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Author Topic: More antenna tuner questions.....  (Read 7884 times)
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kc2ifr
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« on: November 11, 2005, 12:05:55 PM »

I was thinking of building JJ's link coupled tuner but came across a question or two. My ant. is a 130 ft. dipole fed with balanced line. I use a KW matchbox on all of the ham bands but the matchbox does not cover 160 and that is where I want to use JJ's tuner. With that in mind, here are the questions. The antenna will show a very low Z to the tuner on 160 because it is short for that frequency. I was taught that for a Z lower than 50 ohms, u need to use a series  tuner. For a Z above 50 ohms, parallel is the way to go. Now if I look at JJ's tuner....I think it is a parallel tuner. If that is true.....will it tune my antenna on 160. BTW.....here is a web page that deals with antennas and tuners and all sorts of stuff that I was reading. Seems like this guys knows his stuff but a lot of it is over my head..... Embarrassed
http://www.cebik.com/link/link0.html

Thanks in advance,
Bill
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K1JJ
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« Reply #1 on: November 11, 2005, 12:29:50 PM »

Good questions, Bill -

What you say is true. I know that I've heard the Tron say the same thing.

Though, there is a way aound it.  You CAN stick with this same simple parallel configuration if you can add or subtract some feedline, if need be, for 160M. Or, maybe the length you have now is OK...  just gotta try it or calculate it.

Here's the picture...  Lets say that your short dipole on 160M, at the CENTER insulator,  has an input impedance of  25 ohms resistive and a capacitaive reactance of 300 ohms. [if short, it will have capacitive reactance of some value that can be seen exactly on an ant modeling program.]  Anything can be tuned out for a 50 ohm 1:1 match. Now, if you had an exact 1/2 wave length of open wire line from that ant to the tuner, you would see this same 25 ohms +j 300 at the tuner. If it were 1/4 wave, you would see a HIGH impedance at the tuner, say 600 ohms, etc. Any length in between will give you some impedance in between at the tuner.  Not really a concern unless it gets too low to match, as you say.

Actually, it's a little different than I described, but to be technically correct:  You must start at the end insulator of one leg and measure in and down the feedline to 1/2 wavelength away to find the true high impedance point. Then the next 1/4 wave point is the low impedance point. But don't worry about it for now... read on:

So, as you can see, the thing to avoid would be a low impedance point on the line AT the tuner. If you find the tuner tunes up at a high impedance point on 75M, then it will be low impedance on 160M. So, look to get a middle impedance on 75M and the same on 160M and you're OK.

In the real world, just build the tuner and try it out. This tuner has so many combinations of possible adjustments that you will probably match it no matter what length feedline you use and they all are equal in performance as long as you can get a 1:1 match to the rig.

If you build the tuner, I guarantee it will work for you on both bands and I will help you become comfortable with its operation.  I always get a kick out of seeing guys smile at the simplicity of this tuner and how well it works after they've built it. It will handle 10KW no problem - it's just the limitation of the capacitor. If it's a vac variable, then rock and roll.... no arcing worries.  Just mount it high and away from metal objects to maintain its balance.

73,
T
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K1JJ
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« Reply #2 on: November 11, 2005, 01:33:57 PM »

Another thought Bill -

You said the dipole is a 130' flat top fed with open wire, used on 160M.

That is about 50% of full sized - which is still OK -  losses will be no more than 1-2 db *IF* the open wire and tuner are a low loss system. Hopefully you are using #10 or #12 copper wire for the flat top and openwire.  If so, you will never notice the difference between it and a regular full sized 260' 160M dipole on the air.

BTW, we can pre-calculate if you will have low impedance matching problems on 160M. One leg of you dipole is 65' long, so your low impedance point will be about 60' or so down the feedline. If your total feedline length is like, 120' long, it will be fed at a medium  impedance point on 160M.  Get the picture?  Anything roughly in between the high or low points will be perfect for both bands to match at medium impedance..

What length is your present feedline?

It might be an advantage if you could put up only ONE dipole as high as possible that is used on both bands. Lack of interaction and use of your best supports is the advantage. Having to switch between series and parallel tuning is a major hassle, so it can be done with one config and the proper feedline length if possible. Sell the Johnson Matchbox on eBay... Grin

Just running ideas past you that you might use, OM.

T



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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed.  Easily done in DSP.

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There's nothing like an old dog.
Steve - WB3HUZ
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« Reply #3 on: November 11, 2005, 05:29:49 PM »

You can easily make that 130 foot long dipole 190 feet long. Just hang 30 foot vertical sections off each end. You tuner will be much happier on 160 meters, the SWR (thus loss) will be much lower on your feedline, and your taxes will be lower. Been there, done this, it works.



IIRC, the impedance on the line will continue to drop over a length of more than just 60 feet. At a previous location, I modeled the antenna and the feedline and the impedance continued to drop, even at a length of 100 feet. This is why it's very important to get the feedpoint impedance up as high as possible. The tuner will only see a lower impedance, unless you have a very long feedline.

I'll see if I can dig up some of the data I have on this.
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K1JJ
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« Reply #4 on: November 11, 2005, 07:23:02 PM »

You can easily make that 130 foot long dipole 190 feet long. Just hang 30 foot vertical sections off each end. You tuner will be much happier on 160 meters, the SWR (thus loss) will be much lower on your feedline, and your taxes will be lower. Been there, done this, it works.

