Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: [1] 2 Next   Go Down
  Print  
Author Topic: i have another stupid question about my antenna  (Read 3084 times)
KM4IY
Member

Posts: 31




Ignore
« on: March 31, 2013, 06:52:39 PM »

i've been inactive for many years and have forgotten a lot of theory, and i'm trying to recover... right now, i want a better antenna in the air.  for the time being i've chosen a ladderline fed dipole.   from the reading i've done, i was under the impression that a random length wire fed with a random length of ladderline, a balun, and a good tuner could be used on any band.

i put up abt 120' of wire, i didn't measure the ladderline, just used enough to get from the roof to the radio.  i have a 4:1 balun and a dentron 2000a tuner.

the swr on 10/15/20 is fairly decent, close enough that even the internal radio tuner will handle it, however, the lower the frequency, the higher the swr... my analyzer shows the 40/80 meters to be >25 SWR.  i can match that with the tuner, however, is there actually any power getting off the antenna?  is it worth while to use?

perhaps you can make some suggestions?

thank you for taking time for me...
Logged
W2AEW
Member

Posts: 638


WWW

Ignore
« Reply #1 on: March 31, 2013, 07:15:13 PM »

For what it's worth, I have used a similar antenna setup (130' dipole fed with 450 ohm line into the shack, to a good tuner with balun) for nearly 15 years, and it has worked great.  Sure the untuned SWR is very high (>25:1) on some bands, but it still works great once tuned up.
Logged

------------------------------------------------------
My YouTube channel...
...ham radio, basic electronics tutorials, etc.
http://www.youtube.com/w2aew
KM4IY
Member

Posts: 31




Ignore
« Reply #2 on: March 31, 2013, 07:50:08 PM »

thank you, that is encouraging.  i can't seem to get myself to accept that high SWR, all the years i was active, i always worked to get rid of as much as i could, then apply the tuner.   i use the analyzer and find the resonance closest to 3.850 is 2.5XX  to bring this up, shortening the antenna perhaps?  or feedline?  how might this change the higher bands?  

do you use an amplifier?  i have an AL811H that i like to use sometimes.  the little balun i have is rated for 200 watts, i need a bit stronger one... i haven't located one that will handle that much power except for the ones that are used as dipole centers.  this one needs to be in/near the shack.

« Last Edit: March 31, 2013, 08:11:21 PM by KM4IY » Logged
W5DXP
Member

Posts: 3562


WWW

Ignore
« Reply #3 on: April 01, 2013, 05:30:30 AM »

i can match that with the tuner, however, is there actually any power getting off the antenna?  is it worth while to use?

You seem to be confusing the 50 ohm SWR indicated by your SWR meter with the actual SWR on the ladder-line. They are not the same. And ladder-line will tolerate a lot higher SWR than coax. Let me give a worse case example of what is happening.

Assume your 120' dipole presents a feedpoint impedance of 50 ohms to the ladder-line on 80m and assume that your ladder-line length is 58 ft. (1/4WL) on 80m. The SWR on the ladder-line is only 9:1 but the impedance presented to your balun is 9x450=4050 ohms which, after the 4:1 balun, gives a 50 ohm SWR of 20:1 seen by the tuner. If your tuner will match it, there's nothing wrong with that condition.

However, if you want to lower the 50 ohm SWR, you can do that by varying the length of the ladder-line. Here's the optimum length of ladder-line for a 130 ft. dipole.

http://www.w5dxp.com/pnts130.gif

Note the grouping in yellow around a ladder-line length of 100 ft. You might want to try a length of approximately 100 ft. for your ladder-line. On some bands, you may be better off with a 1:1 balun rather than a 4:1. If you want to know more about your antenna, here is the complete article:

http://www.w5dxp.com/notuner.htm
Logged

73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
KM4IY
Member

Posts: 31




Ignore
« Reply #4 on: April 01, 2013, 10:09:35 PM »

thank you, that is more information than i've been able to dig out of the web... where can i find more abt using ladderline feed antennas?  i haven't been able to dig out a whole lot... i'd like to have a better understanding of what you're saying...

this antenna works better than anything i've had in the air, including a ********* folded dipole that i paid lots of $$$ for.  i realize how important propagation is, but for this junkbox antenna to enable me to have ragchew qso's into VK/ZL several nights in a row.  well, it's PSK, but still... amazes me...

i think i will try to add some length to the feedline and see if i can bring the two (antenna/feedline) closer ... thanks!

i can match that with the tuner, however, is there actually any power getting off the antenna?  is it worth while to use?

