I am pretty surprised by the varied amount of information provided
here to the original poster. I think this really shows one reason why
quads aren't popular. Yagis can be designed to be fed directly with
50 ohm coax. Or when you push a yagi for the last bit of gain, you
usually find the impedance is below 50 Ohms, around 20 ohm and various
matching networks are employed. Quads are no different. A 2 element
quad DOES NOT NECESSARILY need a 2:1 transformer. It just depends on
the specific design used (i.e. element lengths and element spacings).
You can very easily predetermine what type of feedpoint impedance you
will be dealing with by using NEC2. NEC2 has no problem at all
modeling quad elements. I have found the output of NEC2 for quads to
highly correlates to the feedpoint impedance found in the real world.
If you don't want to deal with NEC2, then pick a well documented
design.
I agree that designs from W6SAI typically required a 2:1 transformer,
but let's not confuse the issue here, the answer depends on the
specific dimensions of the 2 element quad being built and since that
hasn't been specified, we can't answer this question. And yes,
regardless of the feedpoint impedance some type of (ferrite) choke
should be employed at each feedpoint.
-Scott, WU2X
On Tue, Feb 2, 2010 at 12:55 PM, Craig D. Smith <craig@powersmith.net> wrote:
> I've never built a multi-band quad, Dan, but have done several single band
> loops and quads. You will want a 2:1 impedance transformation at the feed
> point (assuming 50 ohm coax) and also a conversion from balanced to
> unbalanced. As others have mentioned, an easy way to do this is to use a
> 1/4 WL (electrical) section of 75 ohm coax to do the impedance
> transformation. For the balun, I usually just tightly coil the 75 ohm coax
> so that it serves both purposes. You can probably obtain better balun specs
> with ferrite, but I've found that the coiled coax works adequately in this
> application.
>
> 73 Craig AC0DS
>
>
>
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