Building a Fong dua...
 
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Building a Fong dual-band J-pole

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Jed Baer
(@kd0ymg)
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This has been a back-burner thing for me for a couple years. I have all the pieces, and I'm willing to share some coax and window line. My small apartment with the 7ft. ceiling isn't a good place to take measurements, particularly when window line is involved. Possibly, some other foiks would be interested in a group build?

For starters, check out Ed's page. There are download links at the bottom for articles about this antenna. But note that it's changed a bit since the original article was published, I'm assuming at least partly due to the substitution of JSC1321 window line for the 300-ohm twinlead. (I think I'm correct that it's JSC1321, but it could be something else - it could in fact be any sort of line that fits inside the PVC.)

I purchased one of these in Aug 2020, and checked it out with my MFJ 259C and my NanoVNA. I read high SWR on 70cm, which I found surprising, given the positive reviews. Doing a little web searching, I found a few reports of issues on 70cm, but those were pretty few. I talked with Ed on the phone, and exchanged emails with him to try and figure out what was going on. I'm of course open to my measurements being off, but I worked pretty hard at things such as getting it up and away from anything that could influence it. I also discussed it at length with my RF Elmer, Bill -- N0CU. I ended up returning it. I give Ed a lot of credit for the time he spent trying to help me get it figured out.

Other than enjoying building ham gear using plumbing parts, the idea of how to build one held my interest, and I started working on that. But I got hung up on how to do a good job of taking measurements, and having a good space to work on it.

The Fong antenna is cool. Ed made use of the fact that 70cm is really close to being 3 times 2m. The overlap isn't perfect, but it's close enough to work. In his Feb 2003 QST article, he mentions 445Mhz being the 3rd-order harmonic of 146, and that's almost right. The trick is that the matching section can work for both bands, and then there's a trap on the radiator to effectively shorten it for 70cm.

I have all this stuff in my head about the theory, and I'll try to briefly lay it out here.

At some point, I realized that a J-pole antenna is a Zepp. Combine that with understanding the use of transmission lines as impedance transformers (PDF, presentation by Bill -- N0CU), and you get the idea. You folks who know all about tracing circles on Smith charts probably already know what I'm talking about.

Back when I built a 20m Zepp, I made a drawing showing the voltage at resonance for a Zepp.

zeppwave

This shows the typical high-impedance feed point for an end-fed halfwave. The dotted lines show the zero-crossing point. The 1/4 wave section of transmission line transforms the impedance down to something we can work with, without using a 49:1 balun. In fact, the Zepp I built measured 14 ohms at 14.175, at the end of the window line. In theory (I think), in a perfect world, the impedance at the feed end of the transmission line approaches zero, except for resistance.

The Fong antenna works on the same principle. The transmission line part is 1/4 wave at 2m, and 3/4 wave at 70cm, so in either case, with an odd number of 1/4 wavelengths, the high-impedance of the end-fed 1/2 wave radiator is transformed down. The feed point is moved a little in from the end, to where the impedance is as close to 50 ohms as possible, for the desired operating areas. The operating range for 70cm is 440-450Mhz. I don't recall the details for 2m, but for starters, I'll say it's 144.515 - 147.975, with the center at 146.245. Ed mentioned that getting the bandwith wider than 10Mhz would be quite difficult.

I did some numbers. The velocity factor for the JSC1321 window line is .82 or .88, depending on whom you ask.

Length of JSC1321 matching section:

@445Mhz:

 - 1/4 wave: 6.63 in. (C), 5.44 in. (0.82), 5.84 in. (0.88)

 - 3/4 wave:  19.89 in. (C), 16.31 in. (0.82), 17.5 in. (0.88)

@146.245Mhz

 - 1/4 wave: 20.177 (C), 16.54 in (0.82), 17.76 in. (0.88)

You can see that the matchup isn't exact, but it's quite close, so that while the lowest point of SWR won't be at band center on both bands, it should be possible to get a curve below 2 on both. This is possibly quite fiddly.

Of note is the length of the 1/4 wave at 445Mhz. How coincidental that it's the length of the "stub" in the radiating portion of the antenna. Ed calls it a stub, but I think of it more as a trap. And in fact, given that 1/4 wave line "inverts" the impedance, the short at one end of the trap should result in extremely high impedance at the other end. And that's how that little piece of coax at the end of the window line works to shorten the antenna for 70cm.

One final point. Back when I was a new ham, I didn't understand the notch in these antennas made from window line, e.g. the Slim-Jim. That notch disconnects the upper part of the window line, making the part below the notch the matching section, and that above it as the radiator. In the Fong antenna, the disconnected wire still radiates, according to Ed. I assume this is due to capacitive coupling. (The Slim-Jim is a folded J-pole.)

Whew! That's enough for now. Later, I'll post some notes regarding how I want to build one - I have a couple modifications.


   
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Jed Baer
(@kd0ymg)
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Joined: 8 months ago
Posts: 42
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I should clarify that I'm intending to build the DBJ-1 - the version that goes inside a PVC pipe. My understanding from talking with Brad - W0BDT is the club was thinking of doing a group build of the portable version. Well, why not do both?


   
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Jed Baer
(@kd0ymg)
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Joined: 8 months ago
Posts: 42
Topic starter  

Considering the actual construction of this antenna, my initial thought has been to make a 5/4 wavelength section of window line, at 445MHz. Measuring this using a VNA should be straigthforward. But starting off with that length makes cutting the notch a question of physical measurement, and maybe that's good enough. I had the thought of a sacrificial 3/4 length window line, to use as a template for the position of the notch. That window line is cheap enough.

There's a gotcha here, in that to get a really accurate measurement, the VNA has to be calibrated at the measurement point, i.e. the end of the line from the VNA to the antenna, including whatever leads are used to attach to the window line. I've attempted calibrating the NanoVNA to the end of the supplied jumper coax, with poor results. I'll retry that, and also try with RG8-X. The final connection to the window line should be a fixture that's short, rigid, and repeatable, so as to minimize changes in capacitance.

Continuing on, determining the spot to attach the coax feedline can get tricky. I recall, from playing with the Slim Jim, that making one change can affect multiple things. The trick is to find the spot on the window line where the impedance is 50 ohms at the target frequency. My plan is to move it up/down until there's a 50 ohm reading, but getting an accurate measurement of this can get tricky. To do it really really well again requires a good test setup, as with setting the window line length. I'm starting with the thought that the 70cm band is more difficult than 2M, and therefore the initial target frequency should be 445Mhz. My thinking here is that 440-450 is a wider bandwidth than what we need for 2M, and 70cm also varies more along the length of window line.

Once the window line portion is complete, adding the 2M parts shouldn't be too difficult. Ed recommends adding the trap section and trimming for low SWR, then adding the remaining 2M radiator section and again trimming for low SWR. My thought is that with a VNA, it should be possible to figure the optimal length of the trap section using a 2-port impedance measurement. I would probably follow the "trim to length" method for the final length of wire.


   
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