The AM Forum
July 17, 2024, 09:03:30 AM *
Welcome, Guest. Please login or register.

Login with username, password and session length
 
   Home   Help Calendar Links Staff List Gallery Login Register  
Pages: [1] 2 ... 5   Go Down
  Print  
Author Topic: Some words on the Ladderline vs. Coax issue...  (Read 105568 times)
0 Members and 1 Guest are viewing this topic.
kc6mcw
Guest
« on: November 15, 2009, 05:27:08 PM »

A few words on the "Coax vs. Ladderline" issue...
Written by Joe Townsley KC6MCW

If you are interested in constructing a high efficient antenna system, you must follow the rules of mathmatics, physics, and reflection mechanics. I will give a simple explanation and a more complex example of transmission line theory.

The simple rules to remember:

Rule #1

 Use coax only on antennas that are resonate on single bands only! See, 50 ohm coax wants to see a 50 ohm antenna feedpoint impedance.
 
Explanation....
That antenna feedpoint impedance sways away from 50 ohms the moment you QSY away from the resonate frequency. Now say the swr meter on your tuner shows 3:1 and you then simply retune to obtain a 1:1 again. Do you think you are still efficient? Do you think you really have a 1:1 swr on your transmission line? If you answered "yes", then you couldn't be more wrong. The 3:1 swr remains on the transmission line and NEVER goes away! Your tuner is only correcting the mismatch to provide the proper impedance your equipment needs to see (50 ohms) to transfer all the available power. This 3:1 swr on your transmission line is where the loss is! Yes, 3:1 swr is not that bad. This was a simple example. But what you may not realize is when you QSY to another band, the swr on the transmission line may become 40:1, 50:1, or maybe 60:1 swr !!! But wait, why do you only see 2.5:1, 3:1, or maybe a 5:1 swr on your tuner swr meter on these other bands and think that you are still ok? Are you ready? The reason is because you are viewing an swr that also includes all the coax resistive losses.  When there is a 40:1, 50:1, or 60:1 swr on the line, you will NOT see this on your tuner swr meter. The only 2 ways to measure the true swr on the line is to measure the antenna feedpoint impedance directly or calculate it using long math.

Those of you that think there is only half a DB of loss on your coax due to your 2.5:1 swr you are viewing and think your signal is fine and not suffering that much loss to worry about it, I will encourage you to read the book “Reflections II” written by Walter Maxwell. And don’t just skim through it reading the simple stuff and say you read the book. I have read this book 6 times front to back and went through 3 yellow highlighters. This book does have some complicated formulas but if you don’t take the time to understand them and follow what Walter is talking about, you will NOT understand transmission line theory. I know this because that was me. I put the book down after looking through it because it was too complicated. But then in time, I went back to it and learned to understand what I was reading.

When you select a type of coax to use, many of you will look at its losses per 100’. I have heard these quotes when people brag about their low loss cables “I have LMR400...I have hardline…blah blah blah. Sure they exhibit lower losses, yes. But the losses are STILL HUGE when there is a 40:1 swr on the line. (see above). Its simple, don’t use coax of any kind on a multiband antenna system!

Rule #2

Use ladderline, open wire, window line, etc. on antenna systems that will be used on multiple bands! Why? Because of its’ extreme low loss properties.

Explanation….
When there is a 40:1 swr on the transmission line (that never goes away regardless of tuner settings, please see above again) you will want a transmission line that has very LOW losses. My swr on my open wire transmission line is on the order of 19:1 when I operate on 75m. Sure I have a 1:1 between my equipment and the tuner. But the 19:1 swr REMAINS between the tuner and the antenna. My losses are around 2 or 3% and not 38% if I had used the best coax you can buy.

What are the differences of 75, 300, 450, and 600 ohm openwire transmission lines? Here’s the deal, when you construct a multiband wire antenna, you’ll want to put up as much wire as possible. Don’t worry about specific cut lengths. You are constructing a non-resonate antenna system. The antenna feedpoint impedance will change drastically, 200 ohms to 2000 ohms ,when you QSY across the bands, depending on your unique installation. When you are operating on a band that exhibit’s a high complex antenna feedpoint impedance, a transmission line with a higher characteristic impedance will have a lower SWR on it, thus reducing the losses on the line. I have a 19:1 swr on my 600 ohm openwire line when operating on 75m. If I had used 450 ohm, the swr would have been 22:1. If I had used 300 ohm, the swr would have been 26:1...and so on. So my 600 ohm openwire line  is able to handle a much broader impedance swing while keeping the losses low.

