The
"Cizirf-Special" Receiving Antenna
A
40/80m receiving antenna optimized for short and medium distances
Patrick Destrem
F6IRF
Introduction
In a recent
article, I presented a receiving antenna concept for short distance communications
on the 40 and 80m bands using 2 broadband dipoles placed at low height and fed
in various phase relationships(*). This antenna is not a
DX antenna, but an antenna whose purpose is to facilitate
"domestic" communications, which in these times of minimum solar
flux are often more difficult to achieve than DX contacts. This is
especially true on the 40m band. This antenna may also be interesting for those
who are affected by one or more sources of local noise that typically arrive at
low angles such as urban neon signs, transformers or power lines.
The following is
the complete description of this antenna with various available options.
Although this antenna was initially designed with our “national contest” in
mind, it may also be interesting for other applications.
(*) http://mangafight.free.fr/Antenne%20CDR%20part4.htm (original in French only so far.
However you may try an automatic translator or just look at the pictures!)
NB:
If you have never used a separate receiving antenna, please note that a basic
precaution consists in short-circuiting the input of the receiver (for example,
by means of a coaxial relay) during transmission. The RF "collected" by the receiving
antenna can reach dangerous power levels for the receiver. Even in the case of a transceiver having an
" RX-ANT " input, it is recommended that you make sure from
the schematic that this input is disconnected or shorted-circuited before the
first stage of the receiver (this is not always the case...). Also, the
receiving antenna should be located as far as possible from the transmitting
antenna.
The
broadband dipole
Figure 1 the broadband dipole. As you can read it in
the text, it is better for the dipole to be non resonant.
Why use a
broadband dipole of twice15m? The first reason is to maintain an acceptable SWR
on the transmission line. Typically with this dipole, the SWR should hardly
exceed 1.5:1 on 40 and 80m, whatever its height (from 1 to 10m).
W8JI, in his
article http://w8ji.com/small_vertical_arrays.htm explained the
reasons why low-Q antennas should be preferred for this kind of receiving
systems. Please refer to his article
Figure 2 On top SWR versus wire-length (in meters) for the fig1
dipole. At the bottom Gain versus height (in meters) .
In all the cases it is better to avoid resonance...
The minimum length
of the elements (2x 15m) is imposed by the gain-limit of -20dBi, below which
receiving antennas are likely to require preamplification. The length of wire
is not very critical. However, it is better to avoid half-wave resonance, as
the SWR may reach about 4:1, due to the low radiation impedance versus the
loading resistance. Because of its
low-Q, this antenna should not disturb the other antennas located nearby (*).
For short/medium distances, it is not desirable to make the dipole too long,
because with length, the directivity increases, reaching a maximum at 2 x 5/8
λ, beyond which the secondary lobes become dominant. In short, 2 x 15m seems a
good compromise for 80 and 40m bands. For more “broadband” applications,
typically 3 to 10Mhz (i.e. SWL’s) I would recommend to
use a terminated folded dipole (see W4RNL pages) which maintains a
low SWR over a wider bandwidth.
(*)
the opposite may not be true: As mentioned by W8JI in the above mentioned
article, a transmitting antenna may affect greatly the performances of your
receiving antenna system. I did experience this myself: I noticed quite a
difference in levels and noise between my East antenna and my West antenna…
until I detuned my transmitting vertical!
Figure 3: Vertical gain and SWR of the broadband dipole
on 40 and 80m versus height in meters for an average ground (NEC2). For a
receiving antenna the gain is not a determining factor, but a minimum is
required.
The antenna
Figure 4 the antenna consists of 2 broadband dipoles,
such as those of figure 1.
The antenna is
made with two the broadband dipoles like the one
pictured in Figure 1. Optimum spacing
depends on what is desired. If one wishes an extreme NVIS antenna, a half-wave
is a good spacing. If one prefers a directional antenna for the intermediate
vertical angles, a more reduced spacing will be preferable. For use on 40 and
80m it is a matter of compromise, but around fifteen meters of spacing between
dipoles appears to be a good choice. However, this is not very critical.
Figure 5 vertical gain and gain at 45 degrees elevation,
versus dipole spacing, for the two Figure 1 dipoles fed in phase at 7m AGL
(NEC2).
Figure 6: Vertical pattern for spacings of 0.2, 0.5, 0.6 and
0.8 wavelengths (MMANA). A half-wave (in red) is good spacing for a NVIS antenna without a low
lobe on the horizon. However 0.6wl(in blue) provides nearly 10dB more attenuation of the
signals arriving at 40 degrees.
