Multiband ‘trap’ antenna
(Published in Electron #12, 2000 and Electron #1, 2001)
The story is familiar. After obtaining your radio-amateur license you like to be 'on the air' as soon as possible. Already for month you have been looking though piles of information for affordable ham gear and you finally have bought a set a little bit beyond budget. Next question is the antenna and for this you first ask around because antenna practice is still something to be gained and a commercial full-size tower-cum-multiband beam is currently out of reach. Actually, you are looking for a system that will cover a maximum number of (HF-) amateur frequency bands for a minimum of effort requiring as little 'real-estate' as possible. The build-in tuner of your new ham-gear should be able to cover all impedance excursions of this antenna since you like to play with this expensive piece of equipment for quite some time to come. Unfortunately this type of tuner only covers a small range of mismatches, usually not beyond SWR 1 : 3 - 4. Furthermore you would like to feed this antenna with coaxial cable, since this is easiest to handle and relatively insensitive to 'external conditions' like the weather, metal obstacles like roof trimming, birds nests etc.
The above considerations were background to the design of the multi-band trap antenna in this article, that should cover the as many as possible of the 'classical' HF radio-amateur bands.
Simplicity does it
One of the simplest and yet useful antenna designs is the dipole; a center-fed antenna that will resonate on every frequency for which the electrical antenna length is a multiple of 1/2 wave length. At every odd-multiple thereof the terminating impedance is 'real' and of a 'low' value, suitable for connecting to 'low' impedance transmission line and the thing to go for when also a simple antenna tuner is to be applied.
the HF amateur bands are not so nicely odd-multiple related for one dipole to
fulfill the above relationship. As an example let's look at an antenna of 2 x
Next best approach could be an off-center fed dipole like a Windom-variation, using a transformer to 'translate' to a low value. Again, the odd relation of wavelength' will not permit one antenna (or one specific transformer ratio) to 'do the trick' of making the antenna to perform within the framework as described earlier.
An other variation to this theme may be the multi-dipole solution, each cut for a specific frequency (range) and all connected to the same balun, as only the dipole on its fundamental resonance frequency will exhibit a low enough impedance to do most of the radiation (most current). This will certainly lead to a workable situation although tweaking the system for optimal performance on each amateur band may be somewhat problematic as all antenna's are operating in each-others near-field area (mutual influence). Furthermore, a five dipole construction may not be called a simple antenna system anymore.
different approach to the dipole theme is the W3DZZ - type of trap dipole,
originally developed from a series of practical trials to obtain an antenna
for multiple amateur frequencies, to be fed with high impedance transmission
line and be used in connection with tube-type transmitters with a higher
termination impedance than contemporary transistor equipment. When using this
type of antenna with a balun into low-impedance coax, the SWR will not be low
enough on 20, 15 and
Difference of the trap dipole to other dipole solutions is, that the first is consisting of much more parameters to play with, e.g. inside length, trap inductor, trap capacitor, outside length. In principle these four (independent) variables in theory should be sufficient to solve the resonance requirements on four different frequencies, compared to a basic dipole that has only one parameter to play with. It is precisely from this view point the antenna discussion in the rest of this article has been derived.
In these days multiple antenna design programs are available, both at a price or as free-ware, together with personal computers to take the burden out of multiple calculations of the same nature. Using these tools, various approaches have been investigated based on the model in figure 1, all to fulfill the basic requirement of resonating on at least four different radio-amateur bands.
In the modeling phase, the trap is consisting of an ideal, lossless inductor in parallel with an ideal lossless capacitor. Further references in this article will be to this model and notation.
it seemed a good idea to start from basic elements of the W3DZZ, I have
looked into various editions of the design. Looking in detail, one may come across
different designs depending on publication source. For this model the details
as given by the ARRL Antenna Handbook have been used, taken as; L2
the W3DZZ at an antenna height of
freq. reson. R X SWR gain elevation
band freq. (Ohm) (Ohm) (re 50) (dBi) angle
80 3.531 35.7 0 1.4 6.2 38.5*
mid band 3.7 45.8 145 11.1
40 7.271 83.3 0 1.7 6.1 25.5*
mid band 7.05 65.5 -102 5.1
20 15.35 257 0 5.1 7.2 27
mid band 14.175 264.1 -553 28.6
15 22.363 115.7 0 2.3 8.9 18.5
mid band 21.225 134.1 -484 38.0
10 32.525 150 0 3.0 10.9 13
mid band 28.85 931.7 -1491 65.8
Table 1: Performance of the W3DZZ as modeled according to the ARRL Antenna Handbook details
the 80 and
Looking at this table one finds the antenna mostly resonating outside the (radio-amateur) band limits. Also, regarding the behavior at mid-band frequencies it is clear why a simple, build-in antenna tuner will have problems handling the antenna, SWR is (sometimes far) outside the range of these simple devices. Solving these problems with parallel dipoles cut for specific frequencies will limit maximum antenna gain to around 6 dBi, where the W3DZZ is capable of delivering up to 11 dBi of gain, almost one S-point more.
Modeling the antenna at different antenna heights or above different soil types does not solve the problem; SWR and resonant frequency are hardly changing. Radiation angle will vary though, as this parameter is related to the combination of the direct and ground-reflected wave.
Discussing matters with L.B. Cebik (visit his rich web-site!) leads to the conclusion that the W3DZZ has been design for use with the pi-filter output stages of tube transmitters, that were much more permissive to odd termination impedances.
On his web-site L.B. shows more examples of trap-type antenna's. Basic principle usually is that the total length of the antenna is resonating at a lower frequency and the inner side at a higher frequency, with the parallel L-C trap to decouple the lower frequency part. This effectively turns a trap antenna into a two-band system.
discussed earlier, a trap antenna consists of four independent variables, so
it should be possible to have the antenna resonate at four different
frequencies. I have tested this premises modeling the antenna at
# fr. L
2 L 1 l tot. L C
trap (m) (m) (m) μH pF band band
1 6.0 7.1 8.8 31.8 4.5 156.4 23.862 32.840
2 6.4 8.4 7.9 32.6 4.7 131.6 23.072 32.278
3 6.8 9.6 7.3 33.8 5.1 107.0 22.080 31.396
4 7.0 10.1 7.0 34.2 5.2 99.4 21.774 31.034
5 7.2 10.6 6.7 34.6 5.0 97.7 21.390 30.480
6 7.4 11.1 6.4 35.0 5.0 92.5 21.302 30.181
7 7.6 11.6 6.1 35.4 5.2 84.3 21.227 29.801
8 7.8 12.1 5.8 35.8 5.2 80.0 21.073 29.294
9 8.0 12.6 5.3 35.8 5.4 73.3 21.334 29.186
Table 2: Multi-band antenna variations
At a lower trap resonance frequency than 6 MHz., it becomes difficult for the system to fulfill the resonance requirement at the basic amateur bands; with trap resonance above 8 MHz. this again is a problem.
It is clear that the solution space is continuous, although models have been calculated in steps. It is further interesting to notice that trap inductance is only varying marginally.
at the upper two radio-amateur bands, one may notice that the
Antenna gain and elevation angle
To complete this first round of analysis, I further have looked into the antenna gain of the above models. Of cause this (maximum) antenna gain is in a different direction for each amateur band, at the higher bands in a multi-lobe structure with deep 'nulls' in between. Nevertheless it is clear that more antenna gain will be available with more wavelength on the antenna adding to total radiation.
Model # 1 2 3 4 5 6 7 8 9
80 6.2 6.3 5.8 5.8 5.9 5.9 5.9 5.9 5.9
40 6.4 6.3 6.2 6.1 6.0 5.9 5.7 5.6 5.4
20 7.0 6.9 6.9 7.0 7.0 7.0 7.1 7.1 7.1
15 9.7 9.5 9.5 9.3 9.2 8.9 8.8 8.5 8.4
10 10.6 10.5 10.6 10.7 10.7 10.8 10.8 10.7 10.5
Table 3. Antenna gain (dBi) per model and per amateur band.
table 4 the elevation angle of maximum radiation has been calculated. This
angle is the (vector) sum of direct and (ground) reflected energy. At the
fixed antenna height of
Model 1 2 3 4 5 6 7 8 9
80 38* 38* 38* 38* 38* 38* 38* 38* 38*
40 26.5* 26.5* 26.5* 26.5* 26.5* 26.5* 26.5* 26.5* 26.5*
20 29.5 29.5 29.5 29.5 29.5 29.5 29.5 29.5 29.5
15 17.5 18 19 19 19.5 19.5 19.5 20 19.5
10 13 13 13.6 13.6 14 14 14 14.5 14.5
Table 4. Elevation angle.
Comparing table 4 and table 1, very little difference will be noticed, further underlining the statement that the elevation pattern is mainly determined by the height above ground and ground type.
Feed point impedance
A further interesting antenna parameter is feed point impedance as this is an important part of our starting position (low SWR). In table 5 one may find the feed point impedance of the antenna models at the center of each radio-amateur frequency band where the antenna has been designed to resonate (lower three bands), or at the exact resonance frequency (higher two bands).
Model 1 2 3 4 5 6 7 8 9
80 34.9 38 44.9 45.5 46.6 47.3 42.8 48.6 49.6
40 116.3 100.8 87.5 82.5 77.7 72.5 67.7 63.7 59.9
20 151.8 182.7 211 215.8 219 208.7 204.4 195.6 179.7
15 127.3 114.3 112.5 116.9 122.8 134.2 149.5 164.2 183.5
10 133.3 153.2 153.3 144.7 137.8 129 123.7 122.5 127.2
Table 5. Feed point impedance (real value; resonance)
It is interesting to notice a certain structure in the feed point impedances over the frequency bands and the different models. First two rows are showing a constant rising or descending tendency, the next three having a more wavy structure. It is also clear that except for the lowest band, all antenna's are exhibiting an impedance above 50 Ohms, even up to (and a little above) 200 Ohms. To obtain lowest overall SWR when connecting to one of these this antenna's, a good choice may be to have a transformer connecting the antenna to 50 Ohm coaxial line, at an impedance transformation ratio half-way highest and lowest antenna impedance, e.g. 125 to 50 Ohm (2,5 : 1).
Although this appears a somewhat odd ratio, one will find a good proposition in Jerry Sevick's book on "Transmission line transformers" (ISBN 1-884932-66-5) in his 1 : 2,25 model, showing high efficiency over all frequencies of our wish list and above. An analysis of this transformer may be found at "Transmission-line transformers".
multiband antenna we are looking for, is to be used within the tuning range
of a build-in auto tuner, e.g. SWR< 4. Let's see how our models perform within
those limits when connected to the above mentioned 1 : 2,25 impedance
transformer, i.e. related to a system impedance of 112,5 Ω. Since the
lowest model numbers did not perform well on
Model 4 5 6 7 8 9
80 3.591 3.608 3.629 3.563 3.592 3.580
3.821 3.856 3.890 3.802 3.872 3.928
40 6.864 6.890 6.912 6.918 6.946 6.977
7.343 7.314 7.294 7.269 7.254 7.243
20 13.775 13.726 13.732 13.725 13.692 13.658
14.731 14.709 14.700 14.718 14.720 14.715
15 21.289 21.037 20.836 20.693 20.616 20.861
22.345 22.070 21.851 21.692 21.599 21.842
10 30.480 30.076 29.656 29.217 28.789 28.684
31.657 31.236 30.796 30.337 29.875 29.743
Table 6. Boundary frequencies within SWR = 4 limits.
