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Re: [TowerTalk] Name this antenna

To: steve d <>
Subject: Re: [TowerTalk] Name this antenna
From: Eric Scace K3NA <>
Date: Thu, 25 Oct 2007 20:01:05 -0400
List-post: <>
That is the Leesburg FAA control center.  I believe that it is a back-up 
HF facility to talk with FEMA HF facilities, other control centers, and 
possibly for aeronautical HF communications as well.  Those antennas 
went in around 1980 or so.

The antenna is a HyGain rotatable log periodic that covers 4-30 MHz.  It 
is a design that has been sold to customers for over 30 years.  The 
vertical pipe between the two facing towers is both the transmission 
line and the drive pipe for rotating the antenna.  The rotator is at the 
base of the tower.  The antenna can spin anywhere with no stops; a 
rotary joint connects the coax coming out of the ground to the tube 
going up in the air.  That tube has to be kept pressurized with dry air 
or nitrogen to keep moisture out of it.

I happened to get to use one of those antennas for a number of DX 
contests in the late 1970s.  The one I used was installed on top of the 
National Weather Service headquarters building on the border between 
Silver Spring MD and Washington DC.  That was a 17-story building on a 
ridge top, so the antenna was way up there.  I was completely new to DX 
contesting, but there was a Collins HF station on the top floor with a 
kilowatt amp, so I gave it a spin for a year or two (lacking any station 
of my own).  It worked pretty well on 80m!  But, as impressive as it 
looks, it's basically equivalent to a 3-el Yagi on any one frequency.

Eventually the Weather Service scrapped the antenna.  The asymmetric 
wind loading (boom sticks out a lot more in the front than to the rear) 
put a lot of torque on the rotator.  With a full height of tower, some 
of that torque gets taken up in the six-inch diameter drive pipe/coax 
line.  But with the antenna on top of a building, there was only about 
15 ft of drive pipe, so almost all the torque got transfered to the 
chain drive that was clamped on the bottom of the pipe.  The clamp kept 
slipping and scoring the pipe... which needed a welder to come in and 
refill some of the lost metal.  The costs of keeping the system charged 
with dry air were another recurring expense.  It wasn't used often 
enough to justify the maintenance cost... so one day W3NRS's company 
lifted it off the building onto a nearby parking garage, cut it up, and 
hauled it to the scrap yard.

The Weather Service's HF station had a club callsign, too: W3KWB 
(Kilowatt Weather Bureau).

   -- Eric K3NA

on 07 Sep 01 Sat 18:36 steve d said the following:
> I drive a truck part time and came across this 
> ( 
> antenna on Rt. 227 in Leesburg, VA on my trip the past couple days. Does 
> anyone know whos it is and what it is for??? It is a log periodic and is the 
> largest rotatable antenna I have seen with my own eyes. The main boom for the 
> antenna is made out of triangular tower sectios. It appears to be about 70 - 
> 100' in length, from the perspective I had on it. 
>   The funny thing is.. I looked it up on google maps. You can actually see 
> the antenna and the shadow it, and the tower it is mounted to, cast on the 
> land HI. Pretty darn big! (I dont see the two other utility towers in the sat 
> view, they must be new). 
>   73,
>   Steve, KC8QVO
> ---------------------------------
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