Feeding
the dipoles
For the simple
NVIS version (Figure 6), the simplest solution is to mount the dipoles parallel
to each other and use identical feeder lengths to a broadband matching device
(25/50 ohms) such as a UNUN. For the “high angle” directional pattern option
(Figures 7 and 8) and if the goal is a particular distance (1) or direction, it
is possible to use dissimilar lengths of cables to the combining device. If the
dipoles are installed in phase (2), it is the feed line of the dipole nearest
the desired direction which should be made longer (see Figure 7). If the
dipoles are mounted with reversed phase it is the feeding line of the dipole
opposite to the desired direction which should be made longer (3).
If you want to
reverse the desired direction, I have described on my blog a simple
solution, using a single coaxial relay and a UNUN. The article is in
French, but the diagram is quite obvious. This simple solution assumes that the
2 dipoles are absolutely identical, and that the SWR
of each one is low (you may need to adjust the loading resistors and/or the
wire lengths). With this simple solution the main problem remains the mutual
coupling between the 2 dipoles fed in quadrature (or close to it). However due
to the low Q of the dipoles, the degradation of the patterns compared to the
simulation should be acceptable.
Ideally a hybrid
coupler should be use to combine the 2 antennas when they are not fed in phase
or 180 degrees out of phase, but such couplers are generally designed for one
band (like the Comtek PVS-2).
You may also try something like the DH-P or the BB2 power
splitter, used as a combiner. Anyway for receiving, a more flexible
solution is provided by phasing boxes such as such as the MFJ-1025 or the
DX-engineering NCC-1 (see tests below). This said, even a very simple solution
just using switched
coaxial cable lengths and a quickly
made UNUN
can sometimes provide very surprising results, as demonstrated in this 80m recording.
(1)
You may have a look at the relation vertical-angle/distance in this article
(original in French but the diagrams are self-explanatory)
(2)
The two dipoles, with equal feed line lengths may be fed either in phase or 180
degrees out of phase, simply by reversing their connection to the feed line -
one dipole's left side to the center conductor, the other dipole's right side
connected to it.
(3)
For feeding out of phase by more than 90 degrees this solution requires less
cable. Demonstration: starting with 2 dipoles A and B installed 180 degrees out
of phase, adding 1/8wl to the feed line of the dipole A, will create a phase
relation of 180+45 = 225 degrees (in other words -135 degrees), and the
directivity will be A towards B (Our dipole A occupying the same situation as
the reflector of a HB9CV). If the 2 dipoles are in phase, it will be necessary
to add 3/8wl in antenna B feed line to achieve the same result.
Simulated performances on 40 and 80m
The diagrams below
were generated with MMANA (and verified with NEC2) using a spacing of 15 meters
and a height of 7m. As can be seen in Figure 3, there is some gain reduction by
reducing the height. However the absolute
gain is not a determining factor for a receiving antenna and overall, the real
performances should be close, even with a height as low as 1m.
Figure 7 : 40M patterns for various phase
differences between the dipoles. All the horizontal patterns are generated for
45 degrees elevation. For the 2 dipoles in phase the maximum Gain given by NEC2
is minus 2.85dBi and the SWR 1.42. For the 2 dipoles fed in quadrature the max
gain is minus 3.69dBi and the SWR for each dipole is 1.4 and 1.47. For 15m
spacing, the maximum F/B ratio is obtained for a phase-relationship of about
110 degrees (green curve). If the low-lobes on the horizon are a problem, a
phase relationship of 70 to 90 degrees will also provide excellent results
Figure 8 : The same configuration, but on
the 80m band. As you can see, the gain decreases notably when the phase
difference is increased. The maximum gain given by NEC2 is -10.28dBi (SWR 1.44)
for the dipoles in phase and of -15.65dBi (SWR 1.47/1.45) for 140 degrees phase
relationship. For 180 degrees, the maximum gain goes down to -20.83dBi. For 120
degrees the diagram is excellent with no secondary lobe near the horizon.
As can be seen in
Figure 7, the gain variation is minimal on the 40m band. At 45 degrees
elevation, it is the same for the phased-dipoles and the anti-phased dipoles.
That is the ideal case!
On the 80m band
the gain varies more. However the antenna remains usable with good
performances.
Of course, it goes
without saying that continuing to turn around the phase-circle rotates the
diagram by 180 degrees in azimuth.