You seem to be confusing the 50 ohm SWR indicated by your SWR meter with the actual SWR on the ladder-line. They are not the same. And ladder-line will tolerate a lot higher SWR than coax. Let me give a worse case example of what is happening.

Assume your 120' dipole presents a feedpoint impedance of 50 ohms to the ladder-line on 80m and assume that your ladder-line length is 58 ft. (1/4WL) on 80m. The SWR on the ladder-line is only 9:1 but the impedance presented to your balun is 9x450=4050 ohms which, after the 4:1 balun, gives a 50 ohm SWR of 20:1 seen by the tuner. If your tuner will match it, there's nothing wrong with that condition.

However, if you want to lower the 50 ohm SWR, you can do that by varying the length of the ladder-line. Here's the optimum length of ladder-line for a 130 ft. dipole.

http://www.w5dxp.com/pnts130.gif

Note the grouping in yellow around a ladder-line length of 100 ft. You might want to try a length of approximately 100 ft. for your ladder-line. On some bands, you may be better off with a 1:1 balun rather than a 4:1. If you want to know more about your antenna, here is the complete article:

http://www.w5dxp.com/notuner.htm
Logged
K9ZMD
Member

Posts: 170




Ignore
« Reply #5 on: April 02, 2013, 12:20:15 AM »

Searching for "1:1 current balun", or "4:1 current balun", will lead you to some sites where you can learn about building your own balun for high power operation. 

These two sites offer high power baluns for sale (and I use baluns by both manufacturers):
http://www.balundesigns.com/servlet/StoreFront 
http://www.dxengineering.com/search/department/antennas/part-type/baluns-ununs-and-chokes

Don't sweat the high SWR out there on that balanced feed line; in almost all cases, any loss in the feed line is so low that it can be ignored.  Your tuner will take care of providing your transceiver with the necessary impedance match.  Also, don't work too hard to get a resonant antenna length; again, your tuner will take care of the mismatch. 

If you want to find a balanced line length that will make the job easier for your tuner on most bands (or even just one favorite band), then Cecil has already steered you to some excellent information. 
Logged
M6GOM
Member

Posts: 888




Ignore
« Reply #6 on: April 02, 2013, 04:26:01 AM »

thank you, that is encouraging.  i can't seem to get myself to accept that high SWR, all the years i was active, i always worked to get rid of as much as i could, then apply the tuner.  

"SWR makes you stupid" is a term used and an article has been written with the same title.

My inverted L has a SWR of >85:1 on some bands. I use a SGC230 antenna coupler at the feedpoint to tune it. It has absolutely zero problems getting out and I've worked the world with it and 100W as well as contesting with it winning a 1st in World in class in the 2011 CQ-WPX-RTTY contest.

Now that SWR tuned at the shack end of a length of co-ax is a bad thing due to feeder losses but that is less of an issue on ladderline.
Logged
W5DXP
Member

Posts: 3562


WWW

Ignore
« Reply #7 on: April 02, 2013, 04:46:28 AM »

where can i find more abt using ladderline feed antennas?

Here's something else I wrote: http://www.w5dxp.com/goodbad.htm

In particular, IMAXGRAF.EXE is a DOS program that, given a particular velocity factor of a parallel feedline, allows one to vary the length of the dipole and watch what happens to the optimum feedline lengths per band. For instance, if one chooses 90' for the dipole, one sees a grouping of dots around 40' of feedline length. That's a ZS6BKW.

Learning to use the feedline calculator at: http://www.vk1od.net/calc/tl/tllc.php will teach you a lot.

The best thing I could recommend is learning the basics of the Smith Chart so you can carry one around in your head like I do. That is the best approach I know of for being able to visualize what is happening on a transmission line.
Logged

73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
KM4IY
Member

Posts: 31




Ignore
« Reply #8 on: April 03, 2013, 04:09:44 AM »

I have absolutely fallen in love with this antenna.  not that I've had all that many antennas in my time, but no wire antenna I've ever had has compared to this one.  just about anyone I can hear, I can talk to... and the reports are amazing.

