It is currently Tue Mar 31, 2020 1:14 pm

Click the link below to visit the site sponsor

The Ham Radio Shop



Post new topic Reply to topic  [ 16 posts ]  Go to page 1, 2  Next
 Technical Puzzle #22 - Select the right ratio 

Which is the best advice?
Poll ended at Sun Jan 24, 2010 9:11 am
9:1 8%  8%  [ 1 ]
4:1 25%  25%  [ 3 ]
1:1 50%  50%  [ 6 ]
No balun 17%  17%  [ 2 ]
Total votes : 12

 Technical Puzzle #22 - Select the right ratio 
Silent Key

Joined: Fri May 22, 2009 1:23 pm
Posts: 5637
Location: Northampton IO92ME
Feedback: 1 (100%)
Reply with quote
Post Technical Puzzle #22 - Select the right ratio
Folks,

Now for one of the most commonly debated issues I come across!

Pete is planning to build a centre-fed, horizontal, 80m half-wave dipole. He could feed it with coax if it were for use on just 80m, but realises that he will be able to use it as an all-band antenna if he feeds it with 450 ohm ladderline and uses his wide-range unbalanced tuner with a balun. At this stage he's not quite sure which part of the garden the antenna will occupy, so he doesn't know how long the ladderline will be. He is trying to understand what ratio balun he should use between the ladderline and the tuner. He scours a lot of web sites but seems to get conflicting advice, including:

You need a 9:1 because you're stepping down from 450 ohm ladderline to 50 ohms
You need a 4:1 because that reduces the impedances at the end of the ladderline to a range better-suited to the tuner
You need a 1:1 because it's likely to result in the least tuner losses
Don't use a balun - they all introduce too much loss - they're the work of the Devil!


Which of these is the best advice for Pete's situation?

Enjoy,
Steve G3TXQ

_________________
"I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind;" (Lord Kelvin 1883)


Fri Jan 22, 2010 9:11 am
Profile Send private message WWW

 Technical Puzzle #22 - Select the right ratio 
Advanced Member
User avatar

Joined: Sat Jul 21, 2007 9:37 am
Posts: 10570
Location: Manchester IO83TK
Feedback: 60 (100%)
Reply with quote
Post Re: Technical Puzzle #22 - Select the right ratio
my thinking is simply keep it simple.. feed with as much ladderline needed to get to the shack. forget the 4.1 balun in the atu and simply use a 1.1 current balun or choke balun at the ladderline/coax connection point.

if the tuner can cope with the missmatches leave it be, if not my next move would be trim/add a little extra feedline first before trying a stepdown balun.

i have no figures or proof but understand the 4.1 baluns in many atu's especially my favorite brand MFJ are not much cop.

i dont think theres much point potentially reducing tuner losses via a 4.1 or higher balun only to introduce some via the balun itself once thats out of its optimum operating range.

i suspect theres a slightly longer or shorter antenna length thats better suited to all band coverage impedence wise than a halfwave on 80 anyway avoiding high voltage halfwaves further up the bands..


billy

_________________
Voice is for CBers, amateur radio operators, the average citizen, and the military. In other words, voice is for everyone with a mouth. CW is for those who choose this newer mode of communication. Newer? Why yes. Voice has been around for a million years.


Fri Jan 22, 2010 9:36 am
Profile Send private message WWW

 Technical Puzzle #22 - Select the right ratio 
Silent Key

Joined: Fri May 22, 2009 1:23 pm
Posts: 5637
Location: Northampton IO92ME
Feedback: 1 (100%)
Reply with quote
Post Re: Technical Puzzle #22 - Select the right ratio
Rob raises an interesting issue about the type of tuner being used. The choice doesn't have a major impact on the conclusions, but just for the record my answer assumes a High-Pass T-match, which is probably one of the most widely used in commercial tuner designs.