Note:: It is not
as simple as you may imagine: if you consider the two dipoles fed in quadrature
(90 degrees out of phase), adding 180 degrees will provide the
" mirror " diagram...
it is not true any more if you start at 135 degrees (135+180 = 315),
because in this case the phase difference between dipoles will only be 45
degrees.
Performances
on adjacent bands
The performances on the adjacent bands is interesting, as it
gives us an idea of what could be done to improve the design (with regard to
spacing and element lengths).
Figure 9 Patterns on 30m. It is quite
obvious that, if an NVIS antenna is the choice, a half-wave is a good spacing
between dipoles ( 0.6wl is even slightly better). On
the other hand as soon as the phase relation is changed, a back lobe appears
rather low on the horizon. This goes against the desired goal. It can also be
seen that the horizontal pattern (at 45 degrees elevation) becomes resolutely
oval. However the antenna remains usable
for short-range operation.
Figure 10 Patterns on 160m. The antenna
becomes difficult to use due to the insufficient gain 11.
Construction
I built the baluns
on high-AL ferrite beads, initially intended for choking RG58 or similar cables
(
Figure 11 The
two baluns are virtually identical. The resistors used for the tests are two 510 Ohm 1/4W in parallel. It will replace by 3- 820 ohm
3W resistors when available because these are likely to burn if the antenna is
in close proximity to the transmitting antenna.
Figure 12 Checking of the
balun/resistor assembly. SWR 1.2 to 1.3 up to 30MHz... Note my high-class
network analyzer!
Figure 13 Mounting of the SO239 coax
jack.
Figure 14 The assembly is made in a small
electrical box. . Like the Balun, the dipole is constructed with a black and a
white wire (of 0.5mm diameter- not critical ) to allow
easy identification of the phase.
Figure 15 Here, the antenna is installed on a fiberglas mast supported by a
tripod. The dipole apex is approximately 4m AGL and the ends approximately 2m
AGL.
Tests
with one dipole
With reference to
my vertical, the results with only one dipole are already interesting. With the
2 antennas levels balanced on the background noise, the gain in terms of SNR(*) is spectacular, frequently 10 to 15dB on the stations
located at less than 500 kms. Only the most remote stations arrive stronger on
the vertical. The improvement is such that certain stations, completely inaudible
on the vertical, appear above the noise when switching on the low dipole.
Admittedly it is necessary to mitigate this result by the fact that the
comparison is made with a vertical. Compared to a relatively low dipole (<
0.5wl), it is likely that there will be no difference. On the other hand, on
40m, compared to high yagi or a dipole (>.0.5wl AGL) I think that the
results will be similar to this 40m recording (the HB9 station
is about 300 kms away, TM8P is around 400 kms away)
With
this version using two 15m wire lengths, the level is sufficient on the 40 and
80m bands, not even requiring the use of the transceiver preamplifier. On 160m,
it is necessary to use the preamp (for the IC756pro2, preamp position 2 which
is approximately + 20dB gain). In spite of the very negative gain of the
antenna on this band, the results are also still spectacular (the station on
this audio-clip is located about150kms away). The local stations emerged from
the noise as if by magic.
(*)
Signal to Noise Ratio
Tests
with the two phased dipoles
Making a point of
testing a dedicated NVIS antenna, I initially installed the 2 dipoles (still 4m
AGL) with 20m of spacing. The two
antennas were connected by equal cable lengths to the 2 ports of a
"stackmatch" (to allow quick comparison between a single dipole and
two phased ones). On 40m the first immediate result
was a reduction of the background noise from a 380 kV power line by about 7 dB
(compared to a single dipole). Of course on 80m the difference is tiny
(approximately 2dB), because the 2 antennas are too close for this purpose. On
40m monitoring nearby stations (100 to 300 kms), I noted about 20 dB
improvement of the SNR compared to the vertical, partly due to the reduction of
the noise floor. The improvement of the SNR with the 2 phased dipoles is also
very clear when compared to a single dipole.
Figure 16 One of the
NVIS dipoles at 1m AGL. In the background is the vertical, used as reference antenna.
Interested in
testing the limits of this antenna system, I then mounted the 2 antennas at
only 1 meter AGL, with 25m spacing (0.6wl on 40m). Although this configuration
resulted in having to use the transceiver preamp, it is clear that the results
were still there. With the two phased dipoles, the results on 40 and 80m were
surprising. Up to 30dB improvement of the SNR (with reference
to my vertical) when monitoring local stations on the 80m band was achieved.
As it is logical, the difference was reduced as the distance to the
transmitting station increased. However on 80, as on 40m, the difference was
still some 10 to 15 dB in favour of NVIS antenna for stations located between
400 and 600 kms away.
