...OK, I have some nice brand new, manufactured in 1997, CATV hardline that I
want to use to feed the new 20 meter beams....my question....what should I
anticipate as a velocity factor? I will try and cut it so that it is (a 75
ohm) half wave multiple on 14.025.....making its 75 ohmedness transparent.
In order to know roughly how long to cut the coax, I need a starting point
to cut a little long, so that I can cut a little back....then a little
more.......then a little more....and then
OH SH** TOO MUCH!
I will aim for 14 MHz as my target when cutting so that a piece a little too
short will hopefully be at about 14.025, eh?
The CATV line will actually be barreled into some RG11 at the shack end so it
is flex-ie for attachment to either the ground panel or the coax switch 20
meter position.
The coax is only going to be used for 20....so it is ok for it to be one
length only...
The ARRL Antenna Book says VF of 1/2" Aluminum Jacket Foam Dielectric coax is
.81.....my question to you all is:
Have you recently "started" where I am starting, and did you find that the
.81 was a good number? If you have recently gone through this exercise I
would appreciate your input....especially if you found that what really
happened was it ended up 10 foot too short, etc.
Taking it another step....please correct me if I am wrong
The equation (I believe) for converting a frequency to its electrical length
is:
One w/l in Feet = 983.6/F(MHz) * VF
so, for me: One w/l = 983.6/14.025*.81 ....or.... 56.8 feet for one w/l of
the hardline....
I think.....
THEREFORE,
If I am shooting for a multiple of a half wavelength to make the line's
impedance a non-issue I should shoot for a multiple of half the above, a
multiple of 28.4 ft....
Am I on the right wavelength? (couldn't resist)
Since it is about 140 feet from the shack wall to the switchbox for the 20
metre antennae, I could follow this logic, but it may be flawed (this is
where you come in)....
I will aim for overall run of hardline that is four times 28.4, or 113.6
feet.....then
NEXT QUESTION
Since it will then barrel into some flexable 75 Ohm coax, RG11, which has a
different velocity factor.....should I aim to make the tail of RG11 simply a
multiple of 1/2 wavelength.....or
983.6/14.025*.66 (RG11 has a .66VF) = 46.287 feet........
This means that physically I would have 113.6 feet of hardline plus 46.287
feet of RG11 or 159.887 feet of overall feedline length......
----> OR <------
Do I need to worry about each of the types of feedline having their own
lengths in terms of multiples of 1/2 wavelength at their own velocity
factors, OR
Should I just make a hard line run that goes from the switch box at the tower
to the wall-through, no matter what that length is (roughly 140) and just
make my flex piece of RG11 my tuning adjustment for the line as a
whole......accepting it as simply the next 1/2 wavelength longer than the
hardline was.....
I would like to be able to simply mechanically cut the hardline to the length
it takes to go from the switchbox to the shack wall, without looking at v/f
as an issue. I do not know if the RG11 tail on it, which will have a
different velocity factor, mandates the hardline be a multiple of 1/2
wavelngth.....do you get my drift?
If I can simply cut the hardline to a physical length, I can then use the
flex tail as a tuning element inside the shack.....it will be the right
length to make the whole thang look like a half wave at 20 meters....but,
does that flex piece change the behaviour of the .81 v/f hardline....or
should I indeed look for it to be around what the v/f calculations say?
I want this feedline to seem like it was 50 ohms, like the rest of the system
will be....
Thanking you all very much, I will summarize your input to the towertalk
reflector.
de - an appreciative QCAO
Jim, K4OJ
k4oj@aol.com
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search
|