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Multiband ‘trap’ antenna (Published in Electron #12, 2000 and Electron
#1, 2001) Introduction 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. Unfortunately,
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. A
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. As
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
= Modeling
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 On
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. As
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. Looking
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 band 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. In
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". Bandwidth The
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 amateur band 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. Above
we have modeled a four band antenna, based on four variables, for 80, 40, 20,
and
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. Practical designs The
antenna to fully comply with the design goals for a four band antenna with
SWR < 4 is model seven as in figure 3.
This
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.
In
this design, also all frequencies within the 80, 40, 20, and Extended five-band design A
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. Simplest
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.
outside diameter One
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
in At
these low frequencies, parasitic effects will not yet be too noticeable. Capacitor
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
around 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) For
my test antenna I made the transformer by winding (8) each coax separately on
a single 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.
As
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. As
a practical test a have constructed a model 9 type of antenna. As predicted,
all HF amateur frequencies on 80, 40, 20, 15 and As
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. Conclusions 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. Final
design is showing a 'standard' antenna gain of 6 dBi for the lower HF-bands
(80 and 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, |
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