You may listen to
the various recordings made with this
configuration, but the most interesting is probably this one because it shows
how it is possible to "sort" between 2 stations, located in the same
direction but at different distances, when switching from one antenna to the
other. Note, though, that the reference antenna is a vertical, antenna which is
particularly unsuited for domestic traffic (on the scale of a relatively
small country like
Figure 17 Audiogram obtained
on a local station (40kms) when switching from the NVIS antenna to the
vertical. The receiver AGC was turned off and the two antennas balanced in
level with respect to the background noise. As can be seen, the difference can
reach up to thirty dB when switching from the vertical to the NVIS antenna.
Playing
with the Phase
The idea of
varying the phase between two close spaced dipoles is not new. In 1937, Dr.
John Kraus, W8JK, described the principle of bidirectional antenna using 2
close-spaced dipoles fed with 180 degrees phasing. Antennas like the
HB9CV, F8DR and ZL-special use a similar principle (2 close spaced dipoles -
typically 1/8 wavelength), except that in this case the dipoles are not
“anti-phased” anymore but fed with phasing of about 135 degrees, which makes
the pattern more or less unidirectional, but limits the use to one frequency
band.
If you imagine
being able to vary the phase relationship between the 2 dipoles from 0 to 359
degrees, while controlling the amplitude of the currents in the 2 elements, it
becomes possible to obtain a complete palette of patterns going from the
phased-dipoles to the W8JK-array (Figure 7 and following). On the transmitting
side, the control of the currents flowing into the elements is the most
delicate part, which results in using complex feeding systems such as those
used for the 4-square arrays (see " Low-band DXíng " from
ON4UN). On the other hand, for receiving purposes, using resistively loaded
dipoles tends to minimize the impedance variations (and therefore the current
imbalance) to something more acceptable. Ultimately if it becomes possible to
“fine-tune” the phase and amplitude of the 2 dipoles, all the diagrams
envisaged by the models become achievable.
A simple way to vary
the phase is by lengthening one of the feed-lines. By using switched sections
of one feed line (see blog *) it is possible to obtain
front-to-back ratios in the order of 15 dB (or more if you are lucky!). In
practice if we are able to vary the phase from 0 to 180 degrees per 45 degrees
steps, the missing 180 degrees sector can be obtained using a 180-degree
broadband transformer. A single band system does not require more than 2 line
sections. A section of a eighth of the wavelength results in a 45 degrees
rotation and a quarter-wave section results in a 90 degrees phase rotation. With
3 line sections and a transformer we can easily build a system working on 40
and 80 meters (figure 18).
(*)
automatic translation from French provided by Babelfish
(don’t ask too much!)
Figure 18 Draft of a "rustic" 80m system, to
be inserted in one of coaxial lines. It allows phase-shifts from 0 to 337.5
degrees in 22.5 degrees steps. It is
also usable on 40m, but the step becomes 45 degrees (on 40m SW2 becomes useless
since it has the same effect as SW1). All the lines are cut on 80m, taking into
account the cable velocity factor.
The limitation of
the above “simple system" and one of the reason for reduced performance
(compared to simulations), is that it is not possible to balance the amplitude
of the signals arriving at the coupling device. Even if the antennas are
virtually identical, it is possible to note rather important differences in the
amplitude of the signals arriving from one antenna and the other. This
difference can be caused by the terrain configuration, surrounding obstacles,
other antennas present on the site or even by the losses introduced by the
variable-phase system. In short, to
achieve the results envisaged by the simulations it would be necessary to add a
step attenuator (or better a variable gain amplifier) in each line.
Electronics offers
an alternative to us. Without going into the details, an "active"
device allowing continuous phase and amplitude control should produce the
anticipated results. To my knowledge there are currently on the “amateur”
market " 2 magic boxes” allowing such a wonder: The MFJ-1025 (less than
$200 in the
The MFJ-1025,
which I tested personally, suffers from a certain number of weaknesses:
- It allows only a
phase variation from 0 to 130 and 180 to 310 degrees, which is sometimes
insufficient. It had to add an external "switchable" piece of line to
cover the two missing 50 degree sections.
- It has negative
gain (again, see W8JI page dedicated to this device)
- Its noise figure
does not allow the use low output antennas (I had to raise my two broadband dipoles
from 1 to 7m AGL, to get a sufficient level to cover the noise introduced by
the box)
- Adjusting the
phasing potentiometer requires precision, and over time repeating such precise
adjustments becomes tiring as the potentiometers are not smooth enough.
