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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