I'm looking into ways to match a 137 MHz RX dipole antenna to an unbalanced coax + amplifier. I know there are several types of baluns which can be categorized under magnetic baluns (where there are seperate primary and secondary windings) and current baluns which are essentially chokes.

I was hoping I could build a transformer type balun, since this would allow me to choose almost any impedance matching ratio I want. I have FT50-61 and FT50-63 toroid cores, which are advertised to work as wideband transformers from 10-200 MHz.

I tried winding a few 1:1 transformers for measuring purposes and the results were pretty bad, I got at least -3.5 dB insertion loss. I tried using between 2 and 8 windings of different diameters of copper and silver for both primary and secondary.

This is a typical response (depending on how close the windings are and how many I put on it, I can move the peak left and right by a few ten MHz only):

enter image description here

Now I'm wondering if there is a systematic approach to getting good insertion loss, maybe by moving the peak of the curve to 137 MHz

Any tips are appreciated

  • $\begingroup$ There is no need for a blaun. Just like with regular HF antennas you can direct-connect from coax to the elements. But since you mentioned 137 MHz, take a look at this article about the 9A4QV 137 MHz Weather Satellite V-Dipole antenna: lna4all.blogspot.com/2017/02/… which gets a mention here as well: rtl-sdr.com/… - Much simpler than a turnstile, and Adam says he gets great reception - all without a balun. $\endgroup$ – SDsolar Oct 23 '17 at 5:39
  • $\begingroup$ Hi, the whole thing does indeed work without a balun, but since (theoretically) it's needed to achieve a symmetric radiation pattern, I'd like to compare results with and without a balun. Of course, if I don't find a construction with tolerable insertion loss, I'll just leave the whole balun out. $\endgroup$ – Felix S Oct 23 '17 at 7:24
  • $\begingroup$ Understood. I do think you have a real challenge on your hands finding something that will work for VHF any better than the 9A4QV design. The bottom line is that his works. Pretty hard to improve on that. (When I was building this myself I first looked into building a turnstyle and decided that I simply didn't have the mechanical skills. It is more a sculpture than an antenna) Until I discovered the coax-fed horizontal-V I thought I wouldn't have any chance of receiving signals from polar-orbiting WX sats. If you do come up with something I hope you will come back and post it. $\endgroup$ – SDsolar Oct 23 '17 at 8:31

VHF transmission line baluns are very difficult to construct due to interwinding capacitance. As a result, the balun will have an undesirable self resonant point and will not typically reach the desired transformation ratio. Note that a balun does not consist of simple primary and secondary turns but rather primary and secondary transmission lines (2 parallel wires of a specific characteristic impedance). If a balun is made simply with conventional primary and secondary windings then the core flux plays an active role. Such a design will also have a limited number of practical impedance ratios due to the requirement of a low number of turns to minimize capacitance.

Transformer capacitance consists of capacitance between turns, capacitance between windings, capacitance between layers, and stray capacitance. These can generally be modeled as a capacitor in parallel with each winding.

A better approach is to use a choking balun for common mode suppression and a lumped L, T, or Pi network to achieve the desired impedance transformation ratio. In some cases, a transmission line transformer may also be suitable for impedance transformation. In order to minimize losses, the matching network should be placed directly at the antenna feedpoint, followed by the choking balun.

At VHF frequencies, a choking balun consisting of #31 or #43 ferrite mix beads slipped over the outer coax jacket will be the most effective since any attempt to wind the coax through a toroid form will suffer from interwinding capacitance effects. Consider using 20 or more beads on the coax.

  • $\begingroup$ But what about the transformer type baluns, where primary and secondary are completely seperate? Why can I not use on of these? Interwinding capacitance should be negligible there? $\endgroup$ – Felix S Oct 23 '17 at 12:41
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    $\begingroup$ I updated my answer to address your questions. $\endgroup$ – Glenn W9IQ Oct 23 '17 at 15:48
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    $\begingroup$ Will a ferrite choke on the feedline yield a sufficiently high common-mode impedance, or would something like a sleeve or folded balun be more appropriate? $\endgroup$ – Phil Frost - W8II Oct 24 '17 at 17:43
  • $\begingroup$ @philfrost-w8ii I specifically didn't include the sleeve balun because as typically shown it is a radiating element unless its construction is inverted (open end toward antenna). Properly sealing the open end from the elements is problematic. It tends to have a relatively low common mode choking effect in any case partly due to the inclusion of the coax outer jack in the dielectric mix. Multiple sleeves can improve this but it is beyond the average application to do so. Is your experience with non-radiating sleeve baluns better? $\endgroup$ – Glenn W9IQ Oct 25 '17 at 6:34
  • $\begingroup$ No, I've never built a sleeve balun in the non-radiating direction. I've tried a couple ferrite beads and not had good results, but I'd imagine 20 of them would work. $\endgroup$ – Phil Frost - W8II Oct 25 '17 at 17:13

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