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Author Topic: The Truth: How lossy are traps?  (Read 50188 times)
W2ANZ
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Posts: 9




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« Reply #15 on: December 02, 2012, 10:04:40 AM »

I thought the loss at the parallel resonance frequency of the trap was not the issue.  I thought it was the loss at the lower bands where the trap just acts as loading.... loading = lower radiation resistance = higher resistive losses.  This applies more to trapped vertical antennas, not Yagis.
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WB6BYU
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Posts: 13479




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« Reply #16 on: December 02, 2012, 11:33:34 AM »

Quote from: W2ANZ
I thought the loss at the parallel resonance frequency of the trap was not the issue.  I thought it was the loss at the lower bands where the trap just acts as loading.... loading = lower radiation resistance = higher resistive losses.  This applies more to trapped vertical antennas, not Yagis.


That's part of the confusion:  if one antenna has less gain than another, it
is because it uses traps, it is shorter than full sized, or some other reason?
If you are discussing "lossy traps", then you have to compare an antenna
of similar size that uses some form of loading other than traps, in which
case the difference due to the antenna not being full sized doesn't enter
into the equation.  So you need to be very clear whether you are comparing
traps to some other form of loading, or a trapped antenna to a monobander.
(Both are interesting comparisons, but this thread was specifically about
"how lossy are traps.")

A yagi designed for maximum gain often has a low radiation resistance, meaning
that current is higher than in a dipole.  Thus trap losses can actually be higher
than in a trap vertical.

The voltage across the trap is highest at resonance, which is where the
losses will be highest due to high circulating currents.  Designing a trap
antenna with the trap resonant just below the band where it needs to
act as a trap reduces losses, but it also complicates the design somewhat
because the outer sections still affect the tuning to some extent.  The
further the trap is from resonance, the more current flows in the antenna
beyond the trap.

Many antennas that claim "NO LOSSY TRAPS" use instead some even lossier
substitute, such as self-resonant coils ("Iso-Res Inductors") that act like a
trap due to the parasitic capacitance of the coil rather than an intentional
capacitance added across it.  Traps made using linear loading tend to have
slightly higher losses than those using standard coils.

The reason that W8JI's value of 1.6dB for coax traps is not typical of most
commercial offerings is that the loss is due to the poor Q of the coax
used as a capacitor.  The traps in my TA-33jr use mostly air dielectric
(with some plastic spacers) and will be considerably more efficient.
(Unless the antenna comes down in an uncontrolled manner after Field
Day and breaks one of the connecting wires...)


True, there is some loss in performance due to a shorter element, but that
is the case whether one uses a trap or a loading coil.  Actually, performance
is pretty reasonable down to about 60% of full size, after which the losses
accelerate as it gets shorter.

Because the traps are a combination of L and C, the bandwidth of a trapped
antenna will be less than that of one using just loading coils, but there
otherwise might not be much difference in performance.
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N8CMQ
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Posts: 386




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« Reply #17 on: December 02, 2012, 01:43:45 PM »

 Your question is a good one!
Unfortunately, it does not have a simple answer...
The best answer tho, is not enough loss to worry about,
unless inferior materials are used to construct the trap.
 
 Look at what a trap is composed of, a coil and a cap.
Many early trap antennas used a transmitting cap and
coil in their construction. Somewhere along the line,
someone used coax to form the cap, someone else made
the coax into the coil and the cap.
  Antenna manufacturers made traps out of coils
and sections of tubing to make an air dielectric cap.

 Other antenna manufacturers started the 'claims' wars.
Claims like, antennas with traps are lossy, have narrow bandwidth, or
whatever they wanted to say to make their antennas seem better.
 
 Unfortunately, many of the negative claims have taken on
the LAW of TRAP ANTENNA status. It really is too bad, as
many antennas have traps and work very well, and have
good bandwidth.
 
 As far as traps go, they work the same in a vertical as a Yagi.
A Yagi just happens to have two traps as they have two elements,
where a vertical has one trap operating over a ground plane.
 
 Another problem with a trap Yagi, the element spacing is best for
one band and a compromise on the other bands. However, the
antennas do work, and many hams have used them for years
and have been happy with them.
 
 Vertical antennas also work very well with traps.
I have a 4BTV with an 80M resonator on top. The SWR is good
for all four bands, 10M to 40M, while the 80 meter resonator is
a narrow slice of the phones band.
 