Some misconceptions about ladderline, windowline, openwire, etc….
1. Do they radiate?
They only radiate an inch or two. Yes, I have tested this. It is nothing to worry about. The opposing current on the opposite line cancels and keeps the line balanced. Your coax is probably radiating MUCH more due to common mode currents on the outside of the shield.


2. Do they cause RFI in the shack?
No! the RFI is caused my RF current trying to return back to the tuner caused by an incorrect antenna system installation. See, multiband antenna systems are NOT easy. They are not plug and play. You must sharpen your pencil on this type of antenna setup. You are dealing with a complex antenna system that will exhibit very efficient power transfer capabilties on multiple bands.
3. Do they break in the high winds?
Install it correctly and it will be fine. I have had no problems with my openwire setup. And yes, the winds reach 70 mph here at times.
4. Problems with feeding these lines through walls?
Use your head. Its not that difficult. Believe me, it is worth the effort.


So I have attempted to explain a few points to you about transmission lines and their differences. Here are a few more examples to think about:

What kind of transmission line do most high power shortwave broadcast stations use to transfer their power to the antenna systems? Yep, openwire balanced line. Why? Because of the complex impedance their phasing networks exhibit and the long transmission line runs. Remember, low loss and high efficient power transfer is the name of the game here.

So now you have constructed the ultimate multiband antenna system. You have used openwire feedline to minimize the losses due to the high swr on the line. And now you think you are efficient now. Wrong. That nice, expensive,  pretty, commercially made antenna tuner is where your loss is now.  Why? There are baluns in there that exhibit high losses when complex impedances are imposed on them. Remember, baluns work great if they are used within their limits.

This is where a link coupled true balanced tuner comes in. No baluns. No loss.

My opinion on the best antenna system?
A 160m inverted V made with #10 wire up 80 feet, fed with 600 ohm balanced openwire transmission line made with #10 wire at 6” spacing, and matched with a link coupled true balanced tuner. This antenna system will be very efficient on all bands and will exhibit some large gain numbers on the higher bands. And only one support is needed in the center!


I hope this helps anyone who is interested in constructing the ultimate multiband antenna. Why have 5 antennas when 1 will work perfectly!

Joe Townsley
KC6MCW
Logged
K1JJ
Contributing
Member
*
Offline Offline

Posts: 8886


"Let's go kayaking, Tommy!" - Yaz


« Reply #1 on: November 15, 2009, 06:45:54 PM »

 
My opinion on the best antenna system?
A 160m inverted V made with #10 wire up 80 feet, fed with 600 ohm balanced openwire transmission line made with #10 wire at 6” spacing, and matched with a link coupled true balanced tuner. This antenna system will be very efficient on all bands and will exhibit some large gain numbers on the higher bands. And only one support is needed in the center!
Joe Townsley
KC6MCW

Very good summary, Joe.  This is a keeper for the handbook here.

One comment on using a 160M multi-band flat top on the higher bands...  Yes, on 75M there will be about 2.5db of gain over a half-wave dipole in the broadside direction as a sharp figure eight. However, on 40M the 250’ 160M dipole produces a big bi-directional null broadside and splits into a 4-leaf clover pattern. Gain in those four directions is reduced.  On 20M, there are now eight lobes, like an octopus with eight big nulls and negative gain against a dipole. On 10M there are 16 lobes and 16 nulls, etc. It becomes pot luck as to what areas of the world you cover or null out when on 40M or above.

One suggestion is to use a 180’ flat top (fed with open wire) to cover 160-40M only. The pattern stays as a figure eight with a few db gain broadside on 75 and 40M.  Then put up a second center fed flat top that is about 23’ long for 20-10M. This pattern is also a figure 8 on all bands with a few db gain on 15 and 10M.

Another option is to add legs (fan dipole) for 40M and above  (to the 250’ 160M dipole) to help stabilize the pattern on the higher bands.

It's all about keeping control of the pattern by using a broad figure-8 pattern in your favored directions on all bands.

* Of course, if someone didn’t really care about their directional pattern coverage (or nulls) then the single 160M dipole will do FB on all bands.

Again, good job on the summary, OM!

73,

Tom, K1JJ
Logged

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.
KC2IFR
Guest
« Reply #2 on: November 15, 2009, 07:14:01 PM »

Joe,
Rite on..............one of the best descriptions (easy to understand) I have read on the topic.

Bill
Logged
k4kyv
Contributing Member
Don
Member

Offline Offline

Posts: 10037



« Reply #3 on: November 15, 2009, 08:58:55 PM »

I will encourage you to read the book “Reflections II” written by Walter Maxwell.

