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+--- Thread: 9:1 OR 4:1 ?? (/thread-668.html)
9:1 OR 4:1 ?? - VK4CCV - 22-10-2018
I am about to put up a Dipole antenna but unsure what Balun would best suit.
I can only place a wire up that is 88' in length.
At the centre it will be 33' and the ends drop down to 20'.
The Antenna will be fed with 450 Open Wire which will be 33' to the ground where the Balun will sit.
The run back to the shack will be around 40' where I will use LMR195 to the Tuner.
I have purchased a 9:1 but now wondering wether it should have been a 4:1 ??
VK4CCV Pete
RE: 9:1 OR 4:1 ?? - VK4ADC - 22-10-2018
(22-10-2018, 02:52 PM)VK4CCV Wrote: I am about to put up a Dipole antenna but unsure what Balun would best suit.
...
The Antenna will be fed with 450 Open Wire which will be 33' to the ground where the Balun will sit.
...
I have purchased a 9:1 but now wondering whether it should have been a 4:1 ??
VK4CCV Pete
Peter
In reality you have answered your own question. The bits in bold are the main things - you need to match the 450 ohm open wire impedance to 50 ohms hence 9:1 is still the right answer. At some frequency the open wire will act as a 450 ohm transmission line - which frequency - don't know...
A standard dipole has a centre feed impedance of 60-75 ohms but with the length being 44ft per side (13.4 metres / per side) giving a standard resonant frequency around 5.5MHz, and that is regardless of height above ground at centre and ends. That also ignores the effect of the open wire feeder.
What its feed impedance is at 1.8, 3.5, 7 MHz etc is an unknown due to the interaction of the open wire feeder itself. I don't recall the G5RV style tuning / lengths but I suspect that it is getting close to one of those style antennas. Then again, its polar pattern at any of them is an unknown too.
Amateur radio = experimentation.
Build it, erect it, test it.
If needed, adjust the two side lengths to shift the frequencies and hence resulting feed impedances at the ones you want.
An antenna impedance analyser is a great tool while doing this. If you don't have one yourself, a nearby amateur may be able to help out if he/she has one. Ask around.
RE: 9:1 OR 4:1 ?? - VK4CCV - 22-10-2018
Thanks Doug,
Yes mate, it all makes sense. I'll just get it all up and see what happens.
Yea, I have the RigExpert AA-55 ZOOM, which I will use once installed. Should be interesting.
Modeling the antenna with EZNec, I am finding a couple of thousand ohms on some bands, but that is not taking the 450 into account.
Thanks again Doug.
Pete VK4CCV