SWR readings on a folded dipole for 2 meters

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wbswetnam

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Using schematic plans from the internet, I made a folded dipole antenna for 2 meters out of copper wire instead of copper tubing. I am getting rather bizzare readings from the MFJ-269 antenna analyzer, though. Here is a sample of the readings:


MFJ-269 Readings
Frequency / SWR / Rs / Xs
134.000 / 1.1 / 38 / 0
140.000 / 1.2 / 37 / 0
144.000 / 1.5 / 33 / 5
145.000 / 1.7 / 29 / 0
146.000 / 1.8 / 25 / 0
147.000 / 1.9 / 21 / 0
148.000 / 2.0 / 18 / 0
152.000 / 1.9 / 25 / 0
156.000 / 1.5 / 46 / 20
159.000 / 1.5 / 49 / 22

I have no idea what to make of this. Bad soldering job, of where I soldered the shieldings together? Bad RG8X cable (22 feet long)? Is it because I used copper wire instead of copper tubing? Faulty meter? Some combination of these? What did I do wrong?
 
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elk2370bruce

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Using schematic plans from the internet, I made a folded dipole antenna for 2 meters out of copper wire instead of copper tubing. I am getting rather bizzare readings from the MFJ-269 antenna analyzer, though. Here is a sample of the readings:


MFJ-269 Readings
Frequency / SWR / Rs / Xs
134.000 / 1.1 / 38 / 0
140.000 / 1.2 / 37 / 0
144.000 / 1.5 / 33 / 5
145.000 / 1.7 / 29 / 0
146.000 / 1.8 / 25 / 0
147.000 / 1.9 / 21 / 0
148.000 / 2.0 / 18 / 0
152.000 / 1.9 / 25 / 0
156.000 / 1.5 / 46 / 20
159.000 / 1.5 / 49 / 22

I have no idea what to make of this. Bad soldering job, of where I soldered the shieldings together? Bad RG8X cable (22 feet long)? Is it because I used copper wire instead of copper tubing? Faulty meter? Some combination of these? What did I do wrong?
Drop a 4:1 balun in line between the antenna and coax and try again. You may be surprised.
 

wbswetnam

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Drop a 4:1 balun in line between the antenna and coax and try again. You may be surprised.
I already have a coax balun for 4:1, if you look at the photo you can see the loop it makes. It is 26" in length. Each center wire of the balun loop is attached to an end of the wire dipole, with the center wire of the feed cable attached to the "upper" connection to the antenna. The cable shields of all three are soldered together.
 

zz0468

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Drop a 4:1 balun in line between the antenna and coax and try again. You may be surprised.
It's got a balun on it. And the impedance readings he's showing are from 18 to 49 ohms.

To the OP...

If that's from a construction article, it's generally best to duplicate the original published construction details. In other words, if the author used tubing and you used wire, expect your results to be different. Also, your dimensions on the balun could be off. If you cut to the same length as the construction article, and used a cable with a different velocity factor, your balun isn't going to work right. And so on.

My guess is that you have several such errors all adding up to an antenna that doesn't work as expected. Couple that with possible measuring errors, and nothing will make sense. As for the measurement errors, if you're measuring a device with a complex impedance at the end of a length of coax, that coax will have an SWR other than 1:1, and will introduce measurement errors.

Bottom line here is, there's a lot that could go wrong unless you follow the construction plans exactly, or design your own antenna from scratch.
 
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wbswetnam

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I didn't use any construction plans, I relied on a combination of schematic drawings and theoretical plans.

I risked connecting my HT to it and I was able to connect to a local repeater (about 12 miles away) and another one about 20 miles to the south. I also seem to get great reception with it... strange... despite the bizarre antenna analyzer readings, it seems to work pretty well with the HT anyway.
 

mm

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Your close, very close. Just shorten the length up by around 3/4 of an inch at both ends, you can just place a short inward from both of the curved ends to make each side shorter, this should bring your best match in the 146 MHz range.
 

prcguy

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I'll side with ZZ in there could be problems with antenna dimensions vs element diameter and different brands of coax with different velocity factors can really throw a wrench into things.

Without having an antenna analyzer to see if the coax balun is doing its job by terminating with a 200 ohm resistor and adjusting for best match, then, connecting the antenna and tuning for the same its hard to follow a project without having the exact materials the author used.

I know its good to experiment, but why not just make a regular half wave dipole, which will match well to 75 ohm coax or well enough to 50 ohm coax?
prcguy
 

mrweather

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The SWR doesn't look that bad. Any radio should be able to handle a 2:1 or better without ill effects. Also, the fact that the reactance (X) is zero over a fairly wide range suggests the antenna is resonant over that range and has been built properly. The resistance (R) is a bit low that's all.
 

majoco

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I would be inclined to rotate the copper wire so that the the centre point doesn't lie over the top of the phasing link too. I've made a few like this from old TV antennas and they work well. You can still use the old fittings too. (Jeez, I'm cheap!)
 

wbswetnam

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I know its good to experiment, but why not just make a regular half wave dipole, which will match well to 75 ohm coax or well enough to 50 ohm coax?
prcguy
Because I already have one! I made one using this plan on YouTube: Ham Radio 2m/70cm Vertical Dipole Antenna - YouTube

It works pretty well. Initially I made the elements too short, but after I made new, longer elements it worked like a charm.

