The 4 m Forum

[OH1ZAA]

OH1ZAA 50/70 MHz Supercombo (G > 10 dBi F/B > 25dB)

This is a new design serving both 50 and 70 MHz.
It has been derived from 18/24 MHz interlaced yagis
designed by OH1ZAA for the OH5AB_DX-gang.
John/OH5NZ has been a fairly satisfied user of
the HF-version during the last four years. Due
to a similar frequency-ratio the scaling to 50/70
was fairly straightforward, but due to slight ratio
differences, element diameter and further iteration,
antenna gain on 70 MHz was markedly improved. The
next version will probably be a Hypercombo one day,
and it should have a much longer boom. Commercial
use of the design is not permitted unless there is a
written agreement with the designer (OH1ZAA).

The OH1ZAA 50/70 MHz Supercombo is an interlaced
yagi with 4 elements on 50 MHz and 5 elements on
70 MHz. Element diameter is 12 mm. The length of
the boom is 396 cm. Feeding is done with separate
driven elements on each band with a gamma-match.
50 MHz gain is 10.5 dBi (8.4 dBd). 70 MHz gain is
10.1 dBi (8.0 dBd). Front-to-back ratio is better
than 25 dB on the design center. The antenna is
designed more for gain and F/B than for bandwidth
so it covers best the most actively used parts of
the bands (beacons @ CW/SSB-modes).

As gamma matching is ultimately always successful
this is the only challenge reserved to the constructor.
At least with the HF-version it was no problem to
get it tuned on both bands. The tuning sequence
should start with 50 MHz first as changes in the
hardware of that band can severely affect 70 MHz
dimensions, whereas 50 MHz is not much prone to
changes in the structure on 70 MHz. More detailed
feed impedance data will follow, but Z is in the
11 - 16 ohms range, so it can be easily matched
with a gamma (purposely left to the constructor).

Dimensions:

Reflector-50 position 0 length 298
Driven el-50 position 85 length 280
Director1-50 position 242 length 272
Director2-50 position 396 length 268

Reflector-70 position 25 length 210
Driven el-70 position 108 length 202
Director1-70 position 212 length 198
Director2-70 position 290 length 182
Director3-70 position 366 length 190

The front director on 70 MHz is indeed longer
than the second director. These dimensions are
trusted to represent correct values, but stay
alert for possible corrections or remarks. I
have not personally built this antenna as I
have used single band yagis so far, but all
former designs have worked well in practice.

Wishing you good luck on both bands!

73, "Zaba" OH1ZAA/OHoMZA/NNoY

[S59MA]

Hi Zaba,

thanks for quick reply! I have two more questions:
- can you tell me the boom dimension?
- I suppose all elements (including DE) are in electrical contact with boom.

73,
Ales S59MA

[OH1ZAA]

The mathematical impedance at the center of the 50 MHz element
is 18 - j3 ohms (complex) and at the 70 MHz feeder 11 - j3 ohms.
These can be easily matched with a gamma to any coaxial cable
impedance. Since the structure is for two bands, one can not cut
the element at the feedpoint, as the feeder would not present a
predictable parasitic impedance on the other operating frequency
as it is dependent on the feeder length and its termination. With
the gamma the current is just passing by along the element on
the "wrong" frequency, and thus its influence is negligible on the
pattern or tuning of the wanted band. However, make sure to do
the 50 MHz matching first. Sometimes a little adaption of driven
element length is handy to aid gamma tuning, but changing the
50 MHz driven element is a bit risky here for the 70 MHz pattern.

I prefer always isolated elements (less static and less detuning).
For mechanical reasons I do not make holes for the elements in
the boom (no feedthrough). With a square boom (30 x 30 mm)
one can use mounting blocks for hydraulic pipes (e.g. Stauff).

So far I have used 1" titanium tubes and U-bolts that go through
the center of Alu angle stock (about 15 cm long). One can put two
rings of 12 mm inner diameter polyethylene tube around each
element, and use hoseclamps to tighten the elements parallel to
the "outside" of the angle stock to get an isolated fixture. Titanium
is very hard to get, so if you use aluminium for the boom, make
sure it is at least 30 or 35 mm diameter for this 400 cm boom.
The center of the driven elements can be grounded to the boom,
but it is not necessary for proper operation with a flawless gamma.
Certainly the coax braid needs a contact with the element center.

Good luck! More details or views may follow... 73, "Zaba" OH1ZAA

[S59MA]

[OH1ZAA]

Before attempting construction of the antenna the dimensions obtained with Antenna Optimizer were fed into the MMANA 1.2.0.20 (free) simulation tool. The results were identical within 0.1 dB and it did not make much difference to choose either 10 mm or 12 mm diameter elements. The next step is to construct this antenna: a dual band 9-element design with essentially more than 10 dBi gain on both 50 and 70 MHz, each band fitted with its own feeder element with a separate feedline ---- 73, "Zaba" OH1ZAA/OHoMZA ----

[F5DQK]

I confirm the Mmana simulations giving 8.45 and 8.15 dBd respectively on 50.2 and 70.2 MHz on 12 Ohms impedance, and a very beautiful F/B pattern of more than 20 dB on every band ! But perhaps a DK7ZB impedance matching to 50 Ohms'll be more robust against rain and moisture than a Gamma feeding.
Congrats from Marcel F5DQK

[OH1ZAA]

Merci Marcel pour le cross-check! Yes indeed 8.15 dBd = 10.29 dBi --- and your simulations were done 100 kHz higher on both bands.... Regarding the feed method there is a fundamental reason why I am inclined to restrict the choice of feeder configurations on multi-band antennas.

As the radiation pattern of a single-band yagi is essentially almost independent of the state of resonance (or mistuning) of the driven element, there would be also no problem regarding the stability of the radiation pattern if the position of the driven element in a dual-band yagi would be in exactly the same position on both bands, and if it would be fed with the same feedline (I will make a new design for such a situation). However this Supercombo yagi uses two separate feeders in differing element positions.

The problem with an "open" dipole (like in the DK7ZB feed) for dual-band operation is the unpredictable impedance of the dipole element (through projection of the coax impedance and its termination), when it is functioning as a parasitic element for the other band, i.e. on 70 MHz the 50 MHz driven element will be a parasitic element for the "wrong" band. Now, if this is an unbroken element with a gamma-match, then most current will pass the center of this element undisturbed, and very little energy will be coupled to the unmatched gamma and connected feedline on the "wrong" frequency. However, with an open dipole center (DK7ZB feed), all current will be forced to flow into the other feedline (of course all determined through the complex impedances of all the "strings attached"). Therefore the radiation pattern and especially the F/B-ratio will be strongly dependent on the projection of the center impedance of the open element. And if the antenna is used for 70 MHz monitoring during 50 MHz transmit there is additionally a lot more filtering required to keep out the tens of volts of possible TX-power when listening at the microvolt level on the higher frequency.

The above is not a mathematically sound description, but I hope it makes the point in explaining the fundaments behind the choice for a gamma match. However, it will be fun to compare the real-life implications of your proposition as well. And a properly constructed gamma is not a bad performer at all.... However, it will be an interesting challenge to do the other dual-band design with a single feeder ---- Salut/73, "Zaba" OH1ZAA/2 (KP20JE) ----

[OH1ZAA]

The construction of the Supercombo prototype has started. The basic idea of the dual feedline version is to be able to full-duplex listen on 70 MHz while transmitting on 50 MHz. However, MMANA-simulations have revealed that an 8-element version with a combined 50/70 radiator and a single feedline is possible. The search for a new radiator structure with manageable impedances has started ---- The 9-element version provides excellent 50 MHz stacking gain (total gain 16 dBi) and satisfactory pattern clarity with 0.9 wave boom-to-boom spacing for a group of four (2x2). Close spacing is not recommended for this model (F/B ratio weakening issues). Front-to-back ratio is better than 25 dB for a single yagi or a wide stack... On 70 MHz the gain with 1.26 wave spacing (due to 0.9 wave on 50 MHz) is even better (16.5 dBi) and the F/B ratio as good as on 50 MHz ---- 73, "Zaba" OH1ZAA ----

[OH1ZAA]

This project has been in its winter sleep. The 4-element 50 MHz half was up all winter in KP20XW, and the 5 elements for 70 MHz will be added, then allowing the dual-band feeding alternatives and experiments. Progress will depend very much on time-sharing between the KP01RO/KP20JE/KP20XW/KP21SA locations (let alone KP41CA) --- 73, "Zaba" OH1ZAA/OH0MZA/OH2MZA/OH5ZA ---

[G0MJW]

Interesting. I have recently built a dual band shorter yagi based on a design by the late lamented W4RNL. It is only 2m long which is good for me because of the limitations of my location.

In my case the design was further complicated by the other antennas surrounding it on the mast. This design uses 7 Elements in total, 3 on 6m and 4 on 4m. Theoretically in free space it has 7.3 dBi or so of gain on 6m and 8.6 dBi on 4m. Obviously the real gain is less. The useful feature is that there is a single feeder, which prevents destroying the other band receiver which can happen with dual feeders. (With only one feeder, you can't forget to disconnect the unused antenna.) It is not gamma matched, it is designed to match 50 ohm coax directly - with a choke balun made from a coil of coax.

Here is the NEC file...

CM 50/70 MHz (dimensions in Inches)
CE
GW 1 25 -58.4 0 0 58.4 0 0 0.5
GW 2 25 -41.5 10.5 0 41.5 10.5 0 0.5
GW 3 25 -47.9 73.45 0 47.9 73.45 0 0.5
GW 4 25 -40.5 34.2 0 40.5 34.2 0 0.5
GW 5 25 -54.6 40.7 0 54.6 40.7 0 0.5
GW 6 25 -38.5 79 0 38.5 79 0 0.5
GW 7 25 -38.7 48.75 0 38.7 48.75 0 0.5
GS 0 0 0.0254
GE 0
LD 5 1 0 0 24900000
LD 5 2 0 0 24900000
LD 5 3 0 0 24900000
LD 5 4 0 0 24900000
LD 5 5 0 0 24900000
LD 5 6 0 0 24900000
LD 5 7 0 0 24900000
GN -1
EK
EX 0 5 13 0 1 0


TL 5 13 4 13 100 0
FR 0 0 0 0 50.1 0
EN



I found it worked well on 6m but not so well on 4m until I adjusted the driven element length. This is probably because it was being detuned by all the other antennas close by. I simulated all the antennas and, though not as good as well spaced monoband antennas, it does at least get me on the bands.


