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