Isn't it common to use a delta feed (I think that's what it's called)
where the shield of the coax goes to the center of the element (total
length is 1/2 wave, just as a dipole, but not cut in the middle), and the
center of the coax goes part way to the end of the element? I think you
can adjust the point where the center conductor connects to the element to
adjust the radiation resistance as seen by the coax. I imagine reactance
changes too, so a series inductor or capacitor may be required for a
perfect match. Or, you can just adjust for a minimum SWR (smallest radius
on a Smith chart plot of R+jX) and call it done.

This is what I recall from my high school ham radio days of LONG ago...

Harold

>
> Use a stub matching network.
>
> Regards,
>
> Jim KA9QHR
>
> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf
> Of Zik Saleeba
> Sent: Wednesday, June 20, 2007 6:50 PM
> To: Microcontroller discussion list - Public.
> Subject: [EE] Feeding a dipole
>
> I have an RF question which has been bothering me for a while. I have a
> 900MHz 1W transceiver module which I'm planning to put in a model plane
> for telemetry purposes. Every gram counts so rather than using an
> off-the-shelf antenna I'm planning to make a dipole from conductive foil
> tape stuck to the wing. I have a small amount of experience with VSWR
> meters so I think I can get that part of the system working pretty well.
>
> My problem is that the output of my transceiver is 50 ohms unbalanced
> and dipoles are an inherently ~73 ohm balanced antenna. I want to
> connect the transceiver to the antenna in the lowest loss way I can so
> I'd prefer to avoid using a balun if at all possible. Weight is also a
> consideration here.
>
> Is it "good enough" to connect the signal and ground to the two arms of
> the dipole or is there some better low-loss, low-weight way I can do
> this?
>
> Incidentally this is a for-fun project, not a hugely important
> commercial application so some bodginess is fine.
>
> Cheers,
> Zik VK3MHZ
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