73,
Steve G3TXQ

_________________
"I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind;" (Lord Kelvin 1883)


Fri Jan 22, 2010 12:11 pm
Profile Send private message WWW

 Technical Puzzle #22 - Select the right ratio 
Moderator

Joined: Fri Jul 20, 2007 1:02 pm
Posts: 14667
Location: IO83lb, Wrexham
Feedback: 16 (100%)
Reply with quote
Post Re: Technical Puzzle #22
G7IGB wrote:
I'll rule out option 4 as he'll probably need a balun with 450 ohm ladder line and unbalanced tuner, although some amateurs I know don't bother using one.

Not my field but, if you have a balanced feed and an unbalanced input to the tuner, how could you not bother with a balun?

Am I missing something, quite possible :D , or are they missing everything :)

Cheers

Tony

_________________
50MHz and above from IO83lb


Fri Jan 22, 2010 12:24 pm
Profile Send private message

 Technical Puzzle #22 - Select the right ratio 
Contributor 2019
User avatar

Joined: Fri Jul 27, 2007 4:26 pm
Posts: 1507
Location: IO92kx Nottingham
Feedback: 3 (100%)
Reply with quote
Post Re: Technical Puzzle #22 - Select the right ratio
I'm tempted to say try the 1:1 first and if the antenna & chosen feeder length proves difficult to match with the tuner then try the 4:1.
(hedging my bets :wink: )

John - G7SSE

_________________
John - G7SSE

Once you know what the question actually is, you'll know what the answer means...


Fri Jan 22, 2010 7:05 pm
Profile Send private message WWW

 Technical Puzzle #22 - Select the right ratio 
Moderator

Joined: Fri Jul 20, 2007 1:02 pm
Posts: 14667
Location: IO83lb, Wrexham
Feedback: 16 (100%)
Reply with quote
Post Re: Technical Puzzle #22
G7IGB wrote:
gw8asd wrote:
Not my field but, if you have a balanced feed and an unbalanced input to the tuner, how could you not bother with a balun?

Some amateurs I know don't use a balun, they buy one of these 'smart' tuners for £500, roll out 7' of wire and work the world on 1W. :lol:

I don't know how they do it either Tony, but that's what they tell me. :wink:

I see.
I thought you were talking about attaching an antenna with ladder line, as Steve is talking about, attached to an unbalanced tuner. :?

Cheers

Tony

_________________
50MHz and above from IO83lb


Fri Jan 22, 2010 8:15 pm
Profile Send private message

 Technical Puzzle #22 - Select the right ratio 
Silent Key

Joined: Fri May 22, 2009 1:23 pm
Posts: 5637
Location: Northampton IO92ME
Feedback: 1 (100%)
Reply with quote
Post Re: Technical Puzzle #22 - Select the right ratio
Folks,

I hope to post my answer this evening - Grandad duties permitting. By the way, how is it you can go from being a "trendy young thing" in the 1960s to a Grandad so quickly - it's a mystery :) I'm sure time accelerates as you get older!

So still time to cast your vote: 9:1, 4:1, 1:1 or no balun ?

73,
Steve G3TXQ

_________________
"I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind;" (Lord Kelvin 1883)


Sat Jan 23, 2010 11:17 am
Profile Send private message WWW

 Technical Puzzle #22 - Select the right ratio 
Advanced Member
User avatar

Joined: Mon Jan 04, 2010 8:23 pm
Posts: 4221
Location: Still by the sea, but sadly only IO83ls
Feedback: 17 (100%)
Reply with quote
Post Re: Technical Puzzle #22 - Select the right ratio
My tuppence on the subject is a 1:1 balun would be fine for single band operations fed by coax, however the mismatch at anything other than the design frequency would result in greatly reduced effeciency. a 9:1 balun would match feed line to coax, but I don't think it's as simple as that, my guess (and it's just that, a guess) is if a 4:1 balun is used it would allow the antenna to tune up over a greater frequency range with the least variation in impedence.

