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Author Topic: The Truth: How lossy are traps?  (Read 52910 times)
G3TXQ
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« Reply #45 on: December 20, 2012, 03:08:48 AM »

The maximum front-to-back ratio of the 14.1 MHz beam with 15-meter traps was 10.68 dB, while the feedpoint impedance was 50.5 ohms. Under these control parameters, the forward gain was 5.64 dBi. (Note: for all the 2- element beams modeled here, higher forward gains are certainly possible, but at reduced front-to-back ratios.) The gain difference between the full size Yagi and the version with 15-meter traps is 0.49 dB.

Note that not all the 0.49dB gain difference is attributable to dissipation in the traps - about 0.34dB is, the rest is attributable to the changed geometry.

Also note that Cebik assumes a trap Q of 126 for his model.

73,
Steve G3TXQ
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KA7NIQ
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« Reply #46 on: December 20, 2012, 03:57:18 AM »

The maximum front-to-back ratio of the 14.1 MHz beam with 15-meter traps was 10.68 dB, while the feedpoint impedance was 50.5 ohms. Under these control parameters, the forward gain was 5.64 dBi. (Note: for all the 2- element beams modeled here, higher forward gains are certainly possible, but at reduced front-to-back ratios.) The gain difference between the full size Yagi and the version with 15-meter traps is 0.49 dB.

Note that not all the 0.49dB gain difference is attributable to dissipation in the traps - about 0.34dB is, the rest is attributable to the changed geometry.

Also note that Cebik assumes a trap Q of 126 for his model.

73,
Steve G3TXQ
Hi Steve! Good to see you over here! Where do you stand on the Trap Loss Debate ?
I know you do a considerable amount of computer modeling. Is it not also true that any non trap interlaced Yagi for 3 or more bands will have some element interference that may effect it's performance ?
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G3TXQ
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« Reply #47 on: December 20, 2012, 07:27:08 AM »

Hi Steve! Good to see you over here! Where do you stand on the Trap Loss Debate ?

Obviously, real traps dissipate power as heat; but with well-designed traps it's nothing like the loss that "trapless antenna" advertising copy would have us believe.

Folk seem to be forgetting that even a monoband Yagi is a "compromise" antenna: it's a compromise between Gain, F/B, F/S and a feedpoint impedance that can be efficiently matched!

73,
Steve G3TXQ
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WB6BYU
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« Reply #48 on: December 20, 2012, 09:15:21 AM »

And don't forget operating bandwidth, for all of those parameters.

You have to be very careful comparing antennas, because sometimes the parameters that
seem most important really aren't the ones that determine effectiveness.

Gain is a good example:  yes, gain is good, as it makes our signal stronger.  But does
that parameter represent the peak gain at some point in the band, or the average gain
across the band?

F/B can be even more misleading:  I've built antennas with a claimed 50dB F/B ratio, and
they might actually meet that spec at one frequency on 2m.  But 500kHz away it might
only be 20 or 25dB, and less than that at the band edges.  And that presumes you are
measuring a null exactly off the back of the antenna - a more useful figure often is the
Front to Rear ratio, which averages the signal strength across the rear hemisphere of
the antenna.  A beam with a cardiod pattern may have a very sharp rear null but not be
as good at reducing interfering signals that come from 30 degrees off the back.

Reasonable 2-element yagis typically run around 6dBi gain and 10dB F/B ratio.  I've built
them with 30dB F/B ratio by using much shorter spacings (5" to 6" on 2m, or 4' to 5' on
20m.)  But the F/B ratio might only be 10dB at the band edges, and the F/B will vary
as the elements blow in the wind due to the very close spacing.

2-element beams with straight elements only have 3 effective variables for adjustment:
the lengths of each element and the spacing between them.  This makes it difficult to
try to optimize gain, F/B, resonant frequency, and feedpoint impedance at the same
time, because every change to one thing affects something else.  A 3-element yagi has
5 variables, making it easier to find a combination that meets the requirements an all
4 categories.

Trapless combinations aren't always easy to design and maintain, however, due to the
interactions among the elements.  This is more of a problem with 2-element designs
due to the limited range of adjustments.  W4RNL put a lot of work into trying to develop
a design for nested Moxon beams for multiple bands.  Antenna performance depends on
the mutual coupling between the elements, and adding extra bits will change that.
And as you add more elements the wind loading increases.

