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I'm a newbie who is attempting to build a half-wave dipole antenna that will be located in the rudder of a sailplane. 123.3 MHz is the most important frequency to me, although it's reasonable to assume that the antenna should be wide-band enough to work adequately down to 118 MHz and up to 136 MHz. Based on these requirements, and based on a whole lot of internet reading, using some copper tape about 1/2 in wide and 45 inches long (0.47 lambda) is a good starting point for a resonant half-wave dipole antenna centered on 123.3 MHz. I would expect this antenna will be around 73 ohms of inductance (more or less because I expect some coupling to the carbon fiber structure of the aircraft).

So my question is, given that I'm feeding this with around 5 watts of power through 50 ohm RG-400 coax, I'm struggling with how to approach matching the inductance and adding a balun between the coax and the antenna?

It seems like a gamma match might be a good choice, but there are a bewildering number of choices here and it's hard to get a handle on where to even get started with this design. It's probably worth mentioning that the rudder is small, thin, and needs to stay very light weight.

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  • $\begingroup$ You mention copper tape. Taped to what? The conductive carbon fiber? $\endgroup$
    – JSH
    Commented May 3, 2016 at 17:56
  • $\begingroup$ Is the rudder also carbon fibre? It will almost completely short circuit your antenna, performance will be terrible. How about a much narrower tape, on the canopy? $\endgroup$
    – tomnexus
    Commented May 4, 2016 at 4:27

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If you truly have 73 as impedance, I would not further match this, and connect the coax to it.

Lets look at VSWR for a 73 Ohm antenna fed with 50 Ohm coax: (calculation here: http://www.cdt21.com/resources/TechnicalTools/vswr1.asp)

VSWR is 1:1.46, if you look at reflected power, that would be 3.5% (calculation here: http://www.antenna-theory.com/definitions/vswr-calculator.php)

From your 5W in: 4.825W "going into the antenna"; depending on the antenna efficiency (assuming 100%) then it all would be radiated.

From the 0.175W which is reflected, some of that will make it back up to the antenna as well.

Due to this, I would not worry too much about matching, I would tune/prune your dipole to have lowest VSWR at your transmitter, which would include the coax and influence the plane's body, and get it as low as possible.

If you still want to further match, have a look at the Gamma-Match discussion, right here on this site: What is a Gamma match in the context of the driven element of a Yagi antenna?

HTH.

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  • $\begingroup$ Thank you, this is helpful. So any suggestions on how to handle the balun if I'm not going to the trouble of matching the impedance? $\endgroup$
    – Chris Young
    Commented May 3, 2016 at 17:18
  • $\begingroup$ I would just solder the braid of the coax to one leg of the copper tape and the center conductor to the other leg of tape. No balun, as Edwin suggests, just straight onto the antenna from the coax. You may want to make sure that the length of coax is a multiple of an electrical half-wavelength of the intended center frequency. That way you're not adding any potentioal additional mismatches caused by the length of cable. $\endgroup$
    – captcha
    Commented May 4, 2016 at 0:19
  • $\begingroup$ what you could do is use some "clip on ferrites" on the coax near the dipole connection to supress common mode... that is all I would do. $\endgroup$
    – Edwin van Mierlo
    Commented May 4, 2016 at 6:25
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    $\begingroup$ I doubt they would do anything at those frequencies. They're good for mains AC and HF but not really effective on 123MHz. $\endgroup$
    – captcha
    Commented May 4, 2016 at 21:53
  • $\begingroup$ @captcha, fair enough... I still have some on an old wire 2m dipole in my attic, now I honestly don't know if they make a difference. $\endgroup$
    – Edwin van Mierlo
    Commented May 5, 2016 at 6:35

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