What is considered "flat" for a balun?

[GerryR]

Greetings. I posted this on another forum and have had several "lookers" but no responses:

I recently made a 4:1 Balun that I plan on using with a new Carolina Windom that I am building. I took the recipe out of Jerry Sevic's book. I connected a 200 ohm resistor across the output using 6 inch grabbers, and connected it to an MFJ analyzer using a one foot long coax jumper terminated with pl239's, to read the SWR. Starting at 1.8 MHz I got a 1.1 SWR. It changed to 1.2 at ~7.3 Mhz, 1.3 at 14MHz, 1.4 at 18 MHz thru 25MHz, and back down to 1.2 at 30 MHz. Just for completion sake, it was 1.5 from 50 to 54 MHz and went to 2.7 at 70 MHz.

Is this normal, or have I done something wrong that needs to be looked at more closely. Is it possible the grabber hooks and leads are contributing to the error?

Here's a picture; maybe there is something wrong with the assembly(?):

Thank you for any input and insight into this.

Gerry, kk4fpo

 

[W5LZ]

I figure your test set up is showing a 'flat' response. About as 'flat' as can be expected, really. Your results are very typical of a good balun.
So, are you making any mistakes? Yes and no. Your use of a 200 ohm resistor as a substitute for the antenna is going to be misleading! Why? Because it's a 'perfect' "antenna", no reactance present. The readings you will get from any 200 ohm input antenna are going to vary from your test set up, and that variation will get 'larger' as the frequency changes. A 'Carolina Windom' does not have a constant 200 ohm input impedance. That input impedance will change with frequency because of additional reactances resulting from a change in the applied frequency. The further you get away from the design frequency the more reactance ('+' or '-') there will be. If you substitute a resistor of different values you will see the typical changes you will find when connecting that balun to the antenna. How big of a resistance change? I can't give examples (never done that sort of test) but I figure it'll be fairly large ones. Try a 100 and 400 ohm value for that resistor, that should give a reasonable 'spread'.
So why the big 'jump' at 70 cm? Probably because those connection leads are getting close to a large percent of a resonant length at that frequency and thereby adding reactances close to what you'd probably see with the antenna elements there instead of just a resistor.
One thing you might try is to make those connection leads to that resistor of related length to the antenna's element lengths. One 'side' of that antenna is going to be some % larger than the other 'side', right? When those leads get to something approaching the 'actual' lengths for a particular frequency, the more they will mimic a real antenna. At HF, they aren't anything close to that sort of relationship, but at UHF they certainly can be.
Ain't all of this shi... stuff fun??
- 'Doc

 

[GerryR]

Thanks for your input. This "stuff" really is fun. I'm no stranger to electronics, but some of the things in radio electronics still eludes me. Just have to keep digging. I'm hoping to get my antenna up within the next couple of weeks, the good Lord willing. Thanks again.
Gerry, kk4fpo

 

[Robb]

Since you tested it with a dummy load; isn't it time to put it up and see what it does?

Nice work - BTW.

What is the wattage this will hold?

 

[mackmobile43]

appears to be of legal limit

 

[GerryR]

It is supposed to be good for 1 kW continuous power. Recipe is 2 x 10 bifilar turns of 16 awg wire in 17mil wall teflon tubing on a 2.4" OD, #61 toroid (u=125), connected as a Guanella balun from Sevic's book, Transmission Line Transformers, 1996.

I'm working on getting the antenna up and hope to be done in a couple of weeks. Thanks for the input.

 

[W5LZ]

"Here's a picture; maybe there is something wrong with the assembly(?):"

Yes. It's too neat. Being that neat/clean means it probably isn't going to work very well, it has to be messy to work good. Everybody knows that!

------

If I really have to be 'picky', I think I'd recommend putting a couple more bolts into holding that SO-239, not just the one. Considering the stage of construction, I really had to 'stretch' for that criticism.
I do have a question though. Why the insulator between the inner ends of those 'ring' attachment points?
- 'Doc

 

[GerryR]

There are actually 4 screws holding the connector on the box. I tapped the box so three of them do not go through, only the one long screw for the ground connection. There is also an o-ring between the connector and the box; as the connector is on the bottom, I guess it is to keep water in the box if any gets into it!

The fiberglass rod is threaded to accept both eyebolts, and will, hopefully, keep me from pulling the box apart when I stretch the wires between trees. Probably overkill.

That's what is frustrating about radio electronics, the neater it is, the worse it operates, or at least it seems that way

 

[Beetle]

"What is considered 'flat' for a balun"?

Two passes with a steamroller should do it.

 

[wavrider]

Get the antenna installed, as high as you can get it.

when one of those whiz bang wind howling thunderstorms come along and tear the rope out of the tree, the balun falls on the ground, then you will have a "flat" balun

Looks great and if it performs half as good as it looks it should be fine.
Thanks for the picture.

 

[hookedon6]

nice build gerryR,.... I think i'll make one too for an antenna i'm planning.

btw welcome to the forum

 

[W5LZ]

And no matter what's expected, nothing is ever extremely 'flat' with any RF device unless there are gobs of 'additional' circuitry involved. Anytime there's much of an excursion from the design frequency there will be variations. An antenna that's 'flat' from daylight to dark is an antenna that has very low 'Q'. That usable bandwidth is always paid for with some other characteristic, which is usually efficiency in one form or another. The only RF device I can think of that is good for almost daylight to dark is a dummy load, and they make for lousy antennas.
- 'Doc

 

[GerryR]

Well, again, thank you for the input, but before you go off and build one, go read some of the feedback that I am getting on the forum at the link below. It seems that some redesign may be in order. (I hope it is OK to post this link??) It seems that I may need to split the transmission lines by putting them on separate cores.

http://www.eham.net/ehamforum/smf/index.php/topic,79538.0.html

 

[hookedon6]

Well, again, thank you for the input, but before you go off and build one, go read some of the feedback that I am getting on the forum at the link below. It seems that some redesign may be in order. (I hope it is OK to post this link??) It seems that I may need to split the transmission lines by putting them on separate cores.

http://www.eham.net/ehamforum/smf/index.php/topic,79538.0.html

wow,................ Tom kinda tells it like he sees it

 

[GerryR]

I did some temporary mods to incorporate some of the suggested changes to see the differences in performance. The results were, in fact. as described. I ordered some FT-240-43 toroids and will hopefully put this part of my project to bed shortly. When I am done, I will post the SWR readings, including those with each side shorted to ground so the balance can be seen, and the details of the baluns.

I don't profess to understand everything at this point, but I am learning, and that's the point anyway. There is a lot of good information in Sevick's books, however, the info is not complete as related to common mode currents, and people should be aware of that. The results are the proof of the pudding. Thanks to everyone, again.