_________________
My online log

My videos


Sat Jan 23, 2010 11:28 am
Profile Send private message WWW

 Technical Puzzle #22 - Select the right ratio 
Moderator
User avatar

Joined: Sun Aug 31, 2008 4:39 pm
Posts: 4942
Location: Sleaford, Lincolnshire (Bomber County) IO93TA WAB-TF04
Feedback: 0 (0%)
Reply with quote
Post Re: Technical Puzzle #22 - Select the right ratio
G3TXQ wrote:
Folks,

I hope to post my answer this evening - Grandad duties permitting. By the way, how is it you can go from being a "trendy young thing" in the 1960s to a Grandad so quickly - it's a mystery :) I'm sure time accelerates as you get older!

So still time to cast your vote: 9:1, 4:1, 1:1 or no balun ?

73,
Steve G3TXQ


My thoughts are that the 1:1 balun is most suited as it will allow the tuner to tune out the mismatch on all the bands worked, whereas a 9:1 or 4:1 may make things better on some bands, it will make things so much worse on others that the tuner would not cope. I am probably wrong though, so good job my pension cheque is not riding on the answer.


As to your "trendy" to grandad thing Steve, I have a theory about the apparent acceleration of the passage of time as you grow older - If you think about it on the basis of one year in relation to the number of years lived, then it starts to make some sense. One year to a ten year old is one tenth of his lifespan and therefore is proportionally a long time, take that same year in the lifespan of a 60 year old and it becomes a relatively small proportion and therefore passes relatively quickly. I am sure that a mathematician could express it in some very meaningful formula to baffle us all with. :D

Alan

_________________
73 de Alan.

Today I broke my personal record for the most consecutive days alive, despite Covid 19 - I hope to better it again tomorrow.


Sat Jan 23, 2010 2:27 pm
Profile Send private message

 Technical Puzzle #22 - Select the right ratio 
Silent Key

Joined: Fri May 22, 2009 1:23 pm
Posts: 5637
Location: Northampton IO92ME
Feedback: 1 (100%)
Reply with quote
Post Re: Technical Puzzle #22 - Select the right ratio
Folk,

Once again a big 'thank you' to everyone who took part - particularly those brave enough to share their thoughts :)

Let's dismiss the two easy ones first!

It's good engineering practice to install a balun wherever there is a transition from an unblanced to a balanced part of a sytem. In Pete's situation if there was no balun between the ladderline and the tuner, the ladderline would be severely unbalanced, would carry unequal leg currents, and would radiate. If the balun ratio is chosen appropriately, and the balun is well designed and constructed, it should introduce well under a dB loss; so we can dismiss Answer #4

Answer #1 is more subtle and I often hear it being promoted. The fact is that the impedance at the end of the 450 ohm ladderline feeding this doublet will never be 450 ohms, no matter what its length; in fact the only time it could be 450 ohms is if the ladderline was connected to a resonant 450 ohm antenna, and you don't find many of those around. Ironic isn't it - the one impedance you're very unlikely to encounter at the end of 450 ohm ladderline is 450 ohms !!

That leaves us with a choice of 4:1 or 1:1.

----------------------------------------------------------------------------------------------------------------------

The arguments put forward for a 4:1 impedance transformation often assume that the tuner works best when operating with a load impedance close to 50 ohms. That turns out not to be true! This chart shows the losses (%) vs load resistance of a typical T-network tuner on 80m for several values of load reactance; I chose 80m because losses tend to be more evident on the lower-frequency bands.

Image

As we can see the lowest tuner losses occur when the load resistance is in the medium/high range 250ohms-2500ohms; the highest losses occur at low load resistances, particularly where they are accompanied by a large capacitive reactance.

Let's now take the example of Pete's proposed doublet. At modest heights above average ground, the doublet has a feedpoint impedance close to 50 ohms on 80m. That means that the impedance seen at the tuner end of the ladderline could have a resistive component anywhere from 50 ohms to 4050 ohms depending on ladderline length; that range of impedances is indicated by the lower shaded bar in the chart, labelled 1:1. If we now introduce a 4:1 impedance transformation, the range of impedances will be lower by a factor of 4 as indicated by the upper shaded bar labelled 4:1. It's clear that the 1:1 range of impedances will result in the lower overall tuner losses.

