adjustments: J-poles need both length (frequency) and feed point (impedance) tuning.
- Bandwidth: J-poles have narrower bandwidth than dipoles, making precise tuning crucial.
- Material: Use bare wire or tubing for best results; insulation affects performance
- Increased Gain: Stacking elements (like two J-poles) concentrates the radiation pattern, adding gain, often resulting in 3-6 dB more than a single unit, pushing it from a typical 3-5 dBi J-pole to higher figures.
- Lower Take-Off Angle: This design provides a lower angle of radiation compared to a simple dipole, which is excellent for FM voice communication and hitting distant repeaters.
- Performance: You can expect good range, with some users reporting success reaching distant repeaters or even simplex contacts (e.g., 40-80 km) with modest power (3W+).
- Construction Matters: For dual-band 2m/70cm stacked J-poles, the 70cm performance often involves a 2x5/8 wave design, requiring careful tuning and phasing for both bands.
- A single J-pole is often around 3-5 dBi.
- A stacked 2x5/8 wave 70cm J-pole can reach 6-9 dBi (or 4-7 dBd), depending on the design and how well it's stacked.
Images: (Click on images for larger view.)