From table 6 it may be concluded that starting form model 6, all antennas comply well with the 4 -bands target. In fact model 9 also covers a large part of the ten meters band without sacrificing performance on lower frequencies, so this should be the model to go for. It's clear however, that four variables may be selected to cover four bands but we need a fifth variable when we also would like to include this fifth frequency band.
Sensitivity to ground conditions
All models have been designed at ten meters above average ground, i.e. conductivity 5 mS/m and ε is 13. If this is to be an explicit requirement, only those living on such average ground could profit. Let's find out how the best model from our earlier tests is performing above different ground conditions, i.e. 'good' at 20 mS and ε is 20 (flat country and high moist soil) and bad conditions at 1 mS/m. with ε = 5 (rural, densely populated), as in table 7.
good average bad
band soil soil soil
80: fr (MHz) 3.731 3.738 3.752
Zo (Ώ) 39.0 49.6 61.6
gain (dBi) 7.3 5.9 4.4
elevation (degree) 40.5* 38* 33.5*
40: fr (MHz) 7.063 7.069 7.076
Zo (Ώ) 59.6 59.9 59.3
gain (dBi) 6.2 5.4 4.3
elevation (degree) 29.0* 26.5* 23.0*
20: fr (MHz) 14.180 14.173 14.165
Zo (Ώ) 184.7 179.7 173.8
gain (dBi) 7.7 7.1 6.4
elevation (degree) 30.0 29.5 28.5
15: fr (MHz) 21.335 21.334 21.335
Zo (Ώ) 184.7 183.5 183.7
gain (dBi) 8.9 8.4 7.8
elevation (degree) 19.5 19.5 19.0
10: fr (MHz) 29.190 29.186 29.180
Zo (Ώ) 126.8 127.2 126.8
gain (dBi) 10.8 10.5 10.0
elevation (degree) 14.5 14.5 14.0
Table 7. Sensitivity of model 9 to ground conditions
The antenna model is not very much influences by ground-type as far as resonance frequency is concerned or connection impedance. Antenna gain (maximum) is diminishing marginally with worse ground conditions which is to be expected as this is the vector summation of ground and reflected wave energy. In the same manner the elevation angle is influenced.
Sensitivity to antenna height
To find out the sensitivity to antenna height, I have modeled model 9 at different levels above average ground conditions, as may bee seen in table 8.
height (meters) 10 12.5 15 17.5 20
80: fr (MHz) 3.738 3.736 3.734 3.740 3.748
Zo (Ώ) 49.6 57.1 64.1 72.4 79.1
gain (dBi) 5.9 6.4 6.6 6.2 6.2
elevation (degree) 38* 35.5* 32.5* 29.0* 25.5*
40: fr (MHz) 7.069 7.078 7.087 7.094 7.096
Zo (Ώ) 59.3 66.1 67.4 63.3 56.7
gain (dBi) 5.4 5.3 5.5 6.0 6.7
elevation (degree) 26.5* 21.5* 40.0 34.0 29.5
20: fr (MHz) 14.173 14.164 14.127 14.123 14.133
Zo (Ώ) 179.7 155.8 153.4 163.3 166.8
gain (dBi) 7.1 8.1 8.3 8.2 8.3
elevation (degree) 29.5 23.5 19.5 17.5 15.0
15: fr (MHz) 21.334 21.342 21.345 21.322 21.341
Zo (Ώ) 183.5 191.2 182.8 185.4 191.9
gain (dBi) 8.4 8.6 9.2 9.2 9.1
elevation (degree) 19.5 15.5 13.0 11.5 10.0
10: fr (MHz) 29.186 29.169 29.176 29.173 29.162
Zo (Ώ) 127.2 125.2 128.4 127.7 127.1
gain (dBi) 10.5 11.0 11.0 11.2 11.3
elevation (degree)14.5 11.5 9.5 8.5 7.5
Table 8: Sensitivity of model 9 to antenna height
As with table 7, we find no dramatic deviations from the basis antenna characteristics. Main difference are in the elevation angle of maximum radiation, so this parameter should be considered for a particular application.
we have modeled a four band antenna, based on four variables, for 80, 40, 20,
Feeding this new model into an antenna design program, we obtain the following table.
