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[Articles Home]  [Add Article]  

Antenna IQ Brainteaser #2

Glenn (DJ0IQ) and (W9IQ) on September 28, 2011
View comments about this article!

Introduction

Antenna theory can sometimes be a tad dry. For the typical amateur radio operator who is not a practicing engineer in the field of antenna engineering, the association of practice to theory can become a bit strained leading to incorrect assumptions and generalizations that creep into the articles, vernacular, and assertions of our hobby.

To this end, I offer this Antenna IQ series in a fun, brainteaser format. The questions are constructed so that you can easily re-use the brainteaser during your next QSO or club meeting.  They are guaranteed to spark plenty of disbelief, debate, and ultimately enlightenment.  The number of Antenna IQ  Brainteasers produced will be determined by your feedback. I hope you enjoy them.

Consider the Following

Bill has recently upgraded to General class and he is anxious to get on the HF bands. He has managed to collect a nice all solid state HF transceiver, a good quality antenna tuner, 100 feet of Belden 9913, a balun, 150 feet of antenna wire, and all the necessary accessories to put up an antenna.

At a Saturday morning breakfast, Bill’s Elmer, Al asked how Bill’s station plans are going. Bill was excited to report on his research and ideas. “I have the space for a full size 80 meter dipole and can get it about 60 feet in the air using some of the large trees in my yard so I thought I would start there. I plan to cut it for the middle of the band and use the antenna tuner to match the antenna. The SWR without the tuner should be no more than 5:1 at the band edges so it should work out well for both CW and phone,” blurts Bill.

“Hold on just a minute,” interrupts Al. “I have put up and tested a lot of antennas in my 23 years as a ham. An antenna with a 5:1 SWR is just not acceptable. The coax and reflected power losses will make that antenna  very ineffective. Your coax may radiate as well with that high of SWR. Keep the SWR under 2:1 and you can use the tuner from there to keep the transmitter happy.”

“I have to admit that some of this is over my head. I thought the tuner would take care of matching the antenna so the SWR wouldn’t be a problem,”  a humbled Bill retorts. “It seemed like the 5:1 SWR would make no real difference on my signal.”

Now Answer the Following Question

Who would you side with, Elmer Al or newbie Bill?

[ Stop reading here if you don’t want to see the answer just yet. ]

Working Out Your Answer

Probably the first or second piece of test equipment a ham radio operator acquires is an SWR meter. We are conditioned early on to pay attention to the SWR of our antennas. As we peruse the antenna ads, almost all of them make some type of assertion regarding SWR.  Most of the new breed of antenna analyzers have full SWR reporting capability. Clearly, SWR is an important topic to amateur radio operators. To the average ham radio operator a 5:1 SWR doesn’t sound very good.

While SWR is one parameter that we should know and monitor, it is regularly misunderstood and misapplied. Bill has outwitted his Elmer, Al. All of Al’s assertions regarding the antenna Bill has planned  are false.  Read on…

The Detailed Answer

Al was worried about the “reflected power losses” due to the 5:1 SWR. When an antenna is “matched” using an antenna tuner as Bill has planned, the power that has been reflected from the antenna toward the antenna tuner is returned to the antenna – not lost somewhere. Al was completely wrong on this point. The antenna tuner performs what  is called a conjugate match.

Let’s look at a conjugate match from a non-technical perspective (engineers - please forgive the minor foibles of this analogy). Think of the transmitter handing off 100 watts to the antenna tuner. The job of the antenna tuner is to not give any of it back to the transmitter (show the transmitter a 1:1 SWR) and to deliver the full 100 watts to the antenna.  At a 5:1 SWR, the antenna will always consume about 56% of any power given to it and it will always return the remaining 44% to the antenna tuner.

Some people point to the 44% returned power and call it lost power, like Al seems to think. But here is the important bit – the antenna tuner says “No problem, I will just send it back to the antenna again!” The first time, the tuner forwards 100 watts and gets 44 watts back from the antenna. It then forwards the 44 watts and gets back 20, it forwards 20 and gets back 9, and so on.  This goes on until finally the full 100 watts is delivered to the antenna and nothing is returned. At a 5:1 SWR, by the time this exchange has happened  15 times, more than 99.999% of the power has been delivered to the antenna. If we add all the power exchanges the tuner has handled to make this happen, it is 180 watts. This is called the Incident Power. If we put a directional wattmeter right at the tuner on the antenna side, it would actually show 180 watts.  But so far this analogy has ignored the coax, so don’t stop here.

Elmer Al was also worried about “coax losses”. We all know that coax has some loss. Bill chose 9913 coax so at 80 meters his 100 foot length of coax will have a 0.25 dB loss. This means that any signal passing in either direction through the coax will have about 6% of it extracted, mainly as heat.

Let’s now look at the influence this coax loss into our analogy. All the actors stay the same but there is now a middle man between the antenna tuner and the antenna – the coax. No power can go in either direction without the middle man collecting his 6% “cut” of the transaction. So when the tuner passes the 100 watts to the antenna, the middle man take his 6% cut, pocketing 6 watts, and sends the remaining 94 watts on to the antenna. The antenna will still return 44% of whatever it gets so it returns 41 watts. But there is the middle man again so he takes his 6% cut, pocketing 2 watts, and passing on the remaining 39 watts to the tuner. And so it goes like before but with the middle man accumulating a small horde of watts just for being the go-between.

If we make the middle man account for everything  he collected in all of the exchanges, it turns out to be about 13 watts. But of course the middle man is the coax. So the 13 watts is the total power lost in the coax.  Since the coax consumed some of the available power, the Incident Power is reduced to about 167 watts. Another point to note is that since the coax is consuming power, it will throw off the reading of the antenna SWR if we try to measure it by the tuner. In our example, the SWR would show as 4.3:1 instead of 5:1.

Before we wrap up this analogy, let’s look at one more detail about the coax loss. If an antenna has a 1:1 SWR, then it reflects no power – it consumes all of it. In our analogy, the tuner would send the 100 watts to the antenna, the middle man collects his 6%, and the antenna consumes the remaining 94 watts without sending anything back. Did you notice that the middle man only pocketed 6 watts this time? That gives us the ability to say that 6 watts of loss are attributable to normal coax loss that would be there no matter what the SWR and the remaining 7 watts (13 watts -6 watts) are due to the SWR not being 1:1. A meager 7 watts lost for a 5:1 SWR? That is less than 1/10 of an S unit difference - pure decimal dust!

The Incident Power described earlier will always be greater than the transmitter power when the antenna  SWR is not 1:1 and we have a tuner performing the conjugate match.  As a result, we should always check the maximum voltage developed to make certain we are still within the ratings of the coax.  The 9913 coax Bill chose for his project has a maximum voltage rating of 300 volts RMS.  If we calculate the voltage of the Incident Power at 50 ohms, it is 158 volts so we are well within the coax ratings.

Table 1 is  a handy chart showing the effect of various SWRs for Bill’s 80 meter station.

Table 1 - Valid only for 0.25 dB loss cable


Elmer Al also asserted that the SWR could cause the coax to radiate. He might as well have said that high SWR causes aurora borealis, a shift in the magnetic north, and sterility within 5 wavelengths. This is a complete fallacy - a high SWR does not produce antenna currents on the line or cause the line to radiate. Coax radiation is caused by reradiating energy that is coupled into the coax because of asymmetrical positioning of the coax with respect to the antenna or coupling a balanced antenna to an unbalanced coax without the use of a balun. Once again, neither of these conditions are caused by high SWR.

If you would like a handy spreadsheet that can perform all pertinent calculations related to conjugate matching, send me an email at my U.S. call sign at arrl.org. Make sure to put “Antenna IQ Spreadsheet” in the subject line.

Closing Thoughts

SWR is largely a misunderstood and misapplied measurement within the ham radio community.  Here are some additional Antenna IQ truths about SWR.

·         High SWR can be neutralized with a proper tuner and a low loss feed line for the frequencies involved. There is minimal impact on the remote received signal strength. For example:

o   With reasonable coax, the remote station will not be able to detect ANY difference in the signal due to a 10:1 SWR on any HF band if your tuner can match it. Most dipoles are only 3:1 at band edges.

·         Low SWR in itself is not an indication of the quality or efficiency of an antenna system.

·         The higher the frequency, the more effect SWR has on signal loss in a coax cable. Pay careful attention at VHF and up frequencies.

·         Antenna tuners do introduce a small additional loss that can typically be disregarded. Consult the manual from the manufacturer for expected losses and safe operating guidelines.

·         Antenna SWR should only be used to confirm that an antenna is performing as expected when first erected and then to monitor the antenna for any SWR changes (up or down) that may indicate antenna system maintenance is needed.

·         The SWR between the transmitter and the tuner should be monitored to ensure the tuner is performing the conjugate match as intended.

 

- Glenn DJ0IQ and W9IQ

 

Member Comments:
This article has expired. No more comments may be added.
 
E Mail Correction  
by W9IQ on September 28, 2011 Mail this to a friend!
Please note that my correct email address is W9IQ at ARRL.NET.

- Glenn DJ0IQ and W9IQ
 
Antenna IQ Brainteaser #2  
by K5MF on September 28, 2011 Mail this to a friend!
Great Article! Well written and easy to understand. You are a natural teacher. I was never impressed with my professors who just had to show you how much they know. The very best and memorable professors are those who can take a complex subject such as quantum mechanics and antenna theory and make it read like a Dick and Jane primer. Well done!
 
RE: Antenna IQ Brainteaser #2  
by K9MHZ on September 28, 2011 Mail this to a friend!
Nice topic and presentation. Some on here in the past have made the "radiating coax" argument, or the "dummy load antenna tuner" assertion. Finally though, I think most (except one notable, who has thankfully given us all a break from his posts) are moving on to antenna efficiencies, rather than obsessing over the evils of antenna tuners.

You seemed to also implicitly point toward another matter....that being the differences between Bird (or similar) watt meters and cheaper (but not necessarily in price) VSWR reading meters like Daiwa, et al. I've never figured out why those VSWR meters get such high marks in the reviews section, to go along with their high prices. Borderline snake oil.

Well done, and thanks for making it readable!

Best,
Brad, K9MHZ





 
RE: Antenna IQ Brainteaser #2  
by W9PMZ on September 28, 2011 Mail this to a friend!
well antenna theory??

I am sure that if this subject (antenna theory) was treated as theory it would be much more complex and confusing to those who do not have an understand of the complex mathmatical operation needed to understand subject such as triple integrals (be sure to get the duct tape to keep the head from exploding). In graduate school I did not find the subject dry, but rather it was ***VERY*** tedious to arrive at "the answer".

But in reality this example is not that of antenna theory, rather it is the application transmission line theory.

I suppose some people can be apparently Mr. Showoff the Knowldge types, but in my experiece it is because those people fail to understand the real application to the theory; so the blame is applied they are aloof. But, even with my meager education almost all of the time when I ask Mr. Aloof what the heck they were talking about nicely, they are more than earger to share the education.

Never in life fail to ask questions if you don't understand!

73,

Carl - W9PMZ
 
RE: Antenna IQ Brainteaser #2  
by K0BG on September 28, 2011 Mail this to a friend!
I wonder how many amateurs know, that some types of radar antennas have a high SWR designed in?

While SWR is indeed important, it is best to have your facts in order, as explained in this article.

Alan, KØBG
www.k0bg.com
 
Rule of 3s?  
by N9NFB on September 28, 2011 Mail this to a friend!
Some antenna lore I once heard was the "rule of 3s":

1) 3:1 SWR measured at the antenna
2) 3 dB of feedline loss
3) 3 dB of additional SWR loss above the simplistic 3 dB feedline loss (total of 6 dB effective loss)

Is this rule usefully close to reality?

73 de N9NFB EN53ua
 
RE: Rule of 3s?  
by W9IQ on September 28, 2011 Mail this to a friend!
Hello Vince (N9NFB),

First, let me say hello to a close neighbor to where I grew up. I lived most of my early life in Sussex, not far from you. My first ham radio contact as WN9NDM was made from Sussex. My favorite club, the Wisconsin Amateur Radio Club Inc., operates a 2m repeater at WCTC in your backyard. I live in Germany now but I still help to maintain the WARC repeater infrastructure when called upon.

Specifically to your question, if you have a 3:1 SWR at the antenna and a 3 dB loss in the coax cable, you will have just shy of a 4 dB transmit signal loss in total. So it looks like the rule of thumb you quoted is off a bit. There is only 1 dB additional loss due to SWR - not even noticeable on the S meter of the receiving station.

