>
> >Some ballpark calculations:
> >87 ft @ .72 vf = 19.10 meters free space equiv
> >87 ft @ .80 vf = 21.22 meters free space equiv
> >
> >lambda @ 3.625 MHz = 82.70 meters so the lengths of the two feedlines
are:
> >0.231 lambda and 0.257 lambda... neither is a quarter wave, if that's
what
> >you're shooting for.. (I note that the Comtek web page talks about using
75
> >ohm coax for the element feed lines, as well...) At 3.775 MHz, the
> >respective numbers are lambda=79.42 m, 0.240 lambda, 0.267 lambda...
>
>
>
> I think there's something backward in the math above. The lower the VF
the
> shorter a cable must be for a given free-space equivalent, right?
You're right.. everything's upside down (I had it the other way, it didn't
look right at first, so I flipped it.. too early in the morning..)
However, the conclusion is basically the same.. not a heck of a lot of
difference in lambdas, but given (see below) that you're using these as
tuned stubs....
>
> 87 feet x 100/72 = 120.8 feet or ~36.6 meters free space equivalent; 87
> feet x 100/80 = 108.75 feet or ~33 meters in free space. Or am I all wet?
You got it.. 36.84 m one way and 33.16 the other...
3.625 = 82.70 / .445 lambda /.401 lambda
3.775 = 79.42 /.464 / .418
>
> This array design uses 87 feet of standard RG8-X feedline on each
> dipole. The 3 feedlines not being driven are floated (both shield and
> center), presenting an inductive load in the center of their dipoles,
which
> makes them electrically longer, thereby functioning as reflectors. This
is
> not a 4-square, phased array a la Comtek. Modeled forward gain is under 6
> dB, with maximum F/B ~30 dB.
I see.. then, you almost certainly want to make them the right number of
wavelengths, especially since if you swing around the 1/4 wave point, you're
going turn from inductance to capacitance
A fairly substantial change in the R and L with the "wrong length" then...
Impedance of a stub 87 ft long
3.625 MHz, Z(.72) 12.93+139j, Z(.80)4.42+69.4j
3.775, Z(.72) 28.36+212.7j , Z(.80) 6.25+87.3j
>
> I have a fairly good model of this array, and also ran the various stub
> lengths through a transmission line calculator. This indicates that 87
> feet of the lower VF line is a considerably different load than 87 feet of
> the original, higher VF. In my EZNEC model, pattern isn't so much
affected
> as is forward gain -- the long .72 VF feedline seems to cost ~ 1 dB in
> forward gain, presumably doe to losses in the three open stubs.
>
> This is broadly consistent with my empirical experience. I don't seem to
> be quite as loud as I was before, but the F/B is still fairly
> impressive. S9 Europeans almost disappear into the noise when I turn the
> antenna Southwest.
Interesting..
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