The rather complicated NEC file is below is for the stack..

CM NEC Input File of an 8 element yagi for 144MHz sold by Moonraker UK at 7m AGL
CM 6/4m Yagi 50cm below and 19 el 432 MHz 50 cm Above
CM HF Beam 90 degrees offset below
CE
GW 1 25 0 -0.51 7 0 0.51 7 6.4e-3
GW 2 25 0.18 -0.492 7 0.18 0.492 7 6.4e-3
GW 3 25 0.519 -0.46 7 0.519 0.46 7 6.4e-3
GW 4 25 1.039 -0.452 7 1.039 0.452 7 6.4e-3
GW 5 25 1.557 -0.446 7 1.557 0.446 7 6.4e-3
GW 6 25 2.08 -0.44 7 2.08 0.44 7 6.4e-3
GW 7 25 2.59 -0.435 7 2.59 0.435 7 6.4e-3
GW 8 25 3.12 -0.426 7 3.12 0.426 7 6.4e-3
GW 9 25 0 0 6.965 1.4 0 6.965 0.015
GW 10 25 1.4 0 6.965 3.12 0 6.965 0.015
GW 14 21 0.7667 -1.0541 6.6 0.7667 1.0541 6.6 6.4e-3
GW 15 21 1.3636 -1.0287 6.6 1.3636 1.0287 6.6 6.4e-3
GW 16 21 1.7319 -0.983 6.6 1.7319 0.98298 6.6 6.4e-3
GW 17 21 2.5066 -0.9779 6.6 2.5066 0.9779 6.6 6.4e-3
GW 18 21 0.5 -1.483 6.6 0.5 1.483 6.6 6.4e-3
GW 19 21 1.53378 -1.387 6.6 1.53378 1.387 6.6 6.4e-3
GW 20 21 2.36563 -1.217 6.6 2.36563 1.217 6.6 6.4e-3
GW 21 11 0 0.17275 7.4 0 -0.17275 7.4 2.e-3
GW 27 5 0.139 -0.16 7.4 0.139 0.16 7.4 2.25e-3
GW 28 5 0.201 0.152 7.4 0.201 -0.158 7.4 2.e-3
GW 29 5 0.274 0.15025 7.4 0.274 -0.15025 7.4 2.e-3
GW 30 5 0.437 0.14525 7.4 0.437 -0.14525 7.4 2.e-3
GW 31 5 0.62 0.14525 7.4 0.62 -0.14525 7.4 2.e-3
GW 32 5 0.82 0.14275 7.4 0.82 -0.14275 7.4 2.e-3
GW 33 5 1.055 0.14275 7.4 1.055 -0.14275 7.4 2.e-3
GW 34 5 1.325 0.14275 7.4 1.325 -0.14275 7.4 2.e-3
GW 35 5 1.595 0.13775 7.4 1.595 -0.13775 7.4 2.e-3
GW 36 5 1.865 0.13775 7.4 1.865 -0.13775 7.4 2.e-3
GW 37 5 2.135 0.13775 7.4 2.135 -0.13775 7.4 2.e-3
GW 38 5 2.405 0.13775 7.4 2.405 -0.13775 7.4 2.e-3
GW 39 5 2.675 0.13775 7.4 2.675 -0.13775 7.4 2.e-3
GW 40 5 2.945 0.13525 7.4 2.945 -0.13525 7.4 2.e-3
GW 41 5 3.215 0.13525 7.4 3.215 -0.13525 7.4 2.e-3
GW 42 5 3.485 0.13525 7.4 3.485 -0.13525 7.4 2.e-3
GW 43 5 3.755 0.13275 7.4 3.755 -0.13275 7.4 2.e-3
GW 44 5 4.025 0.13275 7.4 4.025 -0.13275 7.4 2.e-3
GW 45 25 -.9 -.75 6.2 3.8 -0.75 6.2 0.015
GW 46 25 -0.8 .75 6.2 3.7 0.75 6.2 0.015
GW 47 25 1.45 -0.75 6.1 1.45 0.75 6.1 0.02
GE 1
LD 4 45 5 5 0 210
LD 4 45 20 20 0 210
LD 4 46 6 6 0 242
LD 4 46 19 19 0 242
LD 5 0 0 0 30800000
GN 2 0 0 0 4 0.003
EK
EX 0 19 11 0 1 0
TL 19 11 15 11 100 0
TL 45 13 46 13 -600 0
FR 0 0 0 0 70.15 0
EN

[PA5DD]

On the subject of dual-band yagis, I compiled a small overview for my own purposes before deciding for an open sleeve feed yagi of my own design:

http://home.hccnet.nl/uffe.noucha/log/antennas.htm

73 Uffe PA5DD