I'd  like to find a copy of the book.  I thought it would be available from some of the online booksellers; they list it as "Currently OUT OF PRINT, new release date TBD".  Amazon.com has some used copies available at prices ranging from $168.30 to $399.89 each!  Alibris.com lists similar prices for its used copies.

I find it hard to believe that this book is really selling on the used market at those prices.  That ranks right up there with audiophoolery and their $200 2A3's, $300 wooden volume control knobs and $600 power cords. I recall Walt saying some time ago that a new edition was in the works, but I have heard nothing further on the subject.

Does anyone know of a source of this book at a realistic price?  I think it originally sold for $19.95 per copy.
Logged

Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

- - -
This message was typed using the DVORAK keyboard layout.
http://www.mwbrooks.com/dvorak
K5UJ
Contributing
Member
*
Offline Offline

Posts: 2814



WWW
« Reply #4 on: November 15, 2009, 09:05:57 PM »

Joe, Very good.

I wonder sometimes if there's an advantage to hb ladder line that is spaced more than 3 inches to get say 900  or 1000 ohms characteristic Z.  Some of the feedlines from Germany that Array Solutions used to sell had spacers that were 5 or 6 inches at one time.  OTOH, the miss match to the f/p of a 1/2 wave dipole on its lowest frequency would be greater.  Probably 600 ohm feedline is the best compromise between low Z feedpoint and high Z.    You make some great points such as sw bc stations using parallel wire feed to their curtains.   It's hard to go wrong doing what professionals do.   I'm not wild about horizontal wire antennas on 160 however because they have to be so high to eliminate ground loss and then you have a 260 foot span with some possible structural challenges there.  However if a ham has a couple of supports at least 100' high (preferably higher) may as well put up a cloud burner.  Otherwise, I advise a separate 160 vertical antenna of some sort working against ground.  Don't like inverted Vs unless they can be put up so high the ends are at least 50' high, otherwise they have a low average height.  BTW, I followed Tom's advice on the high and low band dipoles and they have worked out very well.  
Another great antenna book is the one that came out in the 1950s by Edmund LaPort (or was it Laporte?).  One of the antenna bibles. 

73

Rob
K5UJ
Logged

"Not taking crap or giving it is a pretty good lifestyle."--Frank
KC2IFR
Guest
« Reply #5 on: November 15, 2009, 09:08:25 PM »

Don,
I Have a copy.....make me an offer.......
I got it from the ARRL for 19.95...........

This offer goes out anybody..........Im sure Don will not respond.
Im not looking to make money on this......a lot of the stuff in the book is WAY over my head and Im not afraid to admit that.  

Bill
Logged
W2DU
Contributing
Member
*
Offline Offline

Posts: 490

Walt, at 90, Now 92 and licensed 78 years


WWW
« Reply #6 on: November 15, 2009, 09:37:24 PM »

Nice summary of what I've been trying to get across for a long time, Joe!! Thank you for bringing the subject up to date. Some of the chapters of the 2nd ed are available for downloading from my web page at www.w2du.com.

Yes, it's true that the 3rd edition is in progress by CQ Magazine. It's going rather slowly, however, due to a small editing staff that has many tasks other than publishing a book. There are 30 chapters in the new edition, and about three months ago the editor sent me the first eighteen chapters to proof read. I'll have to contact them again to see how the job is progressing. I'll post any new info I learn.

I appreciate your interest, guys!

Walt
Logged

W2DU, ex W8KHK, W4GWZ, W8VJR, W2FCY, PJ7DU. Son Rick now W8KHK.
KC2IFR
Guest
« Reply #7 on: November 15, 2009, 09:41:16 PM »

Walt.......
What a GREAT book......thanks much (for the stuff I could understand!)
Bill
Logged
Opcom
Patrick J. / KD5OEI
Contributing
Member
*
Offline Offline

Posts: 8281



WWW
« Reply #8 on: November 15, 2009, 09:49:29 PM »

Don,
I Have a copy.....make me an offer.......
I got it from the ARRL for 19.95...........

This offer goes out anybody..........Im sure Don will not respond.
Im not looking to make money on this......a lot of the stuff in the book is WAY over my head and Im not afraid to admit that.  

Bill

I offer $40 shipped to Dallas TX. In view if its scarcity and detailed content.
Logged

Radio Candelstein
kc6mcw
Guest
« Reply #9 on: November 15, 2009, 10:03:08 PM »

I have personally seen a setup using openwire transmission line with 12" spacing and heard of guys using up to 18". I dont think it would be necessary to go that extreme unless you really have complex impedances that are off the smith chart!

Hi Walt!