BTW - I checked this using the spreadsheet I referenced in the article.

- Glenn DJ0IQ and W9IQ
 
RE: Antenna IQ Brainteaser #2  
by W9IQ on September 28, 2011 Mail this to a friend!
Hello Tom (AE5QB),

I am glad that you enjoy the format of the Antenna IQ Brainteaser series. Thank you for the nice compliment.

Having taught at the college level for a couple of years, I do appreciate the perspective of gaining a practical understanding and not simply a theoretical one. In amateur radio circles, I find that most non-engineer hams appreciate getting a practical grounding before tackling the formulas. The structure of this series of articles is designed to address the mainstream of the the amateur radio community.

Any technical / theoretical discussions are always welcomed as part of the article responses. Antenna IQ Brainteaser #1 certainly took a few interesting but valuable turns during the interrogatories.

- Glenn DJ0IQ and W9IQ
 
RE: Antenna IQ Brainteaser #2  
by W9IQ on September 28, 2011 Mail this to a friend!
Hello Brad (K9MHZ),

Thanks for your comments on the article. I am glad you enjoyed it.

I don't know that I want to jump right into manufacturer comparisons but let me say that one usually only has to look at accuracy specifications to understand the price differences. There are however, a handful of nice products for amateur radio that rival their commercial counterparts and don't require plugs...

- Glenn DJ0IQ and W9IQ
 
RE: Antenna IQ Brainteaser #2  
by K9MHZ on September 28, 2011 Mail this to a friend!
>>>>by W9PMZ on September 28, 2011
....but in my experiece it is because those people fail to understand the real application to the theory...Never in life fail to ask questions if you don't understand!....
73,
Carl - W9PMZ<<<<


Hey Carl,

I bought a book.....Antennas: Fundamentals, Design, Measurements, Std. Ed., and it ties together the multi-integrals and practical applications nicely. So, math-rigorous enough for this aero engineer, but basic/practical enough for this aero engineer. ARRL has them for too much, so I found a supplier linked through Amazon who has them for less.

Agree with the second statement....I always ask questions, because I don't understand much!

Cheers,
Brad, K9MHZ

 
Antenna IQ Brainteaser #2  
by G0GQK on September 28, 2011 Mail this to a friend!
Perhaps this 2:1 SWR was acceptable in the days of using valves, tubes to those over the sea, but a tranceiver bought in the last 25 years will start shutting down at 1:5. Radio ham book authors need to be told about this

G0GQK
 
RE: Antenna IQ Brainteaser #2  
by N2EY on September 28, 2011 Mail this to a friend!
Good article!

Two small points and one big one:

Small Point 1) The example given assumes zero balun loss (even at 5:1 SWR) and zero tuner loss. Depending on the actual devices used, these may or may not be reasonable assumptions.

Small Point 2) The statement:

"With reasonable coax, the remote station will not be able to detect ANY difference in the signal due to a 10:1 SWR on any HF band if your tuner can match it"

does not state what "reasonable coax" means. 1 dB overall loss when matched? 2 dB? 3 dB?

It makes a big difference.


Big Point 3) The overall moral of the article (to me, anyway) is:

"Always consider the entire SYSTEM performance, and do not be led astray by blind application of rules-of-thumb."

Perhaps the biggest mistake hams make in antenna systems (and many other things) is not considering the entire SYSTEM performance. They look at one piece rather than the big picture.

In the example given, Al latches onto the 5:1 SWR number as a deal-killer without figuring out what impact it really has. Turns out that the additional loss due to 5:1 SWR is very small *in this case*. A problem not worth fixing because the matched-loss of the coax is so low.

OTOH, if the matched-loss of the coax were considerably higher, the additional loss due to SWR could be very high, and 5:1 SWR would be a problem worth fixing.

It all depends on the SYSTEM.

btw, there are ways to make an 80/75 meter dipole that has less than 2:1 SWR from bandedge to bandedge. The trick is to use a cage of wires rather than a single wire. Problem is the mechanical complexity.

73 de Jim, N2EY
 
RE: Antenna IQ Brainteaser #2  
by NZ5L on September 28, 2011 Mail this to a friend!
Proving yet again that if you are lucky enough to have a good automatic tuner, it belongs on your desk next to your rig. Not out in the rain.
 
Antenna IQ Brainteaser #2  
by N0AH on September 28, 2011 Mail this to a friend!
The whole issue of incident power and how it works with a tuner, etc....is something I never heard of until now- Is this the "Lucy" as far as explaining how tuners really work?? FB article for me, I just love antenna stuff........very much food for thought- glad you wrote it- will be interesting to see the over-all response. 73 Paul N0AH
 
RE: Antenna IQ Brainteaser #2  
by K0BG on September 28, 2011 Mail this to a friend!
Jim, I agree with you 100%.

Some time back, I made a statement which I got severely chastised for. I stated, that depending on the installation, it could very well be that feeding a resonant doublet through a tuner with open wire line, might be more efficient than the exact antenna fed with coax, and requisite balun.

The statement was sort of a jab at those who favor resonant dipoles fed with coax (something I personally prefer to do, as I don't like fiddling with a tuner).

All of this palaver only enforces what you said. One has to look at the SYSTEM, not just a small part or two of it.

Alan, KØBG
www.k0bg.com
 
RE: Antenna IQ Brainteaser #2  
by N2EY on September 28, 2011 Mail this to a friend!
K0BG writes:

"I stated, that depending on the installation, it could very well be that feeding a resonant doublet through a tuner with open wire line, might be more efficient than the exact antenna fed with coax, and requisite balun."

I think you meant ".....feeding a NONresonant doublet...."

And you'd be right.

