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Author Topic: Antenna tuners?  (Read 3314 times)
KA3VEZ
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Posts: 92




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« on: May 19, 2012, 08:31:01 PM »

This may seem odd in asking but let us just say you have a 15m dipole antenna and the lowest SWR you could get was 1.4(now I know most of you would say that is fine)but could you use a tuner such as the MFJ 949e to bring it down lower.  Is the same for the other bands. If you had a 160m, 80, 30m, 20m, 17m, 15m, 12m and 10m and the lowest you could get without a tuner is 1.4 could you use a tuner to get it to 1.0 to 1?


~Kirk, KA3VEZ
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WB6BYU
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Posts: 12973




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« Reply #1 on: May 19, 2012, 10:38:21 PM »

Yes, you can.  Probably.

That assumes the tuner is designed to match a load in that range.  Most modern
tuners are, but some might not.  An "L" network, for example runs into problems
with practical component sizes when matching impedances below about 100 ohms
on some bands.  Other tuner designs might be optimized for 200 to 1000 ohms
for ladder line, and not work as well at 50 ohms.

Whether using an antenna tuner actually gives any improvement in that case
depends on why you want to improve the SWR, and whether that is important
enough to offset the inconvenience of having to adjust the tuner, the potential
losses in the tuner, and the additional cost.  That's something that you have to
decide for yourself.
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W5FYI
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Posts: 1044




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« Reply #2 on: May 20, 2012, 02:02:51 AM »

To put it in perspective, a 1.4:1 SWR means that only 2.78 percent of the forward power is being reflected. According to the conjugate match theorem, it will be rereflected from the transmitter back to the antenna, so better than 99% of the power makes it to the antenna (except that which is lost to transmission line attenuation).

An antenna tuner is usually used to lower the SWR the radio experiences; the SWR on the line from the radio to the tuner can be lowered to 1:1 by the tuner, but if you put an SWR meter at the tuner's output, you'd see you still have the 1.4 SWR, and very close to the same losses.  In other words, a tuner won't change that 99% to 100%. In fact, some would argue, the tuner itself attenuates power, and you may end up worse off than by not using a tuner.

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KG4NEL
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Posts: 373




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« Reply #3 on: May 20, 2012, 10:45:03 AM »

An antenna tuner is usually used to lower the SWR the radio experiences; the SWR on the line from the radio to the tuner can be lowered to 1:1 by the tuner, but if you put an SWR meter at the tuner's output, you'd see you still have the 1.4 SWR, and very close to the same losses.  In other words, a tuner won't change that 99% to 100%. In fact, some would argue, the tuner itself attenuates power, and you may end up worse off than by not using a tuner.

Yep, which is why some don't like the term "antenna tuner" - you have to think of the antenna and feedline as the whole system. Coax under a high SWR condition on the antenna side of the tuner will result in a very inefficient system, even though the radio is happily dumping full power into it.
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AC5UP
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« Reply #4 on: May 20, 2012, 11:11:45 AM »

In fact, some would argue, the tuner itself attenuates power, and you may end up worse off than by not using a tuner.

Give that man a cigar!

As mentioned previously, perfection is an unnatural and unsustainable condition..........    Grin
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W0LEV
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Posts: 10




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« Reply #5 on: May 20, 2012, 11:35:24 AM »

An "ANTENNA TUNER" does NOT tune the antenna.  It is simply a MATCHING NETWORK.  It takes an impedance (real +/- reactance) at its input and transforms that impedance to a desired impedance - usually 50 +/- j0 ohms.   Most commercially available antenna matching networks would likely be able to take anything on the 1.4:1 circle on the Smith Chart to the center of the chart - usually 50 +/- j0 ohms.  However, as previously pointed out, the energy on the feedline still remains on the 1.4:1 circle of the Smith Chart.

Dave - W0LEV
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W5DXP
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Posts: 3525


WWW

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« Reply #6 on: May 20, 2012, 04:28:09 PM »

For all my transceivers, any SWR below 2:1 is acceptable and losses are low. As previously pointed out, a tuner can actually increase net losses.

An "ANTENNA TUNER" does NOT tune the antenna.

It is true that an antenna tuner does not establish an impedance match at the antenna but, ideally, it does indeed establish a conjugate match at the antenna feedpoint which allows maximum available power to be delivered to the antenna feedpoint. Anyone who asserts that an antenna tuner has no effect at the antenna feedpoint is simply wrong. Ideally, an antenna tuner tunes the entire antenna system to resonance which increases all the voltage, current, and power parameters everywhere in the system including at the antenna feedpoint.

http://www.w2du.com/Appendix09.pdf
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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.
KA3VEZ
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Posts: 92




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« Reply #7 on: May 20, 2012, 05:42:25 PM »

In regards to W5FYI's comment about 1.4 is only about 2.78 percent loss.  Is there a chart to that or how did you come up with that percent?


~Kirk, KA3VEZ
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N4NYY
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Posts: 4742




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« Reply #8 on: May 20, 2012, 07:02:16 PM »

In regards to W5FYI's comment about 1.4 is only about 2.78 percent loss.  Is there a chart to that or how did you come up with that percent?


~Kirk, KA3VEZ

http://www.firestik.com/Tech_Docs/SWRLOSS.htm

If I had a 1.4 SWR, I would be popping champagne bottles by now. It is a superb SWR. I have CB friends that won't accept nothing short of 1.1, and they fiddle, fiddle, and fiddle, until they get 1.1. What a complete waste of time. Go fishing or something, instead.
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WB6BYU
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Posts: 12973




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« Reply #9 on: May 20, 2012, 07:47:27 PM »

Don't believe the results of that chart.  It is possible to have a 10 : 1 SWR and still get
90% or more of your power radiated.  You can also loose 50% of your power with a
1 : 1 SWR.


