jonny290
Member
A "bored at work" mini-Christmas gift to the RR community. Happy Holidays! Note that I have no formal electronics training and all hypotheses contained within are purely a result of my experimentation and retarded math. If I need correction or clarification, please give it.
OK, so I've been hitting the books hard lately, working on antenna design and theory. let's attack one of the most popular antennas out there, the J-pole! Today we'll build a 2 meter J-pole and tune it for maximum performance on 146.5 MHz, with bias towards the high end of the 2 meter band, as those 147 positive offset repeaters can tax some lesser antennas.
The above image is horizontally polarized. Let's think vertical!
The J-pole is an end-fed half-wave antenna, fed through a shorted 1/4 wave matching stub. It offers easy construction, good performance, and lots of tweakability, with a few caveats.
A few things to know about the J-pole:
-It is a DC grounded antenna. Since it is fed through a shorted stub, there is a DC path to ground from your element. This reduces atmospheric static on amplitude modulation types (AM, SSB and CW), and provides extra lightning protection. Even though the antenna is 'shorted', you will *not* blow up your radio by transmitting into it. That's the magic of RF!
-Behind every great antenna, every nice SWR curve is a great matching system. The J-pole offers a *very* adjustable matching system, and the matching system is the most important part of tuning this antenna. It must be precisely constructed - parallel elements and precise measurement is required. 1 millimeter of misalignment will give you 50% higher SWR. It gets worse from there, too.
-The 'stub' area _does not radiate_. Only the part sticking out by itself, the half-wave section, radiates. You could rotate the matching stub section 90 degrees and have it stick straight out instead of down and it would work fine.
So, let's load up MMANA (my antenna analyzer of choice) and take a look!
Here's a build plan for a 2 meter J-pole, pretty well tuned.
Edit: I didn't put it in the diagram, but the main and stub elements are parallel and 5.0 cm apart, measured on center.
For feed, two hose clamps and 2mm copper wire soldered to a connector should do the trick. Make sure that the lengths from the center pin of the connector are equal to each leg, going through the wire. Uneven currents will make us unhappy.
Here's the tuning detail:
Pretty straightforward. The tuning stub absolutely should have adjustment capability; the main leg, less so.
Let's set the screws for 1.52 meters and 0.52 meters for the main and stub(4cm screw length, from the top of the pipe cap to the top of the screw), and set the feedpoint at 6cm (measured from tubing center) from the bottom, and see what she looks like.
Hmm, not too shabby for a first attempt. Well under 2:1 SWR for the whole 2 meter band. Let's take a closer look at the impedance curve of this antenna.
This is the typical curve of a J-pole antenna. You can see that the impedance curves smoothly down (If I would have zoomed out, you would have seen a rise below 130 and a 180 ohm high impedance point around 130 MHz), but the reactance (the non-resistive part of an AC load) has a U-shaped curve. We have a perfect 50 ohm impedance match at 146.5, but there's a little reactance in there causing us some grief.
Our goal is to get a 50 ohm match with reactance at a minimum at the goal frequency, and secondly, get as wide a bandwidth as possible. We could tweak for gain, but we'd only go up or down 0.1 or 0.2 dB, so we'll ignore it for now.
The antenna's tuned a little high right now. Let's lengthen the main element 2 cm and see what happens.
Our SWR curve is prettier now. Impedance?
Our resistive impedance has dropped across the board due to the lengthening of the antenna. The reactance, however, is approximately the same at the same frequencies (approx -j6 ohms at 144.5, approx +j4 ohms at 148.5). So, it seems as though once we get minimum reactance at our test frequency, we can just adjust the main element for the best match.
Back to the stock 1.52/0.52 configuration. Let's see what happens when we lengthen the stub element by 1 cm.
Err...wha?
HOLY CATS WHAT IS GOING ON
We raised the reactance a little bit and it's now close enough at 146.5 that I'm happy, but wow, did we ever detune this thing! Fortunately, we just learned that we can now tune the end screw to finalize it.
End screw to 1.49m:
Reactance dropped a tiny bit, let's see here... we'll raise reactance a bit by going long on the stub, and we'll counter the change in resonance by shortening the main some more.
End element 1.483 meters (assuming 1.48 pipe length and 3mm for nut + screw), stub element .534 meters:
BAM
I'm happy with 1.06 SWR and you should be too.
How'd we do on our secondary goal?
SWR curve:
http://img472.imageshack.us/my.php?image=148305346swrsi1.jpg (i hit the image cap in this post)
1.5:1 SWR bandwidth increased by 11%, up to over 4000 KHz! Everything we're interested in- 145 mhz repeater inputs, 147 mhz repeater inputs, and everything in between - is under about 1.3:1! 2:1 ratio is up 420 khz, easily giving us less than 2:1, closer to less than 1.6:1 over the entire 2m band. Not bad!