73 de Jim, N2EY

 
RE: Antenna IQ Brainteaser #2  
by KC3JV on September 28, 2011 Mail this to a friend!
Now this is the type of article you get on eHAM and its great! Way better than QST and plenty of discussion. THANK YOU thank you,
Mark KC3JV
 
Antenna IQ Brainteaser #2  
by K9CTB on September 28, 2011 Mail this to a friend!
Wow ... SUPERBLY done, Glen. As a system engineer myself, I have to applaud the system approach to your article, although I did chuckle when the egg was delivered to the face of the "elmer" instead of the noobie ... very apropos in many ways to the style in which some "experts" appear out of the woodwork here on e-ham, only to have peer review shut them down quickly. Present company excepted, of course! :-)

73,
K9CTB
 
RE: Antenna IQ Brainteaser #2  
by W9IQ on September 28, 2011 Mail this to a friend!
Hello Mel (G0GQK),

You are correct that many solid state rigs have circuits that protect the finals by reducing power as they face higher SWR. While it is also true that a solid state final is less forgiving than its tube (valve) counterpart, the primary reason that a solid state rig cannot tolerate higher SWR like a tube rig is simply the lack of a tunable output stage to produce a conjugate match.

In the end, any solid state rig married with an internal or external antenna tuner can handle a very wide range of SWR and keep those solid state finals in their safe operating zone.

- Glenn DJ0IQ and W9IQ

 
RE: Antenna IQ Brainteaser #2  
by W9IQ on September 28, 2011 Mail this to a friend!
Hi Jim (N2EY),

I am glad you liked the article and I appreciate your comments.

You make a good point that an antenna engineer considers the antenna "system" which in this example would largely consist of the antenna, the balun, the coax, and the antenna tuner. As such, losses in the balun would also need to be considered. There is am upcoming Antenna IQ Brainteaser that deals with baluns in more detail.

You asked the question as to what a "reasonable coax loss" would be when facing a 10:1 SWR. If one accepts that 1/2 of an S unit is an acceptable loss due to SWR then a coax with 1.5 dB matched loss would meet the requirement. In practical terms, this means that RG-58 style coax can be used up to 10 MHz and RG-8 style up to 40 MHz. But caution must be exercised since SWR loss should not be the only consideration when selecting coax.

Your cage dipole example brings me back to the essence of the article - yes, you can with significant difficulty build a proper 80 meter dipole with a lower SWR. But if you have an antenna tuner, there is no practical reason to do so - there will not be a measurable difference in signal strength. This reinforces your early point that the antenna system must be considered - not simply the antenna.

- Glenn DJ0IQ and W9IQ
 
RE: Antenna IQ Brainteaser #2  
by W9IQ on September 28, 2011 Mail this to a friend!
Hi Bryon (N0AH),

I am glad you like the article.

The description given is what really happens within the coax - all within 80% of the speed of light. The exact method by which the antenna tuner returns reflected power to the antenna is a complex topic since, as you might suspect, it must also correct the phase of the reflected component in order to return it to the antenna. Thankfully this all handled behind the scenes by the average antenna tuner.

- Glenn DJ0IQ and W9IQ
 
Antenna IQ Brainteaser #2  
by KH6DC on September 28, 2011 Mail this to a friend!
All I know from elmers is that low SWR is achievable thru making an antenna resonant and higher than 1:1 isn't a bad thing except for the finals in a solid state transceiver or a linear amplifier. I use a tuner to match the antenna just to keep the finals happy. If running my Tokyo Hy-Power HL1.2KFx solid state linear with SWR >2:1, the protection circuit always kick s in and it becomes an annoyance to reset it. To me SWR is no big deal and I don't go out of the way to make my antenna and feedline resonant.
 
RE: Antenna IQ Brainteaser #2  
by KH6DC on September 28, 2011 Mail this to a friend!
PS, great article with lots of good stuff.

73 Delwyn KH6DC
 
RE: Antenna IQ Brainteaser #2  
by KW4JX on September 29, 2011 Mail this to a friend!
They say that a good philosopher is not one who knows the right answers, but one who asks the right questions. However you do both - thanks. Gil
 
RE: Antenna IQ Brainteaser #2  
by VK2FXXX on September 29, 2011 Mail this to a friend!
Gday .
I enjoyed the article,thanks.
One question.
If I have a ~50ohm antenna with an electrical half w/l of 600ohm tx line and no tuner(ie 50 ohms at tx) .
I have a 12:1 swr. The radio puts all 100w into the feedline ,does the reflected power from the antenna re-reflect from the tx just like in the atu case???
thanks.
Brendan
 
RE: Antenna IQ Brainteaser #2  
by LA9XSA on September 29, 2011 Mail this to a friend!
My guess was correct about who was correct in the scenario, but wouldn't there still be a slight improvement in placing the tuner as close to the balun as practicable, like in a box outside? My tuner's manual says to do that.

When it comes to dipoles it says that it's good to have a diploe which is as close to resonant as possible to begin with, but for random wire antennas it likes best random wires which are NOT resonant. I assume that's because an end-fed antenna will present very high impedance near the half wavelenghts - same reason why J-poles are J-shaped.
 
RE: Antenna IQ Brainteaser #2  
by N2EY on September 29, 2011 Mail this to a friend!
To W9IQ:

Continuing the discussion about "reasonable coax":

"what a "reasonable coax loss" would be when facing a 10:1 SWR. If one accepts that 1/2 of an S unit is an acceptable loss due to SWR then a coax with 1.5 dB matched loss would meet the requirement."

I think it may be a bit less.

I went to the VK1OD coax loss calculator and calculated the loss of 109 feet of RG-58 at 10 MHz. The calculator said 1.502 dB loss when matched.

Then I added the 10:1 SWR at the load end. The loss rose to 4.206 dB, which is a bit more than half an S-unit (6 dB per S unit?)

btw, it also said the apparent SWR at the source end was only 3.75 to 1, which makes sense. With enough line loss, any antenna can have low SWR....

"Your cage dipole example brings me back to the essence of the article - yes, you can with significant difficulty build a proper 80 meter dipole with a lower SWR. But if you have an antenna tuner, there is no practical reason to do so - there will not be a measurable difference in signal strength. This reinforces your early point that the antenna system must be considered - not simply the antenna."

Yes - and there's also the point that one must consider all the resources available in a particular case. As you say, if one has a tuner, it can be a very useful tool.

OTOH, there may be hams for whom the cage dipole is not difficult because they have all the materials/tools/skills, but a Transmatch represents a considerable cost and difficulty. Or a Field Day group may want to use the same antenna and rig for 80/75 CW/data/phone, and not have to worry about various ops not using the Transmatch correctly. Etc. All depends on the situation.

