At 07:30 PM 3/7/97 EST, you wrote:
>Here is something very nice you could do for all of us. How about making
>a table of xmission line lengths and capacitor values for folded dipoles
>fed with 300 ohm twin lead? I used to build these when I was a kid (many
>moons ago) and used a parallelmica xmitting cap (surplus of course) to
>match the line to 50 ohms. It could then be easily fed with a 1:1 balun.
> Honestly though... I never used baluns in those days. I just wound up
>the coax around a coffee can and fired away.
I built a 80m inverted vee folded dipole and fed it with an electrical
1/4 wave length section of 93 ohm coax (RG-63). As an inverted vee,
the feed point Z is ~200 ohm and the 1/4 section matches it well for 80m.
I did not use a balun either and it did play well.
>Makes a nice broad antenna... easily matched and very cheap to build. TV
>twinlead is certainly fine at QRP levels and available everywhere.
>What could be easier?
Yes, it was broad banded. Mine was constructed from a good grade of 300
ohm twin lead and worked well at QRO levels, too.
>I made such a table a few years ago but I lost it. I can see how much
>you enjoy antenna building (hot stove and otherwise) and I thought you
>might enjoy providing this table to some of the younger folks who may
>never have heard of this 'trick'.
Yeah, that is some trick!
>By the way, those folded dipoles were fairly easy to tame with a matchbox
>so one cut for 80 would work pretty well on all bands. How about giving
>it a go, Cecil?
I may be wrong here, but I think (uh oh!) that you will find that due
to current distribution on the conductors of the antenna that it will
NOT work well on even harmonics (i.e. 40, 20, 10m). It MAY work fine
on 30m, though, in my case a ladder line fed dipole "seemed" to perform
better. From my experience with the 80m folded dipole, I would say it
it a very good, broad band antenna for the intended frequency, but a
poor choice for multi-band operation.
Maybe someone else can elaborate further on exactly why the current
distribution leads to this cancellation on even harmonics...
Maybe I'm wrong, too!
73, Brian
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