[BULK] - [TowerTalk] Vertical Dipole for UHF

[Steve Katz]

The commercial 4-pole and 8-pole arrays (not including Cushcraft's product,
which is amateur and quite different) use folded dipoles of heliarc welded
aluminum construction, so there's no "hardware" (nuts & bolts, screws or
rivets) in the RF current path.  This is done to minimize noise and maximize
longetivity.  Lacking appropriate welding equipment and experience, the same
thing could be done with copper tubing and solder.  The Cushcraft AFM4D etc.
models use ordinary "Redi-Match" (gamma matched) tubing straight dipoles,
which in theory should work the same, but in practice really don't.  Nothing
magical about the stacking distances used: It's one wavelength,
center-to-center, between dipoles.  At 146 MHz, this is ~78", so a 4-pole
utilizes 3WL of mast, or 19-1/2 feet.  At 446 MHz, it's ~25-3/16", so a
4-pole utilizes 3WL of mast, or 6'3-1/2".  8-poles are obviously larger!
Stagger them at 90 degree intervals around the mast for an omni pattern
~5.2dBd for a 4-pole, or ~7.4dBd for an 8-pole.   After owning a DB Products
4-pole for 146 MHz for a few years, I copied one as a homebrew equivalent,
and it worked the same as (and as well as) the original, but the "copy" also
used heliarc welded aluminum tubing.  After all welding was completed, we
took each dipole assembly to a plating shop and had them hard clear
anodized, for about $10 per unit.  Far as I know, that one is still up,
after about 20 years!  -WB2WIK/6

"Success is the ability to go from failure to failure with no loss of
enthusiasm." -Winston Churchill

> -----Original Message-----
> From:	Rick Wheeler [SMTP:omalleyvalve at yahoo.com]
> Sent:	Monday, December 08, 2003 5:28 PM
> To:	towertalk at contesting.com
> Subject:	[BULK] - [TowerTalk] Vertical Dipole for UHF
> 
> Perhaps not totally on subject but this reflector does talk a lot about
> antennas. I currently have an operational UHF repeater and wish  to
> equip it with a 4 bay or better yet an 8 bay vertical dipole. Being a
> ham it would be out of the question to buy new commercial (about $500
> and $1,100 respectively). 
> 
> So I searched the Internet for plans to build such an antenna but could
> not find any. So I am wondering are they (vertical VHF and UHF dipoles)
> that difficult for the average ham (I do have a machine shop) to build
> or have I just not found the plans? I have built the j poles and such
> but I want a stacked dipole for the gain advantage.
> 
> Rick
> K4LX
> _______________________________________________
> 
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> Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with
> any questions and ask for Sherman, W2FLA.
> 
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