Consider too the following rule:

"Antenna engineering is no more than 10% electrical engineering and no less than 90% mechanical engineering. For many amateur radio antennas, the ratio is closer to 5%/95%"

73 de Jim, N2EY
 
RE: Antenna IQ Brainteaser #2  
by W9IQ on September 29, 2011 Mail this to a friend!
Hello Jim (N2EY),

A small correction on your coax loss calculation that probably explains the difference between what I said and what you calculated. The initial loss of 1.5 dB of the coax is not due to mismatch - it is inherent in the design of the coax. The incremental loss of 2.7 dB is due to the SWR. This is less than 1/2 S unit loss due to SWR (following the standard of 6 dB per S unit).

I like your mechanical verses electrical weighting. Coming originally from Wisconsin winters, I go for a simple dipole and use the tuner. I don't need to prove my mechanical ability with no signal advantage, I just want to stay on the air through the long winter...

- Glenn DJ0IQ and W9IQ
 
RE: Antenna IQ Brainteaser #2  
by W9IQ on September 29, 2011 Mail this to a friend!
Hello Gunnar (LA9XSA),

The advantage of moving the tuner closer to the antenna is really a question of the quality of the coax involved. As this Antenna IQ Brainteaser points out, if you use a good quality coax and you are talking about reasonable lengths of coax then the benefit is marginal. But we need not speculate - you can use my Antenna IQ spreadsheet to play with specific scenarios. For example, with 30 meters of RG 8 type coax and an HF antenna that has an SWR of less then 10:1, there is no practical advantage to moving the tuner closer to the antenna.

As far as building dipole antennas, it of course is better if you can implement a resonant design but unless you are staying in a very narrow frequency band, the typical single wire dipole antenna will not maintain resonance. With this being said, too many people that own antenna tuners spend incredible, wasted efforts in trying to get a perfect match with the misbelief that it will substantially improve the performance of the antenna system. I trust this article disproves this erroneous conclusion.

- Glenn DJ0IQ and W9IQ

 
RE: Antenna IQ Brainteaser #2  
by W9IQ on September 29, 2011 Mail this to a friend!
Hello Brendan (VK2FXXX),

Whatever power the transmitter puts into the line is delivered to the load, less any line losses (including those due to SWR).

- Glenn DJ0IQ and W9IQ
 
RE: Antenna IQ Brainteaser #2  
by AA4PB on September 29, 2011 Mail this to a friend!
"All I know from elmers is that low SWR is achievable thru making an antenna resonant..."

Resonance and low SWR are not related *unless* the particular antenna is designed to present a 50 Ohm load when resonant. Resonance simply means that the load is resistive with no Xl or Xc. That can occur with any impedance or SWR.

Consider a 1/2 wave dipole that is resonant on 20M. It presents about a 50 Ohm load on 20M. It is also resonant on 10M, being a full wavelength on that band. However, that same dipole presents a very high impedance on 10M. In both cases the antenna is resonant.

The bottom line is that provided you can provide a reasonable low loss match to the feed impedance it doesn't make much difference whether the antenna itself is resonant or not.
 
RE: Antenna IQ Brainteaser #2  
by W9IQ on September 29, 2011 Mail this to a friend!
I was asked a question off-line about voltage concerns from running 1500 watts with high SWR. I checked this using my spreadsheet and some quick references to coax manufacturer data. The answer is you can safely run RG-8 style coax beyond a 10:1 SWR with this amount of power.

If you have any other equipment in the line such as relays, unusual connectors, lightning protection, etc. make sure to check out their voltage rating as well.

- Glenn DJ0IQ and W9IQ
 
RE: Antenna IQ Brainteaser #2  
by NU1O on September 29, 2011 Mail this to a friend!
Can you give the formula you are using to calculate voltage? My voltage calculations are not in agreement with the formula I'm using from W2DU's "Reflections" book.

Keep up the great work! These are very educational articles!

73

NU1O/Chris
 
RE: Antenna IQ Brainteaser #2  
by NU1O on September 29, 2011 Mail this to a friend!
I was using incident power and that's what caused the voltage differences. At 1500 Watts, 50 Ohms, and a 10 to 1 SWR I calculate 866 Volts.

NU1O/Chris
 
RE: Antenna IQ Brainteaser #2  
by W5WSS on September 29, 2011 Mail this to a friend!
Hello I enjoyed reading your article. One relative point of interest worth mentioning here is that when an antenna system reflects the incident power back towards the tuner this power is then attenuated and again on the return trip and every trip, that happens to occur very quickly. The amount of additional mismatch attenuation is dependant on the relative lines loss charachteristics.and the amount of trips it takes. Yes there is a difference. I have loaded a 52 ohm Kenwood 820s directly into a 300 ohm line connected to the SO-239 receptacle. The SWR was not my concern, the additional line mismatched loss was. As it turned out The system radiated 95% of the output power presented to the antenna terminals. Mismatched line loss occurs during every trip and adds up. Whether this is counter productive depends on the system.
 
Antenna IQ Brainteaser #2  
by AB0RE on September 29, 2011 Mail this to a friend!
Glenn,

You used 75/80M for your example. I'm curious what effect going higher in frequency would have on the additional feedline losses (particularly the SWR-induced losses) for your example.

Can you let us know what the effect of the mismatch would be if you left all things equal but used 20M or 10M for the band in question instead? (Ya, I know.... *something* would be drastically wrong if one had a 5:1 SWR on a 20 or 10M dipole....).

Thanks & 73,
Dan / ab0re
 
RE: Antenna IQ Brainteaser #2  
by W9IQ on September 29, 2011 Mail this to a friend!
Hello Bob (W5WSS),

I am glad you enjoyed the article. There are more to come.

With regard to the losses in the coax occurring in both directions of travel, I hope the article made this very clear. The loss from the total number of trips is accurately calculated by traditional transmission line SWR loss calculations, including those in the Antenna IQ spreadsheet.

The loss of power in commercial coax is very real. Whether the loss is significant in most cases is what should be examined as this Antenna IQ Brainteaser points out. I trust this is what you meant by "counter productive".

Glad your transmitter, 300 ohm line, and antenna system worked out. I don't understand enough of the situation to comment any further.

