[Milsurplus] NVIS again

Al Klase ark at ar88.net
Tue May 15 14:54:17 EDT 2012


Gang,

Originally, the primary role for low-HF radios like the GRC-9 was for 
medium-range comms, direct wave out to about 10-15 miles, using a short 
(10 ft) whip.  Wire antennas were also provided for situations where 
skywave operation was needed.  By the Vietnam era, this role was assumed 
by VHF FM, a generally better solution.  However, when there was five 
miles of wet triple-canopy rain forest between stations, let alone a 
mountain, it was found that the old HF sets worked better.

We've had a fair amount of first hand experience with NVIS with the MRCA 
in general and the Moose & Squirrel net in particular. 
http://www.mrca.ar88.net/  The basic requirements are that you choose a 
frequency that supports near-vertical reflections from the ionosphere, 
that is less than the prevailing F0F2, and use an appropriate antenna.

There's good info here for choosing the freq.: 
http://www.ips.gov.au/HF_Systems/6/5

Horizontal half-wave wire antennas are the way to go.  The problems with 
a short whip are almost zero vertical radiation, plus a lot less overall 
radiation than a full-size antenna.
Check it out: http://www.mrca.ar88.net/Net/Images/Dipole%20vs.%20Whip.png

The military has a number of broadband wire antennas that allow 
frequency agility, but they are compromises, and the good old dipole or 
end-fed half wave are a better deal for amateur putposes, especially 
when output power is limited to a few watts.

Apologies for the sermon,
Al

On 5/15/2012 8:11 AM, C. Whitaker wrote:
> de WB2CPN
> It was the Viet Nam war that I first heard of NVIS, and I been in
> USAF radio communications since WWII.  One favorite HF antenna
> for almost every use was the Delta Match half wave dipole.  We
> never had fancy names then, most of our technology came from
> the American Radio Relay League Handbook, and a few others.
> Sig Corps had some very good books on How&  Why to do things.
> An excellent Manual was all about HF Rhombics.  It had all the
> math for building them, and aiming them.
> So along comes the Nam war and someone invented a name,
> NVIS, for what had been around for a long time.
> 73
> Clete
> ++++++++++++++++++++++++++++++++++++++++++++++++++++=
>
>
>
> On 5/15/2012 2:40 AM, Hue Miller wrote:
>> They didn't categorize it as NVIS, but every WW2 mobile radio with a wire
>> antenna option, including the
>> W.S. No.19, operated using this propagation feature WHEN USING WIRE ANTENNA
>> and not the standard
>> vertical whip. They simply didn't have masts tall enuff to raise the wire
>> antenna an appreciable fraction
>> of a wavelength, at those frequencies.  Actually, it occurs to me that with
>> a short whip, there is lots of
>> high angle radiation too, so I'd suppose even the whip transmitted a mixture
>> of modes, ground wave
>> and NVIS. It comes to mind, that I'd read in a German manual for a walkie
>> talkie type, I think operating
>> at low HF, that the user could expect LOWERED distance at night.
>> -Hue
>>
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-- 
Al Klase - N3FRQ
Jersey City, NJ
http://www.skywaves.ar88.net/



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