Joe Townsley
Logged
KC2IFR
Guest
« Reply #10 on: November 15, 2009, 10:04:33 PM »

Opcom...
SOLD.......the book is pristine........contact me off the board at wgr@roadrunner.com
Logged
kc6mcw
Guest
« Reply #11 on: November 15, 2009, 10:09:42 PM »

You will love the book as it is good reading for the brain.
Logged
W2PFY
Contributing
Member
*
Offline Offline

Posts: 13290



« Reply #12 on: November 15, 2009, 11:12:43 PM »

Quote
SOLD.......the book is pristine...

A friend called me on the fone who is a swl and wanted me to try to buy the book for $1000.00. When you snooze, you lose.
Logged

The secrecy of my job prevents me from knowing what I am doing.
KC2IFR
Guest
« Reply #13 on: November 15, 2009, 11:15:05 PM »

Quote
A friend called me on the fone who is a swl and wanted me to try to buy the book for $1000.00. When you snooze, you lose.

OH well..............I can always reconsider  Undecided
Logged
w3jn
Johnny Novice
Administrator
Member

Offline Offline

Posts: 4611



« Reply #14 on: November 15, 2009, 11:21:59 PM »

Very good summary, Joe.  This is a keeper for the handbook here.

73,

Tom, K1JJ


I'll give this a few days, prune some of the posts regarding the hosstradin' on Walt's book, and move the thread to the Handbook section.

Logged

FCC:  "The record is devoid of a demonstrated nexus between Morse code proficiency and on-the-air conduct."
W2DU
Contributing
Member
*
Offline Offline

Posts: 490

Walt, at 90, Now 92 and licensed 78 years


WWW
« Reply #15 on: November 15, 2009, 11:34:31 PM »

I should have mentioned in the earlier post that the 3rd ed will have everything that appears in both the 1st and 2nd eds, plus several new chapters. So why don't you all wait for the 3rd? As I said, I'll keep you informed.

Walt
Logged

W2DU, ex W8KHK, W4GWZ, W8VJR, W2FCY, PJ7DU. Son Rick now W8KHK.
W2PFY
Contributing
Member
*
Offline Offline

Posts: 13290



« Reply #16 on: November 15, 2009, 11:47:38 PM »

Quote
OH well..............I can always reconsider

Just kidding Bill Grin Grin Grin
Logged

The secrecy of my job prevents me from knowing what I am doing.
K1JJ
Contributing
Member
*
Offline Offline

Posts: 8886


"Let's go kayaking, Tommy!" - Yaz


« Reply #17 on: November 15, 2009, 11:47:54 PM »

I have personally seen a setup using openwire transmission line with 12" spacing and heard of guys using up to 18". I dont think it would be necessary to go that extreme unless you really have complex impedances that are off the smith chart!

Hi Walt!

Joe Townsley

Yes.

I've used 24" spacing for 75M.  I used no spacers and simply pulled the two wires tight from the top of the tower to the ground. The feeders canceled well. 24" is a very small fraction of wavelength on 75M. However, on the higher bands it may not be so.

BTW, open wire using no spacers is probably THE most effiecent feedline known to man. There is no dieletric to cause loss current across the line. Except for the end insulators to hold the feeders tight, it uses air.  I used a Hartley oscillator transmitter with it and you could hear the whee-o whee-o  of the carrier swing as the wind blew the feeders to and fro... Grin

T
Logged

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.
kc6mcw
Guest
« Reply #18 on: November 15, 2009, 11:54:21 PM »

A few pics of my transmission line with 6" spacing.


* feedline1.jpg (118.18 KB, 1200x900 - viewed 1202 times.)

* feedline2.jpg (270.38 KB, 1200x900 - viewed 1212 times.)
Logged
k4kyv
Contributing Member
Don
Member

Offline Offline

Posts: 10037



« Reply #19 on: November 16, 2009, 12:16:02 AM »

I should have mentioned in the earlier post that the 3rd ed will have everything that appears in both the 1st and 2nd eds, plus several new chapters. So why don't you all wait for the 3rd? As I said, I'll keep you informed.

Thanks, Walt.  I'm looking forward to when the next edition is ready. That says a  lot about a book when used ones are going for that much more than when brand new, that soon after they go out of print.  Usually a book has to be out of print for many years, and be an extremely rare collectors item, to command that kind of price.  I'm not even sure a mint 1st edition/1st printing original ARRL handbook would bring much more than $400.

Back to the subject of tuning antennas and feedlines; some interesting concepts are discussed in a number of technical books, under the subject of conjugate matching.  The basic idea is to use a feedline that will tolerate substantial SWR without excessive loss, and to do all the tuning at the transmitter end of the feedline, allowing the changes in R + j to efficiently reflect down the feedline all the way to the antenna.  In other words, the antenna, feeders and tuner are all part of a system that is adjustable from one convenient point.

