TheRadioBoard

I’m looking for a simple way to measure the magnitude of a standing wave on an open-wire transmission line. (Actually - it’s 300 ohm TV twin lead.)

I’ve found 1 reference to something called a “coupled line reflectometer” (?) ----

http://www.vias.org/radioanteng/radio_a ... _04_11.htm

that looks like what I’m after, but I can’t find find any more references.

Does anyone have more information about such a critter - or can you point me in a different direction.

I want to be able to change the length of an antenna connected to a 300 ohm transmission line and watch the SWR on the 300 ohm transmission line change.

The setup is:

1.5w, 7MHz cw xmitr - 50 ohm coax - Tayloe SWR bridge - balanced line tuner - ~300 ohm transmission line - ~ 300 ohm folded dipole antenna.

This configuration works very well as-is just cutting the folded dipole to length by formula. But I want to play around with trying to make the antenna resonant at my operating frequency - and learn something about transmission lines, matching, SWR, etc. in the process.

73, Dan

Dan,

See "A Reflectometer For Twin-Lead," by Fred Brown, W6HPH, October 1980 QST, with a Feedback in December 1980 QST (I think you're an ARRL member). The archive page is http://www.arrl.org/arrl-periodicals-archive-search

Here's how it's put together:



If you only wanted to resonate the antenna, a dipmeter or antenna analyzer coupled into the shack end of the feedline would do.

73,

_________________

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http://qrp.kearman.com/

Thanks Jim! That looks like what I’m after.

I haven’t been a member of ARRL for years, but I’m joining again ASAP. I’d forgotten what a great resource it’s archives are.

You’re a pretty good resource yourself, HI.

73, Dan

I wasn’t able to get a reflectometer based SWR indicator to work. There’s probably not enough power since the transmitter is only 1.5 watts out.

But a bridge circuit seems to work well. Here’s the balanced line bridge circuit that I’m using.





With an input power of only 0.38 watts to the bridge, I’m getting decent voltages and believable numbers for the VSWR on the balanced feed line. Common mode currents don’t seem to be an issue as judged by reversing the feed line and comparing the VSWR.

It’s been interesting watching the VSWR change when the feed line goes from dry, to damp, to wet, to snow covered and back to dry again.

Using the bridge as an indicator, I’m confident that I can now prune my folded dipole antenna to minimize VSWR on the feed line and therefore minimize feed line loss.

Really interesting and fun stuff.

73, Dan

Dan,

Don't think the VSWR loss will be a problem. A folded dipole has 288 ohms impedance. So matching to 300 ohms won't actually resonate it.

73,

_________________

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http://qrp.kearman.com/

Hi Jim.

When I first made the folded dipole antenna in 2005, I hung it about 6’ off the ground then grid-dipped it and pruned it to resonate at about 7.20Hz - or so I thought. Then I hoisted it up to about 30’ and have been happily using it ever since, primarily on SSB.

Its made by lashing 2 insulated #14 wires tightly together with cable ties. Theoretically the spacing between the wires shouldn’t matter if the wires are the same diameter. But I’m not sure that the impedance is really increased by the theoretical “x4” for this unorthodox arrangement.

The antenna is now used for CW at around 7.05MHz. It seems to work great, but I’ve always wondered what the VSWR looked like across the band.

From the bridge measurements, it looks like the minimum VSWR on the feed line, with the antenna at 30’, is actually nearer to about 7.50MHz. And the VSWR at 7.05MHz is close to 6:1. That’s high.

I don’t think the losses are serious even at a 6:1 VSWR on the twin lead. I calculate for my 55’ long feed line that the total loss is only about 0.6dB for the twin lead operating with a 6:1 VSWR. That says that only about 13% of my power is being wasted heating the feed line. And on-air results are very good. My 1.5 watts routinely makes 1000 mile contacts.

But still, 6:1 is very high and I’d like to bring the VSWR down - just because.

This could all be baloney Jim - but it’s fun.

73, Dan

You may consider line length. I believe that lengthening or shortening it will have a more pronounced effect on twin/balanced lines than coaxial ones.

Indeed! So if you're measuring SWR with a 300-ohm bridge, you're not certain where resonance actually is, only the point where the combination of feedline and antenna look like 300 ohms.

I think I'd ditch the twin-lead antenna and just use your lashed-up twin-lead as the feedline for a normal dipole, cut somewhere around 7.1 MHz. Though I can't say #14 house wire will be any less lossy than coax in this application, at least you'll have balanced feeders. (I recall an old QST article where Jerry Hall tested lamp cord -- zip cord -- which has a characteristic impedance of about 105 ohms. At 7 MHz the loss looks like about 1.5 dB/100 feet. Your feedline doesn't sound much better. But here's the money paragraph:



The article was in QST for March 1979, p. 31: "Zip-Cord Antennas--Do They Work?") (Edit: The article may be downloaded here: http://www.we0h.us/Amateur_Radio.../Zipcord-Antenna-7903031.pdf)

Removing one wire from the antenna will lighten it and reduce stretching. It may have self-tuned itself a few kHz lower in frequency since you put it up.

73,

_________________

http://kr1s.kearman.com/
http://qrp.kearman.com/

Thanks for the thoughts fellas.

AD1OS (Mike?) -- Welcome to the RadioBoard!

My understanding is that while the impedance looking into the feed line will change with feed line length, the VSWR on the feed line is independent of the feed line length (for a “zero loss” feed line). But I’m still learning, so I hope I got that right.

Jim -- there’s a bit of a misunderstanding.

My FEED LINE is fairly good quality 300 ohm TV twin lead from Radio Shack (oval, dark brown covering).

The folded dipole ANTENNA piece is made from lashing #14 insulated wire tightly together. And it’s THIS element that I’m beginning to question.

My understanding was that, ideally, the folded dipole antenna element is expected to have a x4 impedance transformation at its feed point if the wires making up the antenna have the same diameter - independent of the spacing between the wires. (ARRL Antenna Book,1968,pg113)

So, depending on height, nearby structures, etc., I expected the antenna feed point to have a resonant impedance of about 4x50 ohms to about 4x75 ohms or so. This 200 to 300 ohms antenna impedance (at resonance) would be a good match to the approximately 300 ohm TV twin lead feed line. The classic “Folded Dipole”.

I expected a VSWR on the 300 ohm feed line of maybe 2 or 3:1 at worst, not 6:1.

As you point out, the lashed together wires in the antenna (NOT the feed line) may have a transmission line characteristic impedance of ~ 100 ohms, and a low velocity factor -- like “zip cord”.

I’ve recently discovered the works of G3TXQ (Steve Hunt) who analyzed this situation!!. See for example:

http://www.karinya.net/g3txq/folded_dipole/

He has shown 3 things (at least) that I may have screwed up by tightly lashing 2 wires together for the folded dipole ANTENNA element.

a.) The folded dipole impedance transformation may be something much less than x4 when the wires making-up the antenna are very close together..

b.) The bandwidth (delta F for a 2:1 VSWR) may be much reduced when the 2 wires are close together.

c.) There may not be a true resonant frequency (reactance = 0) for the antenna constructed the way I did it. (ie the minimum VSWR achievable may be high.)

And you have pointed out yet another factor - the darn thing may have stretched considerably over the years, further confounding things.

I “THINK” the 300 ohm VSWR bridge is giving me decent numbers for the VSWR on the feed line. At least I get what you’d expect when I put various resistor values at the output of the bridge and measure the VSWR.

All of these things lead me to suspect that the measured high 6:1 VSWR on the feed line might be real.

Guess I'll have to do the following (any other ideas appreciated):

- put some known reactive loads on the bridge output and see if the measured VSWR's make sense;

- measure the VSWR with different feed line lengths to see if what I told AD1OS is really true;

- play with the antenna element -- pruning/ adding and measuring VSWR to convince myself that the numbers make sense - in light of G3TXQ’s work.

Would be nice to have one of those MFJ 369 antenna analyzers. I think you can reference VSWR to impedances other than 50 ohms with that unit. But this simple homebrew setup should work - just takes more fiddling around. And that's the fun of it.

Thanks again guys.

73, Dan

GM Dan. I offered the feedline length angle based on my 160M inverted el. At the feed point, it dipped at 1812, and showed the lowest SWR. At the radio end, it changed. It was also different at the entry point to the shed. I use co-ax, but figured it may show the same for twin lead.
I saw a couple of folded dipole "calculators", and they support the idea that as long as the two element wires are the same diameter, the spacing doesn't matter.
Maybe the standing waves always exist, and the point of measurement determines what you read.
Reminds me of "I didn't know my antenna was bad until SWR bridges were invented". GL
Mike - AD1OS

For impedance, yes. But at any point on the radiating element, the point opposite it on the other conductor is 90 degrees out of phase, right? Leakage between those conductors will reduce radiation resistance and efficiency. Loss makes an antenna system look more broadbanded than it is, radiation-wise. If you've got watts to spare this can be a feature, not a bug. Not so with QRP, though.

73,

_________________

http://kr1s.kearman.com/
http://qrp.kearman.com/

For impedance, yes. But at any point on the radiating element, the point opposite it on the other conductor is 90 degrees out of phase, right? Leakage between those conductors will reduce radiation resistance and efficiency. Loss makes an antenna system look more broadbanded than it is, radiation-wise. If you've got watts to spare this can be a feature, not a bug. Not so with QRP, though.

73,[/quote]

The diagrams in the ARRL antenna books show he currents to be in phase. (1970 and 1988 books)

Interestingly, if you put some absurdly high conductor spacing value into an FD "calculator", the 288 number stays the same. When does it stop being a folded dipole and start to become a square loop?
Mike - AD1OS

_________________

http://kr1s.kearman.com/
http://qrp.kearman.com/

Here’s the results of varying the load at the bridge output and changing the length of the 300 ohm TV twin lead feed line between the load and the bridge output.

Loads were either non inductive resistors, or non inductive resistors in series with either a capacitor or an inductor.



Conclusions:

a.) The simple bridge measures VSWR(294) - the VSWR referenced to R3 = 294 ohms - quite well!

b.) Changing the feed line length does not change the VSWR(294) on the line - just as the books say. (I do see very small changes. But the tv twin lead is not lossless which would account for the very slight reduction in VSWR when 50’ of feedline is added.)

c.) I'm now confident that the bridge can be used to watch the VSWR(294) on the feed line as I make changes to the antenna. The goal being to minimize the VSWR.

Now the fun part can start -- figure out why my antenna feed line has a huge 6:1 VSWR(294) on it.

This sure beats working.

73, Dan

Nice work, Dan. It sure looks as though your antenna thinks it's more a dipole than a folded dipole. Rembember, pruning it for low SWR doesn't necessarily resonate it, which is what matters for good pattern.

73,

_________________

http://kr1s.kearman.com/
http://qrp.kearman.com/