In fact, no matter what the antenna impedance, the range of impedances seen at the tuner end of the ladderline would have a “geometric mean” of 450 ohms - that is they would swing equally below and above 450ohms as the ladderline length is varied, but once we introduce a 4:1 balun the geometric mean will reduce to 112.5 ohms. One look at the loss chart tells you that centering the impedances at the higher value is the preferable option.

----------------------------------------------------------------------------------------------------------------------

Let's now take a look at the specific losses that would occur with our example 132 doublet fed with 450 ohm ladderline.

The following chart was produced by varying the feedline length from 0° to 180° in 10° steps. At each step I calculated the impedance seen by the tuner first with a 1:1 balun and then with a 4:1 balun, and the tuner losses determined using W9CF's T-network tuner simulator. Of course, beyond 180° the chart simply repeats itself. Ladderline losses were ignored.

Image

Apart from a small range of line lengths between 80° and 115°, where the line is a quarter-wave long and has transformed the 50 ohm feedpoint impedance to a very high value around 4000 ohms, the 1:1 balun is the better option; not only that, the worst case loss never exceeds 14% with the 1:1 balun whereas it reaches 21% with the 4:1 balun.

But what about other bands - the doublet wont be used on just 80m!

----------------------------------------------------------------------------------------------------------------

Image

This chart shows the tuner loss plotted against line length for Pete's doublet on 40m. Here the doublet feedpoint impedance is around 4000ohms, so for short ladderline lengths the 4:1 balun shows an advantage. However, as the ladderline length increases and the impedance is transformed to lower values, the 1:1 balun soon shows the lower losses again. Across the whole range of possible ladderline lengths the 1:1 balun is twice as likely as the 4:1 to produce lower losses.

-----------------------------------------------------------------------------------------------------------------

None of tuner load impedances from this antenna, whatever the ladderline length or whatever the band, was outside the matching range of the T-match with either ratio balun; so matching range is not a consideration.

It's possible that if you ran very high power into a high load impedance there may be an over-voltage problem with the tuner; a 4:1 balun would help in this situation by dropping the load impedance and the voltage, but then the balun would have to be designed to handle the high voltage.

The conclusion seems clear: if you have to choose just one balun ratio, unless you know that your combination of doublet/ladderline length falls into the minority of cases where a 4:1 balun has the advantage - around a current minimum/voltage maximum - then a 1:1 balun is the preferred choice to reduce tuner losses.

This analysis was all about transformation ratios; we'll take a look at some of the other issues - Voltage Baluns vs Current Baluns, Balun losses, etc - in future Puzzles.

-------------------------------------------------------------------------------------------------------

This topic crops up so many times on various forums that I thought it worth devoting a web page to it:

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

73,
Steve G3TXQ


Sat Jan 23, 2010 6:44 pm
Profile Send private message WWW

 Technical Puzzle #22 - Select the right ratio 
Moderator
User avatar

Joined: Sun Aug 31, 2008 4:39 pm
Posts: 4942
Location: Sleaford, Lincolnshire (Bomber County) IO93TA WAB-TF04
Feedback: 0 (0%)
Reply with quote
Post Re: Technical Puzzle #22 - Select the right ratio
Well, got that one right as well :wink: I astound myself sometimes. :oops:

Thanks again Steve for a very relevant puzzle again, and thanks for the very precise reasoning behind your solution.

This series of tutorials is proving to be very good and much appreciated by all.

Alan

_________________
73 de Alan.

Today I broke my personal record for the most consecutive days alive, despite Covid 19 - I hope to better it again tomorrow.


