On Thu, 27 Nov 2008 10:10:25 -0800, "Thomas Magma"
wrote:
I'm anxious to get started so I've put my copper pipe design on hold well I
wait for parts and decided to start with a coax approach.
Sigh.
So I hit the
hardware store and got some PVC pipe and mounting bits.
Schedule 40, schedule 80, water, or electrical? They're all
different. Did you at least do the microwave oven test on a small
piece to see if you're headed for a problem? I've had a few surprises
with different vendors and styles.
I understand that
the PVC is not as good as fiberglass because of it's near field effects, BTW
if you can tune those effects out, what is the end result in loss?
No. You can't make the radome (pipe) big enough to get out of the
near field. Minimum is a few wavelengths.
Try a chunk of PVC over your 440 HT or scanner whip antenna and see if
you want to continue blundering along this path.
I plan
on using LMR-200 because of it's slight rigidity and it's high velocity
factor (83%).
The added rigidity doesn't buy you much if you're going to shove it
down a pipe.
I bought 1-1/2 inch rubber washers with a 3/16 hole in the
center that will slide over the coax and then be pulled into the 1-1/4 inch
PVC this will center and support the coax up the length of the pipe.
Why such a large diameter pipe? There's no difference in loss.
I will
try using some clamp-on ferrites that we have laying around to stub the
currents on the feed line and slide them around and see if I can tune the
antenna using the network analyzer.
Got a ferrite that works at 418MHz? Even if the ferrite does work,
the RF its blocking is converted to heat. Wouldn't it be better if
you built a proper matching contrivance to that RF is radiated instead
of absorbed? I suggest you lose the ferrites and band-aids as they
tend to hide design errors and inefficiencies.
I still don't understand what that
quarter wave whip is suppose to do that sits on top of the array
I hate easy questions. If you look at the construction of the
alternating coax sections, the top section will be one with the hot RF
lead eventually connected to the outside of the top coax section. In
other words, the outside of the coax is the radiating element.
http://www.rason.org/Projects/collant/collant.htm
Why bother using another coax section when it would be easier to just
use a piece of wire? Look at the Fig 3 drawing and just follow the RF
path from the coax entry at the left to the 1/4 wave element on the
right. That might also answer your question about odd/even sections.
and I think
I will try to omit that in my first design (unless someone convinces me
otherwise).
Not recommended, but you have the test equipment to determine if it's
a good or bad idea. Ummm... you were planning on testing this thing?
Anyways, time to get my hands dirty and build me an antenna!
Good luck, but first a little math. What manner of tolerance do you
thing you need to cut your coax pieces? Let's pretend you wanted to
get the center frequency accurate to 1Mhz. At 418MHz, one wavelength
is:
wavelength(mm) = 300,000 / freq(mhz) * VF
wavelength = 3*10^5 / 418 * 0.83 = 596 mm
That works out to:
596 / 418 = 1.4 mm/MHz
So, if you want the center frequency accurate to within +/- 1MHz, you
gotta cut it to within +/- 1.4 mm. Good luck. Like I previously
ranted, you'll need a cutting fixture. A steady hand, good eye,
quality coax, and plenty of patience are also helpful.
Incidentally, since the top 1/4 wave element represents something
close to perhaps 50 ohms, it would be interesting to measure the
amount of RF that isn't radiated and actually gets to the top section
of the antenna. If my analysis of the antenna is correct, the first
section (near the coax connector) radiates 1/2 the power. The next
section 1/4th. After that 1/8th, etc. By the time it gets to the top
of the antenna, there won't be much left. However, that's theory,
which often fails to resemble reality. It would interesting if you
stuck a coax connector on the top, and measured what comes out.
Happy Day of the Turkeys.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558