That chart (and others like it) shows the amount of power reflected from the load - but
that power is NOT necessarily lost.  It is reflected back to the source, where (depending
on the equipment, etc.) it is often re-reflected back to the load.

A better source of coax losses is a tool such as VK1OD's Transmission Line Loss
calculator available here:

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

You need to know the load impedance, and the type and length of the transmission line
to calculate the actual loss.

Here's an example:  for a load impedance of 70 ohms (SWR = 1.4 : 1) the loss in 100' of
RG-58 at 30 MHz is 2.8dB (52% efficiency.)  With an SWR of 1.0 the loss would be 2.7dB
(53% efficiency.)  You can see that the SWR makes little difference in the total loss in
this case, and the loss for an SWR of 1.0 doesn't bear any resemblance to what is
quoted in the table.

Here's another example:  Let's take a really bad case:  a load impedance of 100+j200
ohms (that's 100 ohms resistance with 200 ohms of inductive reactance) for an SWR
of over 10 : 1.  Using 10' of RG-213 on 80m, the total loss is 0.02dB (99.56% of your
power reaches the antenna) in spite of the high SWR.  In fact, if the SWR is 1 : 1
with a 50 ohm load, the loss would actually be higher at 0.4dB (only 99.17%
of your power reaches the antenna.)

So in this case, operating the coax at an SWR of 10 : 1 actually reduced
the feedline loss.


You can't trust a simplistic table such as the one linked to.  Use a reputable calculator
that factors in the frequency, load impedance, and the length and type of the feedline.



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WB6BYU
Member

Posts: 12973




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« Reply #10 on: May 20, 2012, 08:07:25 PM »

But while we are providing online tools to enable hams to do their own analysis,
let's look at the tuner as well.  W9CF has a convenient tuner simulator on his
web site here:

http://fermi.la.asu.edu/w9cf/tuner/tuner.html

We can see what the losses are when matching an SWR of 1.4 : 1 on 15m.
Put in 36 or 70 ohms for the load impedance (that sets the SWR), set the
frequency to 21 MHz and punch the "autotune" button.  That shows a loss of
about 2% in the tuner.  (But if you play with the settings, you might get
it down to 1%, or about 2.8% if your capacitors don't go over 100pf.  In
fact, in the latter case the losses are 1% higher using a 36 ohm impedance
instead of a 70 ohm impedance, even though both give the same SWR.)

Now you have the tools to do your own analysis:  you can estimate the tuner
losses (which in practice may be a bit higher than the simulator indicates) as
well as the difference in the coax losses due to high SWR.  But remember that
using the tuner does NOT reduce the SWR on the coax.

You also need to remember that the SWR measured at the rig is going to be
lower than the actual SWR at the tuner.  Using VK1OD's calculator, an SWR of
1.4 : 1 on 15m measured at the transmitter end of 100' of RG-58 would suggest
that the SWR at the antenna is 1.7 : 1.


I know, there is a lot of complexity to all this, but the world isn't as simple as
the reflected power table marketed to CBers would indicate.
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W5FYI
Member

Posts: 1044




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« Reply #11 on: May 21, 2012, 04:19:32 AM »

Quote
In regards to W5FYI's comment about 1.4 is only about 2.78 percent loss.  Is there a chart to that or how did you come up with that percent?

The standard formula is: Percent of power reflected equals 100 times the reflection coefficient, ρ, squared; and reflection coefficient is determined by (SWR-1)/(SWR+1)

Solving for 1.4; 100 x ((1.4-1)/(1.4+1))²  becomes: 100 x (.4/2.4)²,  which is: 100 x (0.166667)²; which equals 2.7777777%

I hope this helps.
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K9IUQ
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Posts: 1620




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« Reply #12 on: May 21, 2012, 05:18:14 AM »

In fact, some would argue, the tuner itself attenuates power, and you may end up worse off than by not using a tuner.

There is NO argument, a tuner will lose power for you. QST has done reviews of many tuners. They always test for loss in the tuner under varying SWR condx. The tuners always lose power from around 5% and up. A 10% loss is very common and even higher loss on unusual loads is common. Add this to the loss you already have on the feedline before it gets to the tuner.

Many times I check the power loss on the internal tuner of my TS-590s. It usually is around 10 to 15 watts. In other words instead of 100 watts going to the antenna, I give 85 to 90 watts because of the tuner being on.

I usually do not use the internal tuner under a 1.5:1 SWR. I leave it out of the circuit because of the tuner power loss.

Stan K9IUQ
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W5DXP
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Posts: 3525


WWW

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« Reply #13 on: May 21, 2012, 06:04:56 AM »

I have an EXCEL file that I generated for my personal use. The first entry is a conversion of loss in dB to loss in percent: For instance 0.5 dB loss is ~11% loss.

http://www.w5dxp.com/hamstuff.xls

There are also other calculations that I find useful such as parallel to serial to parallel impedance conversions and SWR calculations given R, X, and Z0.
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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.
WA3SKN
Member

Posts: 5419




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« Reply #14 on: May 21, 2012, 07:07:29 AM »

Lets think this one out...
You have a dipole, say 70 ohms impedance, with a piece of 50 ohm coax.  The radio sees a 1.4/1 VSWR and supplies full output power.  Loss due to mismatch is tenths of a dB.
You add a tuner.
The loss due to mismatch is still tenths of a dB, but the tuner adds a few tenths of a dB of loss.  Since the transmitter now sees 1.0/1 VSWR, it still supplies full output.
Your loss has increased by a few tenths.
If the transmitter was not putting out full power because of a large mismatch, things would change.
73s.

-Mike.
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