 If you need to have a multi-band antenna, don't worry about
the traps, worry about the materials used to make the antenna.
Read comments from people who own and use the antennas, as well.
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K2TL
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Posts: 40




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« Reply #18 on: December 03, 2012, 10:25:15 AM »

I'm almost certain the ARRL did a study of trap loss in their lab.  I still see references to this study in QST from time to time, like in "The Doctor is In" articles and such.  The ARRL found that even the best traps lose 1.5 db.  A multiband trap antenna can result in quite a bit of loss depending on how many traps RF needs to travel through. As the ARRL does not manufacture antennas,  I would say they do not have a biased opinion.  They have plenty of publications on the use and construction of traps.  As long as you are willing to accept the loss, traps can be very convenient.  But if you can avoid them you might as well.
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G3TXQ
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Posts: 1530




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« Reply #19 on: December 03, 2012, 12:01:10 PM »

I'm almost certain the ARRL did a study of trap loss in their lab.  I still see references to this study in QST from time to time, like in "The Doctor is In" articles and such.  The ARRL found that even the best traps lose 1.5 db. 

That's a much higher figure than I've seen quoted from other reputable sources, unless we are talking about very poor quality traps made from lossy coaxial cable.

An ARRL periodical search threw up nothing; can you point us to a reference?

73,
Steve G3TXQ
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WB6BYU
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Posts: 13479




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« Reply #20 on: December 03, 2012, 12:43:25 PM »

Quote from: K2TL
...The ARRL found that even the best traps lose 1.5 db...


I have a pretty good memory for antenna articles, and I don't ever
remember seeing this in QST, either a full article or even referenced
in the text of another article, at least not in the last 40 years.  I'd
want to see the original article and evaluate their measurement
technique to see what they were actually measuring.


But let's do a quick analysis:  1.5dB means 30% of your power is
lost in the trap.  At 1kW that is 300 watts.  Even if we allow this
for a pair of traps, that means each trap would be dissipating 150
watts.  That certainly would make the trap warm to the touch, as
anybody who has tried to grab a hot lightbulb will know.

I have an old electric heater with 10 curly wires in the element:
a single wire gets hot enough to glow at 100 watts.  That seems
a bit high for a single trap.
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AA4PB
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Posts: 12985




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« Reply #21 on: December 03, 2012, 12:51:21 PM »

In addition, a 3 ele triband Yagi would have a hard time producing any measured gain over a dipole if each trap added 1.5dB of loss.
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K2TL
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Posts: 40




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« Reply #22 on: December 03, 2012, 01:07:34 PM »

Best I can recall, I think I saw a reference to this study in QST within the last  few months.  I don't keep my old magazines anymore so I can't dig through them.  I'm pretty sure it was mentioned in "Doctor is In".. Someone wrote in about traps versus monoband antennas, or something like that.  I have read references to this study several times, but never the actual report itself.  I'll try and dig it up.
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AA4PB
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Posts: 12985




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« Reply #23 on: December 03, 2012, 01:22:57 PM »

I could believe that the difference in gain on 20M between a 3 ele monobander and a 3 ele tri-band trapped Yagi is 1.5dB. Thats a whole lot different than saying that a trap has 1.5dB of loss. Trapped tri-banders are a compromise in a several ways including trap loss and element spacing.
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W0BTU
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WWW

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« Reply #24 on: December 03, 2012, 01:25:23 PM »

Quote from: K2TL
...The ARRL found that even the best traps lose 1.5 db...

But let's do a quick analysis:  1.5dB means 30% of your power is lost in the trap.  At 1kW that is 300 watts.  Even if we allow this for a pair of traps, that means each trap would be dissipating 150 watts.  That certainly would make the trap warm to the touch, as anybody who has tried to grab a hot lightbulb will know.

I have an old electric heater with 10 curly wires in the element: a single wire gets hot enough to glow at 100 watts.  That seems a bit high for a single trap.

In addition, a 3 ele triband Yagi would have a hard time producing any measured gain over a dipole if each trap added 1.5dB of loss.

Precisely. These are words of wisdom from two wise people who have stopped to think about what a dB really is (watts and enough heat to destroy the trap), instead of just echoing yet another useless and harmful belief.

It doesn't take very much common sense at all to figure out that this myth is a bunch of utter nonsense. It was likely started by people trying to sell trapless antennas, so people would spend their money on their antenna instead of their competitor's antennas with traps that worked just fine for what the buyers needed to get a decent signal radiated.

