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Old June 14th 04, 01:20 PM
Frank Gilliland
 
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In [email protected], Lancer
wrote:

snip
Wrong, the coil has very little radiation. The open air coil has less
loss because the losses are less in an open air coil.
(resistive losses, capacitive coupling losses, form loss)
Do a search on coil Q

Care to argue that point?



I would. What happens when you bring a solid sheet of conductive
material close to the end of a coil? Eddy currents -- it has
tremendous losses, the Q drops like a rock, and it's inductance is
unpredictable. It's a royal bitch to design a shielded IF/RF coil or
transformer to be used for high frequencies, and shielding is almost
-never- used for power RF coils and transformers unless there is some
serious space between the inductor and shield, hence the popularity of
toroid cores for those applications. And since a bigger coil makes a
bigger field, you need much more 'free space' to maintain a high-Q.
Vertically mounting a big coil above the sheet metal of a vehicle
results in a very lossy coil.



4) A stainless steel antenna is less efficient than a copper antenna
of the same length.


not if its electrically shorter than the stainless one


Electrically shorter? You meant physically shorter. I can take a 18
foot antenna and make it electrically shorter than 9 feet while still
keeping it 18 feet long. dumbass



The difference between copper and stainless steel is not very
significant at 27 MHz. What -is- significant is that bare copper will
quickly form a layer of corrosion (visible or not) which will render
an excellent conductor useless for RF. So unless you are willing to
polish and degrease the antenna every day, I'll take the stainless.






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Old June 14th 04, 01:23 PM
Lancer
 
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On Mon, 14 Jun 2004 05:20:37 -0700, Frank Gilliland
wrote:

In [email protected], Lancer
wrote:

snip
Wrong, the coil has very little radiation. The open air coil has less
loss because the losses are less in an open air coil.
(resistive losses, capacitive coupling losses, form loss)
Do a search on coil Q

Care to argue that point?



I would. What happens when you bring a solid sheet of conductive
material close to the end of a coil? Eddy currents -- it has
tremendous losses, the Q drops like a rock, and it's inductance is
unpredictable. It's a royal bitch to design a shielded IF/RF coil or
transformer to be used for high frequencies, and shielding is almost
-never- used for power RF coils and transformers unless there is some
serious space between the inductor and shield, hence the popularity of
toroid cores for those applications. And since a bigger coil makes a
bigger field, you need much more 'free space' to maintain a high-Q.
Vertically mounting a big coil above the sheet metal of a vehicle
results in a very lossy coil.


I would think that 4 feet would be more than enough distance.
So you would say that a large air wound coil has more loss than a
small diameter coild wound on a solid form?




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Old June 14th 04, 01:44 PM
Frank Gilliland
 
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In , Lancer
wrote:

On Mon, 14 Jun 2004 05:20:37 -0700, Frank Gilliland
wrote:

In p[email protected], Lancer
wrote:

snip
Wrong, the coil has very little radiation. The open air coil has less
loss because the losses are less in an open air coil.
(resistive losses, capacitive coupling losses, form loss)
Do a search on coil Q

Care to argue that point?



I would. What happens when you bring a solid sheet of conductive
material close to the end of a coil? Eddy currents -- it has
tremendous losses, the Q drops like a rock, and it's inductance is
unpredictable. It's a royal bitch to design a shielded IF/RF coil or
transformer to be used for high frequencies, and shielding is almost
-never- used for power RF coils and transformers unless there is some
serious space between the inductor and shield, hence the popularity of
toroid cores for those applications. And since a bigger coil makes a
bigger field, you need much more 'free space' to maintain a high-Q.
Vertically mounting a big coil above the sheet metal of a vehicle
results in a very lossy coil.


I would think that 4 feet would be more than enough distance.



It would depend on the diameter of the coil. I remember there is an
equation somewhere to determine the loss, but it involves some heavy
calculus and it's too early in the morning for integrals.


So you would say that a large air wound coil has more loss than a
small diameter coild wound on a solid form?



No, not necessarily. If the wire diameter is the same, the smaller
coil might easily be more efficient due to lower DC resistance.
Distributed capacitance isn't much of an issue at 27 MHz unless you
have lots of close-wound turns, and even then it's not necessarily a
loss but a factor that affects the coil's reactance. Regardless, there
are very effective winding techniques to reduce that problem. There
are many low-loss coil form materials available, such as Teflon, some
of the better quality ceramics, and even grooved hardwood makes a
decent core at RF frequencies (however, PVC sucks above a couple
hundred kHz). There is also the issue of weather conditions that can
significantly affect an open coil regardless of whether or not the
wire is insulated -- rain/humidity, snow, air pollutants (ozone can
make a -=BIG=- difference!), etc. Even the speed of the vehicle can
have an effect on the properties of an open coil. For the sake of
consistency and longevity, a smaller coil that is sealed from the
elements (as a whole) is a far better choice for anyone serious about
the issue.






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  #28   Report Post  
Old June 14th 04, 03:00 PM
Lancer
 
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On Mon, 14 Jun 2004 08:14:58 -0500, itoldyouiamnotiamnotgeorge
wrote:

Lancer wrote in news:[email protected]
4ax.com:

I would think that 4 feet would be more than enough distance.
So you would say that a large air wound coil has more loss than a
small diameter coild wound on a solid form?


There are no large coil cb antennas where the coil is 4' from the vehicle
roof, they are all mounted 18" or less.


Bug Catchers aren't.. so all isn't correct.
  #29   Report Post  
Old June 14th 04, 03:02 PM
Chris
 
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Replacing the lower section with pvc for flexibility. Sorry this thread is
hard to follow....too much off topic content.

Chris
"Lancer" wrote in message
news:[email protected] ...
On Sat, 12 Jun 2004 23:52:38 GMT, "Chris"
wrote:

Thanks. I just wanted to prove to myself that I could make my own antenna
and see how well it worked. There are 2 reasons for the large coil. In
comparing a few antennas of roughly equal length, the ones with large

open
coils tend to be more broad-banded. Larger materials also have less
resistive loss and more radiating are. The lower part doesn't really have

to
be 1/2' copper pipe but it was the first thing I thought of. I'm going

with
copper wire wrapped on pvc shortly.

Chris


Chris; Nice looking antenna, large open air coils are more efficient.
Some people see a large open air coil and think its all about power
handling capability. Are you planning on replacing the coil with pvc
wrapped with wire, or the lower section? If you want to beef up the
lower secton you might take a trip to a hamfest and pick up a lower
section of a Newtronics Hustler.



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Old June 14th 04, 03:24 PM
Lancer
 
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On Mon, 14 Jun 2004 14:02:15 GMT, "Chris"
wrote:

Replacing the lower section with pvc for flexibility. Sorry this thread is
hard to follow....too much off topic content.


Chris; yes there is, and for that I apologize.

PVC gets very brittle when exposed to sun or cold. You might need to
think about hooking some kind of line to it keep from losing the top
of the antenna in case it breaks.


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