80 40 20 15 10
fres.(MHz) 3.616 7.002 14.066 20.824 28.648
Zo (Ώ) 48.0 59.9 193.2 181.0 120.6
gain (dBi) 5.8 5.3 7.1 8.4 10.6
elevation (dgr) 38.5* 26.5* 29.5 20 14.5
SWR < 4 3.520 6.918 13.586 20.377 28.176
between: 3.801 7.208 14.587 21.305 29.196
Table 9: Trap-antenna with top-capacitors (model 10)
As in previous sensitivity models, parameters have shifted only marginally and it appears we have designed a truly practical five band antenna, that will exhibit SWR < 4 on and over all of the 'classical' HF-bands.
The antenna to fully comply with the design goals for a four band antenna with SWR < 4 is model seven as in figure 3.
model describes the antenna for 80, 40 20 and
The antenna design to cover much of five HF amateur frequency bands may be seen in figure 4.
this design, also all frequencies within the 80, 40, 20, and
Extended five-band design
full five-band solution may be found in figure 5, which is the antenna system
of figure 4 with top capacitors applied. The
This design will cover all five classical HF amateur bands within SWR < 4 limits. As the design is equal to figure 4 except for the top capacitors, both may be tested using identical components.
Also this design will have to be corrected for the velocity factor of 0,89, bringing dimensions to: L1 – 4,72 m. and L2 – 11,21 m. with total antenna length to 31,87 m., excluding traps and transformer.
method to make traps in general and the traps for this design in particular,
is to take a short piece of pvc drainage pipe e.g.
may check this value by connecting a series resistor of 22 Ω and connect
this series circuit to a HF generator set to 648 kHz. (calibrate against BBC
At these low frequencies, parasitic effects will not yet be too noticeable.
value may be found by resonating with the trap coil at 8.00 MHz., e.g. using
a dip meter, calibrated against the transceiver. A good way to make and tune
this capacitor is to use a piece of RG58 coax cut to resonance, as this will
make a very good high-voltage capacitor. When using RG58U, this piece will be
In my test antenna I drilled a small hole in the end caps to allow a short piece of nylon rope through the trap. A knot in this rope will secure the end-caps while at the same time provide for a mechanical connection of the antenna wire, separating the mechanical from the electrical connection for better mechanical and electrical strength.
Total trap construction may be seen in picture 1. Look at the small dimensions in compare to the match box. Tywraps wave been used for ease of winding and taking the load of the wing-nut electrical connection.
The impedance transformer with a step-up ratio of 1 : 2,25 is a bit out of the usual, but a Jerry Sevick design as in figure 6 is doing a good job.
To get a high enough input to output separation, Jerry configures the transformer on a large K5 (NiZn) type of ferrite toroide by MH&W Inti (TDK) with a permeability of 290, with five turns of good quality RG58 coax. The outer plastic encapsulation has been removed for ease of handling. This is fully allowed as braidings are carrying the same voltage (see figure 6) and the ferrite core has a very high electrical resistance (> 1 MOhm.cm). The picture in figure 7 depicts Jerry's set-up.
Connection details may be taken from figure 6. Feed line is connected between position B and ground; between position A and ground one will find 1,5 times the input voltage, so 2,25 time impedance.