"All traps have enough loss to matter" is simply a myth that just refuses to die, because countless people just keep repeating what they hear and read over and over and over like a broken record instead of using their brain. :-)
« Last Edit: December 03, 2012, 01:29:04 PM by W0BTU » Logged

K2TL
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Posts: 40




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« Reply #25 on: December 03, 2012, 03:01:44 PM »

I agree 1.5 db seems high, and I might be mistaken as to what the ARRL actually measured.  I have read different versions of it.  I wish I could find the original study.  I guess I'll have to make it my life's mission to find it.  When I first read the reports on it, it was depressing because I had really never troubled over trap loss.  Now it has become a big deal as many of the antenna manufacturers hawk "No Lossy Traps!", leading one to believe that trap loss is an issue. The ARRL web site is a misery to try and find old articles.  The previous version was simple.  I can't find anything there now.
Nothing is 100% efficient, including traps.  I suppose there might be some truth to the trap loss drama.  How much depends on what you want to believe or who you listen to?
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K3VAT
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« Reply #26 on: December 03, 2012, 04:03:28 PM »

I agree 1.5 db seems high, and I might be mistaken as to what the ARRL actually measured.  I have read different versions of it.  I wish I could find the original study.  I guess I'll have to make it my life's mission to find it.  When I first read the reports on it, it was depressing because I had really never troubled over trap loss.  Now it has become a big deal as many of the antenna manufacturers hawk "No Lossy Traps!", leading one to believe that trap loss is an issue. The ARRL web site is a misery to try and find old articles.  The previous version was simple.  I can't find anything there now.
Nothing is 100% efficient, including traps.  I suppose there might be some truth to the trap loss drama.  How much depends on what you want to believe or who you listen to?

As this is a technical forum we need to have all of our references clearly designated and available for others to examine.  That 1.5db number seems to be drawn straight out of the hat - I'm sorry, it is meaningless as it is undocumented.  What was documented was posted early on: the W8JI trap study.  We need to go by this unless someone can show another creditable study.

...  How much depends on what you want to believe or who you listen to?

Let's replace myth and hype with solid technical empirically-based knowledge.  IMHO, part of the problem lies in 1) propagating hearsay and this includes biased claims by manufacturers and others mainly concerned with their own self-interest; and 2) not understanding basic laws of radio communications.  73, Rich, K3VAT
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K2TL
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Posts: 40




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« Reply #27 on: December 04, 2012, 01:58:07 AM »

OK Rich.  I did not know you were the moderator of this thread and made up the rules. However, I did clearly say that it was just something I had read from time to time, as a reference, and not the whole report.  If this offends you, then maybe you can do some deep breathing meditation and try and calm down.
You can pick up any issue of QST and see antenna manufacturers claiming better efficiency antennas due to "no lossy traps".  I did not pull that out of my hat either.
You are free to accept what you like, documented or not.  Same as everyone else.  Personally, I have used traps over the last 48 years and I don't have a big problem with their performance.
The 1.5 db claim is something I read regarding an ARRL study.  That's the end of it.
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G3TXQ
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« Reply #28 on: December 04, 2012, 02:46:15 AM »

I just ran a quick EZNEC model of a 20m/15m trap dipole with different Q values for the trap inductors. These were the losses per trap at 21MHz:

Q=300, trap loss = 0.04dB
Q=200, trap loss = 0.08dB
Q=100, trap loss = 0.16dB

To get a total loss of 1.6dB for the two traps, I had to drop the Q to 15; to get a loss of 1.6dB for each trap I had to drop the Q to 4.

73,
Steve G3TXQ

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K2TL
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Posts: 40




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« Reply #29 on: December 04, 2012, 05:13:28 AM »

I would like to see what kind of trap the ARRL used in the study.  I guess it was some kind of crap.  Or maybe those who have quoted the study, and I had the misfortune of reading, did not represent the findings well.
Years back the ARRL prohibited antenna advertising to show gain in db.  They claimed that those figures could be misleading and they did not want to be a party to it.  So you will notice that all gain figures are never shown in a QST ad.  However, I see that they do allow "No Lossy Traps" in their ads.  Thinking about this, I concluded that the ads never really say that traps are lossy.  They just say that their antennas do not include lossy traps.  Kind of sneaky and causes one to infer that traps are lossy.
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