Recently I have been looking in to the workings of NVIS or (Near Vertical Incident Skywave) operating. It is a very intriguing means of local communications on HF bands 7MHz and below and has to do with bouncing a signal of the F layer using an antenna with a vertical or near vertical polarization. 60-90 degrees radiation angle is classed as NVIS. The reason for this kind of operating is as previously mentioned, is to establish local communications, say some 200-300 miles in specific away from the station of origin.

Now some might say an antenna that has a high radiation angle is a complete waste of time as no great distance can be acheived with an angle so high. Well, if distance is what you want then you would be correct but the whole point of the antenna is not for DX work. If I wanted a DX antenna, then, the lower the angle is clearly better. Let me explain.

If I have a stone in my hand and i throw it in straight up in the air at a 90 degree angle then I had better duck, because that stone would fall very close to where I would be standing. If you picture the stone as a signal from your radio bouncing straight back to earth from the ionospere that would be classed as NVIS.

If however, you took the same stone and threw it up and outwards away from you, the stone would surely drop back down to earth some distance away from you. Now again imagine it as a radio signal. It would radiate from your antenna at an angle other than straight up. This means that it has to travel further before it reaches the f layer of the ionosphere. Once it hits the ionosphere it would be reflected back to earth at or around the same angle as it arrived. If you could measure the distance between the take off and landing of the signal (ground to ground) that is classed as the the "hop". Clearly both high and low angles have their pro's and con's in amateur radio. 

Eample: You have a friend who lives 20 miles away and you want to make a contact but have no line of sight. your antenna has a radiation angle of 20 degree's and his antenna was more or less the same. Question: Would you make the contact?. In reality the answer would be no. Even if you could. the contact would be of a poor quality. Plainly the wrong antenna was used, as the signal would have passed over your friend on its way to the ionosphere before it bounced back to ground some thousands of miles away.

Now try the same experiment with NVIS antennas and be astounded. Both antennas are TXing and RXing from the same angle of polarisation. A good strong signal good audio is received. The vertical take off means that the signal is been reflected down to ground in a circular fashion pushing out to some 300 miles away from origin meaning you friend would hear you. 

Now there are certain things you need to know before you string up an antenna. One such thing is the "Critical Frequency" This is to do with ionosandes. Everything boils down to electrons and other items to complex to be talking about here. In essence, during the day, you should be able to work NVIS between 2-10 MHz. Due to changes at night 2-5 MHz should be possible. You can only work below the critical frequency the reason for this is the F layer will only reflect signal back to earth below the frequencies mentioned this is partly due to the effect of a vertical signal on the ionosphere imagine a space ship on re-entry  the effect is less damaging for the ship if it enters at an angle.

So how do I make an NVIS antenna?. Well most people have them without realising. A simple dipole!. Now. Another thing to realize is that:

"A VERTICAL ANTENNA DOES NOT HAVE A VERTICAL RADIATION ANGLE"

The best vertical radiators are horizontaly polarised antenna's which are lowered closer to the ground. Preferably around .15 of a wavelength of the frequency of operation. There are two good reasons for this. One is that as you lower the antenna to ground, the radiation angle increases at around .15 of a wavelength the angle is modeled at 90 degree's. The other factor is that the closer to ground the better the antenna impedence will become. Then to top that the antenna will be less noisy due to the high angle not picking up radiation from other more common antenna's.

The antenna that I use at the moment is a 7 MHz dipole some 67 feet long in total or 33.5 feet per leg. This is at a height of 11 feet above ground. The gain between 2 like antenna's would be approx 6.4 DBi at the angle of elevation.  Please remember that a high to low radiation angle between 2 stations would result in a lower signal. Try it with a friend it's well worth an experiment.