That's not to say that 2-element yagis don't have their place:  certainly the many
users of Moxons and Hex Beams have shown that it is sometimes easier to get a light
antenna up higher than a larger / heavier one, and the increased height (or the ability
to put up an antenna at all) will more than make up for lower gain.  I've assembled my
TA-33jr with just 2 elements before for this reason (and sometimes use the driven
element alone as a dipole) when I was concerned about wind loading.  For Field Day or
other portable operation, having something that is small and light is very convenient:
we put up the TA-33jr easily enough with 2 or 3 people, but, while I have the parts for
two TH6s in my barn, one look at the size of the boom (and the required truss to support
it) immediately eliminates any thought of using those.


So I don't think the real world is as polarized as the debate sometimes makes it seem.
Yes, traps have some loss, and that can be reduced by using high Q traps and/or by
designing the antenna so the traps are resonant slightly off of the operating frequency.

Multiband antennas that don't use traps may be more complex in other ways.  One
disadvantage of traps is that something can break inside the trap and not be as easy
to see on a quick review (especially if the antenna came down at the end of the last
Field Day in a rather... umm... uncontrolled manner.)

Sometimes an antenna that is lighter or has less wind loading is a better choice if it
allows the use of a smaller mast, is easier to put up, or can be put up higher than
a heavier antenna would.  Other times a full-sized antenna gives enough performance
improvement that it is worth the upgrade.


It's not that traps are necessarily "bad" and non-trap solutions are always "good".
(For example, compare a trapped 4BTV to a CHA-250B with "no lossy traps".)
Rather they are different methods of making an antenna cover multiple bands.  And,
as with many other aspects of ham radio, you have to consider your own needs,
limitations, and operating preferences in determining which best meets your needs. 
Most beams can be adjusted to optimize one parameter at a  particular frequency
at the expense of reasonable performance across the band (or, with things like element
spacing, over multiple bands.)

As Steve says, all antennas are compromises of some sort or another.  You have to
decide what trade-offs among the parameters best suite your operating preferences.

So rather than arguing whether traps are "lossy" or not (for which there is no single
answer because it depends on many factors), it makes more sense to discuss instead
factors such as F/B bandwidth, weight, wind loading, beamwidth, and the difference
between Front to Back or Front to Rear ratio as a measure of useful performance.
That would do more to shed light on choosing an appropriate antenna than arguing
over how misleading an advertising slogan is.
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KA7NIQ
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« Reply #49 on: December 20, 2012, 03:35:56 PM »

I agree that putting a smaller antenna up higher is a great idea! That is why Hex Beams are so popular. They are lightweight, and easy to stick up in the air, w/o heavy duty tower considerations.
But, wait a moment ?
I remember reading the best overall height for a triband beam antenna based in Newington, CT was only 52 feet!
I read this in QST
Do any of you guys remember that article ?
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WB6BYU
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« Reply #50 on: December 20, 2012, 03:52:04 PM »

Height above ground affects the take-off angle, and hence the coverage area.

If the ARRL analysis was looking at optimizing coverage to members in the
continental US, it would result in a lower height because so much of the ham
population is on the East side of the country, so would be reached by higher
angle signals.

Optimum height might be very different for DX in that case.
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G3TXQ
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« Reply #51 on: December 21, 2012, 03:02:00 AM »

Height above ground affects the take-off angle, and hence the coverage area.

If the ARRL analysis was looking at optimizing coverage to members in the
continental US, it would result in a lower height because so much of the ham
population is on the East side of the country, so would be reached by higher
angle signals.

Optimum height might be very different for DX in that case.

But the "optimum" height would be different for each of the three bands!

Steve G3TXQ
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W6GX
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« Reply #52 on: December 21, 2012, 08:17:07 AM »

Antenna designs are all about compromises.  Each design has it's own pros and cons.  I'd like to mention four positives on the trapless design.

