trap antennas coaxial trap coax dipole antenna loss resistance

Traps

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This article is from a talk I gave about antenna traps. It contains measurements
of traps, performance of trap antennas using models of traps, and ideas on how
to make trap antennas more efficient. 

Try taking this Trap-Q test! Be honest.

1.)     
Is
it best
to make the trap resonant close to the desired operating frequency?

2.)     
Does bandwidth decrease with increasing trap Q?

3.)     
Do traps create noticeable loss, perhaps one dB per trap
typically?

4.)     
Does higher trap operating Q always mean lower loss?

Coaxial Trap Designer by VE6YP (Tony Fields)

(I do not warrant the actual program. I only offer measurements
compared to the program results.)

This
is a good
program to get you in the ballpark with a trap design. It
was available as freeware. (Unfortunately coaxial traps are relatively lossy on the
trapped frequency compared to other types.)

The
software is available at
http://www.qsl.net/ve6yp/ 

 7.04
MHz      
3.5 inch diameter form 
RG58/U into the VE6YP program yields calculated values of:

Calculated                                      
Actual
Measurement

L=
3.689mH                                       
3.116mH

C=
138.5 pF                                      
164 pF   

64
inches                                          
59 inches

Using
the
program TLA
by N6BV (from ARRL), we would
estimate capacitance of a 59″ RG-58/U cable as:

Measuring
a real-world stub, capacitance was 164pF
(Q=590).

While that Q seems high, remember a typical transmitting-type
air-variable capacitor has a Q of several thousand!   

Coaxial
Trap Articles and Programs use capacitance/ft multiplied times length….

26
pF * 4.917 feet 
= 127.84 pF in trap program

C164
pF measured. This error,
36pF low from 164pF, occurs because the transmission line making up the “coaxial
capacitor” is not actually treated as a transmission line in the modeling
program.

Fortunately
the
error is in a useful direction, because we can shorten the cable! Coaxial
capacitors are really open stubs, and should be treated that way once they are
more than a few degrees long. 

CONCLUSION:
The
difference between TLA and an actual measurement was around 4%. This is very
close, but the result has significant difference from the coaxial trap program
since it only considers pF per foot as the capacitance. A longer cable (in
fraction of a wavelength) results in greater error by using pF per foot. The
error comes because a coaxial cable capacitor is really a stub, NOT a pure
capacitor!!

Trap
Measurements (at resonance)

Highest
R parallel equivalent is best!!
Lower Rp means more loss. 

Trap
Measurement summary:

• Coaxial
  trap poorest

  
  

• Once
  #10AWG
  wire is used, not much improvement

  
  

• Space-wound
  bare wire makes best inductor

  
  

• Transmitting-type
  capacitors noticeably better than capacitors made from coax          

  
  

10 Meter (Tribander) Traps

Traps
are not all that bad when you plug them into models.

15 Meter
(Tribander) Traps

Trap Model

                           
R             
L

                                         
 C

Measured
Values Coax 7 MHz Trap

Measured Values L/C 7 MHz Trap

 

SWR Bandwidth

7
MHz RG-58 TRAP

80 m 75 ohm VSWR

EZNEC
#12AWG
dipole

Coax
trap 80m 2:1 VSWR   ~210 kHz

Total
trap loss = 0.05 dB

RG-58
TRAP, 75 ohm VSWR, 40 METERS

VSWR
BW

 


 Coax
trap 40 meter 2:1 VSWR ~ 80 kHz

Total
coaxial trap loss at
resonance on 40m= 1.6 dB

Total
coaxial trap loss 100kHz off-resonance (at 7.15 MHz)= 1.06 dB

Note that loss is maximum at trap resonance!!! 

Never make a trap resonant on the desired
operating frequency!!  

W2LH ARRL Handbook Trap Design

100pF #12awg Miniductor trap

 40m 2:1 VSWR 
~120 kHz

Total loss =
0.24 dB

 W2LH
ARRL HANDBOOK TRAP 80m  VSWR                   

80m
2:1 VSWR 
BW ~ 200 kHz

Total
trap loss = 0.026 dB

 What
happens if trap is not in band?

VSWR Bandwidth
of 6.51MHz trap in 80/40 dipole  

Trap at 6.51 MHz Q=130

Loss at 7.15 MHz 
= 0.314 dB

Loss at 3.7 MHz   
= 0.324 dB

This is a 104-foot long antenna, with very
poor Q traps, and loss is less than .4dB! The reason loss is low is we have
moved the trap slightly out-of-band.

 6.15 MHz Q=130
TRAP 40m VSWR

 
7
MHz 2:1 VSWR BANDWIDTH ~200kHz

Trap
Q at resonance = 130    
7 MHz loss ~ .3 dB

 6.15 MHz Q=130
TRAP 80m VSWR

80M 2:1 VSWR BW ~130 kHz

Loss at 3.7 MHz  
= 0.324 dB

1.)     
Is
it best
to make the trap resonant close to the desired operating frequency?

NO!
Loss is highest when the trap is resonant at the operating frequency!

2.)     
Does bandwidth decrease with increasing trap Q?

NO! Bandwidth is a function of many variables,
trap Q actually has one of the smallest influences on BW.

3.)     
Do traps create noticeable loss, perhaps one dB per trap
typically?

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

4.)     
Does higher trap operating Q always mean lower loss?

NO!
Loss depends on many factors, including trap resonant frequency.

Conclusions:

• Trap loss has been greatly exaggerated by advertising hype

• Traps should not be resonant at the actual planned operating
  frequency

• Coaxial traps are more lossy than articles conclude

• Coaxial stubs used as capacitors can not be calculated using
  pF/ft unless the stub is a very small fraction of a wavelength long (less
  than a few electrical degrees)

• Coaxial stubs have low Q (are relatively lossy) compared to
  normal lumped components.