- Glenn W9IQ and DJ0IQ

 
RE: Antenna IQ Brainteaser #2  
by W9IQ on September 29, 2011 Mail this to a friend!
Hello Dan (AB0RE),

Thanks for the interesting question. The normal line losses and the SWR induced line losses will both go up in a coax cable as frequency increases. If we stick with the 9913 coax used in the example, the normal line loss goes up to about 0.4 dB at 20 meters and 0.7 db at 10 meters, both for 100 feet of coax. This is due almost exclusively to ohmic line loss caused by skin effect within the coax.

If we then take these losses and add a 5:1 SWR to the equation, we have an additional loss at 20 meters of about 0.55 dB and at 10 meters an additional loss of about 0.9 dB. Neither of the additional losses due to SWR amount to even 2/10 S units of received signal strength loss - pure decimal dust.

You raise an excellent point regarding the SWR bandwidth of a dipole antenna. One should always know what the expected SWR should be and if the measured SWR is significantly higher or lower, investigate the cause.

- Glenn DJ0IQ and W9IQ
 
RE: Antenna IQ Brainteaser #2  
by N2EY on September 29, 2011 Mail this to a friend!
W9IQ:

DOH!

I see my mistake now. Your point was about *added* loss (from SWR), while I was looking at *total* loss (matched loss plus SWR loss).

"Coming originally from Wisconsin winters, I go for a simple dipole and use the tuner. I don't need to prove my mechanical ability with no signal advantage, I just want to stay on the air through the long winter..."

Which proves my point: Almost all of the engineering in your system is mechanical - figuring out an antenna, feedline and support system that will stay up in winter.

You've probably discovered what wire, insulators, ropes, etc. to use and not use, how to make good weatherproof joints, etc. All mechanical stuff.

Meanwhile the electrical end is relatively simple. Dipole, feedline, Transmatch, SWR indicator.

73 de Jim, N2EY
 
RE: Antenna IQ Brainteaser #2  
by VK2FXXX on September 29, 2011 Mail this to a friend!
Hi again.
Thanks for the answer:
Hello Brendan (VK2FXXX),

Whatever power the transmitter puts into the line is delivered to the load, less any line losses (including those due to SWR).
................

However my question using the example was "does the reflected power from the antenna re-reflect from the tx just like in the atu case??? "
in other words ,if there is no atu to reflect off ,does the tx circuitry reflect the same as an atu?
Thanks again.
Brendan
 
RE: Antenna IQ Brainteaser #2  
by N4KC on September 29, 2011 Mail this to a friend!
I do wonder what happened to the fellow down in St. Augustine who used to jump into these type discussions to tell us all how ignorant we were if we used anything but "resonate" antennas. He maintained the antenna tuner was the work of the devil and swore he would never use one.

Yet he was an advocate of and had in his shack a bunch of beautiful old Drake gear. And in every one of those radios, between the final amplifier stages and the antenna connectors, there were circuits made up of inductors and capacitors specifically designed to allow the operator to tune the circuit to match the 50-ohm output of his amplifier stage to whatever impedance was present at the connector. Gosh, that sounds a lot like a transmatch (often referred to as an "antenna tuner").

He also found it impossible to understand that an antenna system that was carefully honed to present "resonance" at 3.8 mHz would be decidedly non-resonant if the operator dared to go tripping off up to 3.995 or, horror of horrors, tried a little CW DXing way down yonder at 3.501. Depending on your definition of resonance, you could need a dozen or more antennas just to maintain resonance on that one ham band alone! Never mind the other nine HF bands at our disposal.

Good article, and I, too, guessed that the Elmer was full of hooey.

73,

Don Keith N4KC
www.n4kc.com
www.donkeith.com
http://n4kc.blogspot.com


 
RE: Antenna IQ Brainteaser #2  
by K9MHZ on September 29, 2011 Mail this to a friend!
Hey Don....that was a great description about our St. Augustine friend! Also loved his shack picture, too....barefoot for the whole world to see right in front of the camera. Ever wonder why people think that Americans have no class?

Ah well, I'll lighten it up.....great thread topic, nice, civil replies.

Best,

Brad, K9MHZ

 
Antenna IQ Brainteaser #2  
by N0AH on September 29, 2011 Mail this to a friend!
Glen,

I go by Paul- Bryon gets messed up with Byron all the time- 73
 
RE: Antenna IQ Brainteaser #2  
by W9IQ on September 30, 2011 Mail this to a friend!
Hello Brendan (VK2FXXX),

Sorry that I didn't fully answer your question the first time.

The short answer is yes, the conjugate reflection in your example occurs at the output circuit of your transceiver and the re-reflection toward the antenna occurs at that point.

Here is a longer answer. As you point out, a 50 ohm load on a transmission line of 600 ohms is a 12:1 SWR. The SWR stays at 12:1 anywhere along the length of the line (less the effect from line loss as described in the article).

What does change along the length of the line is the input impedance of the line (looking toward the antenna) as we move along the line. Even with a 50 ohm resistive antenna load, the input impedance of the line will vary from a highly capacitive impedance, to a highly resistive impedance (in the order of 1000's of ohms), to a highly inductive impedance. However, at multiples of 1/2 wavelength, the line input impedance is a direct representation of the load impedance including any complex component. In other words, in your example, the transmitter would see a load with an impedance of 50 ohms.

In order to perform a conjugate match, the transmitter must present the same impedance as the input impedance of the feedline but with a reactive component that is opposite in sign. If the load is purely resistive, then with the 1/2 wavelength of feedline, the transmitter needs only to present a purely resistive 50 ohm load to perform the conjugate match.

As you might suspect, when you start to change frequency, both the transforming effect of the 1/2 wavelength line and the changes in the complex impedance of the antenna begin to force the transmitter to reduce its output power if the output circuit of the transmitter cannot be adjusted to compensate. For this reason your configuration is typically usable at full power only for a narrow frequency range.

It might be good to mention here that the effect of the input impedance of the transmission line changing over its length when it is not terminated by a matched load, is the basis for adding or subtracting some feedline if the tuner cannot perform a match. However, if the measured SWR in the line changes at this position beyond what can be accounted for by simple line loss, then the antenna system is most likely plagued by common mode current on the transmission line. This is true for coax and open wire style lines.

- Glenn DJ0IQ and W9IQ
 
RE: Antenna IQ Brainteaser #2  
by VK2FXXX on September 30, 2011 Mail this to a friend!
Thankyou Glenn .
I appreciate your extra effort.
re-reflection is an interesting concept.
take care.
Brendan
 
RE: Antenna IQ Brainteaser #2  
by N6AJR on September 30, 2011 Mail this to a friend!


