[TowerTalk] Fwd: [Antennas] cloud warmer

Jepilot@aol.com Jepilot@aol.com
Thu, 19 Nov 1998 10:17:47 EST

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Date: Wed, 18 Nov 1998 14:50:26 EST
To: artm@t-com.com, antennas@qth.net
Subject: Re: [Antennas] cloud warmer
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In a message dated 98-11-18 07:07:18 EST, artm@t-com.com writes:

<< I have been hearing abt a cloud warmer antenna being used on HF, and info
please tks  art w1fji >>

Art and the Antennas List:

A "Cloud Warmer" is ham or HF comms slang for any HF antenna which provides
good medium- and high-angle radiation.
Some people ask why use high-angle...don't all HF communications require low-
angle "DX" antennas? 
High-angle antennas used in 1-8 or 1-12 MHz provide better "local" coverage
within a 300 KM (approximate) radius.  On the other hand, a low-angle or "DX"
antenna such as a vertical may produce skip zones- areas where there is little
or no signal within the desired radius.  

The 1-8 or 1-12 MHz spectrum is more appropriate for high-angle communications
since the ionosphere will reflect these frequencies even at very high angles.
It is not uncommon for ham, military and other HF users to string up a high-
angle antenna and communicate out of gullies, canyons and other poor terrain
where VHF or traditional DX antennas and higher HF band would not work.
The military calls these antennas and practices NVIS, Near Vertical Incidence

Most hams don't realize it, but they may already have an NVIS antenna.
A typical ham dipole for the 2, 4, 7 or 10 MHz band(s) is usually mounted less
than 1/4 wavelength above ground or onjects which form a counterpoise.  All
you need for NVIS or cloud-warmer communications is for your antenna to be 1/4
wavelength or less above ground.  
If you designed a multiband antenna for 2-10 MHz NVIS, it would  have to be no
more than 1/4 wavelength above ground on the highest frequency (10MHz); about
23 feet.  There would be a slight drop in efficiency on lower frequencies
since the antenna is closer to ground, but the antenna will perform.  Ham,
military and other communicators have used end- and center-fed wire antennas
for NVIS communications just a few feet above ground in emergencies.  

Military NVIS antennas are single-frequency dipoles with coax feed,  T2FD
types, or crossed dipoles with different leg lengths and fed with a coupler
(automatic antenna tuner).   L-shaped wires and random wires have also been
used for military and commercial NVIS work, again the rule is keep the antenna
1/4 wavelength or less above ground or counterpoise.

In mobile communications, military units have had some good results by tilting
over their 16, 24 or 32 foot HF whips to 45 near-horizontal.  Note that
military mobile whip antennas have couplers (automatic antenna tuners) at
their feedpoints.  Amateurs have had some results by tilting their 2, 4 or 7
MHz whips at a 45 degree angle.  Those amateurs with antenna couplers (as is
my case) are able to park, remove the HF whip antenna and attach a 50 to 200
foot wire at the antenna ball mount.  The far end of the wire is tied off to
any convenient support between 7 and 30 feet high.

Some experimenters have claimed improvements by laying a counterpoise wire
under the horizontal portion of their NVIS antenna, whether it is end-,
center- or coax-fed.

An added bonus of end- and center-fed NVIS allband antennas is that they work
above the high frequency limit of NVIS; providing medium to low angle
communications as frequency increases above 10 MHz.

For field and emergency allband use, I cannot recommend coax-fed dipoles.
These are restricted to a few Khz on a single band, and may not work as well
when set up in a different site from the one the dipole was pruned for (close
to ground, folded, etc).  
Instead, use an end- or center-fed wire antenna with legs of at least 1/4 wave
on the lowest frequency to be used.  Feed center-fed antennas with balanced TV
line or 450 ohm line for higher powers, and use a tuner with a good balun.
Some hams have used automatic couplers with baluns added.  End-fed antennas
and their counterpoise may be fed with a simple L-network manual tuner, or a
marine/military/ham coupler designed for wire and whip antennas.

Keep in mind that in many emergency, tactical or other situations requiring
300 KM radius communications, DX mindset and antennas won't do the job.  2-10
Mhz and NVIS antennas may be more appropriate.

Jay Coote

Submissions: antennas@qth.net


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