Sat Jan 23, 2010 8:39 pm
Profile Send private message

 Technical Puzzle #22 - Select the right ratio 
Advanced Member
User avatar

Joined: Sat Jul 21, 2007 9:37 am
Posts: 10570
Location: Manchester IO83TK
Feedback: 60 (100%)
Reply with quote
Post Re: Technical Puzzle #22 - Select the right ratio
G3TXQ wrote:
As we can see the lowest tuner losses occur when the load resistance is in the medium/high range 250ohms-2500ohms; the highest losses occur at low load resistances, particularly where they are accompanied by a large capacitive reactance.

Steve G3TXQ


i was going to try and add this on my answer but i had forgot which way it was so didn't bother ..

i always believed a simple 1.1 current or choke was the way to go if the matcher can cope keeping as little out of line as possible.

very well explained as usual steve.

billy

_________________
Voice is for CBers, amateur radio operators, the average citizen, and the military. In other words, voice is for everyone with a mouth. CW is for those who choose this newer mode of communication. Newer? Why yes. Voice has been around for a million years.


Sat Jan 23, 2010 8:57 pm
Profile Send private message WWW

 Technical Puzzle #22 - Select the right ratio 
Silent Key

Joined: Fri May 22, 2009 1:23 pm
Posts: 5637
Location: Northampton IO92ME
Feedback: 1 (100%)
Reply with quote
Post Re: Technical Puzzle #22 - Select the right ratio
m0jha wrote:
i was going to try and add this on my answer but i had forgot which way it was so didn't bother ..


Billy,

The majority of the losses in a tuner occur in the inductor(s) - they have Qs of a few hundred whereas the capacitors have Qs of a few thousand. It's a bit simplistic, but generally the more inductance that is needed the greater the loss. That's why the recommendation is always to use the minimum inductance on a T-match which gives a 1:1 VSWR.

Low frequencies need more inductance, as do capacitive loads; and circulating currents go up as the resistive part of the load decreases. So the real "killer" for tuner loss is:

(Low Frequency + Low Resistance + High Capacitive Reactance)

73,
Steve G3TXQ

_________________
"I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind;" (Lord Kelvin 1883)


Sat Jan 23, 2010 9:55 pm
Profile Send private message WWW

 Technical Puzzle #22 - Select the right ratio 
Silent Key

Joined: Fri May 22, 2009 1:23 pm
Posts: 5637
Location: Northampton IO92ME
Feedback: 1 (100%)
Reply with quote
Post Re: Technical Puzzle #22 - Select the right ratio
Folks,

I forgot to mention that in this application we don't care if the balun causes some small transformation of the load impedance because it will be compensated by the tuner. That avoids the need for any specific characteristic impedance for the balun windings, which in turn allows us to design them to cope with the high voltages we're likely to encounter at a ladderline current-minimum/voltage-maximum. Typically, bifilar windings of Thermaleze wire inside Teflon tubing is used. Balun Designs have a nicely-engineered example on their web site:

http://www.balundesigns.com/servlet/the ... mon/Detail

It's rated at a differential-mode voltage of 12kV; so even when it's positioned at a current-minimum/voltage-maximum it should be good for powers up to many kW. Sound like just the thing for Billy's QRP operating :)

I see that Bob KZ5R has now kindly added a link from that page to the analysis on my web site.

73,
Steve G3TXQ


Sat Jan 23, 2010 10:23 pm
Profile Send private message WWW

 Technical Puzzle #22 - Select the right ratio 
Advanced Member
User avatar

Joined: Mon Jan 04, 2010 8:23 pm
Posts: 4221
Location: Still by the sea, but sadly only IO83ls
Feedback: 17 (100%)
Reply with quote
Post Re: Technical Puzzle #22 - Select the right ratio
Just to like to add that I've throughly enjoyed reading through the questions and answers, I can't say I can understand it all :oops: , but I'd like to add my thanks :D

_________________
My online log

My videos


Sun Jan 24, 2010 1:12 pm
Profile Send private message WWW
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 16 posts ]  Go to page 1, 2  Next


Who is online

Users browsing this forum: No registered users and 1 guest


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Jump to:  
Powered by phpBB © phpBB Group.
Designed by Vjacheslav Trushkin for Free Forums/DivisionCore.