Awwww, Heck.

Just have him put up a Fan Dipole, and not worry about it. 1 antenna, many bands no sweatty-da



good article, But still over my head. I just put them up according to the well used "rule of thumb" and go for it. Actually I mostly use a 3 element steppir, so I make the antenna fit the signal..
 
RE: Antenna IQ Brainteaser #2  
by N2EY on September 30, 2011 Mail this to a friend!
"Just have him put up a Fan Dipole, and not worry about it. 1 antenna, many bands no sweatty-da"

If the mechanical requirements of a multi-dipole can be met, fine.

But in many situations mechanical complexity is not a good thing.

73 de Jim, N2EY
 
RE: Antenna IQ Brainteaser #2  
by KL7IPV on September 30, 2011 Mail this to a friend!
Good info and I have read it before. Do you know Kurt? :)
Thanks for the article.
Frank
 
Antenna IQ Brainteaser #2  
by AD5VM on September 30, 2011 Mail this to a friend!
Awesome article! It cleared up a couple of things I was unsure of. Thank you!

What I would like to know is the question to the real-world scenario of the new ham in question putting up a 40 or 80 meter dipole and using it on 'as many bands as his wide range tuner will allow'

Option one is to feed it with 100 foot of 450 ohm window line to a wide range manual or auto tuner with a built in 4:1 balun or maybe he springs for a fully balanced tuner.

Option two is to put a 1:1 balun at the feedpoint and feed it with 100 foot of RG-8X to the manual or auto tuner.

He finds that his antenna will tune up on everything but say 160 and 30 meters...

What do his losses look like (ballpark) on all those other bands?

Maybe I've been wrong but in the bit of elmering I've done, the new ham wants one antenna that works all bands... no new general class ham I've ever met would be content with just one band. They just shelled out big bucks for a DC to Daylight rig and they would be more inclined to pay a bunch of money for some 'all band' resistor on a stick than to put up a good mono band dipole. I always insist that they put up the biggest loop or doublet possible and feed it with balanced line connected to a tuner and if they can afford it, a balanced tuner...

After a ham has some experience, he might be content with a mono band antenna cut for his favorite band but a new guy doesn't have a favorite band yet and wants to play on them all.
 
RE: Antenna IQ Brainteaser #2  
by W9IQ on September 30, 2011 Mail this to a friend!
Hello Frank (KL7IPV),

I am glad you enjoyed the article.

I am not sure to which "Kurt" you are referring but since it doesn't make an immediate association for me I think a safe answer is no.

- Glenn DJ0IQ and W9IQ
 
RE: Antenna IQ Brainteaser #2  
by W9IQ on October 1, 2011 Mail this to a friend!
Hi Larry (AD5VM),

I am glad you enjoyed the article. There are more to come. It is wonderful to read that you mentor new hams - it is a critical part of our hobby and a very rewarding one.

You pose a series of great questions that put together present enough content for a complete article on the subject. For now, let me pick off a few points as general comments.

It is generally wise to use a current balun when feeding a dipole antenna with coax. This reduces feedline radiation. Whether or not the balun should also transform impedance (e.g. 4:1) is a question of the antenna characteristics. If a balun with a ferrite core is used, regardless of transformation ratios, then the properties of the antenna system must be understood to make certain that the core will not become saturated at the anticipated frequencies and power levels involved.

When selecting the type of feedline, one must look at the anticipated losses (per this Antenna IQ Brainteaser) and the physical limitations of the installation. If all things are nearly equal, it is generally true that using coax is easier for most hams. It is also generally true that ladder line will have lower losses but as this Antenna IQ Brainteaser makes clear, one must analyze if it really makes a practical difference.

The selection of the length of feedline need not be arbitrary. The length of the feedline plays a direct role in whether or not a tuner can perform a conjugate match. We are conditioned to understand that a 1/4 wavelength feedline can be used for impedance transformation and as VK2FXXX exemplified, a 1/2 wavelength feedline mirrors the impedance of the load, but what we then ignore is the effect of all of the other fractions of a wavelength. Part of the reason we disregard this is simply that the antenna tuner can normally adjust for a wide range of input impedances or the antenna in question closely matches the characteristic impedance of the feedline. But this fortuitous result isn't always forthcoming when pressing an antenna into multi band operation.

I know I haven't fully answered your questions but I hope this provides a starting point. I am happy to keep the discussion going.

- Glenn DJ0IQ and W9IQ

 
RE: Antenna IQ Brainteaser #2  
by KC2KCF on October 1, 2011 Mail this to a friend!
AE5QB wrote: "I was never impressed with my professors who just had to show you how much they know. The very best and memorable professors are those who can take a complex subject such as quantum mechanics and antenna theory and make it read like a Dick and Jane primer."

There's nothing wrong with explaining things in a simple way, but the danger is it may be too simple. In that case, a student feels good and thinks to have understood a topic when actually he/she hasn't.

The litmus test is if the student can derive (new) results independently after being taught the basic concepts.

In this case, a simple test whether one understands this article is to independently re-calculate the numbers given in the table. In this specific case, the involved math is very simple - the most advanced concept required is logarithms and exponentials (and those are solely needed for converting to/from dB and S units); the entire "magic" requires nothing more than simple addition, subtraction, multiplication and division. No complex numbers are involved either, so there's really no excuse not to try it.

Some of the questions already coming up in the comments (i.e. how do things change if I change that parameter?) indicate that not a small number of hams would fail this test miserably.

A nice followup to this article might be to actually walk through the mathematical derivation - rather than referring to spreadsheets authored by someone else or formulas dug out from somewhere else. Maybe Glenn left this as homework for the readers.
 
Antenna IQ Brainteaser #2  
by KC2KCF on October 1, 2011 Mail this to a friend!
Glenn,

"The 9913 coax Bill chose for his project has a maximum voltage rating of 300 volts RMS. If we calculate the voltage of the Incident Power at 50 ohms, it is 158 volts so we are well within the coax ratings."

I think you're a bit off here. The maximum voltage occuring along the cable is not the voltage of the incident power (traveling wave); it is the sum of incident and reflected voltages (standing wave, at least as long as the cable is long enough for a voltage maximum to occur) - and this is what the cable needs to withstand.
 