Ferrite type 'K' may not be around too much any more but may be replaced by type 4B1 by Ferroxcube. As stated before, main function of core material is to obtain a high input to output impedance on the transmission lines. Therefore any type of ferrite material and number of turns will do, as long as total impedance is high enough at the lowest operating frequency (around 150 Ohm for a single coil) and self dissipation of the transformer is within material limits (see Ferrite materials, check at highest operating frequency)
my test antenna I made the transformer by winding (8) each coax separately on
Total transformer has been placed inside a box made of drainage pipe, this time of somewhat larger diameter and again closed by end-caps. A simple hook construction provides for a hoisting position and a small piece of 'trespa' is reinforcing this position while at the same time providing for a mechanical attachment for the antenna wires to take the load of the electrical connections (wing-nuts). Picture 2 is showing the construction and also a number of turns of RG58 coax, to ensure separation between the a-symmetrical feed-line and antenna plus transformer as a RF choke.
Picture 3 provides a better view on the back of the transformer box including the 'trespa' re-enforcing and pulling plate.
mentioned above, we are to couple a symmetrical dipole antenna to an
a-symmetrical transmission line. This usually is accomplished by some sort of
balancing device, sometimes a balancing transformer with or without a transformation
ratio. The 1 : 2,25 impedance transformer in our multi-band antenna has no
balancing properties. Balancing the transmission line currents to an antenna
is to ensure that all transmission-line current is going into the antenna and
not anywhere else e.g. to the outside of the feed cable. A good way of
preventing is outer current to become a significant portion of total RF
current, is by means of enhancing the outside impedance by means of a choking
action. A simple way of providing for such a choke is to have a length of
feed-line coiled up; around ten turns of feed-line on a diameter of 10 -
To make this RF-choke effective, the transceiver should have a low impedance to ground as choke to trx-ground impedance effectively makes a voltage divider. Connecting all shack equipment together usually provides for a low enough ground impedance at the same time ensuring equal potential on all equipment as a safety precaution.
a practical test a have constructed a model 9 type of antenna. As predicted,
all HF amateur frequencies on 80, 40, 20, 15 and
a second test I checked for lowest impedance at each HF band without using
the tuner. It showed that this 'resonance frequency' was sometimes outside
the specific amateur band, although the build-in tuner apparently did not
seem to mind. As it happens, my test antenna has been set-up over unknown
ground conditions (presumably poor), was tied to a highest point at around
Modeling the exact situation, the program came up almost exactly on target, enforcing again my confidence in this application and my calculations. Based on this confidence, I re-calculated the trap to have the antenna perform on the original target frequencies, because wires were already cut to size. For those who also like to prune the antenna to specification for their environment, I give the following table based on local experience.
Keeping trap resonance frequency as a constant, I noticed that for every 10 % rise in trap inductance, antenna resonance on
These ratio's were constant over a large range of inductor change. Most remarkable of all: above changes went all in the right direction for my antenna set-up.
In this article we investigated a field of trap antenna's to cover more than two HF amateur band. Out of the solution space we selected three models that satisfied the starting conditions that the design should exhibit good figures on gain and radiation angle and show low enough impedance figures (SWR) to connect directly to 50 Ohm coaxial transmission line of any length without excessive additional losses and to be within range of a simple build-in antenna tuner of modern HF transceivers.
design is showing a 'standard' antenna gain of 6 dBi for the lower HF-bands
It goes without saying that this is the best multiband antenna around with many DX-contacts to prove this. This usually is the claim by most antenna designers and am not very different at that.
This exercise taught me a few practical 'laws' on wire antenna's that I will present without further comments:
- every piece of wire is an antenna,
- antenna gain almost exclusively depend on antenna size relative to wavelength, starting from about 1/10 wave length,
- an antenna system is more effective with characteristic impedance closer to feed line and TRX requirements. Even a high gain antenna will loose its efficiency when much energy is dissipated in cable / transformer losses,
- a dipole antenna is more efficient for DX operation when higher above ground, as this will lower the elevation angle, - for local operation the antenna should be positioned much lower but not (much) lower than about 1/10 wavelength above (not so perfect) ground to prevent excessive ground loss.
Bob J. van Donselaar,