Traps

            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:

R .22  –j143.61 or about 157 pF (Q=650)

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.