I've been biting my lip and gritting my teeth and ignoring the feeling of gloom and doom I have over my shoulder concerning the SWR, but you guys have given me the confidence to enable this...

now, the big question I have, is:  i'm not a power monger, but on occaision I do use an AL811H on 80 meters.  the dipole was cut for 3810 when I was using 9913 coax... it was flat right there... but with ladder and the little 4:1 balun, it still shows a 23 to >25 SWR.  using the advice in this thread, to ignore it and carry on, what happens when I apply the 800 watts? 

will I melt down?  I checked the LL after a long PSK session where I had been using 50 watts to reach a station overseas, and there was no sign of warming up at that power... but 800?  is my brain still in the 50ohm coax mode?

its difficult to break that habit.
Logged
W5DXP
Member

Posts: 3562


WWW

Ignore
« Reply #9 on: April 03, 2013, 05:15:03 AM »

will I melt down?  I checked the LL after a long PSK session where I had been using 50 watts to reach a station overseas, and there was no sign of warming up at that power... but 800?  is my brain still in the 50ohm coax mode? its difficult to break that habit.

Again, you do not know what the SWR on the ladder-line really is and it is probably between 5:1 and 15:1 which is perfectly acceptable. The questions are: Can your 4:1 balun reliably handle its load impedance and can your tuner achieve an efficient match to the impedance presented by the 4:1 balun?
Logged

73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
KE3WD
Member

Posts: 5694




Ignore
« Reply #10 on: April 03, 2013, 05:36:56 AM »

Keep working with your balanced line dipole and you should learn that most situations like this are simply not to be evaluated as a one-input problem. 

In other words, the SWR of a given antenna system has little to nothing to do with its Efficiency at radiation. 

The "Antenna Tuner" - misnamed, actually, should be more properly called a Matching Network IMO - was all but entirely unheard of when I first got licensed the first time around.  That is because all of the available transmitters and transceivers used tube type RF outputs with built in Matching Network between said tube(s) and the antenna.  SWR of the antenna was not an issue. 

It was the advent of the Bipolar Solid State (transistor) output that mandated that necessity to match the rig's output impedance to the load (antenna network).  This is only because a high SWR can destroy the output transistor rather quickly.  Manufacturers soon figured out how to apply Foldback Protection such that a high SWR would be far less likely to destroy the expensive RF Output Transistors, but that did nothing to effect a proper Impedance Match between the outputs and the antenna network.  Since Foldback protected the output transistors by literally folding back the power to them, such that their junctions could survive the SWR mismatch, relative power output to the antenna system would be a lot lower than one would desire.  Use of the Matchbox "Antenna Tuner" thus guarantees a good enough impedance match that maximum power transfer can happen. 

But then the fact that we humans often view a certain experience like this one without taking the time to find out what is actually happening can lead to all sorts of wrong assumptions, in this case the assumption seems to be that an SWR mismatch means that you aren't "getting out" (*another* slang term that IMO ought to be dropped like a hot rock...). 

As long as we had only the tube output rigs, there also were no SWR meters except for a few expensive "laboratory" type instruments of interest to the experimenter.  Hams weren't very likely to need or to buy one. 

So the important reading for your SWR is the one between the rig and the tuner. 

Once the tuner is set properly, the SWR between tuner and antenna system is fundamentally irrelevant, matter of fact this is no place to ever install an SWR meter. 


73

Logged
K8AXW
Member

Posts: 3773




Ignore
« Reply #11 on: April 03, 2013, 10:01:04 AM »

4IY:   
Quote
So the important reading for your SWR is the one between the rig and the tuner. 

Once the tuner is set properly, the SWR between tuner and antenna system is fundamentally irrelevant, matter of fact this is no place to ever install an SWR meter. 

3WD is correct.... way back in the day antenna tuners were not used all that much because most hams used open wire feeders or a single wire.  The tuning was done with the PA components and trimming wire.  Antenna tuners were used, understand, but were not that prevalent.

As for the SWR on the ladder line, I've never heard of it being checked....because it is understood that it will be high, depending on the frequency being used.  The only SWR that is considered or important is between the radio and the antenna tuner.

