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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