I agree that modern day appliance-tuners that insert a balun between an unbalanced tuning network and balanced open-wire resonant feeders work on what is essentially a bogus concept.
Logged

Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

- - -
This message was typed using the DVORAK keyboard layout.
http://www.mwbrooks.com/dvorak
kc6mcw
Guest
« Reply #20 on: November 16, 2009, 12:26:43 AM »

Don,
I believe the concept some people are not grasping is the fact that SWR remains on the transmission line regardless of what you see on a meter in thier store purchased tuner. And this SWR is usually much higher than they think.

"But I can tune all the bands so my coax is fine!"

Yes, a dummy load tunes up good too. It doesn't mean it will be efficient.

Joe
Logged
K1JJ
Contributing
Member
*
Offline Offline

Posts: 8886


"Let's go kayaking, Tommy!" - Yaz


« Reply #21 on: November 16, 2009, 12:40:26 AM »

A real whirl example of extreme loss using coax cable is to listen to the 75M band using a 160M coax-fed dipole. (a 2000 ohm impedance fed with 50 ohm coax)  It's down at least 20db from a well matched dipole.  Or to a lesser extent, listen to the 160M band using a 75M coax fed dipole.  (12 ohms impedance fed with 50 ohm line)

The real loss in db could be accurately calculated.

A shack-located  antenna tuner will have no effect on this mismatch UNLESS it is mounted at the antenna's feedpoint.  I suppose this could be done on a tower with a remote tuner - and is done by some.

Or use 40M and 75M coax-fed dipoles for the same type of tests.  

T
Logged

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.
KC2IFR
Guest
« Reply #22 on: November 16, 2009, 12:41:19 AM »

Hmmmm,
Looks like some folks are trying complicate this post way beyond what it was meant to be. For the average ham....all this tech stuff might be a little confusing. This is not to say it should be ignored......BUT lets not make a big thing out of something thats NOT THAT COMPLICATED. If one reads Joes post.......u can decide what kind of ant THE AVERAGE HAM wants to put up..........no ifs, ands, or butts. It always amazes me that folks want to complicate things to the point that it will scare the average ham. Again I say..........we are talking to the avarage ham........not some genius that has a degree in math.
Let the flames begin.... Tongue  

BTW......I do NOT mean anything bad with this post...........just keep it simple....... Wink
Logged
KX5JT
Contributing
Member
*
Offline Offline

Posts: 1948


John-O-Phonic


« Reply #23 on: November 16, 2009, 03:00:41 AM »

Okay.  I'm considering a "fan dipole" no, really a "parallel dipole" of 75/40/20 meter legs fed by one transmission line.  (Maybe I'll be able to get 15 meters too from the 40 meter legs). 

I do not have a balanced tuner.  I have a T network tuner.  If I use RG-213 to feed the center, I am assuming where I have those legs cut for resonance, I will have a good match with low loss.  If i need to stray up and down the band, I am assuming my swr will rise but shouldn't be a big problem for the amplifier output (SB-200).......  Would I really even necessarily need to use the T tuner unless I was feeding my ricebox directly into the antenna and NOT on the resonant freq?
If I feed the DX-60 or the SB-200 output into the system, wouldn't the output "tune" on those units be enough to match okay?

I have some fairly sturdy (14 awg based) 450 ohm windowline.   But wouldn't I need the balun (and it's losses) or a BALANCED tuner (ideally) to use it?

I'm nearly ready to do something as I feel my g5rv is a real compromise, although it's certainly putting my 100 watts carrier out there and giving some nice 20 to 30 db over 9 reports out a few hundred miles in the "ghetto" around 3.885 Mhz.

John KX5JT
Logged

AMI#1684
WBear2GCR
Member

Offline Offline

Posts: 4136


Brrrr- it's cold in the shack! Fire up the BIG RIG


WWW
« Reply #24 on: November 16, 2009, 09:14:02 AM »



Ok, I am dense.

Explain to me please...

Why does the mismatch of the open wire line to the antenna, resulting in a high SWR not matter when the same SWR from using coax does??

Is not SWR, SWR, and therefore resulting losses due to mismatch??

Ok, ants are not my strong point.

                 _-_-bear
Logged

_-_- bear WB2GCR                   http://www.bearlabs.com
Pages: [1] 2 ... 5   Go Up
  Print  
 
Jump to:  

AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
 AMfone © 2001-2015
Powered by SMF 1.1.21 | SMF © 2015, Simple Machines
Page created in 0.086 seconds with 19 queries.