You're major concern using an amplifier should be, "Will my tuner take the high RF voltage that will usually be found at the tuner?"  RF voltages at the end of open wire feeders can be very high.

 
Logged
N2EY
Member

Posts: 3877




Ignore
« Reply #12 on: April 03, 2013, 10:56:13 AM »

I use the term "Transmatch" because it's more descriptive of a unit that is used at the shack end of the line.

It comes from "Transmission Line Matching Unit" and describes what really happens: it matches the impedance presented by the transmission line - it doesn't "tune the antenna".

73 de Jim, N2EY
Logged
W5DXP
Member

Posts: 3562


WWW

Ignore
« Reply #13 on: April 03, 2013, 11:59:43 AM »

Once the tuner is set properly, the SWR between tuner and antenna system is fundamentally irrelevant, ...

Usually true for low-loss parallel feedlines like we are discussing.

Unfortunately, the reason for 50-ohm transmitters is 50 ohm coax for which the SWR between the tuner and the antenna system becomes quite relevant because of losses in the average grades of coax, e.g. RG-8x, used by hams. Here is a handy feedline loss calculator:

http://www.vk1od.net/calc/tl/tllc.php

- it doesn't "tune the antenna".

From The IEEE Dictionary:

"tuning - the adjustment of a circuit to resonance."

"resonance - the rapid increase ... of the amplitude as the excitation frequency approaches one of the natural frequencies of the system."

Jim, one can hang an RF voltmeter at the feedpoint of an antenna and observe the "rapid increase of the amplitude" as system resonance is achieved by adjusting the shack tuner for a match at the transmitter frequency. The antenna is thus part of a system made resonant by the tuner in the shack.
« Last Edit: April 03, 2013, 12:14:18 PM by W5DXP » Logged

73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
WB6BYU
Member

Posts: 13168




Ignore
« Reply #14 on: April 03, 2013, 12:42:50 PM »

Quote from: K8AXW

As for the SWR on the ladder line, I've never heard of it being checked....because it is understood that it will be high, depending on the frequency being used...




In common ham use, perhaps, where they are operated as tuned lines, but
commercial practice "back in the day" often used matched open wire lines, and
they did actually measure the SWR on them.  It isn't difficult with open wire
line - you just run a voltmeter or ammeter (depending on whether you want
to know the VSWR or the ISWR) along the line and note the maximum and
minimum readings.

When used as matching stubs, and for calculating the impedance at the shack
end that the tuner needs to be able to match, the SWR is an important
consideration, even for open wire lines.   So while many hams ignore it, then
there are times when it is important - especially when their tuner won't match
the feedline on one or more bands.



Quote

The only SWR that is considered or important is between the radio and the antenna tuner.



If you are only interested in whether or not the rig can deliver power to the coax,
and are willing to ignore whether the tuner can match the feedline, or the losses
in the open wire line, that might be the case. 

However, in practice, the losses in open wire lines can be much more than one might
think.  One reason they work well for random length doublet antennas is their higher
characteristic impedance, so, on average, the SWR is lower on such lines than on
coax.    The higher characteristic impedance also means that currents are lower for
the same power level, and the RF current flowing through conductor resistance is
the main cause of feedline loss.

A particular source of loss is where the feedline uses Copperclad(R) wire because
if the copper coating isn't sufficiently thicker than the skin depth, a significant
portion of the RF current can flow through the steel core, which has higher RF
losses due to hysteresis.  This usually shows up on 80m and 160m, particularly
with thinner wires (and especially stranded Copperweld, where the copper on
each individual strand is much thinner than for solid wire.)

Note that this effect is NOT modeled in VK1OD's transmission line loss calculator.

It isn't difficult to get considerable loss in an open wire line, particularly when
matching a low impedance load impedance on the lower frequency bands (an
80m doublet used on 160m, for example).



Quote

You're major concern using an amplifier should be, "Will my tuner take the high RF voltage that will usually be found at the tuner?"  RF voltages at the end of open wire feeders can be very high.



Not just at the end of the open wire feeders, but along them as well.  The maximum
voltages (and their location along the wires) can be calculated knowing the load
impedance at the antenna end - another reason for paying attention to your open
wire line rather than just focusing on the coax portion.
Logged
Pages: [1] 2 Next   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!