Near Vertical Incident Skywave
How to make the most of localized ham radio contacts within one state during a QSO Party.
For those of you lucky enough to be in a state where the QSO Party rules allow county to county contacts, you will most certainly need to read this.
While the sun spots have been at their minimum, 40 and 80 meters have been about the only real choice for close in HF contacts. The following table shows my club’s activity during the 2007 Virginia QSO Party…
| Band | QSOs Total | Percentage |
|---|---|---|
| 0.23 | 1 | 0.0% |
| 0.70 | 110 | 2.1% |
| 1.25 | 17 | 0.3% |
| 2 | 652 | 12.4% |
| 6 | 67 | 1.3% |
| 10 | 58 | 1.1% |
| 15 | 46 | 0.9% |
| 20 | 933 | 17.7% |
| 40 | 1116 | 21.1% |
| 80 | 2253 | 42.7% |
| 160 | 25 | 0.5% |
80 meters was clearly the winner here and many of those contacts were made within the state of Virginia. Antenna systems need not be complex and, actually, there is an advantage of keeping the radiators low to the ground.
First, though, just what is NVIS communications? The web site http://www.athensarc.org/nvis.asp tells it best…
- Near-Vertical Incident Skywave
- A combination of radio hardware, skywave radio propagation, operating procedures, cooperation, and knowledge used by a group of radio operators who need reliable regional communications. It fills the gap between line-of-sight ground-wave and long-distance “skip” skywave communications.
Simulations can help visualize the nature of how antennas behave with respect to height above ground. Using EZNEC I created a typical 135 foot antenna arranged in an inverted V. Initial height of the apex is an impressive 80 feet. Here is a view of the antenna…
Here is the view of the antenna pattern along its maximum radiation lobes.
There is a lot of energy heading to the horizon. This is fantastic for DX work. However, to capitalize on the close in advantages of NVIS we need to send energy almost straight up to reflect off the ionosphere. Let’s take the same antenna and lower it to 40 feet at the apex and spread out the wires a bit so they are flatter…
Here is the radiation plot…
The 80 foot height pattern is in blue and our new NVIS pattern is in red. The antenna is a bit less efficient, but is sending more energy towards the ionosphere… just what we need to make NVIS communications practical.
You will find the lower you go with your antenna the less signals to and from the horizon will be present. This may yield some benefits in eliminating noise from distant sources like thunderstorms in another part of the country. However, the lower you go the less signal you will send straight up as the efficiency of the antenna suffers.
The ideal height varies, but a good rule of thumb would be as tall as you can go before the red trace above turns into the blue DX trace. Here is the same antenna with the apex 20 feet above ground…
The 20 foot high antenna puts a little more energy straight up. Finally let’s check arm height of 8 feet…
The antenna maintains a good shape for NVIS, but begins to lose efficiency. For 40 meters 20-40 feet is about right. This will also work just fine for 80 meters albeit with some less efficiency.
All that said, I did work the 2007 Virginia QSO Party out of my car with a 135 foot dipole tossed in low hanging branches about 6 feet up. It worked with QSO distances to my friends back home about 200 miles away.
Some folks try loops as well, but simulations suggest loops maintain a null overhead even when low to the ground. We will examine loop NVIS in another installment on this web site.