Steve,

Now that's a good idea. You could even get the impedance up high enuff to use coax doing that.

I've always wanted to try the loaded coils idea on 160M.  Put up a 130' flat top like Bill has and then add two coils at the mid point of each leg. Wind #12/#10 insulated wire on an ABS light weight 4" diameter plasdick form. Good high Q wid the large diameter and 1:1 form factor. Even wind on string each other turn to keep a space between wire turns for even better Q.  Play with the turns number until you get the feedpoint to 50 ohms. That antenna surely is within 1db of any full sized non-loaded dipole.  This keeps the whole antenna up high and horizontal.  The feedpoint center must be supported to pull this off.

Guys run "shortie 40" coil loaded rotary Yagis and small 75M loaded Yagis and are darn close to full sized antennas. Same with dipoles.

Keeping that center feedpoint impedance up over 30 ohms or so is important to minimize system losses, especially with wire antennas/feedlines that have low cross sectional area..  You can get away with more using large diameter aluminum elements, and heavy duty matching hardware, but impractical on 160/75M.

T
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed.  Easily done in DSP.

Wise Words : "I'm as old as I've ever been... and I'm as young as I'll ever be."

There's nothing like an old dog.
Steve - WB3HUZ
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« Reply #5 on: November 11, 2005, 08:10:43 PM »

Loading can work. But only if it's low loss, like you said. I've used linear loading successfully. The loading element was ladder line. Two 30 foot sections in each leg added a lot of "length" to the short dipole.




Joe, N2YR took my design and played with it some until he got it to work on 160 meters with coax feed. Then he added dipole elements for the other bands. Cool stuff.

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Vortex Joe - N3IBX
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« Reply #6 on: November 13, 2005, 06:36:45 PM »

Yo Bill,
         I like the ideas mentioned and think it would work FB for you. Your signal was the same on 1885KC as it was on 1930KC last night; though I suspect the Q of your tank is higher on the higher frequency.

Let us know how you make out and I'm always available for a signal check and audio report on 160M.

Regards,
           Joe Cro N3IBX

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Joe Cro N3IBX

Anything that is Breadboarded,Black Crackle, or that squeals when you tune it gives me MAJOR WOOD!
kc2ifr
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« Reply #7 on: November 14, 2005, 06:27:18 PM »

Thanks for all the replys guys. The antenna ideas look interesting Steve. I think the tooner I have now Tom is the same as yours. I got it from Jamie a couple of years ago. The tooning of 160 with this thing is VERY sharp but it does work. I am including a bunch of pics u guys can dl......but they are BIG files. (U know me....I hate small things!!) The only thing I am worring about if I modify the antenna is having to tune the feeders for 160. My feedline is about 67 feet now and this allows me to tune the antenna perfectly in 80 thru 10 with the KW matchbox. I dont want to screw up the antenna function on these bands. Anyway here are the links to my tooner.
www.kc2ifr.com/photogallery/tuner1.jpg
www.kc2ifr.com/photogallery/tuner2.jpg
www.kc2ifr.com/photogallery/tuner3.jpg
www.kc2ifr.com/photogallery/tuner4.jpg
www.kc2ifr.com/photogallery/tuner5.jpg
www.kc2ifr.com/photogallery/tuner6.jpg
www.kc2ifr.com/photogallery/tuner7.jpg
www.kc2ifr.com/photogallery/tuner8.jpg
www.kc2ifr.com/photogallery/tuner9.jpg
www.kc2ifr.com/photogallery/tuner10.jpg
www.kc2ifr.com/photogallery/tuner11.jpg

There they are........the washer and drier are just props!!!!!!
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K1JJ
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« Reply #8 on: November 14, 2005, 06:54:39 PM »

Yep, that tuner should work FB, Bill.

I see that Jamie used a split tuning capacitor [BC-610 cap?] that has the rotor grounded.  My design used a single section cap and it floats. Other than that, the design looks the same and should perform just fine.  There's many ways to get the job done..

Glad to hear the 160M antenna worked out with that feedline length. Evidently a hi-Q tuning if it's sharp. As a result expect to see a little more warming of the coil vs: other bands, but no problem.

Later -

T
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed.  Easily done in DSP.

Wise Words : "I'm as old as I've ever been... and I'm as young as I'll ever be."

There's nothing like an old dog.
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« Reply #9 on: November 14, 2005, 10:35:17 PM »

Hi Bill,

    I tune a similar antenna on 160. Way too short. Get an R.F. Ammeter and hook it in series with one leg. Tune it up under a carefully measured carrier level like 1000 Watts.
Then, lose those clipleads and hook some taps on to the coil that would start a car.
Measure again. More Ant. current? OK. Now swap the feedline for some #8 and see how many more Amps of Antenna current you get. Bet you can increase that antenna current a whole bunch. Yeah I know I sound like a broken record.  Tongue
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« Reply #10 on: November 18, 2005, 01:05:57 AM »

try lining the inside of your hat with aluminum foil, then grounding the rotor of the the split stator cap in the tuner, after you follow Dave's suggesitions. the voices in your head should drop a good 20DB and you'll have better audio on the outside speaker too.
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