Decreased reflector size from 700x710 to 600x700 mm
Advantages of 75Ω output compare to 300Ω.
Any balanced output need balun to feed coax line.
Most popular 4:1 balun is Guanella/Ruthrof, because it's frequency independent (wideband).
Full diagram of 4:1 balun include 4:1 transformator (only partially simmetrical, upper arm is +1.5V, lower arm -0.5V if +1V applied to 75 Ohm input) + 1:1 current balun
1:1 balun is excluded for simplification and to avoid losses, since ferrite is most popular choice and have high losses @ UHF.
75Ω can feed single 1:1 current balun, which has lower loss than 4:1 balun using the same ferrite coil, because of much lower core flux.
And coax side load will be truly unbalanced and avoid coax outer shield currents and radiation.
I have some ideas how to make this 1:1 non-ferrite and how to fit into antenna for convenient F-connector or LNA mount.
Still looking for 1:1 balun ideas.
For optimum performance, symmetrical line of 1:1 balun must have Zw=R=75
For balanced lines its extremely low impedance, very hard to achieve.
Typical bifilar wiring has Zw=150 Ohm, it perfectly suits 1:4 ferrite transformer, but doesn't suite 1:1 ferrite transformer.
To verify behaviour of a such kind transformer, I created model with air-gap strip line.
It's not feasible from construction point of view:
1) very tiny gap (2.5 mm gap for 8.1x1.0 mm strips, 5.0 mm for 16.2 strip etc) and tight tolerances.
2) 1/4L @ 470 MHz = 160 mm, straight 160 mm line cannot fit 111 mm reflector offset gap. Slope line is not good, as it distorts radiation pattern. Line should go away from radiatior perpendicular, but cannot go too close to reflector, so it's double-curved in my drawing (corner turns aren't good either, as this irregularity adds reactance)
3) for outdoor antenna tiny gap is not prone against moisture, dust, ice. Insulating from environment is not easy either.
Hi Yurii
I have a question about your air-gap strip line balun. It looks to me like the longer leg, of the strips fastens to the reflector, just past it's attachment to the ground side of the coax? If this is the case, would it work to turn this longer leg 90 degrees and form the end into a small loop. This loop could be sized to fit over the end of a chassis mount F connector and be held between the nut and body of the connector. This would form the connection between the longer leg and the reflector, as well as attaching to the ground on the coax. The coax cable would connect at the back side of the reflector. The shorter line could then be shaped into a point, that would be soldered to the center connector, of the F connector.
The two strips could be glued to the sides of a 2.5mm thick piece of plastic, or other insulator, shaped to fit between the two strips. The whole assemble could be weatherproofed with heat shrink tubing. The tubing would both weatherproof the assembly, as well as holding everything in place. Let me know if you understand what I'm describing, and let me know if you think this would place all of the parts close enough to what you modeled, that it will work.
One dimension that I don't see on your page is the feed gap. Could you give us the measurement of the feed gap?
it's not mandatory, but good if balun ground (end of one leg) is connected to antenna ground/mast (reflector is a ground in this antenna). it's better for EM model and is convenient for mechanical reason.
I don't bother to draw actual coax connectors or direct coax soldering in HFSS model.
End of this symmetrical line is expected to fit either:
- connector (F-type female connector)
- direct coax soldering (still need some support pipe/fitting structure for coax going through reflector plate).
- LNA
I'm interested only in LNA-version, so actual drawing depends on LNA board design and how you fit it.
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