RE: Antenna IQ Brainteaser #2  
by W9IQ on October 1, 2011 Mail this to a friend!
Hello Wulf (KC2KCF),

Nice to see you again on this thread. I enjoy your contributions.

I am not clear on the difference of what you propose. Please help me understand by describing what the voltage maxima is on the standing wave in this example and how you arrive at that value.

While I approached it from the standpoint of the incident power, a more classical approach is the formula Emax = (P * Zo * SWR)^0.5 In this example it would be (100 * 50 * 5)^0.5 or 158 RMS volts which agrees with what I derived using my method.

Where do we differ?

- Glenn DJ0IQ and W9IQ
 
RE: Antenna IQ Brainteaser #2  
by KC2KCF on October 1, 2011 Mail this to a friend!
Hi Glenn,

the result of 158 Vrms is correct, but this is not the voltage of the incident power. The RMS voltage of 180 watts incident power on a 50 Ohm line is V=sqrt(Z0*P)=sqrt(50 Ohm * 180 W) ~ 95 V.

Similarly, the reflected power (80 W) has a RMS voltage of ~63 V.

It is thus the superposition of the incident and reflected wave, i.e. the standing wave contribution, that increases the RMS voltage to (95+63) V = 158 V somewhere along the line.

So, the result you gave is correct - but I disagree with "If we calculate the voltage of the Incident Power at 50 ohms, it is 158 volts". It is the voltages of the incident and reflected power together that combine to 158 volts (and reduce the voltage to (95-63) V = 32 V a quarter wavelength further down the line).

In any case, thanks for these articles. It is refreshing to see more well-founded articles on Eham.
 
RE: Antenna IQ Brainteaser #2  
by W9IQ on October 1, 2011 Mail this to a friend!
Hello Wulf (KC2KCF),

Aha! I was so focused on the thought of a wrong calculation, I didn't pay attention to what you were actually saying. Thanks for that correction - I definitely teed up the wrong description. Now I am stumped as to how I ever typed such a lousy sentence!

- Glenn DJ0IQ and W9IQ
 
RE: Antenna IQ Brainteaser #2  
by G8WWD on October 3, 2011 Mail this to a friend!
NZ5L says "Proving yet again that if you are lucky enough to have a good automatic tuner, it belongs on your desk next to your rig. Not out in the rain."

Hi Norman,

Think about what you are saying and where the tuner really should go. If the tuner is at the rig end, then the losses due to the tuner repeatedly sending the reflected power back and forth to the antenna in the way that Glenn explains are multiplied each time they traverse the coax feeder. If the tuner is at the antenna end, then the coax loss is a one time loss as the power goes there once and does not get reflected down the feeder. I have looked at quite a few broadcast station setups over the years where many tens or hundreds of kilowatts, or even megawatts are being generated. I have never seen one where the tuner is closer to the transmitter than the antenna. It is usually sited in a little hut close to the base of the antenna for LF, MF and HF installations. If they sited it closer to the transmitter, then they would be likely to cause the feeder to catch fire due to the heat build up.

Have a look at this article and you'll see what happens when they get it wrong with high power. I believe it started when a section of feeder caught fire!

http://www.bbc.co.uk/news/world-europe-14168281

73

Gordon G8WWD
 
Antenna IQ Brainteaser #2  
by W4HV on October 9, 2011 Mail this to a friend!
Very good article in many ways. GOD HELP US GUYS USING 1500 WATTS WITH 9913. Where did you get it can only handle 300 volts rms? I would have sent mine to the great copper pile in the sky long ago!! While not my idea of safe, I have seen many an amp with a separate power supply using it to take 4kv to the deck!
 
RE: Antenna IQ Brainteaser #2  
by AF6AU on October 11, 2011 Mail this to a friend!
I find it truly amazing that you can cut the "perfect" mid-band dipole on 80, get it up high, even use 10 gauge copperclad wire, balun, good coax, and by tuning to the band ends the SWR goes to 5:1. Yep, true. And that's why so often a 80M antenna needed to be a "Phone" or a "CW" length. And this for a full size antenna. Load it to shorten the length, and the bandwidth is even narrower. turn it into a mobile vertical like a hamstick, and the bandwitch is about 5Khz, maybe..

Sometimes physics just sucks...

AF6AU
JML
 
Antenna IQ Brainteaser #2  
by KC5PIE on October 25, 2011 Mail this to a friend!
This might be a dumb question, but from what I have read in the ARRL antenna handbook, when any antenna in question is resonant at the target frequency, it is at maximum efficiency for transmitting, right?
Often the point of maximum resonance presents an unusual feed point impedance to the given transmitter that might not be close to a 50 ohm match. Enter the "antenna tuner" that allows that transmitter to excite the resonant antenna with a perfect "simulated 50 ohm" match and it should transmit at a very high efficiency level, shouldn't it?
73 de kc5pie
 
RE: Antenna IQ Brainteaser #2  
by W9IQ on October 26, 2011 Mail this to a friend!
Hi Ron (KC5PIE),

The first part of your question cannot really be answered because it is the antenna system (tuner, feedline, and antenna) that must be analyzed, not just the antenna in isolation.

Your second question is more to the heart of the topic. Yes, if you have an antenna that is resonant at some impedance other than 50 ohms, such as a 70 ohm dipole, then the tuner can perform the conjugate match. The only loss of efficiency is in the minor additional SWR loss of the coax cable as the article points out.

- Glenn DJ0IQ and W9IQ
 
RE: Antenna IQ Brainteaser #2  
by KC7CEX on November 1, 2011 Mail this to a friend!
My last job we had a lot of high voltage power supplies for generating high power RF. The HV conductor of choice was RG-8 coax. Used a special connector but we routinely ran 8,000 VDC without incident. The braid acted as a safety shield to ground to short out the power supply if the dielectric of the cable ever failed. In over 10 years I never heard of the coax dielectric ever failing.
 
RE: Antenna IQ Brainteaser #2  
by KC7CEX on November 1, 2011 Mail this to a friend!
My last job we had a lot of high voltage power supplies for generating high power RF. The HV conductor of choice was RG-8 coax. Used a special connector but we routinely ran 8,000 VDC without incident. The braid acted as a safety shield to ground to short out the power supply if the dielectric of the cable ever failed. In over 10 years I never heard of the coax dielectric ever failing.
 
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