"Jeff Liebermann" wrote in message
...
On Wed, 26 Nov 2008 10:50:37 -0800, "Thomas Magma"
wrote:

Thanks for all the good points, but you haven't scared me away yet

Good. I don't mind spending the time if you're willing to build it.
None of my objections are particularly fatal. However, I would
suggest you at least investigate alternatives, which in my opinion,
work better (and are easier to build). For example, the 4 bay stacked
vertical folded dipoles, with coaxial power dividers, is far less
complexicated, and methinks works better. I was building these for
463/468MHz in about 1968(?) out of strips of 1/2" wide aluminum and
pop rivets. If you're interested, I'll see if I can find some photos
or scribblings. There are a few in this photo:
http://802.11junk.com/jeffl/pics/Old%20Repeaters/slides/LoopMtn02.html
but I can't distinguish mine from the stock dB Products antennas.
Incidentally, that's a great example of how *NOT* to install antennas.
Those are all transmit antennas with no ferrite isolators. The
intermod generated was monumental.

Some more examples of commercial versions:
http://www.radiowrench.com/sonic/so02202.html
http://www.radiowrench.com/sonic/ (look for dB Products PDF's)

My
target frequency is around lets say 418MHz (that's not really it, I like
to
remain anonymous).

Y'er no fun.

It was interesting what you said about the radome and how
it detuned the antenna. Do you think it was mainly the PVC or the urethane
foam that caused the issue.

Both. I suspect you have a sweeper and some means of measuing
reflection coefficient or VSWR in real time (on a scope). If not, the
reflection coefficient bridge is easy to build. Take any antenna you
look at the VSWR curve on the scope. Then, shove the pipe over the
antenna and watch what happens. If the tubing were fiberglass, some
thin plastics, or glass, nothing will change on the scope. PVC and
ABC will detune the antenna. So will common fence post compound
(urathane foam) but to a lesser degree. Packing the empty space with
styrofoam or styroam peanuts seems to work well enough and result in a
repairable antenna. Real fiberglass tubing (masts or marine hardware)
is easy enough to obtain, that I wouldn't bother with PVC. Besides,
fiberglass is nice and stiff, while PVC flops around in the wind.

I plan to use a fibreglass tubing and spacers so
hopefully I don't see as much near field effects as you did.

Yep. There's always hope.

I have learned
that some PVC pipes have certain conductive additives and are not so good
for antenna use, plus it might be tough trying to sell a 'poop pipe'
antenna
commercially if it ever became a product of ours.

If you want to try a real disaster, try black drainage PVC pipe.
Carbon filled. For a good acid test, try putting a pipe section in a
microwave oven. If it stays cold, you win. If it gets hot, thing
again. If it melts and catches fire, forget it. It's also fun to
take an ordinary 440 yackie talkie or scanner, shove a piece of PVC
pipe over the antenna, and listen to the signal change. I like to do
this demo at radio club meetings. The best of the bunch is
fiberglass. A close 2nd is white ABS (acrylo-nitrile butadene
styrene) which is a bit difficult to find. It's commonly used in
vacuum forming and commonly found on GPS antennas and such.

Do you happen to know if I should be using a odd or even number of half
wave
elements in my design? I'm beginning to think it doesn't really matter.

It matters quite a bit. However, I can't offer an answer. Some
designes use the bottom section as a matching transformer or
counterpoise. That mangles the count. I would have to see what
you're doing to make the determination.

Also, I could probably figure it out, but it's midnight and I'm beat.
I spent 5 days last week fighting a kidney stone and am still kinda
wiped from that. If you can't figure it it, bug me and I'll do the
dirty work. A good clue is that the center conductor of the input
coax connector must connect to the center wire which goes to the 1/4
vertical whip section at the top.

Also, there are patents worth reading:
http://www.google.com/patents?id=JMweAAAAEBAJ&dq=6947006
http://www.google.com/patents?id=qDYWAAAAEBAJ&dq=6947006
http://www.google.com/patents?id=XpgfAAAAEBAJ&dq=6947006
etc. The accompanying explanations are usually sufficient to figure
out how it works. You might notice that one of the construction
methods is applicable to your copper tubing idea.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

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. So I hit the
hardware store and got some PVC pipe and mounting bits. 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? I plan
on using LMR-200 because of it's slight rigidity and it's high velocity
factor (83%). 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. 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. I still don't understand what that
quarter wave whip is suppose to do that sits on top of the array and I think
I will try to omit that in my first design (unless someone convinces me
otherwise).

Anyways, time to get my hands dirty and build me an antenna!

Thomas Magma