1. The 'interactions' from an interlaced design could be positive (i.e. more positive gain).  The higher frequency elements act as directors on the low bands.  So just because there are 'interactions' it doesn't mean the 'interaction' is a compromise.  The positive interactive phenomenon is explained in the book 'Array of Light' by N6BT.
2. Trapless tribanders also work very well on the WARC bands.  The one I have experiences with has low SWR (less than 3:1) on 12 and 17, and moderate SWR (5:1) on 30.  It has some gain and a definite pattern, although the pattern is reversed (i.e. max gain is on the opposite side).  Running WARC bands on a trapped antenna is not advisable as the traps would self-destruct due to heat build-up.
3. Since there are no traps one could run legal limit power on all modes and all bands including the WARC bands.  This is a big advantage in a digital mode contest.  The same couldn't be said for most commercially available trapped designs (see my previous post on the power derating on high duty cycle modes)
4. Most trapless designs utilize a direct feed with no matching devices.  No matching devices means no loss, less mechanical complexity, and fewer maintenance issues.

73,
Jonathan W6GX
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KA7NIQ
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« Reply #53 on: December 21, 2012, 10:34:36 AM »

Antenna designs are all about compromises.  Each design has it's own pros and cons.  I'd like to mention four positives on the trapless design.

1. The 'interactions' from an interlaced design could be positive (i.e. more positive gain).  The higher frequency elements act as directors on the low bands.  So just because there are 'interactions' it doesn't mean the 'interaction' is a compromise.  The positive interactive phenomenon is explained in the book 'Array of Light' by N6BT.
2. Trapless tribanders also work very well on the WARC bands.  The one I have experiences with has low SWR (less than 3:1) on 12 and 17, and moderate SWR (5:1) on 30.  It has some gain and a definite pattern, although the pattern is reversed (i.e. max gain is on the opposite side).  Running WARC bands on a trapped antenna is not advisable as the traps would self-destruct due to heat build-up.
3. Since there are no traps one could run legal limit power on all modes and all bands including the WARC bands.  This is a big advantage in a digital mode contest.  The same couldn't be said for most commercially available trapped designs (see my previous post on the power derating on high duty cycle modes)
4. Most trapless designs utilize a direct feed with no matching devices.  No matching devices means no loss, less mechanical complexity, and fewer maintenance issues.

73,
Jonathan W6GX
LOL, and my Guy wires "act as reflectors and directors" for my Beam, but wait, there's more! I have a metal scarecrow (Obamacrow) on the very top of my antenna mast. And, the dimensions of this metal scarecrow was arrived at by computer simulation, to act as a director at multiple HF frequencies!~
You can read all about it, in my book, called "A Bunch Of Bullchit"
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W7VO
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« Reply #54 on: December 21, 2012, 11:38:35 AM »

I don't know what all the fuss is about. I use the power from my AL-1500 to melt the ice off my TH6DXX's traps, which radiates out through thermal conduction to the rest of the elements. Sure beats me climbing the tower to knock the ice off.  Wink

It is a win-win!

73 and Merry Christmas!

Mike, W7VO
Scappoose, OR
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KA7NIQ
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« Reply #55 on: December 21, 2012, 02:36:38 PM »

I don't know what all the fuss is about. I use the power from my AL-1500 to melt the ice off my TH6DXX's traps, which radiates out through thermal conduction to the rest of the elements. Sure beats me climbing the tower to knock the ice off.  Wink

It is a win-win!

73 and Merry Christmas!

Mike, W7VO
Scappoose, OR
Merry Christmas to you too Mike! No "fuss" really, just a bunch of bored old guys, talking about design compromises in our antennas.
As G3TXQ  correctly pointed out, trap, no trap, or full size trapless monobanders, ALL are a compromise, in one way or another.
I think it as all about finding the right compromise for YOU, and your operating habits.

I own the Hy Gain Explorer 14, with the 40 meter add on kit for instance Mike. I wanted a small (no more then 14 ft boom) tribander, with the best 10 meter performance I could buy, on that boom length. The Hy Gain Explorer 14 is almost a "monobander" on 10 meters, compared to any other tribander I could buy, on that boom length. It has dual driven elements on 10 (no traps) and a trapless, full size separate 10 meter reflector. Only the director element has a trap in it, for 10 meters.
W4RNL felt the dual driven element feed system used for 10 meters in the Explorer 14 (Hy Gain calls it a Para Sleeve) give a small amount of extra gain vs a single dipole feed.
It also gives great 10 meter bandwidth as well!
I think the Hy Gain Explorer 14 is overall the best 14 foot boom tribander money can buy, and certainly the best on 10 Meters. Like I said earlier, 10 meter performance is a MUST for me. I feel Roger Cox did an awesome job on the Explorer 14 design, and made the right compromises for me, and what I want and expect, from a small tribander.

I am up at 50 feet, pretty much the same height as my friends, one who has the 24 foot boom Hy Gain 10 meter monobander, and one who has the 4 element SteppIR.
WE are all within 20 miles of each other.

DO I kick their butts ? Heck no, the 24  foot boom computer optimized 10 meter Hy Gain 10 meter monobander is king, followed by the 4 element SteppIR, then my little Explorer 14.

But, I am "right in there" with the big boys. Both of my friends constantly comment on the clean pattern the little Explorer 14 has, on 10 meters.
I live near Tampa, and often have 1/2 of South America trying to bust into the back of my beam. Often, I can hear stations with less interference then they can, but I just can not match the firepower of the 24 foot 10 meter Hy Gain monobander.
The 4 element SteppIR does not have the firepower of the Hy Gain 10 meter monobander. The signal differences between him and me are not nearly as great as me and my Exporer 14 vs the Hy Gain 10 meter monobander.
So, I feel like I made the right choice, for me/

Now, interestingly enough, we have another Ham Friend who has a small Hex Beam.
He is only up at 38 feet (we are all at 50 feet) though. He has truly excellent rejection on his little Hex Beam.
In fact, this is his SECOND Hex Beam! He started out with a Traffie Hex Beam, and did very well with it too. But, he was less then thrilled with the lack of front to rear rejection, on 10 meters. I told him all about the KIO Hex Beams, and how Steve Hunt was able to get better rejection with his design. So, he took the Traffie Hex Beam to a Ham Radio Tailgate at the last Tampa Hamfest. It sold right away!
USED Hex Beams are nearly impossible to find here in Florida. WE have many retired Hams (winter only residents) who want a small, decent performing lightweight beam antenna, they do not have to worry about in every thunderstorm.
He was able to get 400.00 for the used Traffie Hex Beam, put a few hundred dollars more with it, and ordered the K4KIO Hex Beam.
I remember both antennas well, and the KIO Hex Beam did exactly what he expected, it greatly reduced the South American interference.

So, getting back to the thread, it is all about compromises. Like me, my Hex Beam owning friend found the right compromise for him.

Maybe because he is 12 feet lower in elevation then us, the signal from the KIO Hex Beam is nearly always from 1/2 to 1 S Unit lower then we are. I am sure the little 2 element Hex Beam just does not have the gain the 3 of us have, but it was the right compromise for him.
So many antenna makers IMHO seem to always push Gain as their reason for existence. IF I had told my Ham Friend that the KIO Hex Beam had more gain then his Traffie Hex Beam, I feel quite sure my friend would have kept the Traffie.
But, because Steve Hunt chose to offer better front to rear rejection in his KIO Hex Beam design, his design was chosen as a compromise my friend was willing to accept, and BUY!

The Force 12 owning ham in this thread bought the N6BT Force 12 design, because it does what HE wants it to do.

He appears to be as happy with his choice of antennas, as I am with my Hy Gain Explorer 14 semi trap design.

And, my other friend, with the 24 ft long Hy Gain 10 meter monobander, bought his Yagi, as the ONLY antenna on his tower! He sacrificed all the other HF Bands, to be "King Chit" on 10 meters.

Guess what, not surprisingly, it worked! It truly "messes with him" though, that he does not "kill us" with the big monobander.
But, never underestimate a determined Ham!
The last time we talked, he sent me to the M2 web page, to look at the 42 foot long M2 7 element 10 meter monobander  Cry













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W6GX
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« Reply #56 on: December 21, 2012, 10:58:01 PM »

And, my other friend, with the 24 ft long Hy Gain 10 meter monobander, bought his Yagi, as the ONLY antenna on his tower! He sacrificed all the other HF Bands, to be "King Chit" on 10 meters.

No offense to your friend's decision but why did he pick 10m and not 20m or 15m?  What does he do when 10m is closed?  If I could only have one band I'd have picked 20m.

But, I am "right in there" with the big boys.

The reason for this is because the law of diminishing returns.  Remember that most of the gain from a yagi comes from the first two elements.  When you add a third element and double the boom length you only add one additional db of gain.  Obviously three elements buy you more than just gain; it buys you a much better F/B ratio.

The last time we talked, he sent me to the M2 web page, to look at the 42 foot long M2 7 element 10 meter monobander

Tell your friend that instead of a M2 42' monobander he should get a long mast and stack two 24' boom monobanders on the same mast.  The stacked antennas will kick the M2's butt.

In summary, many folks often go overboard with large antennas and then realize the performance gain over small antennas is actually quite small.  Then they decide to get an even larger antenna.  The vicious cycle continues.

73,
Jonathan W6GX
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KY6R
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« Reply #57 on: December 29, 2012, 08:45:16 AM »

In my situation, stacking two 2 element yagi's (home brewed and designed using EZNec and also cross checked with YW, Yagi stress and even HFTA), and you will have what I show on my QRZ.COM page. I have the plots and the pictures there.

I chose 17M because I am 1 away from Honor Roll and wanted 6 dBd (+ if possible) on one band, and for the ones that I still need - and given that this cycle is as anemic as it is - 17M is the best band until we head down the cycle in a couple of years. (VOACAP used with all of the "rumors" that I know of as far as when certain entities that I need will be activated in the next year or so).

The stacking distance of my 17M monoband "2x2" is 1/4 wl - 54' for the top and 27' for the bottom.

No traps and the element spacing is optimized. I needed height and gain more than I needed F/B (look at the terrain map I have on my QRZ.COM page to see why I say this . . . .
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KA7NIQ
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« Reply #58 on: December 29, 2012, 09:43:58 AM »

In my situation, stacking two 2 element yagi's (home brewed and designed using EZNec and also cross checked with YW, Yagi stress and even HFTA), and you will have what I show on my QRZ.COM page. I have the plots and the pictures there.

I chose 17M because I am 1 away from Honor Roll and wanted 6 dBd (+ if possible) on one band, and for the ones that I still need - and given that this cycle is as anemic as it is - 17M is the best band until we head down the cycle in a couple of years. (VOACAP used with all of the "rumors" that I know of as far as when certain entities that I need will be activated in the next year or so).

The stacking distance of my 17M monoband "2x2" is 1/4 wl - 54' for the top and 27' for the bottom.

No traps and the element spacing is optimized. I needed height and gain more than I needed F/B (look at the terrain map I have on my QRZ.COM page to see why I say this . . . .
Wow, the bottom antenna is pretty low ? Would you not have been better off to use a single 4 or 5 element Yagi at 54 feet, vs the stack of 2's ?
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KY6R
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« Reply #59 on: December 29, 2012, 01:24:46 PM »

In my situation, stacking two 2 element yagi's (home brewed and designed using EZNec and also cross checked with YW, Yagi stress and even HFTA), and you will have what I show on my QRZ.COM page. I have the plots and the pictures there.

I chose 17M because I am 1 away from Honor Roll and wanted 6 dBd (+ if possible) on one band, and for the ones that I still need - and given that this cycle is as anemic as it is - 17M is the best band until we head down the cycle in a couple of years. (VOACAP used with all of the "rumors" that I know of as far as when certain entities that I need will be activated in the next year or so).

The stacking distance of my 17M monoband "2x2" is 1/4 wl - 54' for the top and 27' for the bottom.

No traps and the element spacing is optimized. I needed height and gain more than I needed F/B (look at the terrain map I have on my QRZ.COM page to see why I say this . . . .
Wow, the bottom antenna is pretty low ? Would you not have been better off to use a single 4 or 5 element Yagi at 54 feet, vs the stack of 2's ?


That's a great question. The AB-952 military mast is an absolutely incredible antenna "launching" system. It is way better than a push up mast, but not as good as a tri-legged tower. Anything large and top heavy - and you have to be careful how high you can go - for safety sake. I used to have an A3S up at 45' - and with a rotator, that was the limit.

I took the A3S down because it did what I wanted it to - but while I didn't worry about trap loss, I did worry about trap failure and the fact that all of those hose clamps would surely degrade and issues would pop up.

I found a C3S for $150, and that is going up as high as I can safely put it up - probably in the next couple of months.

The stack offers a way for me to get the higher antenna 55' up. There is no way in the world I can go that high with the C3S, even if I for go the rotator - which I will - to save on the "top heavy" factor.

I'm guesing the C3 will also go up at about 45', which is OK - but not as impressive as having this stack the way I have it now.

But that won't matter - because my "one solar cycle" goal of making DXCC Honor Roll will already be in the bag.
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