[TowerTalk] Bobtail question

[w8ji.tom]

Hi Bob, 

A trivia question!  It's not quite a Bobtail, but it still worked well.

> With all the talk about the bobtails, I have a quick question.
> At the VOA site (before it was dismantled) along I-75 north of
> Cincinnati OH, they had an antenna
> that was strung between 2 large towers.  It appeared to have supports
> strung from the top and bottom of the towers which supported a large
> number of vertical wires running between the supports, about like a
> curtain.  Was this a modified bobtails or something else?

That antenna is common at SW BC stations. It was popularized through use at
VOA stations, and even earned the name USIA Curtain  (US Information
Agency) or jokingly among a few people as the "CIA curtain". (It was
rumored when certain VOA songs were played in order, certain revolutions
would start.) 

It is nothing like a Bobtail, it was a series of collinear dipoles stacked
one above the other in multiple vertically stacked bays fed in phase
through equal length open wire lines that reached common points, that were
again coupled with open wire lines to more common points of other dipole
collinear stacks. The additional stacks are placed next to (collinear) and
above (broadside) each other.

To top it all off, they hang the array in front of a screen reflector and
pick up another 3 dB gain. 

Since all "cells" of the array are fed through equal length lines that
branch from common point, phase is ALWAYS correct no matter what frequency
is used. Arrays like this operate with VERY high line standing wave ratio,
but have excellent gain and efficiency over a very wide frequency range. By
changing phase a bit along the curtain, the pattern can be skewed off to
either side or different elevation angles. Sort of like a RF flashlight.

The gain of arrays like this can be 18 dBd and higher, depending on size.
The "small" curtain you saw at the Bethany relay station was dwarfed in
area by huge Rhombics at the site, but it produced more gain than the much
large and more famous Rhombics!!! 

This type antenna takes less area and produces more gain than any other
type of large array because it combines end fire (the reflector),
collinear, and broadside gain in a nearly optimum combination..and does so
over wide frequency ranges because of the open wire feeder and frequency
independent in-phase feed system.

Cushcraft once sold a two meter antenna like this, and I worked the east
coast on 2M SSB with mine from Ohio night after night. 
 
What an excellent engineered antenna! I can't wait til I get my 40 meter
and up model finished. ;-)

73 Tom

--
FAQ on WWW:               http://www.contesting.com/towertalkfaq.html
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm