by Martin E. Meserve
Multi-Band Coax Trap Antenna Design
This page is intended to help design Coaxial Cable Antenna Traps for a multi-band trap antenna.
The first part is just a short introduction into trap antennas. Next is a section that provides
dimensions and material specifications for a set of Coaxial Cable Traps. Following that is a section
that allows the user to create a custom trap using their specified frequency, coax, and coil form.
That can be useful when the user wants to change the trap frequency, use a different coaxial cable,
or has different coil form material than what is specified in the second section. The program behind
the web page then calculates the necessary physical dimensions to realize the trap.
A trap antenna uses L/C parallel tuned circuits to provide multiband
dipole antenna that effectively switches between bands automatically. The input impedance on each
of the design bands will be such that a tuner will not be needed when going from one band to another.
A trap dipole, in it's simplest form, might be a two band antenna for say, 80/40 Meters, as in Fig. 1.
The total length of the 40 Meter section of the antenna is calculated by the usual formula, listed on the left and right.
These simplified equations are based on the standard wavelength formula of
λ(m) = 299,792,458/Frequency (Hz) and accounts for a Velocity Factor
of 0.95. For 7.1 MHz that would be approximately 66 feet or 33 feet for each side of
the Balun. The trap, which is tuned to 7.1 MHz, will present a high impedance and effectively isolates the 40 Meter
section of wire from the extended 80 Meter wire. Leaving you with a standard 40 Meter dipole.
However, at 80 Meters, the trap, which is no longer resonant, acts as a loading coil and shortens the wire required to get
resonance on the 80 Meter band. Depending on the coax used, this might shorten each side of the dipole by 4 to 5 feet. This may
not seem like a big deal to some, but when your antenna space is limited, it may be the difference between having an antenna
for 80 Meters, or not. A trap dipole for 3, 4, or 5 different bands, in a Inverted-V configuration, could very easily be
the solution you need.
So the trap serves two purposes. One, to divorce one section of wire from another. And two, to effectively shorten the
overall length of the antenna. It's not all good and no bad. The traps have loss and are not perfect at divorcing one
piece of wire from another.
A common multiband trap dipole might be one for 80, 40, 20, 15, and 10 Meters. This antenna would require eight traps.
Overall, the antenna works pretty good. I've built and used one myself. While the antenna is almost full size on 10 and
15 Meters, the other bands are shortened considerably. The antenna is shortened by almost 12 feet on each side. Instead
of needing 133 feet of space for 80 Meters, you would only need a little over 100 feet. In a Inverted-V configuration,
even less space would be needed. That could mean the difference between having a 80 Meter antenna or not.
There are some losses associated with having this many traps. But with the segments adjusted properly,
you should be able to get a good match on each band. This means easy band switching without the need for a tuner.
However, if you do have some available space you might want to think about the configuration on the right. The configuration
shown get around some of the problems that the previous configuration might have. By having a separate antenna for
80/40/15 Meters and 20/10 Meters the number of traps is reduced and their associated losses. This also maximizes the amount
of wire that is actually in the air, giving you the best signal possible. Of course, this does mean you need a bit more space
and the extra lines need to be tied down.
Relative advantages and disadvantages of trap antennas compared with separate antennas per band.
The table below provides dimensions for eight traps between 3.5 and 30 MHz. These dimensions assume RG-8X/9258 Coaxial
cable and 2" PVC stock (2.375" OD). The form lengths provided include 1" beyond each side of the coiled coax.
All traps are close wound and should be tight, to ensure mechanical stability.
The traps in the table are designed for the approximate center of each band.
Traps are normally designed for the for the low
You could simply use the traps defined in the table, in the previous section. But this might
limit some users who are trying to work with what is on hand. The calculator below allows you to create
a trap using almost any available coaxial cable and coil form, within reason.
The sections that follow will calculate the physical properties for an antenna trap made from coaxial cable.
It calculates Inductive Reactance (XL), Capacitive Reactance (XC),
Inductance (L), and Capacitance (C) values.
Plus the cutting information for the coil form, the coax, and the "Effective Length" of the trap. The "Effective
Length" is the length that can be deducted from wire for the next lower band. Cutting and trimming dimensions for
the coax are also included. Inputs required are coax type, coil form diameter and trap frequency.
Start by deciding which bands you would like to use and then start with the trap for the highest frequency band.
This is because the highest frequency band operates as a full size dipole, whereas the wires for other bands are
effectively shortened by the inclusion of one, or more, traps (Effective Length).
Note: For each design frequency you will need to make two traps. One for each side of dipole.
Coax Trap Physical Requirements
From the drop down list, select the coaxial
cable you wish to use for the coax trap. "*" denotes Foam dialectric.
RG-6/8215, 20.5 pF/ft., 0.275" OD
RG-6/U/1189, 20.5 pF/ft., 0.27" OD
RG-8/9913, 24.6 pF/ft., 0.405" OD
RG-8/102, 24.0 pF/ft., 0.405" OD
RG-8/DRF-BF*, 24.5 pF/ft., 0.405" OD
RG-8/WM CQ*, 24.5 pF/ft., 0.405" OD
RG-8/9914, 24.8 pF/ft., 0.405" OD
RG-8/8237, 29.5 pF/ft., 0.405" OD
RG-8X/118*, 25.0 pF/ft., 0.242" OD
RG-8X/9258, 25.3 pF/ft., 0.242" OD
RG-9/8242, 30.0 pF/ft., 0.42" OD
RG-11/8213*, 17.3 pF/ft., 0.405" OD
RG-11/8238, 20.5 pF/ft., 0.405" OD
RG-58/124, 28.5 pF/ft., 0.195" OD
RG-58/8240, 28.5 pF/ft., 0.193" OD
RG-58A/8219*, 26.5 pF/ft., 0.198" OD
RG-58A/8259, 30.8 pF/ft., 0.193" OD
RG-58C/8262, 30.8 pF/ft., 0.195" OD
RG-58U/7806, 20.5 pF/ft., 0.195" OD
RG-59/8212, 17.3 pF/ft., 0.242" OD
RG-59B/8263, 20.5 pF/ft., 0.242" OD
RG-62A/9269, 13.5 pF/ft., 0.26" OD
RG-62B/8255, 13.5 pF/ft., 0.26" OD
RG-62B/9857, 9.7 pF/ft., 0.405" OD
RG-142B/83242, 29.2 pF/ft., 0.195" OD
RG-174/8216, 30.8 pF/ft., 0.101" OD
RG-213/8267, 30.8 pF/ft., 0.405" OD
RG-214/8268, 30.8 pF/ft., 0.425" OD
RG-216/9850, 20.5 pF/ft., 0.425" OD
RG-217/217, 30.8 pF/ft., 0.545" OD
RG-218/218, 29.5 pF/ft., 0.87" OD
RG-223/9273, 30.8 pF/ft., 0.212" OD
RG-303/84303, 29.2 pF/ft., 0.17" OD
RG-316/84316, 29.0 pF/ft., 0.098" OD
RG-393/393, 29.4 pF/ft., 0.39" OD
RG-400/400, 29.4 pF/ft., 0.195" OD
LMR200*, 24.5 pF/ft., 0.195" OD
LMR240*, 24.2 pF/ft., 0.242" OD
LMR400*, 23.9 pF/ft., 0.405" OD
LMR500*, 23.9 pF/ft., 0.5" OD
Enter the Trap Frequency:
Choose any one of the coil forms listed in the table, or select "Manually Enter Diameter" and enter you own.
The form radius should not exceed the cable minimum bend radius.
Prepare the Coil Form - The drawing on the right,
Fig. 4, shows the form used for winding the Coax Trap. The coil form
in the drawing is shown as split, to indicate an indefinite length. But it's also to indicate
that the ends may also be rotated from each other. The number of turns may not be exactly an integer
number of turns. The dimensions are brought down from the previous section,
Coax Trap Outputs and shows the Coil Form
and a Support Strip.
The Support Strip is made from a section of PVC pipe, cut length wise into four
parts. The Coil Form and Support Strip are curved but
one should fit into the other without a problem. In the drawing, the Support Strip
looks to be about 1 inch wide, but the actual width is not important. The strip provides some extra support
for the form and makes it a little easier to wire and connect to.
In Fig. 4, I show 1/4-20 hardware used
on the support strip and 6-32 hardware used to secure the coil form to
the support strip. Use whatever hardware that is conveinent to you. To secure the coil form to the
support strip I use a 6-32 screw, nut, washer, and solder lug. The solder lug is
on the inside, where I attach the wire from the coil. I then loop a short wire to the
The intent is that, a length of coax, longer than is required for the actual trap, is used as a
sample winding to determine the exact position of the second coax hole. Wind the necessary number of turns,
like 6.5 turns, and mark the position of the second coax hole.
I start by preparing the coil form and the support strip.
Prepare the Coax - Fig. 5, below, is a cutting
diagram for the coax. As with the previous drawing, the calculations are brought down from the previous
section. The estimated coil form length is calculated by rounding up the number of turns to the nearest whole turn.
Then I multiply that number by the coax diameter. To account for the support holes, I add 2 inches. For the
trap listed above, the number of turns, rounded up to the nearest whole turn, is
and the coax diameter is x.
After adding another 2 inches, the estimated length of the coil form is approximately
Start by measuring and cutting a section of coax as specified in Fig. 5
listed as Length of Untrimmed Coax. Then carefully remove the jacket
as specified in the drawing as Trimmed Shield Length. Note that the
"In" end and the "Out" end are different
lengths. The reason is due to the wiring of the trap. The center conductor on the
In is wired over to the Out end shield.
This provides the space necessary to accomplish the wiring.
Personally, I just cut all of the forms to around 6 inches and then trim them down
after the trap is wound. - K7MEM
This program is based on the Ham Calc Program called "COAXTRAP.BAS - Antenna Trap Design",
written by George Murphy, VE3ERP. George Murphy's program was an adaptation of a program by Larry East, W1HUE, as it appears
in the ARRL Antenna Compendium, Volume 2, page 100.
I also gleaned some information for this page from a article in the October 1981 issue of Ham Radio Magazine, named
"Trapping the Mysteries of Trap Antennas" by Gary E. O'Neal, N3GO.