-=> On 09-27-97  06:19, Jeff Edmonson said to Ivy Iverson,<=-
-=>"About High Gain Antennas for 2M...,"<=-
 
Hi, Jeff;
 
 JE> For example, would there need to be a shorting bar across the
 JE> resonant point of the "J" connection for the  directors?
 
 II> That's an interesting question.  Perhaps a 50 Ohm resistor?  (Like
 II> terminating a rhombic - the terminating resister has to absorb one
 II> half of the transmitted power, but it's presence controls
 II> the directional characteristics of the antenna.)
 JE> Now, just where are you going to find a 750 watt, 50 ohm resistor? 
 JE> };->
 
There are such things.  They are used in some commercial BC XMTRs.  They
don't have wire leads, they plug into a socket that looks like a large
fuse holder.  The resister itself looks very much like the carbon rods
used in #6 dry cells except they have metal caps on the ends.  IIRC, they
are called Glowbars.
 
HOWEVER, I really doubt that you would need resistors which approach half
the dissapation of the XMTR output.  That's for a rhombic antenna, which
is a horse of a different color, (or antenna of a different cut).   :-}
If a resister is needed in a J-pole array, I doubt it would have to
dissipate more than a few percent of the applied RF.  I could, of course,
be wrong.
 
 II> OTOH, say if you are using 2-element colinear 5/8 wave J-poles in
 II> an array, wouldn't simple 5/8 wave elements, (insulated?), + 5% for
 II> reflectors, and - 5% for directors be effective?
 JE> Probably, and as you said, it might look better, than all the J-pole
 JE> assemblies up there, but you just -never- know until you try, right?
 
RIGHT!  If I had the finances and equipment to measure the results, I
would be experimenting with this.
 
 JE> And, if it DOESN'T work, you've got some extra J-poles to give/sell to
 JE> other  hams there in the area, or Driven Elements for later Beam
 JE> Projects ;-) 
 II> It would be interesting to see what kind of insulators would
 II> be used between elements if it's a horizontal array!
 II> (Don't say, "Wooden dowls!")   :-}
 
 JE> Fiberglass Dowels? };->
 
Fiberglass or possibly nylon might work.  I would suggest a thick wall
tube rather than a solid rod, especially if the antenna is horizontally
polarized.  (A rod bends, but a tube has more bending strength because it
goes into compression and tension modes.)
 
I wonder what the RF characteristics the new carbon fiber materials have?
They are very strong, and should be able to support the weight, even when
mounted horizontally.
 
A 3-element, horizontally polarized, Co-linear J-pole beam anyone?   :-?
(Actually 12 active elements.)
 
           "=" is insulators.
 
       ----===----===O===----===----  Directors (4 insulated elements)
                     O
       -----=-----|--O--|-----=----- Driven J-poles (4 elements)
                ~~~  O  ~~~
      ------=------==O==------=------ Reflector (4 insulated elements)
 
If each J-pole has a gain of 6dB, and the director & reflector elements
are the usual 1/4 wave + and - 5%.  This should make a potent, if large,
beam for 2M or above!  (Can you imagine building one for 160M?)   };->
 
Question: How would the driven elements be phased?  I would think that
they would be fed 180 degrees out of phase.
 
Comments anyone?
 
73 DE KB9QPM
   Ivy
 
 
 
... Spread The Spectrum!!!
 
         --
 
          ,-----> Ivy's WALL BBS Ä Home of Lakeshore Net <-----,
          |---------------> Ivy Iverson, KB9QPM <--------------|
          `-> Netmail me a request for info on Lakeshore Net <-'
 
___ Blue Wave/QWK v2.20 [NR]
--- TriToss (tm) 1.03 - (Unregistered)
---------------