Thanks Nate and Richard - very interesting. I'll have quite a lot of
sensitive equipment on this plane so your ideas sound preferable to
leaking RF into everything. I also saw something on the net about
sleeve baluns so that's another possibility. Thanks for the suggestion
of the ARRL antenna book - it sounds like a good way to go to learn
more about this topic.

Cheers,
Zik VK3MHZ

On 6/21/07, Nate Duehr <nate@natetech.com> wrote:
>
> On Jun 20, 2007, at 5:49 PM, Zik Saleeba wrote:
>
> > 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.
>
> There's a number of ways to make a matching network, but the advice
> to do it with a transmission line match is probably the sanest for
> the little tiny spaces and weight considerations you're working
> with.  You could also look at doing a Gamma match to the dipole or
> similar, but I'm not sure you'll really need it on a setup like
> that.  (If you were pushing 100W into it, we'd be recommending
> something else.)
>
> As the other person said, you might just be able to "try it" and get
> away with it too... most transmitters that low in power aren't going
> to freak out too much with that little of a mismatch.  Dipoles are
> nice in that they radiate pretty darn well, even when mismatched with
> the transmitter.
>
> Many of these little transmitters come with a built in isolator on
> their output -- to keep the transmitter from caring much about
> mismatched antenna systems.  If yours doesn't, this is something else
> you could also do, but it'll weigh more than other options.
>
> An isolator consists of a circulator which is a ferroresonant device
> (magnetic... could cause you other problems in the aircraft) with at
> least three "ports" that "forces" RF to only go one direction around
> it (in a circle, kinda) and one of the ports has a 50 Ohm dummy load
> on it.  RF going out to the antenna gets there, RF coming back from
> the antenna system goes into the load, and your transmitter always
> "sees" a nice stable 50 Ohm load.
>
> In this case, it's probably total overkill for your application, I'm
> just explaining it to show that there's a whole ton of options if you
> feel like being a perfectionist.  LOL!
>
> The biggest problem you'll have (as the other person mentioned in a
> round-about way also) is RF standing waves on the transmission line
> that could bother your transmitter and make it cranky and possibly
> noisy/non-linear.  That RF standing wave could also try get into
> other things as it trys to find a way out of the antenna system.
>
>
> > 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?
>
> Probably, but you'll have to try it and see.  :-)  It's really not
> that big of a mismatch.  Fire up the math and see what the
> theoretical SWR will be... shouldn't be a problem.
>
> > Incidentally this is a for-fun project, not a hugely important
> > commercial application so some bodginess is fine.
>
> Sounds like fun.
>
> If you really want a LOT of good reading about this, the ARRL Antenna
> Book is excellent... whole chapters dedicated to these topics.
>
> I don't claim to be an expert of any kind -- learned all of it the
> hard way...
>
> --
> Nate Duehr
> nate@natetech.com
>
>
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