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

[Dennis]

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