The Truth: How lossy are traps?

[AF5C]

I have seen all sorts of different figures and discussion on how lossy traps are in a yagi.  Where can I find the definitive source to settle this issue for once and all?  Anyone know the truth on this issue?

John AF5CC

[W0BTU]

From http://www.w8ji.com/traps.htm :
Q:  Do traps create noticeable loss, perhaps one dB per trap typically?

A: NO! Even the worse traps (coaxial traps) in the worse possible condition of operation are only 1.6dB loss for BOTH traps!

Conclusions:

    Trap loss has been greatly exaggerated by advertising hype.

[K0OD]

NO! Even the worse traps (coaxial traps) in the worse possible condition of operation are only 1.6dB loss for BOTH traps!

But trap Yagis may have 6 or more of them.

[K3VAT]

I have seen all sorts of different figures and discussion on how lossy traps are in a yagi.  Where can I find the definitive source to settle this issue for once and all?  Anyone know the truth on this issue?
John AF5CC

John,

W0BTU is correct, both in his conclusion and the creditable reference (W8JI site).  Many high quality and top performing yagi manufacturers (such as Mosley is probably the best example) have been using traps in their antennas for decades.  Just ask someone who has a Pro57 or Pro67.

I've used Unadilla traps for years.  They are well made and inexpensive.  I have three rotary dipoles: 12&17M (uses one pair of 12M traps); 20&30M (uses one pair of 20M traps); and at my old QTH 40&80M (one pair of 40M EURO Design).  I've worked DXCC on all of these bands.  In comparing my trap dipole with a fullsize dipole, performance wise I see no difference.  I also have a 40&80M vertical here using a custom Unadilla trap.

You can order custom-made traps from Unadilla (cost about $20 more).  My 40&80M vertical has a trap resonant at 6.5mhz.  If you're wondering why I chose this you can get the theory behind it at the same website URL as W0BTU noted.

One caveat with using the Unadilla traps: During times when running full power 1.5KW for continuous modes (such as RTTY), one must be careful not to exceed a transmit time of approximately 20 to 30 seconds AND then an off-period of at least the same time (20 to 30 seconds) before retransmitting again.  Forget about a rag-chew on RTTY with full power.

One of the few downsides of traps is limited bandwidth, but this isn't a problem for me as 1) I have a Palstar 2K tuner to touch-up those freqs outside my 1.5:1 SWR, and 2) I operate 95% CW so simply built my dipoles and vertical for that portion of the spectrum.

As pointed out in another one of my postings ... "traps have been given a bad rap, mainly by antenna manufactures who want you to think that their "trapless" contraptions are far superior to any antenna using traps."

GL, 73, Rich, K3VAT

[K3VAT]

NO! Even the worse traps (coaxial traps) in the worse possible condition of operation are only 1.6dB loss for BOTH traps!

But trap Yagis may have 6 or more of them.

Do you mean on one element or for the entire frequency coverage of the antenna?  Of course it depends on the number of elements and which elements have traps.  Which trap yagi are you referring to?

[AF5C]

Any of the common triband yagis like a Cushcraft MA5B.

John AF5CC

[WB6BYU]

NO! Even the worse traps (coaxial traps) in the worse possible condition of operation are only 1.6dB loss for BOTH traps!

But trap Yagis may have 6 or more of them.

That doesn't mean you lose 1.6dB in every pair of traps, however.

If you have your power divided among three dipoles in parallel and each
has a 1 dB loss, the total loss is still only 1dB.  That's because each
dipole only handles 1/3 of the power.

It's true that currents are probably higher in a yagi than a dipole,
so you may have 2dB of loss in a 3-element yagi if you had 1.6dB
of loss in a dipole.  But remember that 1.6dB was the worst case,
with coaxial traps (having a lossy capacitor) tuned to resonance
at the operating frequency (as opposed to slightly below it, which
reduces losses.)

By comparison the traps in a common tri-band yagi will have higher
Q capacitors, and lower losses.  A realistic number is probably
around 1dB, depending on the design.


Often the biggest difference in performance between a trap yagi
and a monobander is the compromised element spacings of the
former to get it to work well enough on all three bands.  This can
make more of a difference than the power lost in the traps.

[K3VAT]

Any of the common triband yagis like a Cushcraft MA5B.  John AF5CC

The Cushcraft MA5B has only ONE pair of traps per element http://www.cushcraftamateur.com/pdffiles/MA-5B.pdf.  See WB6BYU's posting.

Even the MFJ Cushcraft has traps much more efficient than a coax trap.

73, Rich, K3VAT

[M6GOM]

NO! Even the worse traps (coaxial traps) in the worse possible condition of operation are only 1.6dB loss for BOTH traps!

But trap Yagis may have 6 or more of them.

They might but they only have two on each side of the DRIVEN element. Those on the reflector and directors have little effect on loss.

[G3TXQ]

Those on the reflector and directors have little effect on loss.

Why would that be, when the Reflector and Directors typically flow similar current magnitude to the Driven element?

Steve G3TXQ

[K0OD]

"But trap Yagis may have 6 or more of them. "

Let's define our "trap Yagi" for this discussion as being a venerable Mosley TA-33: 3 elements each with two traps, one on each side of the boom.  

Surely a TA-33 would perform somewhat better on say 15 meters if you replaced the trap director and reflector with straight aluminum tubing appropriately tuned for 15. (yes, some tweaking of the trap DE might be required)

[WB6BYU]

Let's define our "trap Yagi" for this discussion as being a venerable Mosley TA-33: 3 elements each with two traps, one on each side of the boom.  

Actually (having just repaired my TA-33jr) there are two traps in a single
housing on each side of the boom.  The aluminum trap cover serves as the
radiator between the two traps as well as the capacitor plate for each of
them.

Surely a TA-33 would perform somewhat better on say 15 meters if you replaced the trap director and reflector with straight aluminum tubing appropriately tuned for 15.

Possibly, but you have to be clear how much of the difference is due to
losses in the trap and how much is due to a different radiator length.

The basic yagi element spacing isn't too bad for 15m.  The late W4RNL
showed that yagi elements can be shortened to 2/3 of full size with
little effect on efficiency (assuming that they have some type of loading
to maintain the proper reactance at the operating frequency.)

But the differences due to element length are NOT a factor of the trap
losses, which is the topic of discussion.  You can model such a yagi
using lossless inductors, then replace them with a realistic model of a
trap to get some sense of the actual difference.

So there really are at least two sets of changes that we see when we
make a trapped yagi:  the performance shift due to changes in the element
lengths and spacings, which are NOT a function of the trap efficiency,
(but rather a shift in the antenna pattern) and the loss of signal strength
due to actual power lost in warming the traps.  Traps are often accused
of having low efficiency due to the former, but only the latter should be
considered. 

That's why I said that the difference in performance between a trap yagi
and a monoband design is due to much more than the trap efficiency.

[WX7G]

Let's think about this. If a pair of traps have loss of 1.6 dB that is 230 watts per trap at 1500 watts RTTY. 230 watts would very quickly smoke any trap. Yet we see triband Yagi-Uda antennas rated for 1500 watts RTTY.

So is 1.6 dB loss per pair of traps a valid number? No.

[W6OU]

There was a ham (N6NB) who had a crank-up tower mounted on a trailer. He would drive to various ham stations and set up a reference antenna at the same height as the station antenna and measure relative antenna gain. He then compared the relative gains for different station antennas. His main purpose was to compare quad antennas vs yagi antennas but he also made measurements on tri-band trap yagi antennas.  The results were published in one of the ham magazines. He concluded that the trap yagis had a narrower bandwidth so when adjusted for optimum gain in the phone portion of the band, the gain was much poorer in the cw portion.

[G3TXQ]

Please let's not mis-apply W8JI's figures!

His loss figure of 1.6dB applies to a pair of RG58 coaxial 40m traps operating at resonance, for which he measured Rp as 17,800 Ohms; compare that to the Rp of 140,000 he measured for HyGain 10m and 15m traps!

The 1.6dB figure is not really relevant to the discussion unless you're going to build a Yagi using RG58 coaxial traps.

Steve G3TXQ