This known… it is
inexpensive and it does work, as attested by the videos and audio recordings
published on the blog. Typically 20 to
30dB of rejection of an unwanted signal are made possible (of course the
desired signal must come from a direction and/or a vertical angle different
from the useful signal).
Figure 19: One of the "broad band"
dipoles used for the tests of the MFJ-1025. It was necessary to raise the
dipoles height from 1 to 7m to obtain a sufficient level to cover the noise
generated by the device. By the way, you
can see in the background the power line passing approximately 200m away from
my aerial which makes DXing on the low bands so difficult.
Figure 20 MFJ-1025 in test with
F6IRF
The NCC-1, tested
on the "Cizirf-special" antenna at F6KNB radio club, seems much
easier to use than the MFJ-1025. Moreover, it includes two 20dB preamplifiers
resistant to strong signals. It covers 0
to 360 degrees without any hole and with an easy to use “phase button”. With
this device it should be possible to use the broadband dipoles as low as 1m
AGL.
Note:
In a high power environment (i.e. a contest station) it is highly advisable to
check if the protection is sufficient for your phase-box inputs..
..
Figure 21 The NCC-1 under test at
F6KNB. F6CIS, F5JZA and F6IRA are
pictured.
Summary
and conclusions
My initial goal
was to design a receiving antenna to improve the quality of short and medium
communications on 40 and 80m: Mission accomplished!!
According to your
personal requirements, you may choose from the options described above.
- If NVIS is your
main requirement, 2 phased dipoles with 0.6wl spacing on the highest band (i.e.
25m on 40m), is the best option. Assuming your receiver has sufficient gain
capability, the height of the 2 dipoles can be reduced as low as one meter. It
is also the best option for those who live in a noisy environment (In this last
case, reducing the spacing to a half-wave will eliminate the secondary lobes,
low on the horizon). For this option a simple UNUN will do a perfect job.
- For those who
want a longer range antenna offering some front to back ratio, the spacing can
be reduced to fifteen meters and the dipole in the preferred direction, fed
with a "fixed" phase delay ranging from 70 to 140 degrees (see
Figures 7 and 8). For this option and though a simple UNUN may do the job, I
would recommend a combining device providing some isolation, such as the BB2 .
- If one wishes an
antenna offering a more extended range of possibilities (choice of direction and
vertical angle) and typically a ten to twenty dB's of front to back ratio, then
I would recommend fifteen meters spacing and something similar to the simple
phasing system of Figure 18 to be inserted in one of the feed line after a
combining device.
- Ultimately, if
the very best performance is desired (typically 20 to 30dB rejection of
unwanted signals), you will have to use a box such as the MFJ-1025, or even
better the NCC-1 (or design and build a phasing box ).
Finally your
individual use should be considered if an adjustable device is used.
If you are a
“ragchewer”, no problem. You have all
the time necessary to adjust the rejection of undesirable signals.
On the other hand,
in a contest situation, the QSO will be already finished before you finally
find the optimum adjustment. Personally, for contest use, I would appreciate
having a few “presets” on the phasing-box to make it really useful. This said, it may still be very useful over
longer periods to reduce or eliminate annoying key-clicks or splatters from a
station too close in frequency (our bands are not wide enough, especially
during SSB contests!). However nothing yet replaces the individual operator’s
skill to pull out weak signals from the noise and QRM.
.
73' S - Patrick
F6IRF/CN2WW
A summary of all
the tests carried out with this antenna is available by following this link . Videos and
audio recordings, are used to illustrate the topic.
Copyright
F6IRF January 2008. Thank you for contacting me for any reproduction even
partial of this document.
Main references
The W8JI
pages: http://w8ji.com/
especially the receiving pages http://w8ji.com/receiving.htm
The W4RNL pages: http://www.cebik.com/
“Low band DX’ing”
book by ON4UN
“Transmission line
transformers” book by W2FMI
http://www.dxing.info/equipment/rolling_your_own_bryant.dx Rolling Your Own: building antenna splitters
that perform better than most commercial units by John Bryant and Bill Bowers
Software
MMANA by JE3HHT
NEC2 for MMANA by UA3AVR
4NEC2 by Arie Voors
VOACAP by NTIA/ITS
Audacity “a free digital audio editor”
Acknowledgments: I would like to
warmly thank all who have actively contributed in a way or another to this
publication, namely F4AJS, F6CIS, F6IRA, the F6KNB team, N4ZR, W7ZR and many
others who demonstrated their interest or prefer to remain “anonymous”.