Next up: We touch the feedpoint. Be scared. Be very scared.
OK, so I've been hitting the books hard lately, working on antenna design and theory. let's attack one of the most popular antennas out there, the J-pole! Today we'll build a 2 meter J-pole and tune it for maximum performance on 146.5 MHz, with bias towards the high end of the 2 meter band, as those 147 positive offset repeaters can tax some lesser antennas.
The above image is horizontally polarized. Let's think vertical!
The J-pole is an end-fed half-wave antenna, fed through a shorted 1/4 wave matching stub. It offers easy construction, good performance, and lots of tweakability, with a few caveats.
A few things to know about the J-pole:
-It is a DC grounded antenna. Since it is fed through a shorted stub, there is a DC path to ground from your element. This reduces atmospheric static on amplitude modulation types (AM, SSB and CW), and provides extra lightning protection. Even though the antenna is 'shorted', you will *not* blow up your radio by transmitting into it. That's the magic of RF!
-Behind every great antenna, every nice SWR curve is a great matching system. The J-pole offers a *very* adjustable matching system, and the matching system is the most important part of tuning this antenna. It must be precisely constructed - parallel elements and precise measurement is required. 1 millimeter of misalignment will give you 50% higher SWR. It gets worse from there, too.
-The 'stub' area _does not radiate_. Only the part sticking out by itself, the half-wave section, radiates. You could rotate the matching stub section 90 degrees and have it stick straight out instead of down and it would work fine.
So, let's load up MMANA (my antenna analyzer of choice) and take a look!
Here's a build plan for a 2 meter J-pole, pretty well tuned.
Edit: I didn't put it in the diagram, but the main and stub elements are parallel and 5.0 cm apart, measured on center.
For feed, two hose clamps and 2mm copper wire soldered to a connector should do the trick. Make sure that the lengths from the center pin of the connector are equal to each leg, going through the wire. Uneven currents will make us unhappy.
Here's the tuning detail:
Pretty straightforward. The tuning stub absolutely should have adjustment capability; the main leg, less so.
Let's set the screws for 1.52 meters and 0.52 meters for the main and stub(4cm screw length, from the top of the pipe cap to the top of the screw), and set the feedpoint at 6cm (measured from tubing center) from the bottom, and see what she looks like.
Hmm, not too shabby for a first attempt. Well under 2:1 SWR for the whole 2 meter band. Let's take a closer look at the impedance curve of this antenna.
This is the typical curve of a J-pole antenna. You can see that the impedance curves smoothly down (If I would have zoomed out, you would have seen a rise below 130 and a 180 ohm high impedance point around 130 MHz), but the reactance (the non-resistive part of an AC load) has a U-shaped curve. We have a perfect 50 ohm impedance match at 146.5, but there's a little reactance in there causing us some grief.
Our goal is to get a 50 ohm match with reactance at a minimum at the goal frequency, and secondly, get as wide a bandwidth as possible. We could tweak for gain, but we'd only go up or down 0.1 or 0.2 dB, so we'll ignore it for now.
The antenna's tuned a little high right now. Let's lengthen the main element 2 cm and see what happens.
Our SWR curve is prettier now. Impedance?
Our resistive impedance has dropped across the board due to the lengthening of the antenna. The reactance, however, is approximately the same at the same frequencies (approx -j6 ohms at 144.5, approx +j4 ohms at 148.5). So, it seems as though once we get minimum reactance at our test frequency, we can just adjust the main element for the best match.
Back to the stock 1.52/0.52 configuration. Let's see what happens when we lengthen the stub element by 1 cm.
Err...wha?
HOLY CATS WHAT IS GOING ON
We raised the reactance a little bit and it's now close enough at 146.5 that I'm happy, but wow, did we ever detune this thing! Fortunately, we just learned that we can now tune the end screw to finalize it.
End screw to 1.49m:
Reactance dropped a tiny bit, let's see here... we'll raise reactance a bit by going long on the stub, and we'll counter the change in resonance by shortening the main some more.
End element 1.483 meters (assuming 1.48 pipe length and 3mm for nut + screw), stub element .534 meters:
BAM
I'm happy with 1.06 SWR and you should be too.
How'd we do on our secondary goal?
SWR curve:
http://img472.imageshack.us/my.php?image=148305346swrsi1.jpg (i hit the image cap in this post)
1.5:1 SWR bandwidth increased by 11%, up to over 4000 KHz! Everything we're interested in- 145 mhz repeater inputs, 147 mhz repeater inputs, and everything in between - is under about 1.3:1! 2:1 ratio is up 420 khz, easily giving us less than 2:1, closer to less than 1.6:1 over the entire 2m band. Not bad!
Next up: We touch the feedpoint. Be scared. Be very scared.
Last edited:
