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[TowerTalk] Traps measured data

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Subject: [TowerTalk] Traps measured data
From: W8JI@contesting.com (Tom Rauch)
Date: Sat, 9 Feb 2002 15:36:41 -0500
There is freeware available for designing coaxial traps. I tested it on 
several traps, and it is pretty close. The only error it makes is it 
considers the capacitance in pF per foot! That means it calculates 
the trap slightly low in frequency, but that is a good thing anyway! 

The software is at:

http://www.nucleus.com/~field/

> I confirm Dec 84 QST includes an excellent article about traps
> constructed of coax.  I built a pair for 40M with RG-58 on 2.5"
> plastic pipe and they have been in service since 94.  The bandwidth is
> narrow (I don't have the figures in hand), which implies high Q and
> low loss.

Narrow bandwidth does not mean loss is low. That is a very popular 
but **very** incorrect Ham's tale.

I measured various traps, the frequency of the trap in MHz is first, 
the equivalent parallel resistance in ohms at resonance is next. 
Higher equivalent parallel resistance means lower loss. The 
resistances at resonance were:

Coax RG-58
7.034
17,800

Same trap as above but using fixed mica 
capacitor instead of coax for capacitor
  
7.040
21,660 (up from 17,800)

Coax UT-141-75 semi-rigid cable (copper tubing 
coax)
7.045
45,330

(from handbook)
100pF 7.5kV doorknob & #12 wire airdux
7.040
99,850

(from handbook)
60pF 15 kV doorknob & #10 wire airdux
7.040
250,000

(homebrew)
60 pF vac & Copper tubing
7.040
300,000

(samples constructed and submitted by 
towertalkian)
Coax RG-58
3.700
23,200

Conclusions:
1) Coaxial trap poorest
2) Once #10 wire is used, not much improvement 
going larger
3) Space-wound bare wire makes best inductor
4) Transmitting-type capacitors noticeably 
better than capacitors made from coax           

For 10 Meter traps in tribanders (samples 
submitted by towertalkians):

Coax RG-58
29.00
13,800

Mosley TA-33
30.64
43,100

Mosley Pro-57 
27.46
66,080

Cushcraft A3
28.78
110,000

Hy-Gain TH-3
29.67
140,200

You can see a Mosley TA33 trap has significantly better 
performance than a coaxial trap, and isn't all that bad.

Models using measured trap parameters: 

EZNEC #12 dipole
Coax trap 80m 2:1 VSWR   ~210 kHz
Total trap loss = 0.05 dB
(Insignificant loss on lower band where the trap 
is passive. ALL traps behave that way as you 
will see)

When actually trapping or decoupling he antenna 
ends, things get worse:

Coax trap 40 meter 2:1 VSWR ~ 80 kHz
Total trap loss = 1.6 dB

Loss off-resonance (7.15 MHz)  = 1.06 dB
Loss is maximum at trap resonance

W2LH ARRL Handbook
100pF #12 Miniductor trap
40m 2:1 VSWR  ~120 kHz
Total load loss = 0.24 dB

(Notice the W2LH trap on 40 meters is 
significantly BROADER, yet has significantly 
less loss!! So much for lowloss=narrow bandwidth 
rumors)

W2LH trap design pass-through performance on 80m:
80m 2:1 VSWR  BW ~ 200 kHz 
Total trap loss = 0.026 dB

Trap loss is maximum at the frequency where the trap is resonant, 
and decreases off resonance. It is actually better to set the trap 
high or low in the band or even outside the band, away from the 
operating frequency, and "prune" the antenna for low SWR without 
touching the trap's resonant frequency.

Setting the trap for resonance INSIDE the band increases trap loss 
and reduces power rating!


73, Tom W8JI
W8JI@contesting.com 

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