[MRCA] Ft. Mott Ft. Miles Ground Wave Test was actaully NVIS

[mac]

All,

At our MRCG field days at Ft. MacArthur in San Pedro, CA,  we usually  
have participants operating from the USS Midway and elsewhere in the  
San Diego area, a distance of approximately 80 statute miles with the  
path all over open water.  No problem communicating reliably over that  
distance at low power levels if the stations at both ends are using  
antennas with a horizontal radiating component.  We use a 200 ft.  
inverted "L" at the Ft MacArthur end.  The USS Midway has a choice of  
vertical antennas and a large fan-like  array mounted high up on the  
superstructure which has both vertical and horizontal radiating  
components.  On 75 and 40M and with power output levels of 20 - 25  
watts on both ends of the circuit we have no trouble maintaining  
daytime communications with the MIdway when the ship is transmitting  
on the "fan" array but don't do nearly so well when they go to any of  
their verticals.  Not surprisingly, GRC-9s and other low powered sets  
operating out in the field near Ft. MacArthur with short whip antennas  
are NOT usually  heard in San Diego.  In Aug 2009, however, we  
experienced some kind of a propagation condition where all our  
GRC-9s,  SCR-284s, WS-19s and the like were able to make solid 2-way  
contacts with the Midway and another San Diego station over a period  
of several hours.  Haven't had a replay since, unfortunately.

Dennis D.  W7QHO
Glendale, CA

*****************

On Oct 26, 2011, at 10:16 AM, Dennis wrote:

> Breck, per your ground wave test, I suspect you might be in gave  
> error in the type of propagation you were using. First, it wasn't  
> ground wave. Second, while you dismissed the use of NVIS out of hand  
> because of the short distance, that is in fact what you were using,  
> & that would have been the preferred military application for an  
> NVIS system.
>
> First let me explain. NVIS, combined with inverse "wave  
> propagation", is a method use to communicate over distances to far  
> for use of VHF/UHF line of sight type equipment. I/E about 10-100  
> miles. Or to transmit signals over obstacles that would prevent  
> normal line of sight communications. Like a mountain. Read about the  
> inability to communicate 20 miles experienced by the British at  
> operation Market Garden & it's relation to NVIS experimentation.
>
> NVIS as you know is simply radiating a signal at a near vertical  
> angle causing it to bounce right back to earth much like an  
> umbrella. Wave propagation, greatly simplified, is that higher HF  
> frequencies go farther in the daylight hours, while at the same time  
> lower frequencies work shorter distances. The reverse is true when  
> the sun goes down.
>
> So, if a person wanted to transmit a signal only out to about 50  
> miles in the day time, he would choose a low one(75 or 60 mtrs),  
> then transmit that signal nearly straight up. This time of day  
> atmospheric absorption will only let you signal bounce once.
>
> Simply put, all of the antennas you listed in your test, were NVIS  
> antennas! Any horizonal wire is going to radiate a signal pretty  
> much straight up unless it is elevated at least 1/4 wave above  
> ground. On 75 meters, that's 60 feet.
>
> Had you been using vertical antennas, your ground wave experiment  
> would have been valid. Given a vertical antenna, on 75 meter, with a  
> power level of around 20 watts, your ground wave coverage might be  
> expected to be about 5-10% greater than line of sight, over water.
>
> In my youth I was involved in experimenting with Naval HF inter task  
> force communications. Normally VHF/UHF systems were used in this  
> capacity, but the Navy wanted something that was harder, or  
> impossible, for the enemy DF(directon find). We used the above NVIS/ 
> Inverse Wave Propagation technique, combined with lower power, to  
> link shipping in a task force strung out over about 125 miles of  
> ocean. It worked, but never really caught on.
>
> The low power transmitters we used were in fact URT-23's capable of  
> 1000watts. We installed an step attenuator between the exciter & amp  
> so they only putting out about 500mw.
>
> Today, NVIS is used an MANY HF communications applications other  
> than military. The oil industry, exploration, arctic & remote  
> communications(both US & Canada), CAP, telephone companies(as a back  
> up), National Guard armories are linked with NVIS stations as well  
> as Army Corps of Engineers offices(damns, rivers, etc). Nearly every  
> US government agency has a an HF communication back up plan that  
> depends on NVIS propagation to work.
>
> KB0SFP
>
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