I just like to experiment and I didn't see any plans on the internet for making a folded dipole out of heavy copper wire so I decided to try it myself.

Next experiment: a hand-held 2m/70cm yagi for satellite use! That should be fun.
 

wbswetnam

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Yep I watched it today. That's what I plan to build.

BTW I flipped the element around on the folded dipole so it faces outward, but I still get the same SWR measurements on the MFJ 269.
 

nanZor

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Part of the reason for the errant readings is the 22 foot transmission line.

Use the 269 to help you cut a half-wave of coax to make your swr measurements. For a quickie setup, just multiply a half-wave for 2meters by the velocity factor of the line (essentially it will be a bit shorter) and that way you'll get a more accurate swr, as they repeat at multiples of a half-wave. BUT you either have to use the 269 to do it (try the open or closed stub technique) or just calculate it and cut it.

Once the antenna is trimmed properly, then attach your real feedline.
 

wa1nic

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Worrying about SWR is over-rated, especially at VHF.

It is actually near dang impossible to measure unless it is 50ohms j 0 at the antenna terminals.

As soon as you put even an inch of cable or adapter or whatever on there the cable length makes the impedance change, so you wont really be measuring what you think what you are.

A practical antenna has coax, so while it would be nice to get 50j0 at the antenna, what the radio sees on the other end is what matters of you are concerned about a perfect match to the radio.

I was messing around with an antenna I recently installed on my pickup. I came the the realization that adding or subtracting even a short amount of feedline length changed the SWR/impedance readings enough that it started to become irrational to try to optimize it. Of particular interest is that my radio, like many made nowadays, has the SO-239 connector attached to a 6" pigtail to make it easier to mount it in a confined space, so no matter what measurements I made the end of the coax look like, the transmitter is another 6 inches or so away.

Now, of course if the antenna has a perfect 50 ohm resistive load, and if the coax and connectors are all exactly 50 ohms (they arent I assure you) then the impedance wont change as the coax length changes. I am convinced that the best way for an antenna manufacturer to make a "50 ohm" antenna is to make one with tiny loading coils to introduce enough resistive loss that it swamps out all the radiation resistance and imaginary components and looks a bit like a dummy load.

Once I came to this conclusion I put my antenna analyzer into the field strength mode and spent about 5 minutes adjusting for maximum field strength. As long as the radio in question can handle the SWR and not shut down - that's all that really matters, isn't it?

The results were awesome. I am hitting repeaters full quieting on low power with this roof mounted 1/4 wave whip that I always needed high power to simply stay connected with the 5/8 wave, low SWR antenna that I had on my last pickup.

I flipped thru an old (1948) ARRL handbook yesterday. All they cared about back then was field strength, not SWR. Of course, they had adjustable matching networks in the transmitters back then, something that have disappeared. More modern solid state radios have a more limited output impedance that they can work into, but it sure doesn't have to be anywhere near close to 1:1 to work well.

If the radio can still work into it, if you adjust things for maximum field strength you will be working on the most important variable.

Rick
WA1NIC
 

popnokick

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Agree that max field strength is what you want. You wrote, "f the radio can still work into it, if you adjust things for maximum field strength you will be working on the most important variable."
What "things" are you adjusting, and how? Are you using an inline antenna tuner? Cutting the whip and measuring field strength again? Adding/subtracting coax cable (and how is that being done on a reasonable basis with the radio installed in your pickup)? Had to ask these questions because there might be some good techniques hidden in your post.
 

wa1nic

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In the case of the antenna that I just did, which I admit is a lot easier to adjust than yours is (a 1/4 wave whip), I temporarily replaced the somewhat expensive stainless steel rod with a really cheap piece of .062 brazing rod. I put my field strength meter on top of a ladder about 50 feet way. I found that if I stuff my radio's mike into the door handle it would hold the radio in the keyed position so I could walk out and look at the field strength reading. I clipped an inch off at a time at first when major changes were happening... and then clipped 1/2 inch at a time when changes slowed down.

I then replaced the brazing rod with the stainless rod (cut to the same length), verified that I was at the same amplitude, and life was good. Total elapsed time - about 5 minutes.

An FYI if you do this... rotate the field strength meter 90 degrees left or right so that you can read it without being in front of or behind it... that would probably mess the readings up.

Rick
 

LtDoc

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Baluns are really misunderstood. They can do two things, convert from a balanced to an unbalanced feed line, and by the impedance conversion feature, get that input impedance close to 50 ohms. From the numbers sited it isn't easy/possible to say if it's too long or too short. They are not normal 'numbers'. I would expect the SWR to be higher becuase of that impedance mismatch.
- 'Doc
 
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