how to design balun (transformer) for radio manpack??

[SU-4]

i have a problem, i want design system for radio manpack which have operation for frequency at VHF (30 MHz - 90 MHz) or the otherwords i hope that system radio manpack can work (have good SWR = 2) for all frequency in that range VHF, but i have to use whip antenna because i only have it, which has length 1,2 meter for antenna, can u help me or give me some reference for design of balun to make impedance match for all frequency in that range???

kindly regards,

thanks

 

[kb6nu]

What you want is an antenna tuner, not just a balun. Googling "VHF antenna tuner" yielded this link:

**broken link removed**

The author claims it will work from 1.8 MHz to 150 MHz.

73, Dan KB6NU

 

[SU-4]

thanx before KB6NU, for the reply,

i'm using a ferrite core for the balun but i don't know about mu, can u give some valid reference for what kind of ferrite for mu, that suitable for VHF frequency and i need formula for the winding ratio of balun, can u give some valid reference for that too because i got a lot of reference but each of that reference is not same for formula for the winding ratio of balun.

kindly regards,

thanx

 

[kb6nu]

You're not going to be able to do it with a single balun. You're going to need a circuit, such as the antenna tuner I mentioned in my previous reply that you can adjust over a wide range of impedances.

73, Dan KB6NU

 

[enjunear]

thanx before KB6NU, for the reply,

i'm using a ferrite core for the balun but i don't know about mu, can u give some valid reference for what kind of ferrite for mu, that suitable for VHF frequency and i need formula for the winding ratio of balun, can u give some valid reference for that too because i got a lot of reference but each of that reference is not same for formula for the winding ratio of balun.

kindly regards,

thanx

Apparently you didn't read Dan's message. You don't need a balun (balanced-to-unbalanced transformer), you need a tuner, also know as an antenna coupler.

To get you started, you need to first define what impedances your antenna looks like over the frequency range. Go look around your lab ands see if you can find an old vector impedance analyzer. Go outside with your antenna; get away from buildings, trees, cars, etc. They will mess up the near-field radiation of the antenna and give you goofy impedance results. Install the antenna on top of a large metal plane (copper or aluminum will work best). Then use your impedance analyzer to look at the input impedance and make yourself a table. Depending on the severity of the reactive impedance you're measuring (generally at low freq's), you may need to add a tuning coil in parallel to the antenna, so you can resonate out part of the capacitance and bring the impedance that the analyzer sees back into a reasonable range (often < 80 degrees). Measure the antenna+coil impedance, then disconnect the antenna and just measure the coil's impedance, then you can do the math and find the antenna's true impedance (typically only a few ohms of real resistance in parallel with a LARGE capacitive reactance).

Once you know the range of load impedances, you can begin to define what your tuning network will look like. In the case of the article linked, the final implementation was series C, shunt L, series C. Notice the shunt L... that's because a large chunk of metal (the antenna) isolated from a nearby ground plane looks like a capacitor, especially down at HF. You might be able to reduce the range of your coil and caps, depending on how much VSWR you can handle.

This isn't going to be a simple task, so put on your thinking cap. This was an exercise I went through for a similar situation back when I first got out of school and into the workforce. Best of luck.

 

[SU-4]

thankyou enjunear and kb6nu,

i have to use only balun becasue i just only have a little space at radio manpack, so i think for implementation antenna tuner, it can be done...i don't know what i have to do now

 

[enjunear]

thankyou enjunear and kb6nu,

i have to use only balun becasue i just only have a little space at radio manpack, so i think for implementation antenna tuner, it can be done...i don't know what i have to do now

If you don't have room for much, then you're tuning range will be very limited. As for the balun, I doubt you'll need it, as the output of your PA should be unbalanced, and you can drive a whip-monopole with an unbalanced coax cable. The balun will simply be acting as an impedance transformer. You'd still benefit from knowing the input impedance characteristics of your antenna, then you can try several transformer turns ratios to see what gives you the best performance over the freq range. Additionally, you could add a few series and shunt elements to the feed of the antenna and make a much simplified "line flattener"-like circuit. This could also potentially improve your matching, but it, too, will be fairly narrow band compared to an active tuning circuit. Get some data (self-measured, or from the vendor) and then start doing some work in your favorite circuit simulator.

 

[SU-4]

i try to use only one ferrite with impedance ratio 1:4 to match all frequencies range 30 Mhz - 90 Mhz with antenna length 1.5 M and for the result, i can get VSWR below 2.5 for all range frequencies :razz: